Geologic Map of the Breckenridge Quadrangle, Summit...

OPEN-FILE REPORT 02-7

Geologic Map of the Breckenridge Quadrangle,Summit and Park Counties, Colorado

ByC.A. Wallace, John W. Keller, James P. McCalpin, Paul J. Bartos, Erik E. Route,

Natalie N. Jones, Francisco Gutierrez, Cindy L. Williams and Matthew L. Morgan

Colorado Geological SurveyDivision of Minerals and GeologyDepartment of Natural Resources

State of ColoradoDenver, Colorado / 2003

Bill Owens, Governor, State of Colorado

Russell George, Executive Director, Department of Natural Resources

Ronald W. Cattany, Director, Division of Minerals and Geology

Vince Matthews, State Geologist Colorado Geological Survey

OPEN-FILE REPORT 02-7

Geologic Map of the Breckenridge Quadrangle,Summit and Park Counties, Colorado

ByC.A. Wallace1, John W. Keller2, James P. McCalpin3, Paul J. Bartos4, Erik E. Route5,Natalie N. Jones5, Francisco Gutierrez6, Cindy L. Williams7 and Matthew L. Morgan2

1Wallace Consulting, LLC, Morrison, CO

2Colorado Geological Survey, Denver, CO

3Geo-Haz Consulting, Inc., Crestone, CO

4Colorado School of Mines, Geology Museum, Golden, CO

5Geology Department, Adams State College, Alamosa, CO

6Departamento de Cienciasdeta Tierra, Facultad de Ciencias,

Universidad de Zaragosa, Zaragosa, Spain7

Newmont Mining Company, Denver, CO

Colorado Geological SurveyDivision of Minerals and GeologyDepartment of Natural Resources

State of ColoradoDenver, Colorado / 2003

Bill Owens, Governor, State of Colorado

Russell George, Executive Director, Department of Natural Resources

Ronald W. Cattany, Director, Division of Minerals and Geology

Vince Matthews, State Geologist Colorado Geological Survey

The Colorado Department of Natural Resources ispleased to present the Colorado GeologicalSurvey Open File Report 02-7, Geologic Map of theBreckenridge Quadrangle, Summit and Park Counties,Colorado. Its purpose is to describe the geologicsetting, mineral resource potential, and geologichazards of this 7.5-minute quadrangle located inthe mountains of north-central Colorado. Fieldwork for the mapping project was led by C.A.Wallace, consultant, John Keller, CGS staff geolo-gist, Jim McCalpin, consultant, and Paul Bartos,consultant, and was completed during thesummer and early fall of 2001.

This mapping project was funded jointly by theU.S. Geological Survey through the STATEMAP

component of the National Cooperative GeologicMapping Program which is authorized by theNational Geologic Mapping Act of 1997, Agree-ment No. 00HQAG0119, and the Colorado Geolog-ical Survey(CGS) using the Colorado Departmentof Natural Resources Severance Tax OperationalFunds. The CGS matching funds come from theSeverance Tax paid on the production of naturalgas, oil, coal, and metals.

Vince MatthewsState Geologist

Ronald W. CattanyDirector, Division of Minerals and Geology

iii

FOREWORD

iv

CONTENTS

FOREWORD............................................................................iiiACKNOWLEDGEMENTS.........................................................vINTRODUCTION ......................................................................1PREVIOUS STUDIES ..............................................................3PRESENT STUDY ...................................................................5STRATIGRAPHY......................................................................6

Proterozoic Rocks..........................................................6Paleozoic Rocks ..........................................................10Mesozoic Rocks...........................................................16Cenozoic Rocks and Deposits.....................................19Quaternary Deposits....................................................22

STRUCTURE..........................................................................27Structural Setting .........................................................27Faults ...........................................................................27Folds ............................................................................29Interpretation of Structures ..........................................30Structural Evolution......................................................31

ECONOMIC GEOLOGY.........................................................32Breckenridge Mining District ........................................32Other Mines and Prospects .........................................34

GEOLOGIC HAZARDS ..........................................................39Landslides and Rock Slides ........................................39Floods and Debris Flows .............................................39Abandoned Mines and Placer Deposits ......................39Seismicity.....................................................................40

REFERENCES .......................................................................41

We are indebted to the staff at Breckenridge SkiCorp. for providing access to parts of Brecken-ridge Ski Area. Rick Sramek, general manager ofthe ski area, was particularly helpful in securingaccess for this project. The staff at the U.S. ForestService office in Silverthorne provided access toareas within the Arapaho National Forest thatwere closed for administrative reasons. JamieConnell, District Ranger for the Dillon District,provided permission for special access to a closedportion of the Forest. C.L. Williams and J.B.Turner assisted in Asarco’s exploration effort inthe Breckenridge quadrangle. Permission to

release data was generously granted by Asarco,Incorporated. Asarco is now a division of GrupoMexico. Karl Kellogg of the U.S. GeologicalSurvey provided a beneficial geological tour ofthe Frisco quadrangle at the beginning of themapping season. Helpful technical reviews byVince Matthews (Colorado Geological Survey)and Karl S. Kellogg (U.S. Geological Survey)improved the map and report. Jane Ciener editedthe text, map, and cross sections. Karen Morganand Jason Wilson prepared the GIS data andCheryl Brchan completed the digital cartographyof the map and cross sections.

v

ACKNOWLEDGMENTS

The Breckenridge quadrangle is located in south-central Summit County and northwestern ParkCounty, Colorado, mostly west of the ContinentalDivide (Fig. 1). The quadrangle includes nume-rous peaks that have elevations of 13,000 to morethan 14,000 ft. The Breckenridge ski area is in thenorthern part of the intensely glaciated TenmileRange. Quandary Peak, at 14,265 ft above sealevel, in the southwestern quadrant of the maparea, is a popular peak for hikers and climbers.The town of Breckenridge is located in the north-ern part of the quadrangle. The Blue River flowsnorth through the middle of the map area in aglacial valley. The Breckenridge area has a long

history of mining and mineral processing; lodemines produced gold, silver, zinc, lead, andcopper, and placer mines produced large amountsof gold. Nearly $300 million worth of metal wereproduced from the Breckenridge mining district atapproximate current metal prices.

Access to the quadrangle from the north is viaI-70 and State Highway 9 south through Frisco toBreckenridge. From the south, U.S. Highway 285provides access to State Highway 9 to the north viaAlma and Hoosier Pass.

The oldest rocks are exposed in the westernpart of the quadrangle where Early and MiddleProterozoic metamorphic and igneous rocks are

1

INTRODUCTION

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C O L O R A D O

BreckenridgeQuadrangle

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Map Area and Reference

Georgetown quadrangle (Widmann and Miersemann, 2001)Dillon quadrangle (Kellogg, 2002)Frisco quadrangle (Kellog and others, 2002)Keystone quadrangle (Widmann and others, 2003)Montezuma 15-minute quadrangle (Lovering, 1935)Breckenridge quadrangle (this publication)Mt. Lincoln 15-minute quadrangle (Tweto, 1974)Minturn 15-minute quadrangle (Tweto and Lovering, 1977)SW1/4 of Dillon 15-minute quadrangle (Bergendahl, 1969)Vail West quadrangle (Scott & Others, 2002)Copper Mountain quadrangle (Widmann and others, 2003)Vail East quadrangle (Kellogg and others, in preparation)

This map CGS 1:24,000 scale map

USGS 1:24,000 scale map USGS 1:62,500 scale map

Explanation

Figure 1. Location map and index of selected published geologic maps in the vicinity of the Breckenridgequadrangle. Inset map shows the location of the Breckenridge quadrangle within the Colorado MineralBelt.

widespread. Proterozoic rocks are overlain uncon-formably by Paleozoic clastic and carbonate sedi-mentary rocks, which are exposed mainly in theeastern part of the map area. Mesozoic rocks occurmainly in the northeastern part of the map areaand are composed of shale and subordinate lime-stone and sandstone. Intermediate and siliceousTertiary sills and dikes intrude Paleozoic andMesozoic sedimentary rocks, and lode and dissem-inated mineral deposits are commonly associatedwith these intrusive bodies. In the western part ofthe map area, Proterozoic rocks are intruded by aTertiary stock and numerous Tertiary dikes.Quaternary deposits are common in valleys andare dominated by till, outwash, and lacustrinedeposits. The topography of the Tenmile Rangewas sculpted by at least two alpine glacial events.

Northwest- and north-trending faults and shearzones that cut Proterozoic through Tertiary rocks,and in some cases Quaternary deposits, are thedominant structural feature of the Breckenridgequadrangle. At least two of these northwest-trending faults, in the western and northern partsof the map area, have large vertical separation.Many other faults in Phanerozoic rocks form amosaic of intersecting faults of small separation. Aless obvious but important east-northeast-trendingzone of faults, dikes, and fracturing in the vicinityof the Humbug stock in the Tenmile Range likelyrepresents a structural expression of the ColoradoMineral Belt and may have been active periodicallysince Proterozoic time. This zone appears to becontinuous with several faults of similar trend inthe Illinois Gulch area in the Breckenridge miningdistrict east of the Blue River. Most faults east ofthe Blue River trend northeast, with a subordinatenumber of faults that trend northwest. Lower Pale-ozoic rocks rest unconformably over the Protero-zoic basement and are exposed on east-facing dip-slopes on the east side of the Tenmile Range onlyin the southern half of the quadrangle. In theeastern part of the map area the Upper Paleozoic

and Mesozoic sequences are generally homoclinal,dipping eastward, except where small fault blockshave locally rotated beds to other orientations.Prominent folds of regional scale occur in Paleozoicrocks in the Blue River Valley; these folds arecovered by Quaternary deposits. Metamorphicrocks in the Tenmile Range exhibit strong foliationthat has a well-defined and consistent north-north-east orientation.

The Breckenridge area has a well-documentedhistory of placer and lode mining. Placer mineswere active for one hundred years, from 1859 to1959, with the greatest production occurringduring the period 1906 to 1924 (Parker, 1974). Lodemining commenced in 1869 and eventually 25 to 30mines produced ore of at least 1000 tons. TheWellington Mine, in French Gulch, was the mostprolific producer, and gold, silver, lead and zincwere the principal metals extracted. None of themetal mines in the quadrangle are currently active.

Analysis of Quaternary deposits has identifiedfour categories of geologic hazards in the Brecken-ridge quadrangle: (1) landslides and rock slides; (2)floods and debris flows; (3) abandoned mines andplacer deposits; and (4) seismicity. Landslides arewidespread in the map area, most commonly in tilland in glaciated cirques. Rock falls and rock slidesare common in rugged valleys of the TenmileRange. Areas of incipient slope failure are identi-fied by “sackung” structures, which are deep-seated tension gashes in bedrock caused by gravi-tational spreading on high ridges and mountains(Varnes and others, 1989). Floods are local hazardsof known extent in the map area, but debris-flowhazards can occur in ephemeral streams or onHolocene alluvial fans. Abandoned mines pose ahazard from collapse, and areas that were dredgedcan form cavities at the surface. Mine water alsoposes a hazard because this water is usually acidicand can carry toxic elements. If it percolates intosoil it may be corrosive to metal and concrete infoundations.

2

Early geologic studies by the U.S. GeologicalSurvey and by the Colorado Geological Survey inthe Breckenridge area were mainly related todescription and analysis of mineral depositsduring the period that mining became the prin-cipal economic force in Summit County.

Regional studies of mining districts in the late1800’s and early 1900’s established the stratigraphicand structural framework in which mineraldeposits occurred. Emmons (1886) described rockunits, structure, and mineral deposits of theLeadville district, located to the southwest of thismap area (Fig. 2). Patton and others (1912)described the Alma mining district, and studies inthe Alma district were continued by Singewald andButler (1930, 1941). Singewald (1942, 1951)continued to study geology and mineral depositsof the Boreas Pass region and the upper reaches ofthe Blue River, as did Butler (1941). Bergendahl andKoschmann (1971) described the geology andmineral deposits of the Kokomo district to the westof the Breckenridge quadrangle. Mulienberg (1925)reported on the geology and mineral deposits ofthe Tarryall mining district.

Several studies concentrated on the Brecken-ridge quadrangle and adjacent areas. Ransome(1911) described the geology and ore deposits of

the Breckenridge mining district, and Lovering(1934) followed that work with a more detailedanalysis of rock units, structure, and mineraldeposits of the Breckenridge district. Taranik (1974)produced a Ph.D. dissertation at the ColoradoSchool of Mines that presented results of extensivegeologic mapping, regional correlation, sedimento-logic studies, and structural analysis of a large areathat includes the Breckenridge quadrangle.Taranik’s work provided a foundation for ourquadrangle map, and we incorporated some datafrom his dissertation on our map. De Voto (1980)discussed in detail the stratigraphic and structuralrelations in the area of the Breckenridge quad-rangle and established the regional framework forcorrelating these complex rock units. Bergendahl(1963) mapped the northern part of the TenmileRange at a scale of 1:24,000. This map and reportincluded detailed descriptions of the Proterozoicrock units of the Tenmile Range. Tweto (1974)published a reconnaissance geologic map at1:62,500 scale of the Mt. Lincoln quadrangle, thenortheast quadrant of which included the Brecken-ridge 1:24,000-scale quadrangle. More recently,Kellogg and others (2002) of the U.S. GeologicalSurvey mapped the Frisco quadrangle directlynorth of the Breckenridge quadrangle.

3

PREVIOUS STUDIES

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Figure 2. Index map showing historic mining districts in the Breckenridge region.

The present study focuses on geologic mapping inthe Breckenridge quadrangle at a scale of 1:24,000.Geologic mapping was completed in July andAugust, 2001. John W. Keller and Erik E. Routemapped most of the Tenmile Range in the westernpart of the map area. Paul J. Bartos was respon-sible for compilation in the Breckenridge miningdistrict in the northeastern part of the quadrangle,and he mapped between the Breckenridge miningdistrict and Indiana Gulch. During the early1980’s Paul was in charge of exploration of theBreckenridge mining district for ASARCO, Inc.,and was assisted by Cindy L. Williams. Parts ofthe map and report were modified from thisearlier work. Paul also provided descriptive andinterpretative information about mineral depositsin the Breckenridge quadrangle. C.A. Wallace andNatalie N. Jones mapped mostly in the south-eastern and south-central parts of the Brecken-ridge quadrangle, and they mapped the isolatedfault blocks of lower Paleozoic rocks scatteredthrough the central part of the map area. James A.McCalpin mapped surficial deposits in the Breck-enridge quadrangle and completed the geologichazards analysis, and Matt Morgan and FranciscoGutierrez participated in part of the surficialmapping. C.A. Wallace and John Keller compiledthe geologic map from data provided by these co-

authors, and they assembled descriptive geologicdata, descriptions of mineral deposits, and inter-pretations from these colleagues to produce thisreport.

Rock names in this report are field terms. Sedi-mentary rocks are named according to the schemeproposed by Pettijohn (1957), metamorphic rocknames follow the system proposed by Best (1982),and volcanic and igneous rocks were namedaccording to the I.U.G.S. classifications proposedby Strekeisen (1973, 1978). Field data were plottedon color aerial photographs from the U.S. ForestService (1983) and from IntraSearch (1999), at anominal scale of 1:24,000.

Thick forest cover below timberline, much ofwhich is overgrown since Ransome (1911) andLovering (1934) completed their pioneeringstudies, obscure rock units. In addition, Quaternarydeposits and residual deposits combine to makeoutcrops rare in many areas of the Breckenridgequadrangle, except where road cuts provide freshexposures. Exposure is better above timberline,except in areas where felsenmeer and solifluctiondeposits are widespread. Where felsenmeer andsolifluction deposits are common, most of the rockis frost-riven blocks that are not in place. Protero-zoic crystalline rocks are, in general, better exposedthan the softer Phanerozoic sedimentary rocks.

5

PRESENT STUDY

PROTEROZOIC ROCKSThe Tenmile Range, which is west of the BlueRiver Valley, is underlain by metamorphic andigneous rocks of Early and Middle Proterozoicage. Migmatitic (Xmg) to locally non-migmatiticbiotite gneiss (Xb) is the most abundant of theProterozoic rock types of the quadrangle. Onesmall area of felsic gneiss (Xf) was mapped in thenorthern part of the map area. These units arepart of the regionally extensive sequence of meta-morphosed sedimentary and volcanic rocks thatunderlie much of the mountainous region ofnorthern and central Colorado. This sequence ofmetamorphic rocks is known as the Proterozoicgneiss complex (Tweto, 1987) . The regional meta-morphism and deformation occurred concurrentlywith the emplacement of syntectonic plutonsduring the Early Proterozoic in Colorado (Reedand others, 1987). Amphibolite and hornblende-plagioclase gneiss that were mapped over largeareas by Kellogg and others (2002) in the Friscoquadrangle to the north do not occur in the Breck-enridge quadrangle.

Proterozoic intrusive rocks in the Breckenridgequadrangle include small to medium-sized dioriteand granodiorite intrusives (YXdi and Xgd), graniticplugs and small stocks (Xgg, Xg, Yqm), mafic dikes(YXm), and numerous small, irregular masses ofpegmatitic granite, pegmatite, and aplite (YXp).These rocks were intruded during the late EarlyProterozoic and the Middle Proterozoic. An EarlyProterozoic intrusive unit of granitic gneiss that isexposed extensively in the Frisco quadrangle(Kellogg and others, 2002) was mapped in only onesmall area in the central part of the Breckenridgequadrangle. Intrusives of the Routt Plutonic Suitewere emplaced about 1,700 Ma, and those of theBerthoud Plutonic Suite were emplaced about 1,400Ma (Tweto, 1987). No intrusive rocks related to thePikes Peak Batholith (1,000 Ma) are known to occurin this area (Tweto, 1974).

PROTEROZOIC GNEISS COMPLEX

MIGMATITIC BIOTITE GNEISS

The most widespread Proterozoic rock unit in theBreckenridge quadrangle is migmatitic biotitegneiss (Xmg), of Early Proterozoic age (Fig. 3). Aphotograph of an exposure of migmatite in PacificGulch, west of Pacific Peak along the westernedge of the map area, was taken by Bergendahland Koschmann (1971) and later used as a primeexample of migmatite in an undergraduatepetrology textbook (Ehlers and Blatt, 1982).

Migmatitic biotite gneiss is generally mediumto dark gray, dark brownish gray, to locally darkreddish or rusty brown. Red or rusty coloration isfrom weathering or alteration of biotite, whichresults in the formation of iron-oxide minerals. Therock is well foliated (usually wavy), mediumgrained, hypidiomorphic to xenomorphic, andinequigranular to granular. Light-colored (felsic)pegmatitic to granitic layers and irregular veinsconsisting mostly of quartz and feldspar (locallyquartz-only) are interlayered with the darkerbiotite-rich material. The felsic layers are typicallycoarse grained and range from about 0.5 cm to 20cm thick. They constitute from 20 to 70 percent ofthe overall rock mass (Kellogg and others, 2002).The high percentage of the felsic material distin-guishes the migmatite from non-migmatitic biotitegneiss. These felsic layers often show ptygmaticfolding and boudinage structures. The felsic layerswere caused by the processes of igneous injectionand metamorphic segregation. At some places,especially near diorite or granodiorite intrusivebodies, some of the dark layers have an intrusivetexture, with relict plagioclase phenocrysts as longas 1 cm. These intrusive-looking layers may be sillsthat emanated from the nearby intrusives prior toor during metamorphism.

Dark layers consist primarily of quartz, plagio-clase, biotite, and microcline in varying propor-tions. Biotite is locally altered to chlorite. Thin silli-manite-bearing layers are present at a few places.Sillimanite is present as light-gray or colorless,fibrous to bladed clots and masses within the

6

STRATIGRAPHY

layers. Phlogopite and muscovite are locallypresent in small quantities (Kellogg and others,2002) and may be alteration products of biotite.Traces of magnetite, garnet, zircon, and apatite alsooccur in the migmatite. Garnet, where present,occurs as fractured porphyroblastic crystals(Bergendahl, 1963). Epidote coats fractures at someplaces.

A few thin, discontinuous layers (<2 m) ofmetaquartzite occur within the migmatitic gneiss,especially in the Peak 10 and Crystal Peak areas.The metaquartzite was not mapped separately butserves as an additional indication that the biotitegneiss and migmatite have sedimentary protoliths.In the Front Range, biotite gneiss has been inter-preted to be derived from the metamorphism ofsandy shale or graywacke (Sheridan and Marsh,1976).

BIOTITE GNEISS

Biotite gneiss (Xb) is Early Proterozoic in age andis lithologically and mineralogically similar to thedark layers of migmatitic biotite gneiss. However,biotite gneiss lacks the high percentage of felsiclayers and degree of ptygmatic folding and boudi-nage exhibited by the migmatitic gneiss. The

contact between biotite gneiss and migmatiticbiotite gneiss is somewhat arbitrary and is basedon guidelines provided by Kellogg and others(2002) for the Frisco quadrangle. Biotite gneisswas designated where the overall rock containsless than 20 percent felsic layers. Compared to themigmatitic gneiss, biotite gneiss has more parallel(less wavy) layering, often weathers in a moreblocky fashion, and is less folded, at least atoutcrop scale. Non-migmatitic biotite gneiss is lesscommon than migmatite gneiss in the quadrangle.Biotite gneiss is most common in the lower,eastern side of the Tenmile Range.

Biotite gneiss is hard, medium-gray, medium-grained, and well-foliated. This unit consists ofquartz (30 to 50 percent), plagioclase (20 to 30percent), and biotite (10 to 15 percent). Microcline,muscovite, hornblende, and sillimanite occur asaccessory minerals in variable proportions (Kelloggand others, 2002).

FELSIC GNEISS

Felsic gneiss (Xf) was observed as a mappable unitin only one small and poorly exposed area nearPeak 6 in the northwest part of the quadrangle.Felsic gneiss is light-tan to light-gray, fine- to

7

Figure 3. Proterozoic-age migmatitic biotite gneiss (Xmg) exposed on the south-eastern flank of Peak 8 in the Tenmile Range.

medium-grained, moderately well-foliated gneissconsisting primarily of quartz and feldspar. Thin,dark-green, chloritic layers are present and appearto be derived from the alteration of biotite-richlayers. Tweto (1987) suggested that most felsicgneiss in the Front Range and Park Range wasderived from silicic volcanic tuff protoliths.

ROUTT PLUTONIC SUITEThe Routt Plutonic Suite (Tweto, 1987) comprisesthe oldest of the igneous intrusive rocks inColorado. Intrusives of this group were emplacedbetween about 1,660 and 1,790 Ma, during EarlyProterozoic time (Reed and others, 1987). TheDenny Creek Granodiorite (Barker and Brock,1965), a foliated granodiorite of the Routt PlutonicSuite, occupies a large area stretching from a pointabout 18 km south-southwest of the Breckenridgequadrangle in the Mosquito Range southeast intosouthwestern South Park, and west into thecentral Sawatch Range. Granodiorite, graniticgneiss, alaskitic granite, and pegmatite dikes onthe Breckenridge quadrangle have been assignedto the Routt Plutonic Suite. A few mafic dikes mayalso belong to this suite.

GRANODIORITE

Foliated granodiorite (Xgd), of Early Proterozoicage, forms small to medium-sized plutons in theProterozoic terrane of the Tenmile Range. Thisunit is more common in the southern part of themap area. Plutons of granodiorite to locally quartzmonzonite or quartz diorite composition areusually well foliated, with foliation parallel to theregional foliation of the migmatitic gneiss. Muchof what we map as granodiorite was mapped byBergendahl (1963) as “porphyroblastic migma-tite”. Our field work showed that this unit is notmigmatitic in character. Near the margins of thewell-foliated granodiorite bodies, concordantlayers within the migmatitic gneiss (Xmg) take ona porphyritic or porphyroblastic texture. Theselayers are possibly metamorphosed sills thatemanated from the intrusive body. This wouldindicate that this intrusive activity occurred priorto the regional metamorphic event. Feldsparphenocrysts as long as 4 cm are common alongthe outer margins of some of the granodioriticintrusives. The texture and composition is similar

to Early Proterozoic rocks referred to as DennyCreek Granodiorite by Tweto (1987), which locallygrades to biotite quartz diorite and quartzmonzonite and is common in the Mosquito Rangeto the southwest of this quadrangle.

The granodiorite is medium to dark gray,medium to coarse grained, porphyritic, and ismoderately to well foliated. It contains 15 to 35percent biotite, 15 to 40 percent quartz, 30 to 60percent feldspar (mostly plagioclase), and onlyminor amounts of hornblende and pyroxene.Accessory minerals include apatite, sphene, andrutile. Epidote is locally present as an alterationproduct, and biotite is often chloritized.

GRANITIC GNEISS

Granitic gneiss (Xgg) was mapped in one smallarea in the central part of the quadrangle. Graniticgneiss was mapped over a large area in the Friscoquadrangle and is interpreted to be intrusive inorigin and part of the Routt Plutonic Suite(Kellogg and others, 2002). Bergendahl (1963) hadpreviously interpreted the unit to be the strati-graphically lowest unit of a metasedimentarysequence. Our mapping and interpretationsconfirm that the unit is intrusive and not metased-imentary. Only one good exposure of graniticgneiss is present in the Breckenridge quadrangle,along a small landslide headscarp in CarterGulch. The granitic gneiss is younger than biotitegneiss. The granitic gneiss is light gray, lightpinkish gray, to pinkish green, medium to coarsegrained, and moderately foliated to locally wellfoliated. It consists principally of 50 to 65 percentfeldspar (mostly pink to white k-feldspar), 25 to35 percent quartz, and 10 to 15 percent biotite,which is usually partially or completely altered tochlorite. Biotite aggregates commonly form clotsand lenses. Small amounts of muscovite andopaque minerals are also present. Kellogg andothers (2002) noted garnet, zircon, and apatite asaccessory minerals. Thin (0.2 – 2 cm) shear planescomposed mostly of chloritized biotite arecommon and give the rock a streaky appearancein places.

ALASKITIC GRANITE

A few irregularly shaped intrusions of alaskiticgranite (Xg) are present in the Tenmile Range. The

8

largest body of alaskitic granite is located on thenorthern side of Peak 10. Numerous, irregular,dike-like bodies of alaskitic granite emanate fromthe larger intrusive masses and intrude themigmatite gneiss. The granite is white to verylight gray to light tan and consists of microclineand orthoclase (50 to 60 percent), quartz (30 to 40percent), muscovite (5 to 10 percent), and minorplagioclase. Under the microscope, quartzdisplays prominent undulatory extinction. Berg-endahl (1963) noted fine-grained magnetite,pyrite, chlorite, and biotite as very minor constit-uents. The texture is fine to medium grained andgenerally xenomorphic, and the rock sometimesappears aplitic. This rock has not been definitivelyage dated, but the authors speculate that it isEarly Proterozoic in age due to its weak butconsistent foliation.

BERTHOUD PLUTONIC SUITEThe Berthoud Plutonic Suite (Tweto, 1987)comprises the middle age-group of igneous intru-sive rocks in Colorado. Intrusives of this groupare thought to have been emplaced about 1,400Ma, during Middle Proterozoic time. In the Breck-enridge quadrangle, quartz monzonite, diorite,mafic dikes, and pegmatite/aplite dikes havebeen assigned to the Berthoud Plutonic Suite. TheSt. Kevin Granite comprises a batholith and asso-ciated smaller intrusions that extend from nearthe southwestern corner of the Breckenridgequadrangle southwest through the Sawatch Rangeand into the Elk Mountains (Tweto and others,1978). The extensive Silver Plume Granite in theFront Range east of the Breckenridge quadranglealso belongs to the Berthoud Plutonic Suite.

QUARTZ MONZONITE

Intrusive rock of quartz monzonite to granitecomposition (Yqm), of Middle Proterozoic age,closely resembles the “Silver Plume Granite” (Ball,1906). These bodies are present as irregular stocksalong the western and southwestern margin of thequadrangle. Tweto (1974) also recognized thesimilarity and mapped the plutons as MiddleProterozoic granite. These rocks were laterincluded as part of the St. Kevin Granite batholithby Tweto (1978).

The rock is light to medium gray to pinkish tan,medium grained, and porphyritic. It contains bothbiotite and muscovite, with biotite usually domi-nant. From a distance and in air photos, outcropscommonly appear to be rusty-pink colored. Euhe-dral, elongated, tabular phenocrysts of mostlymicrocline feldspar up to 2 cm long comprise 30 to40 percent of the rock mass. These phenocrystscommonly display a rough alignment, especiallynear pluton margins. This phenocryst alignmenthas been attributed to primary flow structure(Tweto, 1987). The phenocrysts commonly displayCarlsbad twinning. Plagioclase crystals, typicallymuch smaller than the microcline phenocrysts,make up 25 to 35 percent of the rock. Quartz(sometimes smoky-colored) varies from 15 to 30percent. Biotite and muscovite together compriseabout 5 to 10 percent of the rock. Apatite and rutileare reported to occur as accessory minerals(Bergendahl, 1963). In small plugs, dikes, and irreg-ular intrusions lying away from the larger stocks,quartz and muscovite generally increase inpercentage, and overall grain size diminishes. Therock unit is not deformed by regional metamor-phism.

Numerous irregular apophyses and dikesemanate from the quartz monzonite stocks andplugs. These often grade into pegmatitic granite,pegmatite, and aplite. In some places, such as theCrystal Peak area, these dikes are so numerous asto constitute nearly 50 percent of areas otherwisemapped as migmatitic biotite gneiss. In such cases,only the largest and most conspicuous dikes weremapped separately. Bergendahl (1963) had a sepa-rate map unit for this mixture of migmatite gneissand younger granite and pegmatite.

PEGMATITE AND APLITE

Numerous discontinuous dikes and irregularmasses of pegmatitic to aplitic rock (YXp) occur inthe Tenmile Range, and these masses are espe-cially numerous in the southern half of the rangenear mapped bodies of Proterozoic quartzmonzonite. Pegmatites and aplites are undatedbut are probably Middle and Early Proterozoic inage. The long axes of the pegmatites arecommonly oriented in preferred directions, espe-cially east-west. Exposures often show a variety oftextures, with coarse pegmatitic zones grading

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into medium-grained pegmatitic granite and fine-grained aplite. Pegmatites and related bodiesrange from 2 to 25 m in width, averaging about 7m. Pegmatitic rocks in the map area are typicallywhite to light pink and consist primarily ofquartz, microcline, plagioclase, and muscoviteand/or biotite. Rarely, large (~ 2 to 7 cm) massesof magnetite occur in pegmatite. Reddish-browngarnet locally is a minor constituent. Severalpegmatites in the Mount Helen area containseveral percent of a light-green mica, possibly thechromium-rich mariposite. Because they are sonumerous and are often only exposed onextremely steep mountain faces, no attempt wasmade to differentiate the different types ofpegmatites, aplites, and pegmatitic granites on themap. Due to their high color contrast withsurrounding rock, these rocks were easilymappable from aerial photographs.

MAFIC DIKES

In the Breckenridge quadrangle, narrow, mafic,fine-grained lamprophyre dikes (YXm) that discor-dantly intrude gneiss were mapped at a fewplaces. At most places, the exposures were toosmall and discontinuous to be shown at mapscale. These dikes are typically massive to weaklyfoliated. Their composition is variable, and manyhave undergone sausseritic alteration to varyingdegrees. In many exposures, rounded masses ofrecrystallized phenocrysts give the rock a spottedappearance. The spots can be either lighter ordarker than the enclosing fine-grained material.The spots are not individual crystals, but fine-grained aggregates of alteration products. Darkaggregates are often hornblende that has replacedpyroxene. Relict plagioclase has in some casesbeen replaced by potassium feldspar. Uralite-chlo-rite intergrowths are present in most samples asan alteration product of mafic minerals.

The dikes are dark gray to black and displaysomewhat variable mineralogy and texture. Thevariation is due partly to alteration and partly toprimary compositional differences. They consistprincipally of hornblende and/or biotite (30 to 60percent), plagioclase and/or secondary potassiumfeldspar (20 to 40 percent), variable amounts ofsecondary epidote and chlorite (up to 15 percent),remnant pyroxene (less than 10 percent), rounded

and resorbed quartz grains (5 to 10 percent),dolomite (up to 10 percent) which may besecondary, magnetite and other opaque minerals (2to 3 percent), and traces of apatite. Secondarypotassium feldspar is present in the groundmass insome dikes. A whole-rock geochemical analysis ofone sample from a mafic dike showed it to behighly potassic, with almost no sodium. Alongsome mineralized fault structures in the southernpart of the map area, mafic dikes contain up to 3percent disseminated pyrite and have beenexplored by short mine adits. Migmatitic gneissand pegmatitic material is locally present as inclu-sions within the dikes. Tweto (1987) grouped mostnarrow, mafic and intermediate dikes that arepresent in the Front, Sawatch, and Park Rangesinto the Berthoud Plutonic Suite. Some of the dikesdisplay more foliation than others and may be ofthe Routt Plutonic Suite. Kellogg and others (2002)mapped a small body of ultramafic material in theFrisco quadrangle and assigned it to the RouttPlutonic Suite.

DIORITE

Small intrusions of diorite (YXdi) are present in thenorthern and north-central Breckenridge quad-rangle. The diorite is dark gray, medium to coarsegrained, and massive to very weakly foliated.Kellogg and others (2002) mapped similar intru-sions in the southern part of the Frisco quad-rangle. The lack of strong foliation indicates thatthese intrusives are probably of Middle Protero-zoic age, but no definitive age date or cross-cutting relationships prove it. The diorite consistsprincipally of plagioclase (40 to 60 percent), horn-blende (30 to 55 percent), biotite (5 to 10 percent),and quartz (5 to 10 percent). Opaque minerals(mostly magnetite; 2 to 4 percent), pyroxene(trace), and variable amounts of secondary chlo-rite and epidote. Accessory apatite and sphenewere noted in thin section.

PALEOZOIC ROCKSRegionally, the complete stratigraphic sequence is,in ascending order, the Sawatch Quartzite andPeerless Shale (Late Cambrian), Manitou Lime-stone (Early Ordovician), Harding Quartzite(Middle Ordovician), Fremont Dolomite (Upperand Middle Ordovician), Parting Quartzite and

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Dyer Dolomite Members of the Chaffee Formation(Late Devonian), Leadville Limestone (LowerMississippian), Belden Shale and Minturn Forma-tion (Pennsylvanian), and Maroon Formation(Pennsylvanian and Permian). The SawatchQuartzite and Peerless Shale, and the PartingQuartzite and Dyer Dolomite Members arecombined on cross-section B-B’.

In the map area, the Harding Quartzite andFremont Dolomite are absent from the stratigraphicsequence; these units were not deposited in themap area or they were eroded before the ChaffeeFormation was deposited. The Leadville Limestonethins from south to north and it disappears nearthe south border of the Breckenridge quadrangle.Most of the lower Paleozoic rock units are absent inthe northern part of the map area where theMaroon Formation was deposited directly onProterozoic rocks. Lower Paleozoic rock units areseparated by disconformities that represent longperiods of non-deposition, or after deposition,erosion. Lower Paleozoic rocks occur mainly westof the Blue River on local fault blocks uncon-formably above Middle Proterozoic metamorphicrocks. The Belden Shale is not mapped in theBreckenridge quadrangle. The Minturn Formationoverlies the Manitou Limestone in exposures westof the Blue River, but nowhere is a completesection of Minturn present. The Maroon Formationis the uppermost Paleozoic unit, and it overlies theMinturn on an intertonguing contact in the sout-hern part of the map area. The Maroon Formationis exposed mainly in the southeastern part of thequadrangle where this unit thickens drastically.The exposed Paleozoic sequence is estimated to beabout 1,765 m thick in the map area, but nowhereis the sequence continuous.

SAWATCH QUARTZITEIn the south-central part of the Breckenridgequadrangle the Sawatch Quartzite (Cs), of LateCambrian age, overlies Middle Proterozoic meta-morphic and igneous rocks. The Sawatch Quart-zite is as thick as 43 m in the map area. This unithas a prominent basal conglomerate that is over-lain by fine- and medium-grained quartzite. Thebase of the Sawatch Quartzite is marked by a feld-spathic conglomerate that contains well-roundedquartz pebbles in a matrix of medium- and

coarse-grained sand. The basal conglomerate is 18to 32 cm thick, and it contains planar crossbeds.Most of the Sawatch Quartzite is a medium-grained, well-sorted quartzite with sub-roundedgrains. Some beds are graded from medium-grained quartzite at the base to fine-grainedquartzite at the top. The amount of feldspardecreases upward. The lower part of the quartziteis grayish white and the upper part of the quar-tzite is reddish brown and contains thin, lami-nated black shale interbeds. The Sawatch Quart-zite has been eroded by a pre-Pennsylvanianunconformity in the northern part of the maparea.

PEERLESS SHALEThe Peerless Shale (Cp), of Late Cambrian age, is asequence of coarse-grained quartzite inter-beddedwith black shale at the base and fine- andmedium-grained quartzite, interbedded withdolomite-cemented sandstone and sandy and siltydolomite. The Peerless Shale is as thick as 12 m inthe map area. This unit is generally poorlyexposed and it forms a recessive-weathering zonethat contains clasts of rusty-weathering dolomiteand dolomitic quartzite in soil above the resistant-weathering quartzite of the Sawatch Quartzite.The Peerless Shale disappears south of the Breck-enridge quadrangle. Taranik (1974, p. 19) regardedthe Peerless Shale as a transitional unit betweenthe Sawatch Quartzite and the Manitou Limestone.

MANITOU LIMESTONEThe Manitou Limestone (Om), of Early Ordovicianage, overlies the Peerless Shale on a disconformityand is mainly a dolomite in the map area. TheManitou Limestone is as thick as 27 m in the maparea. This unit is composed of dark-, moderate-,and light-gray, thin- to thick-bedded dolomite andcherty dolomite, and rare beds of dark-gray lime-stone. Grayish-pink and light-grayish-red dolo-mite beds occur in the upper part of the unit.Taranik (1974) reported that the upper 6 m of theManitou Limestone contains casts of crinoids,shell fragments, and stromatolites. Sand grainsand sandstone beds increase in the upper part ofthe Manitou Limestone. Distinctive grayish-whitechert nodules and rare black chert nodules are

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elongate parallel to bedding planes and mostlyoccur in the upper part of the Manitou Limestone.Silicification of dolomite is not common in theBreckenridge quadrangle, but silica replacementof dolomite beds and dolomite breccia is commonabout 90 km south of this map area between TroutCreek Pass and Salida, Colorado,(Wallace andothers, 1997; Wallace and Keller, 2001). TheManitou Limestone is absent in the northern partof map area where this unit was eroded beforeMiddle Pennsylvanian time.

CHAFFEE FORMATIONThe Chaffee Formation (Dc) is of Late Devonianage and rests disconformably on the ManitouFormation (Taranik, 1974). This unit thins towardthe north in the Breckenridge quadrangle and isabsent north of McCullough Gulch. The ChaffeeFormation is composed of the Parting QuartziteMember at the base and the Dyer DolomiteMember at the top.

The Parting Quartzite Member is a grayish-red,gray, dark-reddish gray, grayish-orange, and blackweathering, fine-, medium-, and coarse-grained,silica- or dolomite-cemented orthoquartzite andslightly feldspathic quartzite. The base of theParting Quartzite is marked by a conglomerate.Planar crossbeds are more common in the lowerpart than in the upper part of the unit. Generallybeds are 3 to 30 cm thick. The Parting Quartzite isas thick as 6 m thick in the map area.

The Dyer Dolomite Member is a laminated, tan-weathering dolomite that is as thick as 14 m in themap area. This unit is light yellowish gray, grayishyellow, light brownish gray, mottled and containswavy microlamination. Dolomite is very finegrained to fine grained and the rock has aconchoidal fracture. Where outcrops occur in thesouth-central part of the quadrangle, the DyerDolomite Member forms a prominent light-coloredband above darker colored rocks.

MINTURN FORMATIONThe Minturn Formation (IPm) is of Middle Penn-sylvanian age (Des Moinesian) (De Voto, 1980),and either disconformably overlies the DyerDolomite Member in the southern part of the maparea or unconformably overlies Proterozoic rocksin the northern part of the map area. The Belden

Shale, which underlies the Minturn Formationwest and south of the map area, was not recog-nized in the Breckenridge quadrangle. In theBreckenridge quadrangle the Minturn Formationis composed of interbedded conglomerate, andgreen, black and gray, arkose, micaceous shale,micaceous siltstone, limestone, and dolomite.Calcite cement is common in most of these clasticrocks. The minimum thickness estimated for thisunit is about 460 m, but nowhere in the map areais a complete sequence exposed.

Conglomerate is light grayish pink, olive drab,grayish green, and greenish black. Framework-supported and matrix-supported conglomerate iscomposed of rounded to subangular clasts ofgranite, gneiss, schist, quartzite, and vein quartz.Pebble conglomerate is most common, but somebeds contain cobble-sized clasts. Conglomeratebeds are arkose. Although conglomerate beds areprominent in the poorly exposed Minturn, congl-omerate and conglomeratic sandstone form onlyabout 20 percent of the unit, based on a measuredstratigraphic section (Taranik, 1974, p.63, andSection 4). Conglomerate beds contain prominenttrough and planar crossbeds and channeledcontacts. Carbonaceous matter occurs in someconglomerate beds.

Sandstone beds in the Minturn are light-grayish-tan, grayish-pink, light-gray, moderategray, reddish-gray, grayish-red, and grayish-purple, micaceous, coarse-, medium-, and fine-grained arkose. Individual sandstone beds may belenticular or laterally continuous across an outcrop,and beds are 10 cm to 1 m thick. Sandstone bedsform composite sets that are about 12 m thick.Most individual sedimentation units are normallygraded, although reverse and central gradingoccurs in some beds. Primary bedding structuresare planar and trough crossbeds, ripple cross-lami-nations, and shallow channels. Carbonaceous mate-rial, which represents plant debris, occurs locally insandstone beds.

Siltstone and shale are prominently micaceous,and are dark gray, black, olive drab, greenish black,and grayish tan. Siltstone and shale commonlyoccur in zones 2 cm to 4 m thick. Compositionally,siltstones are arkose, and many siltstone beds havea clay matrix. Shale beds are dominantly argilla-ceous, but most shale contains silt- and sand-sized

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grains of quartz, feldspar, and rock fragments ascontaminants. Siltstone and shale are laminatedand microlaminated, and siltstone contains ripplecross-lamination. Micaceous black shale is a promi-nent component of the lower part of the MinturnFormation north of Hoosier Pass.

Thin limestone and dolomite beds occur atsome places in the Minturn Formation. Carbonatebeds in the lower part of the Minturn Formationare poorly exposed in the Breckenridge quad-rangle, but limestone and dolomite are exposed inroad cuts between Monte Cristo Gulch and McCul-lough Gulch. Most carbonate beds appear to belaterally discontinuous. Limestone and dolomiteare moderate-gray, dark-gray, and grayish-black,fine-grained, micritic beds that range from a fewcentimeters thick to 3 m thick. Carbonate beds arelocally mottled in moderate-gray and dark-graycolors, and these beds are commonly associatedwith black shale and dark-colored siltstone. Lime-stone and dolomite are indistinguishable withoutusing dilute HCl. According to Taranik (1974, p 75)fossils in an upper limestone bed, which he corre-lated with the Robinson Limestone Member asdescribed in detail by Tillman (1971), are stromato-porids, several types of foraminifera (Komia sp.,Bradyina sp., Tetrataxia sp., Fusulina sp., and Apter-rinella sp.), phylloid and dasyclad algae, and oncol-ites that encrusted echnoid spines and gastropodfragments. Primary bedding structures are notobvious in carbonate rocks of the Minturn Forma-tion. Taranik (1974) emphasized that correlation ofcarbonate members from the type area at Minturn(Tweto and Lovering, 1977) to the Breckenridgequadrangle is fraught with considerable uncer-tainty because (1) the limestone members changelithofacies vertically and laterally over shortdistances; (2) the limestone zones are lenticular anddisappear into clastic and evaporitic rocks laterally;and (3) carbonate lithofacies are diachronous andlimestone beds are time transgressive in the strati-graphic sequence. On the basis of these substantialuncertainties, we did not extend the Robinson andJacque Mountain Limestone Members into theBreckenridge quadrangle.

MAROON FORMATIONThe Maroon Formation (PIPm) probably spans theperiod from Late Pennsylvanian to Early Permian

(Missourian-Virgilian) according to De Voto(1980). In the southeastern part of the Brecken-ridge quadrangle the Maroon Formation overliesthe Minturn Formation on an apparent inter-tonguing contact according to Taranik (1974).However, in the northeastern quadrant of the maparea the Maroon Formation directly overliesMiddle Proterozoic rocks. The Maroon Formationis a redbed sequence of calcareous, coarse congl-omerate, arkose, micaceous siltstone and shale,and limestone that has been traced southward tothe area of Trout Creek, southeast of Buena Vista,Colorado, and is widely exposed in South Parkand the central part of the Mosquito Range. TheMaroon Formation is about 180 m thick in thenortheastern part of the Breckenridge quadrangleat Gibson Hill, where the Maroon may contain afew tens of meters of Chinle Formation at the top.The Maroon Formation thickens greatly to thesouth in the Breckenridge quadrangle to about1,200 m thick. The apparent thickness of theMaroon Formation has been inflated about 180 mby intrusion of hornblende-biotite monzoniteporphyry (Tmp) sills.

In the Breckenridge quadrangle conglomerateis a common component of the Maroon Formation.Conglomerate is light reddish gray, reddish tan,moderate reddish gray, dark red, and grayishpurple. Most conglomerate beds are dispersed-clast, polymict rocks that contain well-rounded tosubangular cobbles and pebbles in a matrix ofgranules and coarse-grained arkose. Channels arecommon at the base of the conglomerate beds andwithin conglomerate beds, and some channels cut 1to 2 m into underlying beds. Commonly channelbases are defined by a zone of cobbles or largepebbles. Large clasts and enclosing matrix arecommonly normally graded, although reverse andcentral grading occur in some conglomerate beds.Equigranular and porphyritic granitic clasts arecommon, as are biotite schist, biotite and horn-blende gneiss, vein quartz, and large feldspar crys-tals. Feldspar composes as much as 40 percent ofcoarse-grained arkose. Primary bedding structuresare planar and trough crossbeds, ripple cross-lami-nations, cuspate and linguoid ripple marks, andparting lineations. Secondary bedding structuresare deformed cross beds and load-casts. Individualconglomerate beds are generally fining-upward

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sequences, and conglomerate passes upward intocoarse-grained, medium-grained, and fine-grainedarkose; ultimately arkose grades into siltstone andshale.

Sandstone beds in the Maroon Formation arelight reddish gray, reddish tan, moderate reddishgray, moderate red, grayish red, dark red, andgrayish purple. Channeled basal contacts occurmost commonly in coarse-grained arkose, althoughcoarse-grained arkose can have basal contacts thatappear flat on outcrop scale. Arkose is commonlynormally graded. Single crystals of feldspar aregranule size, and feldspar can reach about 40percent of the rock. Planar and trough crossbeds,ripple cross-lamination, ripple marks, and partinglineation are the most common primary beddingstructures, and secondary bedding structures aremainly deformed crossbeds and load casts of sandbeds into argillaceous siltstone and shale.

Siltstone and shale beds of the Maroon Forma-tion are richly micaceous, and these fine-grainedrocks are darker in color than coarse-grained rocks.Dark-reddish-gray, dark-grayish-red, dark-red,moderate-red, and reddish-tan colors predominatein fine-grained rocks. The dark red colors of thesefine-grained rocks results from a larger percentageof interstitial clay that is stained with iron oxide.Siltstone is feldspathic and much of the siltstone isclassed as arkose. Shale beds are rare in theMaroon Formation, and most of the beds that arenot resistant to weathering are argillaceous silt-stone and argillaceous fine- and very fine-grainedsandstone. Argillaceous siltstone and sandstonebeds are calcareous. In these fine-grained rocksprimary bedding structures are ripple cross-lami-nations, cuspate and linguoid ripple marks, rillmarks, and flute casts. Secondary sedimentarystructures are small load casts and convolutebedding.

Limestone beds in the Maroon Formation arelenticular, and limestone beds consist of calcareoussiltstone and shale, and moderate-gray andblackish-gray fossiliferous, fine-grained limestone.Limestone beds consist of (1) nodules of moderate-gray and grayish-red, fine-grained limestone inshale and siltstone; (2) nodular moderate-gray anddark-gray limestone beds; (3) grayish-red andmoderate-gray limestone-pebble conglomerate; (4)moderate-gray and dark-gray limestone edgewise

conglomerate; (5) moderate-gray and dark-grayoölitic limestone; and (6) gray sandy and silty lime-stone. Limestone can form from aggregates of later-ally adjacent nodules that form beds as thin as 3 to5 cm thick, or limestone beds can be massive-weathering units as thick as 5 m. Thick limestonebeds consist of numerous, thinly bedded,commonly argillaceous limestone sedimentationunits. Taranik (1974) described the limestone bedsas prominently lenticular, and most could not betraced for more than 600 m. In the Breckenridgequadrangle, a discontinuous limestone, 10 to 50 cmthick, in the Red Mountain area was tentativelycorrelated by Taranik (1974) with the Jacque Moun-tain Limestone Member of the Maroon Formationmainly on the basis of stratigraphic position 1,500ft above the tentatively identified Robinson Lime-stone Member and on occurrence of poorlypreserved oolites and straight cephalopods.Taranik (1974) recognized that the occurrence ofoolites and straight cephalopods in a limestone onRed Mountain do not provide unique identificationcriteria for the Jacque Mountain LimestoneMember and that equating a single, lenticular, thinlimestone bed of many limestone beds that occur inthat area introduces much uncertainty regardingthe correlation.

STRATIGRAPHIC NOMENCLATUREAND CORRELATION OF THEMINTURN AND MAROON FORMATIONSStratigraphic nomenclature and correlation ofPennsylvanian and Permian rock units that havecomplex stratigraphic relations in the CentralColorado Trough have been controversial subjectsamong geologists for the last fifty years (Tweto,1949; De Voto, 1971, 1972; Tweto and Lovering,1977; De Voto and Peel, 1972; De Voto, 1980;Taylor and others, 1975; Lindsey and others,1985). Rapidly subsiding, fault-bounded sedimen-tary basins commonly have rock units that thickenand thin drastically, show rapid lateral changes oflithofacies, and exhibit strongly diachronousformation contacts. Stratigraphic nomenclature inthese Pennsylvanian and Permian rocks in theCentral Colorado Trough is confusing because thecurrent system is a mixture of time-stratigraphicunits and rock-stratigraphic units; concurrence

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among geologists regarding formation boundariesand correlation is elusive.

Tweto (1949) defined the stratigraphic andnomenclature relations of the Minturn and MaroonFormations in the upper drainage of the EagleRiver near Pando, Colorado; his stratigraphic revi-sions abandoned many previously used andconflicting names, such as “Weber Grit,” “Webershale,” “Weber formation,” “Wyoming formation,”“McCoy formation,” and “Battle Mountain Forma-tion,” and replaced those names with Belden Shale,and the Minturn and Maroon Formations. TheBelden Shale was used as Brill (1944) defined thisunit, Minturn Formation was a new name takenfrom exposures near the town of Minturn on theEagle River, and Maroon Formation was redefinedin Tweto’s (1949) report, the name having beenimported from the Maroon Bells area near Aspen,Colorado.

Tweto (1949) proposed the name “MinturnFormation” for the dominant clastic sequenceabove the Belden Shale as defined by Brill (1944).Tweto (1949) placed the boundary between theMinturn Formation and the Maroon Formation atthe top of the Jacque Mountain Limestone in theVail and Minturn area; the Jacque Mountain Lime-stone of Tweto (1949) was oölitic and containedscattered large straight and coiled cephalopods andsome large gastropods. Tweto’s 1949 definition ofthe Minturn and Maroon Formations placed a localtime line in the northern part of the CentralColorado Trough, and the location of the top of theMinturn Formation became a de facto time-strati-graphic boundary. Tweto’s time-stratigraphic defi-nition of the Minturn and Maroon Formations metthe standards of the stratigraphic code at the time(Ashley and others, 1933). Tweto and Lovering(1977) designated a type section for the MinturnFormation and provided a composite measuredsection assembled from exposures near Gilmanalong the Eagle River Valley and in the GameCreek area. The type section of the Minturnretained the Jacque Mountain Limestone as theupper limit of this unit (Tweto and Lovering, 1977).

Subsequent mapping in the Central ColoradoTrough has shown that the limestone beds thatformed his markers are local units that can bemapped and correlated reliably only in thenorthern part of the Central Colorado Trough near

Vail, Minturn, and Kokomo (De Voto, 1971, 1972;De Voto and Peel, 1972; Tillman, 1971). The JacqueMountain Limestone, if it exists south of the townof Breckenridge, is 10 to 50 cm thick and lenticular(Taranik, 1974), and south of Hoosier Pass andacross South Park, limestone beds cannot be corre-lated with the marker beds described and mappedby Tweto and Lovering (1977) according to Navas(1966), De Voto (1972, 1980), and Taranik (1974).The Jacque Mountain Limestone, which is themarker between the Minturn and Maroon Forma-tions, can be identified over an area of about 1,900sq km which leaves no effective marker betweenthe Minturn and Maroon Formations over about22,700 sq km of the Central Colorado Troughwhere the Minturn and Maroon Formations havebeen mapped in South Park in the Antero quad-rangle (De Voto, 1971). Moreover, limestone beds inthe Minturn exhibit abrupt changes in lithofacies,thickness, and stratigraphic position (Tillman,1971), so correlation based on lithologic characterof limestone in the Minturn and Maroon Forma-tions is uncertain outside the region of the EagleRiver, Vail Pass, and the Continental Divide.

The current time-stratigraphic subdivisionapplied to the Minturn and Maroon Formations byTweto (1949) and Tweto and Lovering (1977)clearly requires modification to bring nomenclatureinto conformance with the current North AmericanStratigraphic Code (North American Commissionon Stratigraphic Nomenclature, 1983). Lithologiccriteria define rock units under the current code, soidentification of suitable lithologic criteria isneeded to separate these units beyond the smallarea of the Central Colorado Trough to whichTweto’s original definition applied. In the past, thehindrance to using lithologic criteria has been thediachronous nature of similar lithologies in thequickly subsiding Central Colorado Trough.Although revision to the stratigraphic nomencla-ture of the Minturn and Maroon Formations isbeyond the scope of this report, two criteria havebeen proposed to separate these two units wheremarker limestone beds are absent.

De Voto (1972, p. 161) proposed that name“Minturn” be assigned to the interval of “domi-nantly gray, green, to tan sandstones, siltstones,conglomerates, shales, and several interbeddedlimestones which occur stratigraphically above the

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black shales of the Belden and below an overlyingdominantly red bed sequence of Pennsylvanianand Permian age.” The definition proposed by DeVoto (1972) is principally one of a color differenceinasmuch as “both units contain sandstones, silt-stones, shales, and several interbedded lime-stones,” but color is regarded as a supplementaryfeature rather than a distinctive lithic feature of aformation (North American Commission on Strati-graphic Nomenclature, 1983, p. 858). In addition,De Voto’s (1972) definition of the Minturn is notapplicable to the entire Central Colorado Troughbecause the Belden Shale is absent between theKerber and Minturn Formations southeast ofBuena Vista, Colorado. Although De Voto’s newdefinition of the Minturn and Maroon Formationssolved part of the problem created by using alocally occurring, time-stratigraphic boundary(Tweto, 1949), his definition was not applicable tothe entire Central Colorado Trough and the solu-tion did not focus on the distinctive lithic featuresthat characterize each formation.

Taranik (1974, p. 96) suggested that a significantchange in grain size occurred at the boundarybetween the Minturn and Maroon Formations inthe Breckenridge area, and this lithologic distinc-tion is compatible with the North American Strati-graphic Code (North American Commission onStratigraphic Nomenclature, 1983) and lithologicdistinctions that were applied two decades later inthe central and southern part of the CentralColorado Trough. In the Breckenridge quadrangle,Taranik (1974) showed that the Minturn Formationis a finer grained unit than the Maroon Formation,and he showed that the change in grain size iscommonly accompanied by a color change fromgray and black colors of the Minturn Formation tored colors of the Maroon Formation. The colorchange may not be a material feature for regionalseparation of these two rock units in the EagleRiver and Vail Valley areas, because the colorchange from dark-gray to red rocks is not coinci-dent with the change from pebble conglomerate tocobble-bearing conglomerate at approximately1,600 m above the base of the measured section ofTweto and Lovering (1977). Taranik (1974) clearlydemonstrated that the contact between the MinturnFormation and the Maroon Formation is diachro-

nous where he traced thin, discontinuous limestonebeds across the change in grain size.

The difference in grain size between theMinturn Formation and the Maroon Formationmight be a regionally consistent difference that isworth evaluating. Taranik’s (1974) data show thatin the Breckenridge area, conglomerate accountsfor about 20 percent of the sequence in theMinturn, and most of that conglomerate is in thelower part of the unit. Similarly, his data show thatthe Maroon Formation contains about 50 percentconglomerate, and in the Breckenridge quadranglewe have determined that, in general, clasts arelarger in diameter in the Maroon Formation than inthe underlying Minturn Formation. In the maparea the Minturn Formation contains more siltstoneand shale than does the overlying Maroon Forma-tion, and the Maroon Formation contains moresandstone and conglomerate than does theMinturn Formation. In South Park, much of theMaroon Formation is reddish-gray shale, siltstone,and sandstone (De Voto, 1971), but the grain-sizecontrast is still valid because the Minturn Forma-tion contains thick zones of shale and siltstone,some limestone, and lesser amounts of coarse-grained arkose. Coarse-grained arkose beds arerare in the Minturn Formation near Buena Vista,Colorado (Wallace and Keller, 2001), but in adjacentSouth Park, sandstone and conglomerate arecommon components of the Maroon Formation (DeVoto, 1971).

MESOZOIC ROCKSThe Mesozoic sequence in the region of the Breck-enridge quadrangle is represented by the ChinleFormation (Upper Triassic), Entrada Formation(Middle Jurassic), Morrison Formation (UpperJurassic), Dakota Sandstone (Lower Cretaceous),Benton Shale (Upper Cretaceous), NiobraraFormation (Upper Cretaceous), and Pierre Forma-tion (Upper Cretaceous). Tweto (1974) groupedthe Benton Shale, Niobrara Formation, and PierreFormation, and he grouped the Dakota Sandstoneand the Morrison Formation on his reconnais-sance geologic map of the Lincoln quadrangle(scale 1:62,500), which includes the Breckenridgequadrangle (scale 1:24,000). Tweto (1974) did notshow the Chinle or Entrada Formation on the

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Mount Lincoln map. Taranik (1974) included theChinle Formation with uppermost the MaroonFormation, and he mapped the Garo Formationwhere we show the Entrada Formation. In thenortheastern part of the map area, the strati-graphic interval that should represent the ChinleFormation is covered with slope wash orresiduum, so in the area of Gibson Hill we addthe Chinle Formation to the uppermost part of theMaroon Formation. In exposures east of RockyPoint where the Entrada and Morrison Forma-tions overlie the Maroon Formation, the ChinleFormation appears to be absent. Taranik (1974)showed Cretaceous rocks as one undifferentiatedunit, but we subdivided Cretaceous rock units.The thickness of the Mesozoic sequence is about865 m.

CHINLE FORMATIONThe Chinle Formation (TRPcm), of Late Triassic age,is problematic in the map area because it is notexposed at the surface, but the Chinle Formationis exposed north of the Breckenridge quadranglein the southern part of the Frisco quadrangle(Kellogg and others, 2002). Most authors havegrouped the Chinle Formation with the MaroonFormation as a single map unit because exposuresare generally poor and both are redbed sequences(Taranik, 1974, Kellogg and others, 2002).

Where the Chinle Formation is exposed at thesurface in the Frisco quadrangle it is a brightgrayish-orange and reddish-orange, calcareousshale, siltstone, and fine-grained sandstone. TheChinle Formation apparently thickens to the north;at Dillon dam in the adjacent Frisco quadrangle,the Chinle is about 61 m thick (Kellogg and others,2002). Coarse-grained beds, such as medium- tocoarse-grained arkose, and pebbly sandstone that iscommon in finer grained sections of the MaroonFormation are absent in the Chinle Formation. TheChinle Formation is combined with the uppermostMaroon Formation in the vicinity of Gibson Hill,but exposures are poor and this identification is notcertain. About 1 mi east of Rocky Point, which isnorth of Indiana Creek, the Entrada Formationoverlies a thin sequence of Maroon Formation andthe intervening Chinle Formation appears to beabsent. Again, exposures are poor in this area, butthe distinctive reddish-orange soil color, and

reddish-orange chips of siltstone and fine-grainedsandstone are absent below the Entrada, whichsuggests that the Chinle Formation is absent. TheChinle Formation is not present south of IndianaCreek in the Breckenridge quadrangle.

ENTRADA SANDSTONEThe Entrada Sandstone (Je) is Middle Jurassic inage, and it is composed of light-gray to pale-yellowish-gray, well-sorted, fine- to medium-grained, crossbedded, calcite- and dolomite-cemented orthoquartzite. Sand grains are frostedand most are subrounded to rounded. Small,brown spots are common in this sandstone andthey probably result from weathering of iron-richcarbonate cement. The Entrada Sandstone rangesbetween 5 and 11m in the Breckenridge quad-rangle.

Assignment of this sandstone to the EntradaSandstone is controversial. De Voto (1965) indi-cated that the sandstone intertongued with redshale inferred to be in the upper part of theMaroon Formation in South Park, and he con-cluded that the sandstone was correlative with theGaro Sandstone of Permian age. Taranik (1974)followed De Voto’s (1965) correlation and showedthe Garo Sandstone on his map. Brennan (1969),working in the northern Vail area, identified asandstone as Entrada Sandstone that uncon-formably overlies the Chinle Formation (UpperTriassic) and underlies the Morrison Formation(Upper Jurassic). The sandstone is 12 to 30 m thickand is prominently crossbedded. Kellogg andothers (2002) mapped the Entrada Sandstonebetween the Chinle Formation and the MorrisonFormation in the Frisco quadrangle that adjoins theBreckenridge quadrangle on the north. Where theChinle Formation is identified in the Frisco quad-rangle, assignment of the sandstone to the Entradais unequivocal, and we follow the assignment ofKellogg and others (2002) in this map area.

MORRISON FORMATIONThe Morrison Formation (Jm) is Upper Jurassic inage and is mostly light-gray and light-greenish-gray, locally calcareous claystone and siltstone. Inthe lower half of this unit, light-grayish-yellowand light-grayish-white, medium-grained sand-stone beds are interbedded with claystone and

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siltstone, and the sandstone beds may be as thickas 5 m. Sandstone beds contain iron-rich bandsparallel to joint surfaces and limonitic spots arecommon. A prominent moderate-gray limestoneand dolomitic limestone bed occurs about 10 mabove the base of the Morrison Formation. Thislimestone bed is micritic and is mottled in light-and dark-gray colors. Limestone is nodular, andbedding is poorly defined. Regionally, theMorrison Formation is about 55 to 79 m thick inthe Blue River Valley north of Silverthorne,Colorado (Holt, 1961), and at Dillon Dam this unitis about 70 m thick (Wahlstrom and Hornback,1962). In the map area, the thickness of theMorrison Formation is estimated to be about 65m, but the basal contact of the Morrison Forma-tion is intruded by porphyry sills and the base isnot exposed.

DAKOTA SANDSTONEThe Dakota Sandstone (Kd), of Early Cretaceous age,consists of three informal members: an upper sand-stone unit, a middle shale zone, and a lower sand-stone unit (Ransome, 1911; Lovering, 1934). In theBreckenridge quadrangle the sandstone beds arecompletely lithified by silica cement, and commonlysandstone beds are metamorphosed by porphyrysills and dikes which led some authors to refer tosandstone beds as quartzite (Singewald, 1951).

The Dakota Sandstone is 52 to 69 m thick in theBreckenridge quadrangle (Lovering, 1934), and thebest exposure of this unit is at Rocky Point, in thecentral part of the map area, where Lovering (1934)and Taranik (1974) measured sections. Taranik(1974) measured the upper sandstone at 10 m thick.Lovering measured the middle shale as 18 m thick;but an estimate of the thickness of the middlemember based on Taranik’s data is 33 m. The lowerquartzite member is 12 m thick. To the north of thismap area in the Frisco quadrangle (Kellogg andothers, 2002) indicates that the upper sandstoneunit is 6 to 20 m thick, the middle shale unit is 6 to28 m thick, and the lower sandstone is 20 to 26 mthick. South of the map area at Boreas Pass theDakota Sandstone is 62 m thick (Taranik, 1974).

The upper sandstone unit is composed of light-gray and yellowish-gray, fine- to medium-grained,feldspathic, silica-cemented sandstone. Thin, blackshale interbeds occur at the top of fining-upward

sand, silt, and shale sequences. Black shaleinterbeds are commonly carbonaceous. At the baseof the upper unit is a massive-weathering, silica-cemented sandstone bed. Primary bedding struc-tures are mainly ripple marks and trough andplanar crossbeds. Worm tubes and casts of leavesand branches are common on some beddingsurfaces.

The middle shale member is composed ofinterbedded moderate-gray to black, locallycarbonaceous, micaceous shale, moderate-gray,black, light-gray, and tan-weathering micaceoussiltstone, and gray to light-gray, fine- and medium-grained, silica-cemented sandstone. Ripple markand planar lamination are common sedimentarystructures, and carbonaceous material occurs insome shale beds.

The lower quartzite member of the DakotaSandstone is composed of resistant-weathering,moderate-gray to light-gray, fine- to medium-grained quartzite that contains interbeds of thinlybedded, dark-gray shale and siltstone. Shale andsiltstone are micaceous. Chert-pebble conglom-erate, which is common in the lower member of theDakota Sandstone elsewhere, is not present in theBreckenridge quadrangle. Crossbed sets are asthick as 30 cm, and trough and planar sets occur inthe lower quartzite member.

The upper and lower quartzite units in theDakota Sandstone are severely fractured in thenortheastern part of the map area, and thequartzite beds of this unit are preferred zones forsulfide occurrences. Commonly, fractures and jointsare coated with iron-oxide minerals. Quartzite bedsin the Dakota Sandstone are lenticular and abruptlateral facies change characterizes these members.

BENTON SHALEThe Benton Shale (Kb) is of Late Cretaceous age,and it consists of interbedded dark-gray to blackshale, rusty-weathering siltstone, and dark-graylimestone. The Benton Shale occurs mainly in thenortheastern part of the Breckenridge quadranglewhere this unit is 95 to 110 m thick. This unit isgenerally poorly exposed, except at Rocky Pointwhere the upper part of the unit is exposed in afaulted sequence.

The name “Benton Shale” is applied by Kelloggand others (2002) in the Frisco quadrangle to the

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black-shale dominated sequence that occursbetween the Dakota Sandstone and limestone bedsof the Niobrara Formation. Kellogg and others(2002) retained the term Benton Shale in the Friscoquadrangle because of historical use, even thoughparts of the Benton Shale can be correlated region-ally with other units.

The Benton Shale consists of moderate-gray,dark-gray, and black, laminated and microlami-nated shale that is locally wavy-bedded, andinterbedded light-gray, tan-weathering siltstone.Limestone beds occur at the top of the BentonShale, and thin, gray, quartzitic sandstone bedsoccur at the base of the unit. The uppermost lime-stone beds consist of 1.5 m of thin-bedded, blackand dark-gray, fetid, resistant-weathering finelycrystalline limestone that exhibits pinch-and-swellstructures and contain interbeds of dark-gray,siliceous siltstone. Kellogg and others (2002) corre-lated this uppermost limestone zone with the JuanaLopez Member of the Carlile Shale of Berman andothers (1980). Below the uppermost limestone is 5m of dark-gray, fetid limestone and dark-brownand moderate-gray, calcareous, rusty-weatheringsiltstone and shale above 3 m of resistant-weath-ering, brownish-gray, fine-grained, rusty coloredarkose. This arkose is bioturbated at the base, andit contains chert pebbles locally. Kellogg and others(2002) correlated this sequence of limestone, silt-stone, shale, and arkose with the Codell SandstoneMember of the Carlile Shale of Berman and others(1980). The lower 25 m of the Benton Shale, equiva-lent to the Mowry Shale in Wyoming (oralcommun. from W.A. Cobban, 1998, cited in Kelloggand others (2002), consists of wavy-bedded blackshale that contains fish-scales; the lower 3 mconsists of thinly bedded, fine-grained, grayquartzite. The Benton Shale appears to be conform-able above the Dakota Sandstone in the map area.

NIOBRARA FORMATIONThe Niobrara Formation (Kn) is Late Cretaceous inage, and it consists mainly of shale and shalylimestone. Regionally the Niobrara Formationconsists of the Smoky Hill Shale Member at thetop and the Fort Hays Limestone Member at thebase, but these units were not separated in themap area. In the Frisco quadrangle, to the north ofthis map area, the thickness of the Niobrara

Formation is 148 m. In the map area, the NiobraraFormation is estimated to be about 150 m thick.

The Smoky Hill Shale Member consists ofmoderate-gray and light-gray, calcareous shale andinterbedded shaly limestone. Limestone is finegrained and fetid and becomes more shaly upward.The limestone is platy weathering and has a char-acteristic very pale-gray patina. In the Frisco quad-rangle to the north, this member is 138 m thick.The Fort Hays Limestone Member is composed ofblocky weathering, moderate-gray and light-gray,micritic limestone that is resistant to weathering.Limestone beds are 5 to 15 cm thick, and limestoneweathers light gray. Inoceramid bivalves (rudistidoysters) occur in some limestone beds. The FortHays Limestone Member is 6 to 10 m thick in themap area, and this member appears to be conform-able above the Benton Shale.

PIERRE SHALEThe Pierre Shale (Kp) is Late Cretaceous in age,and this unit consists mainly of black and grayish-brown shale and brownish-gray sandstone. In theFrisco quadrangle to the north, the Pierre Shale issubdivided into three informal members (Kelloggand others, 2002), with a combined thickness ofabout 1,000 m. Only the lowest member occurs inthe Breckenridge quadrangle. The lower shalemember is about 400 m thick in the map area, andthe top is not exposed.

The lower shale member consists of dark-gray,brownish-gray, and black shale and mudstone. Thebase of this member is calcareous, and calcite veinsare common within about 10 m of the Niobraracontact. Weathered outcrops show bedding fissility.In the Breckenridge quadrangle the lower contactof the Pierre Shale is apparently conformable withthe underlying Niobrara Formation.

CENOZOIC ROCKS AND DEPOSITSCenozoic rock units are represented by Tertiaryintrusives including sills, dikes, and stocks thatintrude Paleozoic and Mesozoic sedimentaryrocks in the eastern part of the quadrangle, andintrude Proterozoic basement rocks in the westernpart of the quadrangle. These intrusives aremiddle to late Eocene in age. Cenozoic surficialdeposits are mainly Quaternary units that formresidual and alluvial deposits east of the Blue

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River, and glacial deposits in the Blue River Valleyand in the Tenmile Range. Gold-dredge tailingsleft over from the early 1900s cover large portionsof the Blue River and French Gulch valley floorsin the northeast part of the quadrangle in andnear the town of Breckenridge.

TERTIARY INTRUSIVE ROCKSSix types of Tertiary intrusive rocks were identi-fied in the Breckenridge quadrangle. Intrusivebodies occur as dikes, sills, stocks, plugs, andlaccoliths. They are mostly intermediate calc-alkalic in composition, predominantly quartzmonzonite. Sills and a possible laccolith are mostcommon in the eastern half of the quadranglewhere they intrude Pennsylvanian through Creta-ceous sedimentary rocks. In the Tenmile Range ofthe western half of the quadrangle, where Protero-zoic rocks predominate, Tertiary intrusives mainlyform dikes and small stocks or plugs. In theBreckenridge mining district in the northeasternpart of the quadrangle, Ransome (1911) thor-oughly described the petrography of threeigneous rock types — two varieties of quartzmonzonite porphyry (our Tqp and Tqpm) and horn-blende-biotite monzonite porphyry (our Tmp) —and concluded that they all were emplaced closetogether in time. Lovering (1934) also describedthe intrusives and reached the same conclusion.

The map area contains three additional Tertiaryintrusive types not described by Lovering orRansome because these intrusives are present onlyoutside the main mining district. A quartzmonzonite stock (Tqm) with phaneritic texturemapped as the Humbug stock by Tweto (1974) anddescribed by Bergendahl (1963) intrudes theProterozoic basement rocks in the Tenmile Rangearound Peak 9. Quartz monzonite intrusion breccia(Tqmb) related to the Humbug stock is present in afew places. A very light-colored, very fine-grainedquartz latite (Tql) unit occurs as sparse, narrowdikes cutting the Proterozoic basement in theQuandary Peak area. Singewald (1951) also notedthis unit in his study of the Upper Blue River area.

HORNBLENDE-BIOTITE MONZONITE PORPHYRY

The hornblende-biotite monzonite porphyry(Tmp), of Eocene age, was described and referredto as monzonite porphyry by Ransome (1911) and

Lovering (1934). This porphyry is distinguishablefrom other intrusives in the area by its lack ofquartz phenocrysts and darker color. This rockunit is medium to dark gray to greenish gray,with 5 percent plagioclase phenocrysts 1 to 3 mmlong, 5 percent orthoclase phenocrysts 1 to 3 mmlong, 3 percent hornblende as subhedral blades 1to 6 mm long, and 3 percent biotite as pseudo-hexagonal phenocrysts 1 to 2 mm wide in anaphanitic groundmass. The hornblende andbiotite are often altered to chlorite (penninite),epidote, quartz, and calcite to varying degrees.Feldspar is commonly altered to varying degreesto calcite, sericite, and epidote. The groundmassconsists of abundant orthoclase and plagioclase,and minor quartz, biotite, augite, hypersthene,hornblende, magnetite, apatite, allanite, andzircon (Lovering, 1934).

The hornblende-biotite monzonite porphyryoccurs mainly as thin dikes and sills and as a largeintrusive mass (laccolith?) in the northeasterncorner of the quadrangle. The monzonite porphyryis intruded by, and thus is older than, megacrysticquartz-monzonite porphyry (Tqpm), although bothphases are considered close in age (Ransome, 1911;Lovering, 1934).

QUARTZ MONZONITE OF THE HUMBUG STOCK

The Humbug stock, of middle Eocene age, is aquartz monzonite (Tqm) and quartz monzoniteintrusion breccia (Tqmb) that is 8.7 sq km in areacentered around Peak 9 in the west-central maparea. Several smaller plugs occur adjacent to themain Humbug stock. Another small stock of fine-to medium-grained quartz monzonite in MonteCristo Gulch was shown as “Silver Plume” graniteof Precambrian-age by Singewald (1951), but thisstock is petrographicallly more similar to thequartz monzonite of the Humbug stock than toProterozoic “Silver Plume” quartz monzonite. Adefinitive isotopic age is lacking for the quartzmonzonite in Monte Cristo Gulch, but field rela-tions suggest that it is Tertiary in age. We havetentatively correlated it with quartz monzonite ofthe Humbug stock.

Rocks of the Humbug stock are very light gray,sparsely mottled with black mafic minerals, mostlymedium to course grained, and phaneritic. Theyhave an hypidiomorphic-granular crystalline

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texture. Bergendahl (1963) examined thin sectionsof quartz monzonite from the Humbug stock andfound the principle minerals to be quartz, oligo-clase-andesine, and microcline microperthite.Biotite, hornblende, and magnetite form less thanten percent of the rock, although biotite commonlygives the rock a black-mottled appearance. Apatiteand sphene are common accessory minerals.Phenocrysts of euhedral plagioclase and potassiumfeldspar are commonly present, but these mineralsare not always conspicuous due to the overallphaneritic texture of the rock. The rock is some-times weathered to a light-tan-orange color fromoxidation of small amounts of disseminated andfracture-fill pyrite. On the northeast margin of theHumbug stock, small exposures of Lower Paleo-zoic sedimentary rocks (xenoliths?) display strongcontact metamorphism. Quartzitic sandstone hasbeen recrystallized to quartzite, and dolomite isnow whitish-gray, coarse-grained marble.

The Humbug stock was previously mapped byTweto (1974) as Tertiary-Cretaceous in age. Bergen-dahl (1963) showed a younger dike of quartzmonzonite porphyry (Tqpm in this study) thatintruded the Humbug stock. A new 40Ar/39Ar ageof 42.05 ± 0.23 Ma on biotite was made fromsample K01-059 collected from the Humbug stockon the south flank of Peak 9 during this mappingproject [analysis by Richard Esser, New MexicoGeochronological Research Laboratory, Socorro].This analysis indicates that the quartz monzonite ismiddle Eocene in age. Four potassium-argon datesreported by Marvin and others (1974) for biotite inthe Humbug stock yielded an average age of 40.1 Ma.

Intrusion contact breccia (Tqmb) was mapped intwo areas along faulted contacts between quartzmonzonite and Proterozoic rocks. The intrusionbreccia consists of angular to sub-angular clasts ofcountry rock (mostly migmatite) up to 60 cm indiameter in a matrix of fine- to medium-grainedquartz monzonite. The matrix material is finergrained than the typical quartz monzonite andindicates more rapid cooling than the main body ofthe Humbug stock. This breccia occurs only on, oradjacent to, northwest-trending faults, whichsuggests that intrusion was contemporaneous withtectonic movement. The breccia does not occuralong igneous contacts away from faults. Slicken-

sided fault surfaces that cut through the brecciaclasts and matrix at one exposure on Peak 8 alsoshow that post-intrusive movement has occurredon northwest-trending faults in the quadrangle.Clasts of Cambrian-age Sawatch Quartzite wereenclosed in the breccia in Monte Cristo Gulch. TheSawatch is exposed only a short distance to thenorth of the breccia, and prior to erosion theSawatch was only a short distance vertically abovethe breccia exposure.

BIOTITE-HORNBLENDE QUARTZ

MONZONITE PORPHYRY

Two varieties of buff- or light-brown-weathering,porphyritic quartz-plagioclase-orthoclase-biotite-hornblende quartz monzonite porphyry (Tqp andTqpm), of probable Eocene age, are widely distr-ibuted in the map area. Distinction between thetwo varieties is based on the presence of largephenocrysts (megacrysts) of orthoclase, quartz,and plagioclase in Tqpm, or the lack of suchmegacrysts in Tqp. Compositionally, the two vari-eties of quartz monzonite appear identical,although field relations suggest that megacrysticquartz monzonite (Tqpm) crosscuts quartzmonzonite porphyry without megacrysts (Tqp).Quartz monzonite porphyry without megacrystsis present in the Rocky Point area as well as innumerous dikes and less common sills in theTenmile Range; at most places east of the BlueRiver the quartz monzonite porphyry is themegacryst-bearing variety, and it forms mainlysills and possibly laccoliths. Lovering (1934)referred to the quartz monzonite variety that lacksmegacrysts as “quartz monzonite porphyry (inter-mediate type).” Both authors referred to themegacryst-bearing unit as “quartz monzoniteporphyry.” Singewald (1951) referred to themegacryst-bearing variety as “Lincoln Porphyry,”and correlated these rocks with Lincoln Porphyryin the Leadville area. However, the Lincoln Porp-hyry in Leadville is considerably older than thequartz monzonite porphyry in the Breckenridgearea (Pearson and others, 1962; Marvin andothers, 1989).

In the megacrystic variety of quartz monzoniteporphyry, prominent orthoclase phenocrystscomprise as much as 20 percent of the rock, andquartz phenocrysts (commonly partly resorbed and

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embayed) comprise 2 to 10 percent of the rock.Megacrysts are typically 1 to 4 cm in length;Lovering (1934) reported orthoclase phenocrysts aslong as 8 cm. Euhedral biotite forms 1 to 3 percentof the rock and is usually much more abundantthan hornblende. Grayish-white, chalky plagioclasephenocrysts are also common and can be up to 1cm in length. Typical phenocryst size exclusive ofthe megacrysts is 1 to 5 mm. The groundmass isfine grained (less than 0.5 mm). Similar quartzmonzonite porphyry from a large intrusive mass inthe Frisco quadrangle to the north of Breckenridgeyielded a potassium-argon date from biotite of 44.1( 1.6 Ma (Marvin and others, 1989) and a rubidium-strontium whole-rock age of 44 Ma (Simmons andHedge, 1978).

Quartz monzonite porphyry without mega-crysts contains 10 to 30 percent phenocrysts offeldspar, quartz, and biotite in a fine-grainedmatrix. Phenocrysts of feldspar (mostly plagioclase,less orthoclase) comprise 10 to 25 percent of therock, are usually chalky and altered, and are 1 to 6mm in length. Quartz phenocrysts comprise 2 to 8percent of the rock, are often rounded and partiallyresorbed, and can be as large as 8 mm in diameter.Biotite phenocrysts up to 3 mm in length arecommonly altered to chlorite (penninite) andcomprise 2 to 10 percent of the rock. Hornblende israre or absent in most samples. At most places therock is partially altered. Where alteration is mostintense, it is difficult to distinguish the phenocrystsfrom the groundmass without the aid of a petro-graphic microscope. On freshly broken surfaces,the fine-grained groundmass is often a medium todark bluish- to greenish-gray color from alterationof mafic minerals to chlorite. The feldsparphenocrysts are commonly partly altered to sericiteor clays and minor carbonate.

QUARTZ LATITE

Intrusives of quartz latite (Tql), locally as felsic asrhyolite, were mapped only as a few narrow,north-south-trending dikes in the Tenmile Rangeon and near Quandary Peak. Quartz latite is theyoungest and least common of the Tertiaryigneous rocks in the quadrangle and is late-Eocene in age. Singewald (1951) correlated thisunit to rocks he had previously mapped in 1941 as“later white porphyry” in the vicinity of the

Lincoln Amphitheater and Wheeler Lake about 1mi south of the Breckenridge quadrangle. Heshowed that this unit was younger than intrusiveunits he labeled “gray porphyry group”, whichare the quartz monzonite and monzonite porp-hyries of the present study. Quartz latite dikeswith a similar description (Mach and Thompson,1998) on Tucker Mountain, which is two mileswest of the Breckenridge quadrangle, yielded afission track date from zircon of 38.3 Ma (Naeser,C.W., in written commun. referenced in Mach andThompson, 1998).

The quartz latite is very light gray or tan tonearly white, very fine grained, and contains smallphenocrysts (1 to 2 mm) of quartz, plagioclase, andpotassic feldspar (sanidine?). Sparse biotite pheno-crysts are usually altered to fine-grained musc-ovite, chlorite, and iron oxide. The small, equantphenocrysts comprise 5 to 20 percent of the rock.Quartz phenocrysts are rounded and partiallyresorbed. The groundmass is dense and locallyporcelain-like. K-feldspar stain on thin sectionsshows that potassic feldspar is a large componentof the groundmass, except where sericitic alterationhas destroyed most feldspar, including pheno-crysts. Intrusives of this type are easily distinguish-able in the field because of their light color anddense, fine-grained groundmass.

QUATERNARY DEPOSITSQuaternary units are mainly glacial tills and theirassociated outwash terraces, preglacial terracedeposits, landslides, and a suite of periglacialdeposits above treeline in the Tenmile Range. Ingeneral, the oldest Quaternary geomorphicsurfaces, and corresponding deposits, are found atthe highest elevations above the modern valleyfloor. Landslide deposits are found throughoutthe quadrangle, at all elevations, and range fromsmall slumps in Quaternary deposits to large,quasi-intact masses of bedrock that slid off wallsof the glacial valleys. Alluvial deposits are presentin most large drainages, and colluvium mantlesmany of the hillslopes below treeline. Age esti-mates for most Quaternary deposits are basedmainly on correlation of the youngest, massivemoraines with the Pinedale glacial advance in theRocky Mountains (dated at 16–23 ka), and correla-tion of adjacent subdued moraines to the Bull

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Lake glacial advance (dated at 130-150 ka). Thiscorrelation is based on the similarity of morainemorphology, weathering characteristics, surface-boulder frequency, and amount of stream dissec-tion, compared to Pinedale and Bull Lake typedeposits (Chadwick and others, 1997).

GLACIAL AND PERIGLACIAL DEPOSITS

The oldest glacial deposits (Qto) exist on the pied-mont west of Breckenridge, on which the ski areais developed. This map unit probably includesboth Bull Lake and pre-Bull Lake till deposits thatcannot be subdivided. The unit was also mappedat high levels on a ridge between Spruce Creekand McCullough Gulch. This high-level deposit ishighly eroded and is possibly non-glacial inorigin. Directly north of downtown Breckenridge,the westernmost part of older (pre-Bull Lake)gravels (Qgo) contains boulders up to 2 m in diam-eter in a poorly sorted matrix. This part of theolder gravel deposit may be till rather than allu-vium, and the original morainal morphology maysimply have been flattened by erosion.

Moraines of the Bull Lake glaciation (Qtb),which range in age from 130 ka to 150 ka, occurdirectly downstream of, or adjacent to but upslopeof, Pinedale moraines in the Blue River, SawmillGulch, Cucumber Creek, South Barton Creek,Indiana Creek, and Pennsylvania Creek. The BullLake terminal moraine of the Blue River glacierwas severely eroded by later Pinedale outwash andnow exists as two disconnected ridges. The easternridge is in the southernmost part of downtownBreckenridge between lower Illinois Gulch andEdwin G. Carter Memorial Park; the ridge is sepa-rated from the Pinedale terminal moraine by thealluviated valley of Illinois Gulch. Illinois Gulchwas not glaciated in the Pleistocene but adopted anice-marginal position during the Pinedale glacia-tion. The western Bull Lake moraine ridge formsthe interfluve between the Blue River and LehmanGulch (Warrior’s Mark area). The proximity of BullLake and Pinedale terminal moraines suggests thatthe Bull Lake glaciers were only slightly moreextensive than the Pinedale glaciers, which istypical in the Rocky Mountains (Porter and others,1983).

Pinedale moraines cover the largest area of allQuaternary deposits in the quadrangle, covering

the floor and sides of the Blue River Valley fromBreckenridge to the southern map boundary, alinear distance of about 10 km. Pinedale morainesalso cover much of the piedmont northwest ofBreckenridge and the lower parts of deep glacialvalleys in the Tenmile Range. The Blue Riverglacier at its maximum Pinedale extent was 18 kmlong (measured from the head of the Monte Cristovalley) and up to 250 m thick. The glacieradvanced to within 0.5 km of downtown Brecken-ridge at the maximum Pinedale advance at about23 ka, and the glacier deposited a massive terminalmoraine (Qtp1). This Blue River glacier was fed byfour large tributary glaciers in the Tenmile Range -the Monte Cristo Creek, McCullough Gulch,Spruce Creek, and Crystal Creek glaciers, as well asby a small amount of ice that spilled over HoosierPass, directly south of the quadrangle, from thePlatte River glacier. Although the tributary glaciersin the southern three drainages merged with mainBlue River ice in the normal fashion, the CrystalCreek glacier was forced to turn abruptly north bythe combined Blue River and Spruce Creek icemass, and ice spilled out of Crystal Creek valleyonto the interfluve between Crystal Creek andCarter Gulch. At that location the ice mass mergedwith a smaller glacier flowing down Carter Gulch,and together they built a terminal moraine justsouth of the confluence of Carter and LehmanGulches. This moraine complex is separated fromthe Blue River terminal complex by a ridgecomposed of Bull Lake till (Qtb). A separate, 4.5 km-long glacier in Sawmill Gulch terminated in theBlue River valley about 1 km west of the terminusof the Blue River glacier, where it built a morainecomplex that contains Old Town Lake and theBreckenridge Outdoor Education Center.

Pinedale glaciers east of the Blue River, inPennsylvania and Indiana Creeks, did not advancefar enough to merge with the Blue River glacier.The Pinedale terminal moraine of the PennsylvaniaCreek glacier (Qtp) nearly merges with a bulginglateral moraine of the Blue River glacier thatpushed eastward into the mouth of PennsylvaniaCreek. The Pinedale moraine of the Indiana Creekglacier occurs 2 km upstream from the easternPinedale lateral moraine of the Blue River glacier,which blocked Indiana Creek and formed a short-lived ice-marginal lake. Early Pinedale moraines

23

are composed of bouldery till deposits, withsubround to round boulders up to 4 m in diameterembedded in a massive, unstratified matrix ofsand, silt, and clay.

After the early Pinedale ice advance, glaciersretreated and then readvanced, forming a youngerbelt of moraines (Qtp2) that can be differentiated interminal moraine complexes of the Blue Riverglacier, the Crystal Creek and Carter Gulchglaciers, and the Sawmill Gulch glacier. In thesmaller glacial deposits of Indiana Gulch, Pennsyl-vania Creek, Lehman Gulch, Cucumber Gulch,Cucumber Creek, and South Barton Gulch,moraines of early Pinedale age cannot be distin-guished. The last major Pinedale readvance, whichoccurred at about 16 ka, is represented by theyounger Pinedale till (Qtp3). In the Blue Rivervalley, glacial ice retreated south of Goose PastureTarn, forming a lake behind the Qtp1 or Qtp2

moraine, but then ice readvanced over the lakebed, plowing up silt and sand to form the boulder-poor moraine that presently impounds the tarn(Qtp3). In the Crystal Creek valley, youngerPinedale ice spread as a thin body onto the CrystalCreek and Carter Gulch interfluve and then stag-nated, creating an extremely complex topographyof small ridges, hummocks, and closed depres-sions. Young Pinedale till is also mapped inCucumber Creek and South Barton Creek.

The youngest till in the quadrangle (Qtn) formsvery stony, unvegetated moraines in the cirques ofMcCullough Gulch, Spruce Creek, and CrystalCreek. These sharp-crested, steep moraines occurwithin 0.5 km of cirque headwalls, and themoraines are assigned to the early Neoglacial stadeof the Holocene, at about 3 to 5 ka.

Lacustrine deposits (Ql) of sand, silt, and claywere mainly deposited during and shortly after thePinedale glaciation, when moraines dammed theBlue River and its tributaries to create temporarylakes. The largest area of lacustrine deposits under-lies Goose Pasture Tarn and the Goose Pasture inthe Blue River Valley, where a lake was dammedby the younger Pinedale terminal moraine (Qtp3).Another small lake existed at the mouth of IndianaCreek, where the eastern Pinedale lateral moraineof the Blue River glacier blocked the drainage.

The glaciated valleys of the Tenmile Range arerimmed with a variety of periglacial and mass-

movement deposits, including rock glaciers,protalus ramparts, talus cones, colluvium, andseveral ages of landslides. The typical pattern is foractive and inactive talus (Qta and Qti), respectively,to cover the oversteepened valley sideslopes belowthe Pinedale glacial trimline. If these talus depositsclearly emanate from a single gully, are distinctlycone-shaped, and contain an axial channel flankedby debris-flow levees, they were mapped as talusfan deposits (Qtf). At most locations where talusdeposits are shaded by high valley or cirque walls,the talus has been transformed into active or inac-tive rock glaciers (Qra or Qr). The most commonrock glaciers, such as occur against the southernvalley walls of upper Spruce and Crystal Creeks,are lobate rock glaciers composed of talus bouldersand interstitial ice. In contrast, the tongue-shapedrock glaciers occupying the cirques of McCulloughGulch, Spruce Creek, and Crystal Creek were prob-ably formed when talus buried wasting Neoglacialcirque glaciers, so they probably contain a solid icecore beneath a veneer of rubble. Other periglacialdeposits include solifluction deposits (Qs) that arecommonly above treeline; these are defined bystone stripes and terraces that show the surficialfrost-shattered rubble is (or was) moving downs-lope. In contrast, where bedrock is covered by athin veneer of frost-shattered rubble (felsenmeer)that shows no signs of downslope movement, thearea is mapped as the underlying bedrock.

LANDSLIDE DEPOSITS

Landslide deposits are widespread above andbelow treeline and have been subdivided by agebut not by type. Most larger landslides (Qls) arerotational slumps derived from bedrock and rangein size from the 1 km-diameter slump on theeastern flank of Peak 8 to slumps less than 50 mwide (about the limit of mapping at this scale).The deposits are composed of angular rock rubblefrom sand to boulder size embedded in a finermatrix of crushed rock material. The largerbedrock slumps consistently occur near treelineon the eastern end of the major ridges of theTenmile Range. Each landslide has created aneast-facing amphitheater that contains landslidesof mid-late Holocene age (Qlsy), early Holocene tolatest Pleistocene age (Qlsi), or pre-Pinedale age(Qlso). The two former deposits are composed of

24

rock rubble, whereas the latter is generally largeblocks of quasi-intact bedrock that slid off theheadscarp and were subsequently overrun andsmoothed by Pinedale-age cirque ice. Thus, itappears that the landslide amphitheaters on thenorth side of McCullough Gulch and at the headsof Carter Gulch, Sawmill Gulch, and CucumberCreek were formed by pre-Pinedale slumping andwere later occupied by Pinedale cirque ice.

Landslides derived from Quaternary depositsare typically smaller than the bedrock slumps andare most commonly derived from till on steepvalley sidewalls. The largest such slides occur onthe west valley wall of the Blue River betweenMcCullough Gulch and Spruce Creek. Only asingle historic landslide deposit (Qlsh) was mappedin the quadrangle. This 30 m-wide by 100 m-longslump in Pinedale till formed shortly before 1999and buried Aqueduct Road near the southernboundary of the map area. However, that slide isonly the latest manifestation of slumping that hascontinued since Pinedale deglaciation.

COLLUVIAL AND ALLUVIAL DEPOSITS

Colluvial deposits and mixed alluvial-colluvialdeposits are common in the nonglaciated north-eastern part of the quadrangle. Colluvial deposits(Qc of Holocene age and Qco of Pleistocene age)are composed mainly of silt and sand and aretypically mapped at the foot of hillslopes and inbroad swales that may extend nearly to hilltops.Mixed alluvial-colluvial deposits (Qac of Holoceneage and Qaco of Pleistocene age) are also sandybut are better sorted and stratified than colluviumand occur in deeper swales or on piedmonterosion surfaces (such as the southern Brecken-ridge ski area) that occasionally carry runningwater.

Because glacial deposits are so widespread inthis quadrangle, alluvial deposits are somewhatunderrepresented. The largest areas of alluvialdeposits are the aggraded valleys of French Gulchand the Blue River north of its terminal moraine.The Blue River Valley downstream from theterminal moraines is dominated by five fluvialterraces, the younger three of which (Qob, Qop1,

Qop2) can be traced directly back to the terminalmoraines. For example, downtown Breckenridge isbuilt mainly on the proximal outwash terraces

derived from the Pinedale and Bull Lake terminalmoraines. East of Ridge Street, downtown Brecken-ridge is on the Bull Lake outwash terrace (Qob),which lies 12 to 14 m above the modern Blue River.Between Ridge Street and Main Street, the city isbuilt on the narrow older Pinedale outwash terrace(Qop1), which is 5 m above stream level, whereaswest of Main Street and west of the Blue River thedeveloped parts of downtown are on a youngerPinedale outwash terrace (Qop2), which is 2 to 3 mabove stream level. This latter terrace was severelydisturbed by placer mining. Holocene alluvium(Qal) is mapped along the courses of most tributaryvalleys, as well as in the Blue River valley south ofthe Goose Pasture.

The two older fluvial terraces of the Blue Rivercannot be traced to moraines, so their age is esti-mated based on height above modern streams andon degree of clast weathering and soil-profiledevelopment. The younger of these terracedeposits (Qgo) is 36 m above the modern Blue Riverand forms the interfluve between the Blue Riverand French Gulch (Weisshorn subdivision); acorrelative terrace lies north of French Gulch(Highlands subdivision). This terrace is nearlythree times as high above the Blue River as is theBull Lake outwash terrace (36 m versus 12 to 14 m),but its weathering is not significantly greater thanthat of Bull Lake deposits. Thus, we assign amiddle-to-lower Pleistocene age to this deposit.

The oldest terrace deposits (QTgg) underlie anextensive surface that is best preserved west of theBlue River. The top surface of these terrace depositsis 115 m above stream level at the northern mapboundary but slowly converges to a height of only60 m above stream level in southern Breckenridge.Thus, the top of this deposit forms a nearly hori-zontal surface, in strong contrast to all youngerterraces, which slope northward at 15 m/km. Thisterrace deposit includes two facies, an older BlueRiver alluvium and a younger (or coeval) alluviumand colluvium from tributaries. Blue River allu-vium is well stratified sand and round-subroundcobble gravels derived from Proterozoic crystallinerocks. The alluvium underlies terraces west of BlueRiver between Sawmill Gulch and the northernmap boundary. Tributary alluvium and colluviumunderlie the southern slopes of French Gulch northof Barney Ford Hill between elevations of 9700 and

25

9880 ft (up to 55 m above French Gulch) and theslopes northeast of Gibson Hill. Tributary alluviumis generally massive to poorly stratified gravellysand that contains angular cobbles and pebblesfloating in sandy clay matrix. The deposit iscapped by a strong red soil with textural B hori-zons greater than 2 m thick, which compares favor-ably to the type Rocky Flats paleosol of theColorado Front Range (Birkeland and others, 1996),which is estimated to be about 1 Ma. The BlueRiver gravels were mapped as “older terracegravels” by Ransome (1911), whereas he mappedthe tributary facies as “older hillside wash.”Kellogg and others (2002) mapped both facies inthe adjacent Frisco quadrangle as “Boulder gravel

of Gold Run,” from which we adopted their termi-nology and age estimate of lower Pleistocene tolate Tertiary.

Alluvial fan deposits (Qf), although small, arewidespread throughout the quadrangle. Fans typi-cally form where large tributary streams enter theBlue River valley or where smaller tributaries enterthe large tributaries. The oldest fan deposits (Qfp)accumulated during the Pinedale glaciation fromglacial outwash and ice-marginal streams, but theirsource streams have been captured or incised sothey no longer receive significant deposits. Post-glacial alluvial fans are divided into an older groupof incised fan surfaces (Qfi) and a younger group ofactive fan surfaces (Qfy).

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STRUCTURAL SETTINGThe Breckenridge quadrangle is within a struc-turally complex part of the Southern RockyMountains. The quadrangle straddles the crest ofthe Tenmile Range, the upper Blue River valley,and the western-most slopes of the Front Range.Elements of five large-scale tectonic and tectono-stratigraphic features are recognized within thequadrangle: Early Proterozoic regional metamor-phism of crystalline rocks (Reed and others, 1987),Middle Proterozoic east-northeast trending shearzones (Tweto and Sims, 1963; Shaw and others,2001), the Central Colorado Trough of Pennsyl-vanian-Permian time (DeVoto, 1980), theLaramide-age Colorado mineral belt (Tweto andSims, 1963), and the northern part of the Neogene-age Rio Grande rift system (Tweto, 1979; Kellogg,1999).

Regional metamorphism during Early Protero-zoic time resulted in the formation of amphibolite-grade gneisses derived from sedimentary protolithson the Breckenridge quadrangle. The metamor-phism that produced the foliated rocks probablyoccurred simultaneously with the emplacement oflarge plutons during the Early Proterozoic (Reedand others, 1987). During the Middle Proterozoic, aseries of east-northeast-trending shear zones devel-oped that deformed the older Proterozoic rocks(Shaw and others, 2001).

The Central Colorado Trough (DeVoto, 1980) isa complex graben system into which thick depositsof clastic sediments were deposited during theuplift of Ancestral Rockies in Pennsylvanian andPermian time. The faults and folds of the troughwere recurrently active from Proterozoic time untilpossibly Quaternary time (Fig. 4; DeVoto, 1980).The ancestral Front Range uplift bounds the troughto the northeast of the quadrangle.

The Colorado mineral belt is a 400 km longnortheast-trending zone characterized by nume-rous Laramide (Late Cretaceous-Early Tertiary)igneous intrusions, associated ore deposits, andfaults. The Colorado mineral belt follows a zone ofstructural weakness defined by major northeast-trending shear zones that originated duringPrecambrian time (Tweto and Sims, 1963).

The Rio Grande rift system is a regional exten-sional tectonic feature expressed as a series of northto north-northwest-trending normal faults andgrabens that extend from New Mexico to theWyoming border through the heart of the southernRocky Mountains in Colorado. The Blue Rivergraben north of the Breckenridge quadrangle is thelargest of the northernmost structures of the RioGrande rift system (Kellogg, 1999).

FAULTSIn the Tenmile Range, northwest-trending high-angle faults are the most obvious tectonic features.At least two faults of large separation are presentin the map area. Both are northwest-trending,steeply east-dipping normal faults. The faultexposed near the Briar Rose Mine and Peak 9 inthe Tenmile Range juxtaposes Pennsylvanian-agesedimentary rocks with Proterozoic gneisses andalso cuts the 42 Ma Humbug stock. The otherlarge fault is buried beneath Quaternary glacialdeposits in the north-central part of the quad-rangle near Breckenridge and probably juxtaposesCretaceous sedimentary rocks with Proterozoicbasement. This fault projects into a fault mappedin the Frisco quadrangle (Kellogg and others,2002) and may have moved as recently as theQuaternary. Most northwest-trending fault planesmeasured in the field have dips greater than 70degrees. Co-genetic, generally north-trendingfaults and tension fractures with small separationthat run between the larger northwest-trendingfaults in the Tenmile Range provided zones ofweakness and open spaces that were filled byintrusive dikes and mineralized veins in mid-Tertiary time. These north-trending faults, dikes,and veins are prominent in the southern part ofthe map area in the Tenmile Range but not in thenorthern part. The northwest-trending majorfaults have been active from before Eocene time,and some have been active possibly into theHolocene. Eocene-age intrusive breccia bodiesaligned along the faults indicate that the faultswere probably active during Eocene time. Post-intrusive faulting was also noted on these faults.Northwest-trending faults have possibly

27

STRUCTURAL GEOLOGY

displaced Pinedale-age (late Quaternary) glacialdeposits on the eastern flank of the TenmileRange. This is indicated by linear features andpossible scarps in the glacial deposits. Some of thelinear features (identified from aerial photo-graphs) in Quaternary glacial deposits extendfrom mapped brittle faults in adjacent bedrock.Kellogg and others (2002) also noted northwest- tonorth-trending faults of Quaternary age in theFrisco quadrangle. There is some evidence for pre-Tertiary movement on these northwest- andnorth-trending faults. The presence of Tertiary-agemineralization and silicification on the faultsshow that some of the faults existed prior to theEocene intrusive and mineralization event. On Mt.Helen, between Crystal Creek and Spruce Creekin the Tenmile Range, a northwest-trending shearzone that is probably contiguous with the fault at

the Bryn Mawr Mine exhibits shear fabric, chlo-ritic alteration, and cataclastic texture indicative ofductile or brittle-ductile deformation indicatingthat the shear zone was active during MiddleProterozoic time, concurrent with the MiddleProterozoic deformational event of the HomestakeShear trend (Shaw and others, 2001). Movementon north-northwest-trending faults may be relatedto movement on the very large Mosquito Fault,which lies about 2.5 km west of the Breckenridgequadrangle boundary. The latest movement on theMosquito Fault is probably late-Pleistocene(Widmann and others, 1998). The younger move-ment on these north and northwest-striking faults,including Quaternary-age faulting, may be tecton-ically related to the Rio Grande rift system(Kellogg, 1999).

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Central Colorado Trough

Uncompaghre Uplift

Ancestral Front R

ange Uplift

Saw

atch Uplift

Central Colorado Trough

Apishapa Uplift

Sangre de C

ristoU

plift

Figure 4. Sketch map showing tectonic uplifts (dark gray) in relation to the Central Colorado Trough ofDe Voto (1980) during Pennsylvanian time. Modified from De Voto, 1980, Fig. 3.

An east-northeast-trending set of faults, frac-tures, Tertiary intrusive dikes, and low-gradehydrothermal alteration transects the TenmileRange in the vicinity of the Humbug stock. Thisstructural zone is interpreted to be originallyProterozoic in age but has been reactivated duringlate Paleozoic, Laramide, and possibly mid-Tertiarytime. The intersection of this east-northeast struc-tural zone with the younger northwest-trendingfaults may have provided a significant zone ofweakness that was exploited by magma thatformed the Humbug stock. This east-northeaststructural zone is interpreted to be an expression ofthe Colorado mineral belt (Tweto and Sims, 1963)and the Middle Proterozoic Homestake Shear trend(Shaw and others, 2001). The zone appears to be astructural link between the Breckenridge miningdistrict to the northeast and the Kokomo-Tenmiledistrict to the southwest. This fault zone may alsocontinue beyond Breckenridge northeast to theKeystone quadrangle, where Widmann and others(2003) mapped the southwestward continuation ofthe Montezuma Shear Zone. Figure 5 is a rosediagram of the strike attitudes of faults as meas-ured at the outcrop in the Tenmile Range of theBreckenridge quadrangle.

In the eastern part of the Breckenridge quad-rangle a mosaic of closely spaced faults have smallseparations, and these faults show five directionsof preferred strikes. From oldest to youngest, thestrikes of faults are (1) northwest, (2) east, (3)north-northeast, (4) northeast, and (5) north. North-west-striking faults are relatively rare east of theBlue River in comparison to the Tenmile Rangewest of the Blue River where the northwest-trending faults may be Proterozoic in age. In thenortheastern part of the Breckenridge quadranglethe dominant structural grain is north-northeastand south-southeast, but faults of this orientationare rare in the southeastern part of the map area.East of the Blue River, the northeast-striking faultsappear to be ladder or crossover faults betweennorth-northeast-trending faults. North- and north-west-striking faults appear to have the largest sepa-ration and the longest strike length.

Taranik (1974) showed the Blue Valley faulttrending north along the Blue River Valley fromHoosier Pass. The Blue Valley fault was shown asbeing covered by Quaternary deposits along most

of the trace. At Hoosier Pass he showed MinturnFormation on both sides of the fault, with bedsdipping toward the fault on opposite blocks.Taranik’s cross section B-B’ showed nearly 275 m ofseparation on a reverse fault that was down on thewest block. Our structural solution using foldsshown on our cross-section B-B’ does not requireseparation along a fault to explain the distributionof rock units and dip directions of rocks, so we donot show the Blue Valley fault on our map.

FOLDSLarge-scale folds dominate in Paleozoic rocks inthe eastern part of the map area, but small-scaleductile folds related to Early Proterozoic deforma-tion are common in metamorphic rocks of the

29

Key

N

EW

S

Rose Diagram Bi-DirectionalTotal Data Points = 67Bucket Size = 9 degrees Error Size = 0 degrees

0 10

Figure 5. Rose diagram of strike direction offault planes measured on the outcrop in thePrecambrian area of the Tenmile Range,Breckenridge quadrangle. The strong struc-tural trends of N30W to N45W, and N50E toN80E is evident.

Tenmile Range and in small exposures in thenortheastern part of the Breckenridge quadrangle.

Several large-scale folds were mapped in theeastern part of the Breckenridge quadrangle. Northof French Gulch and east of the Blue River, a large-scale anticline was mapped by Bartos east ofGibson Hill (see cross-section A-A’). This anticlinetrends northwest into the southern part of theFrisco quadrangle (Kellogg and others, 2002) wherethe east flank of the anticline is exposed east of theBlue River. Taranik (1974) showed the Monte Cristosyncline trending northward from Hoosier Passdown the Blue River Valley, and he showed theHoosier anticline trending northward along theeast side of the Blue River Valley. Both folds aredepicted on our map at similar locations to thoseshown by Taranik (1974). The Monte Cristosyncline and Hoosier anticline are broad, open,

north-plunging structures (cross-section B-B’) thatare covered by Quaternary deposits over most oftheir traces.

Small-scale folds related to Early Proterozoicregional metamorphism are common in thegneisses of the Tenmile Range. Ptygmatic folding offelsic layers and quartz segregations is common inthe migmatitic gneiss. The attitudes of folds in themigmatite were not recorded because they usuallyappeared chaotically oriented on the outcrop scale.

METAMORPHIC FOLIATIONNumerous attitudes of metamorphic foliationwere measured in the Proterozoic rocks of theTenmile Range. The rose diagrams in Figure 6show that the foliation has a remarkably consis-tent north-northeast strike and southeast dip. Thiscontrasts with metamorphic foliations in the

30

Figure 6. Rose diagrams of metamorphic foliation strike and dip directions in theTenmile Range, Breckenridge quadrangle. A dominant foliation strike direction ofN10E to N35E is evident. East-southeast dips are also indicated. Most dips aresteep.

Key

N

Rose Diagram Bi-Directional

Total Number of Points = 198

Bucket Size = 6 degrees Error Size = 0 degrees

0 26

Key

N

EW

S

Rose Diagram Uni-Directional

Total Data Points = 198

Bucket Size = 3 degrees Error Size = 0 degrees

0 14

Metamorphic Foliation Strike Direction Metamorphic Foliation Dip Direction

Keystone quadrangle (Widmann and others,2003), which show a much wider variation instrike and dip. The rose diagrams (Fig. 6) alsosuggests that foliation in the Breckenridge quad-rangle has locally been either rotated parallel witheast-northeast- and north-northwest-trendingfaults or shear zones, or possibly has developedas primary foliation in localized shear zones thatwere active in the Middle Proterozoic (Shaw andothers, 2001). In the northern part of the TenmileRange, within and north of the northeast-trendingstructural zone centered near the Humbug stock,foliations are less consistently oriented than theyare in the higher and better exposed area to thesouth of the Humbug stock.

STRUCTURAL EVOLUTIONThe oldest structures on the quadrangle areobserved in the early Proterozoic gneisses of theTenmile Range. Primary metamorphic foliationand folding in the gneisses trends dominantlynorth-northeast. This foliation is thought to havedeveloped contemporaneously with the emplace-ment of large intrusive masses between 1,790 and1,660 Ma (Reed and others, 1987). The east-north-east faults and dikes that transect the northernpart of the Breckenridge quadrangle are anexpression of the Middle Proterozoic (1.4 Ga)Homestake Shear trend (Shaw and others, 2001)and the Colorado mineral belt (Tweto and Sims,1963). Faults of this structural trend were rejuve-nated during Laramide time, and possibly Quater-nary time, and intrusives exploited these struc-turally weak zones during early Tertiary time.

North-northwest-trending faults show evidence ofslip as early as Proterozoic time and were rejuve-nated during and after Eocene time. Post-Eoceneslip on north-northwest-trending faults was alsoobserved in the Tenmile Range. Some north-north-west-trending faults have possibly been active asrecently as the late Pleistocene.

In the eastern part of the map area, the locationof range-front growth faults that governed erosionand deposition of the thick Pennsylvanian andPermian sequences remains a mystery; the thick-ness of the Maroon Formation increases drasticallyeast of Rocky Point, but specific faults responsiblefor the thickness change were not located. Afterlithification and later uplift of the Paleozoic andMesozoic sequence, large-scale folds formed in theeastern part of the map area, possibly duringLaramide deformation; the anticlinal structure eastof Gibson Hill, the Hoosier anticline, and theMonte Cristo syncline pre-date most faults of thefive different orientations described above. Thisfolding event post-dated deposition of the LateCretaceous Pierre Shale and pre-dated late Eoceneplutonic activity. In the eastern part of the maparea, as in the Tenmile Range, faults were activeduring Eocene igneous activity. Clearly, faultscontrolled the emplacement of dikes, and dikesand sills are also cut by faults of all orientations.Prominent north-south trending faults may berelated to extensional tectonics of the Rio Granderift (Erslev and others, 1999; Kellogg, 1999), whichis generally considered to have begun around 29Ma (Tweto, 1979).

31

The Breckenridge area has a long and productivehistory as a mining center. The principal focus ofmining was gold, and extremely rich placers, andto a lesser extent lode deposits, were mined from1859 to 1959. Most of the mines were located inthe northeastern part of the Breckenridge quad-rangle, and these mines are the main subject ofthis section on economic geology. Other minesand adits are scattered through the southeasternand western part of the map area. Mines in theBreckenridge quadrangle have had extensiveproduction, and significant mineral resourceslikely remain in the ground. However, conversionof the local economy from resource based to recre-ation based, with accompanying widespread high-cost residential development, will likely precludeany future development of metallic resources inthe Breckenridge area.

The dominant mining operation in the quad-rangle was placer dredging, focused in the BlueRiver and French Gulch drainages. More than $15.5million worth of gold (at historical prices of $17.50to $35 per oz) was recovered from these placers(Parker, 1974). Placer dredging in the quadranglelasted for one hundred years (from 1859 to 1959),with most production occurring between 1906 and1924 (Parker, 1974).

Lode mining commenced in 1869 and duringthe main phase of lode mining twenty-five to thirtyveins were exploited over a period of about 60years. Although the initial exploration interest inthe Breckenridge district was gold, arrival of therailroad in the Blue River Valley allowed transportof base-metal ores to smelters in Denver, and prod-uction of silver, lead, and zinc became economi-cally feasible. The Wellington Mine was the largestlode mine in the Breckenridge district. Limitedmining activity also occurred on scattered localesthroughout the Tenmile Range (Upper Blue RiverDistrict) from sulfide veins that cut Paleozoic andProterozoic rocks (Singewald, 1951).

The northeastern part of the Breckenridgequadrangle, specifically Gibson Hill, was exploredby Asarco, Incorporated, in 1990 under the supervi-sion of Paul J. Bartos; descriptions of mine geology,

alteration, and geochemistry are derived from thiswork.

BRECKENRIDGE MINING DISTRICTThe main part of the Breckenridge mining districtis located in the northeastern part of the Brecken-ridge quadrangle. Ransome (1911) and Lovering(1934) provide detailed descriptions of the geol-ogic setting of the mining district and of the minesthat were in operation at the time they completedtheir studies. Placer mining operations and lodemines were active in the northeastern part of theBreckenridge quadrangle from 1859 to 1959.

The dominant mining operation in the quad-rangle was placer dredging in the Blue River andFrench Gulch drainages. Initial mining effortsproduced substantial rewards. The Denver RockyMountain News issue of July 24, 1861 (referencedin Parker, 1974) stated that placers at the mouth ofwhat is now known as Barney Ford Gulch yielded$3.64 to the pan or more than 0.1 oz per pan. Deve-lopment of the district was rapid; by 1870, FrenchGulch had 27 km of ditches, 2,000 m of flumes, and5 hydraulic monitors mining the stream gravels(Parker, 1974).

More than $7 million in gold was generated inthe district’s first twenty years, most from simplesluice box and hydraulic washing methods ongulch and bench gravels (Lovering and Goddard,1950). Attention then turned to the deep gravelsbeneath the stream level in broad valleys such asFrench Gulch and the Blue River. Starting in 1898several hydraulic elevator plants were operatednear the mouths of French and Indiana Gulch. Thelargest hydraulic operation was the Gold Pan minein the town of Breckenridge. By 1901, the Gold PanMining Company had constructed a 4.8-km ditchand a 2,400 m pipeline, which delivered water thathad a 107-m head to the monitors and fourhydraulic elevators (Parker, 1974). A gravel thick-ness of more than 22 m was mined. This operationlasted until 1905 when it closed because gradeswere lower than expected and the very large boul-ders presented an insurmountable mining problem(Parker, 1974).

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ECONOMIC GEOLOGY

The technical answer to the presence of largeboulders was the use of dredges to mine placergold, and dredges were successfully employed inthe district beginning in 1906. The initial dredgewas constructed by the Reliance Gold Company ata cost of $35,000 and was soon yielding $1,000 aday in gold (Parker, 1974). Dredging continueduntil 1959; the remains of the last dredge can stillbe seen in French Gulch about 300 m west of theCountry Boy Mine.

Lode mining in the Breckenridge miningdistrict commenced in 1869. Twenty-five to thirtyveins in the Breckenridge mining district eachproduced at least a thousand tons of ore (Lovering,1934). At first, the interest was principally in gold;significant production of lead and zinc did notoccur until a railroad to Denver was completed in1880, which allowed access to smelters andmarkets (Lovering and Goddard, 1950). TheWellington Mine, on the north side of French Gulchapproximately 2.5 km east of Breckenridge, was byfar the largest lode mine in the district. From 1887,when production started, to 1929, over 737,000 tonsof ore were produced, and the ore yielded 6,500 ozof gold, 750,000 oz of silver, 41 million lbs of lead,and nearly 165 million lbs of zinc (Lovering andGoddard, 1950). At the Wellington Mine there wereover 20,000 m of tunnels and shafts through avertical range of 238 m. The Wellington mine wasinactive from 1929 to 1948. Between 1948 and 1958production occurred in the portion of the mineabove the water table. According to smelter sheetsfrom the Resurrection Mill, where the ore wasshipped, over 58,000 tons of ore were mined duringthis period with grades of about 0.04 oz/ton gold,3 oz/ton silver, 10 percent lead, 14.5 percent zinc,and 0.4 percent copper.

Veins at the Wellington Mine strike northeast-erly and dip 60º to 65º, typically to the southeast.The main vein consisted of irregular shoots ofsphalerite and galena scattered through a strikelength of 900 m (Lovering, 1934). At depth, themineralized zone became pyritic and non-commer-cial. Vein widths varied from a few cm to 10 m; 1.5m was the approximate average. The largest oreshoot was on the Great Northern vein, whichextended 213 m along strike and extendeddowndip from the surface to 15 m below the fifthlevel. According to Lovering (1934), major ore

deposits occurred where brittle rocks of themonzonite porphyry or the Dakota Formationformed one wall, which was favorable for forma-tion of open space, as opposed to soft, easilydeformed shale. In addition, ore shoots tended tooccur where veins steepened or made a strikechange (Lovering and Goddard, 1950). The generalgeology of the Wellington Mine consisted of a veincomplex within the hornblende biotite monzonitestock; sections drawn by Lovering (1934) throughthe workings show considerable structuralcomplexity.

Veins in the district are typically sulfide richand consist of aggregates of pyrite, sphalerite, andgalena in varying abundances, plus commongangue minerals of ankerite, quartz, sericite, andcalcite (Lovering, 1934, p. 27). Gangue composed ofsiderite and ankerite appears to be late relative tosulfide minerals, and the gangue fills cracks inearly sulfide mineral deposits. Sulfides tend to bemassive with no crustification or depositionalbanding (Lovering, 1934). Much of the sphalerite isdark brown to black and iron-rich (marmatite).Most of the rich gold ore came from the shallowoxidized zone that generally extended 30 to 90 mbelow the surface. Both Ransome (1911) andLovering (1934) concluded that much of the gold inthe vein ores was supergene enriched. The upperportions of many veins were also enriched in leadas well; much of the enrichment is attributed toresidual enrichment from dissolution of sphalerite(Lovering and Goodard, 1950), although somehypogene zonation of galena relative to sphaleriteprobably occurred. Adjacent to the veins, theigneous wallrock is typically converted to anaggregate of sericite, quartz, and ankerite andminor pyrite (Ransome, 1911). Locally, thequartzitic sandstones of the Dakota Formationshow evidence of leaching, and the bright-redsandstone and siltstone of the Maroon andMorrison Formations are commonly bleached to agray-green color near veins.

Skarns occur locally in the Breckenridge miningdistrict, and these contact metamorphic mineraldeposits had some minor production. Garnet-epidote-magnetite skarn occurs in the calcareouslower portion of the Morrison Formation onGibson Hill and on Prospect Hill west of theWellington Mine. Accompanying the skarn are

33

local occurrences of pyrite-chalcopyrite-(gold),which were mined on a small scale. The cluster offaults at Gibson Hill, along with the occurrence ofskarns and hornfels, suggests that an intrusionoccurs at depth in this area.

Mining essentially ceased in the Breckenridgemining district in 1958, although the Country BoyMine on French Gulch operates as an historicalmine park for tourists. This mine, located south ofthe Wellington Mine across French Gulch,produced about 20,000 tons of ore. In 1947 produc-tion was about 500 tons/month with gradesapproximately 0.1 oz/ton gold, 3 oz/ton silver, 1percent lead, 0.2 percent copper, and 30 percentzinc from a 1 m wide vein (Asarco private report).Production at the Country Boy Mine ceased shortlyafter 1947 and the mine was idle until 1993 when itwas rehabilitated and opened for the tourist trade.

Exploration continued in the Breckenridgemining district until 1990. The Gibson Hill area, inthe northeastern part of the Breckenridge quad-rangle, was explored by Asarco Incorporated in1989-1990 under the supervision of Paul J. Bartos.Thirteen angled reverse-circulation drill holes,ranging in depth from 32.6 to 120.4 m, totaling1,132 m, were drilled following geochemistry andground magnetic surveys. The target was a largedeposit of disseminated gold in the sandstone ofthe Dakota Formation. The Detroit Mine, on thesouthern end of Gibson Hill, had extensive high-grade stopes that contained veinlets, replacements,and disseminations within bedding-plane shears infractured quartzitic sandstone of the DakotaFormation; these were known as “blanket”deposits in the district (Lovering and Goddard,1950). Shock Hill and Little Mountain areas havesimilar types of “blanket” deposits. Dump samplesfrom the Detroit Mine contained stockwork vein-lets of pyrite-sphalerite-galena-quartz-calcite,which were oxidized to goethite and quartz;disseminated pyrite and quartz occurred awayfrom the veinlets. Alteration was typically obscure,although local silica dissolution and leaching of theDakota sandstone occurred. In general, the goldand silver were distributed among many smallfractures or segregated into local pockets, whichcould contain high concentrations of gold andsilver. Lovering (1934) described “mud” seams thatconsisted of altered rock selvages along bedding

plane slips in the Detroit Mine, and these selvagesgraded from 0.5 to 2 oz/ton gold; the adjoiningquartzitic sandstone contained varying amounts ofdisseminated gold that could be detected only byassay. Fifty-eight samples of Dakota Sandstonewere collected from the dump of the Detroit Mine;these samples did not show sulfide minerals, butthey assayed nearly 0.1 oz/ton Au. At the DetroitMine, ground induced polarization and magneticanomalies were found to correspond closely withgeochemical anomalies detected from surfacegeochemical samples, and these anomalies outlinedthe area of old stopes. The drilling programshowed interesting intercepts locally, the best being27.4 m of 0.048 oz/ton Au, but a fence of holesspaced 22.9 m apart along the edge of the Detroitworkings only intersected low-grade or marginalmineralization. Overall, the mineral depositsencountered were perceived to be insufficient to bemined in bulk and too low in grade to be selec-tively mined underground.

OTHER MINES AND PROSPECTSSome mining activity occurred in the TenmileRange before the close of the nineteenth century,mostly in the upper Blue River area (Fig. 2), butmining in the Tenmile Range ceased by 1940.Production of gold, silver, lead, and copper wasfrom mixed-sulfide veins that cut Proterozoic andPaleozoic rocks (Bergendahl, 1963; Singewald,1951). Small mines and adits are scattered widelyin a band west of the Blue River that extends fromthe southern boundary of the map area northwardto the Briar Rose mine southwest of the town ofBreckenridge. Several small mines and adits arescattered along terrain east of the Blue River.

Typically, veins in the Tenmile Range consist ofquartz-pyrite with subordinate to absent sphalerite,chalcopyrite, and galena. Gold is found both nativeand in auriferous pyrite (Singewald, 1951; Alaric,1952). Other reported minerals include magnetite,specularite, molybdenite, tetrahedrite, hubnerite,bismuthinite, tetradymite, and argentite (Singe-wald, 1951; Alaric, 1952; Parker, 1974). Most veinsin the Proterozoic rock of the Tenmile Range occuralong north-south or north-northeast-trendingfaults or fractures. A few (such as at the Briar RoseMine) are on north-northwest-trending structures.Geochemical samples K01-139, K01-179, and K01-

34

254 (Table 1) were taken from veins hosted byProterozoic rocks in the Tenmile Range.

Veins centered on North Star Mountain tend tobe rich in gold and commonly contain minormolybdenite (Singewald, 1951). These veins arebelieved by Parker (1974) to be the source of thenative bismuth nuggets reported from the placersfrom the upper reaches of the Blue River (Engi-neering and Mining Journal, 1877, v. 23, p.353).These veins are typically thin (0.1 to 1 m wide onaverage) and widely spaced. The North Star veinswere discovered in about 1875 and intermittentlyworked until the 1940’s. Limited sampling andreturns from smelter sheets of the North Star veinssuggest metal concentrations of 0.1 to 2.5 oz/tongold and 1 to 6 oz/ton silver; these grades werealmost certainly from hand-selected material.Northward from Quandary Peak, the veins aremuch more silver dominated, with considerablymore sphalerite and galena. Barite and jasperoidare reported from the northernmost mines, whichwas interpreted by Singewald (1951) as distal to themain heat source centered on North Star Mountain.Production from each mine ranged from $10,000 to$80,000 at historic prices (Singewald, 1951). Totalproduction for the upper Blue River area is esti-mated by Singewald (1951) to have been greaterthan $1 million, chiefly from gold and silver.

Several prospects, adits, and mine dumps occuralong the south side of Monte Cristo Creek; thesewere probably related to the Vanderbilt Mine (nearsample site 01080) (Singewald, 1951). Singewald(1951) reported the Vanderbilt Mine was aproducer of “noteworthy quantities of gold ore”. Amain adit, stopes, and drifts were developed in theupper part of Cambrian quartzite, most likely incalcareous sandstone and interbedded shale of thePeerless Shale (Singewald, 1951). Singewald (1951)described mineralized zones as ore beds and hedescribed replacement bodies in Cambrianquartzite. Sulfide minerals, primarily pyrite,galena, and sphalerite form most of the mineraldeposits, and Singewald (1951) described fracturesas the main conduits for ore-forming fluids. Themain adit is caved, but samples of sulfide ore werecollected from mine dump for chemical analysis.The results are shown in Table 1.

On the north side of Monte Cristo Gulch exca-vations at the Monte Cristo Mine opened sulfide

replacement zones in the Dyer Dolomite andParting Quartzite. Singewald (1951, p. 46) reportedthat gold was the principal objective of the miningoperation, but the grade was too low to be prof-itable, and there was “no substantial commercialoutput.” The ore body is on the southeast down-thrown block of a fault that has a few meters ofseparation (Singewald, 1951), and the main orechannel is about 23 m wide and trends at N. 55º W.Singewald (1951) described a zonation in ore-forming minerals: (1) pyrite-rich ore occur at thewestern corner of the ore body in, and adjacent to,the fault; and (2) galena and sphalerite, and minorchalcopyrite and pyrite in a gangue of manganeseand iron carbonate. Chalcopyrite decreases inabundance toward the southeast and sphaleriteincreases in abundance toward the southeast.Manganese and limonite coat rock surfaces and fillveins. An analysis of two samples in the main orebody indicated 15.9 percent and 16.0 percentmanganese (Singewald, 1951, p. 47). Chemicalanalyses of two samples (01059A, B) collected fromoutcrops are reported in Table 1. Near the MonteCristo Mine a sulfide sample (01064) from theSawatch Quartzite contains anomalous concentra-tions of gold.

East of the Blue River and south of Pennsyl-vania Creek, several mines and prospects are sitedin sedimentary rocks of the Minturn and MaroonFormations. The Fredonia Mine is located attimberline in Fredonia Gulch, and the Hunter BoyMine is located in an unnamed gulch south ofFredonia Gulch (NE1/4 Sec. 6, T. 8 N., R. 77 W.).

The Fredonia Mine is located in the lower partof the Maroon Formation as mapped in this reportand as mapped by Taranik (1974). This mine hadbeen mined for silver, and it produced lead andzinc as well. Singewald (1951, p. 57) indicated thatit produced approximately $25,000 in metals.Several limestone beds occur in the vicinity of theFredonia Mine, one of which Taranik (1974) corre-lated with the Robinson Limestone Member. Singe-wald (1951) reported that the ore body occurred inthe thickest limestone where porphyry had beenfractured along a fault. Pre-ore alteration consistedof extensive silicification of limestone and alter-ation of limestone to dolomite. Shattered rockprovided permeable areas for circulation of ore-forming fluids (Singewald, 1951). According to

35

Singewald, high-grade ore consists of vuggy silici-fied rock (jasperoid) permeated with greenish-graysilver-chlorite stain and minor amounts of limonite.The metal concentration of a dump sample ofmineralized limestone (01122A) and silicified lime-stone (01122B) is reported in Table 1.

The Hunter Boy Mine is an adit in a sill ofquartz monzonite megacrystic porphyry. According

to Singewald (1951) prospecting was confined tooxidized zones along minor faults, and gold wasreported to be greatly in excess of silver. Littlemineralization is seen at the surface in the area ofthe adit, but oxidized copper minerals occur in aprospect pit near the top of the porphyry sill. Themetal concentration of a grab sample taken fromthat prospect pit is reported in Table 1.

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Table 1. Trace-element geochemistry of rock samples taken in the Breckenridge quadrangle.[All samples were prepared and analyzed by Bondar Clegg, Inc. of Sparks, NV and Vancouver, B.C. All concentra-tions are in parts per million (ppm) unless otherwise noted. Gold was analyzed by fire assay (30 g charge). All otherelements were analyzed by inductively coupled plasma optical emission spectrometry (ICP) with acid digestion (HCland HNO3). <, less than; >, greater than; ppb, parts per billion]

SampleNumber Description Au (ppb) Ag Cu Pb Zn Mo Ni

01122C Fredonia Mine, ore grab sample 37 200 2224 17,900 15,000 64 14

K01-139 Mine dump sample (North Star area) - 1061 4.4 4 35 39 128 13quartz-pyrite vein

K01-179 Solitary Mine (upper Crystal Lake), 2640 188.2 110 926 273 277 20quartz-pyrite vein

K01-254 Last Dollar Mine (McCullough Gulch) 13,070 65.9 2439 2468 2741 <1 81quartz w/ pyrite, other sulfides

01007 Iron-stained sandstone, Morrison Fm. <5 2.1 10 126 812 3 14

01059A Monte Cristo Mine, mineralized 1724 13.6 1728 1795 448 5 10sandstone of Parting Quartzite

01059B Monte Cristo Mine, mineralized 3326 45.8 4781 177,100 2134 2 7sandstone of Parting Quartzite

01064 Near Monte Cristo Mine, sulfide 874 3.2 5 101 20 <1 33replacement in Sawatch Quartzite

01080 Vanderbilt Mine area, sulfide 4550 480.3 48,600 1980 737 <1 15replacement in Peerless Shale

01122A Fredonia Mine, replacement in 29 1344.6 1983 8230 73,800 9 26limestone bed, Maroon Fm.

01122B Fredonia Mine, silicified and 19 390.5 727 9382 3521 12 12mineralized limestone, Maroon Fm.

01181 Hunter Boy area copper oxide <5 3212.5 14,300 116,500 4140 13 14mineralization of Tqpm

37

SampleNumber Description Co Cd Bi As Sb Hg Fe(%)

01122C Fredonia Mine, ore grab sample 5 51.5 <5 209 298 1.088 1.15

K01-139 Mine dump sample (North Star area) - 28 0.3 9 <5 <5 0.071 10quartz-pyrite vein

K01-179 Solitary Mine (upper Crystal Lake), 6 1.1 <5 <5 <5 0.021 2.28quartz-pyrite vein

K01-254 Last Dollar Mine (McCullough Gulch) 96 13.5 95 <5 <5 0.037 7.45quartz w/ pyrite, other sulfides

01007 Iron-stained sandstone, Morrison Fm. 3 5.2 <5 13 <5 0.014 1.42

01059A Monte Cristo Mine, mineralized 11 1.4 12 <5 <5 0.026 >10sandstone of Parting Quartzite

01059B Monte Cristo Mine, mineralized 7 12.1 13 <5 16 0.055 >10sandstone of Parting Quartzite

01064 Near Monte Cristo Mine, sulfide 48 0.3 7 <5 <5 <0.01 >10replacement in Sawatch Quartzite

01080 Vanderbilt Mine area, sulfide 165 4.9 >2000 <5 6 0.027 >10replacement in Peerless Shale

01122A Fredonia Mine, replacement in 46 220.6 20 268 259 2.246 4.46limestone bed, Maroon Fm.

01122B Fredonia Mine, silicified and 4 26 <5 107 143 0.308 1.3mineralized limestone, Maroon Fm.

01181 Hunter Boy area copper oxide 3 50.3 38 3146 >2000 6.973 1.11mineralization of Tqpm

SampleNumber Description Mn Te Ba S(%) Cr V Sn

01122C Fredonia Mine, ore grab sample 340 <10 195 0.08 563 15 <20

K01-139 Mine dump sample (North Star area) - 45 <10 12 10 275 <1 <20quartz-pyrite vein

K01-179 Solitary Mine (upper Crystal Lake), 35 <10 54 1.78 363 5 <20quartz-pyrite vein

K01-254 Last Dollar Mine (McCullough Gulch) 904 <10 26 5.56 319 15 <20quartz w/ pyrite, other sulfides

01007 Iron-stained sandstone, Morrison Fm. 10861 <10 185 <0.01 583 14 <20

01059A Monte Cristo Mine, mineralized >20000 <10 13 1.85 285 <1 <20sandstone of Parting Quartzite

01059B Monte Cristo Mine, mineralized >20000 <10 9 7.19 183 <1 <20sandstone of Parting Quartzite

01064 Near Monte Cristo Mine, sulfide 172 <10 12 10 391 11 <20replacement in Sawatch Quartzite

01080 Vanderbilt Mine area, sulfide 386 15 16 10 204 4 <20replacement in Peerless Shale

01122A Fredonia Mine, replacement in 3295 <10 64 0.28 362 4 <20limestone bed, Maroon Fm.

01122B Fredonia Mine, silicified and 1444 <10 33 0.07 595 16 <20mineralized limestone, Maroon Fm.

01181 Hunter Boy area copper oxide 117 <10 33 1.85 430 6 <20mineralization of Tqpm

38

SampleNumber Description W La Ti(%) Mg(%) Ca(%) Na(%) K(%)

01122C Fredonia Mine, ore grab sample <20 1 <0.01 0.03 0.05 <0.01 0.09

K01-139 Mine dump sample (North Star area) - <20 <1 <0.01 0.04 0.02 <0.01 0.35quartz-pyrite vein

K01-179 Solitary Mine (upper Crystal Lake), <20 7 <0.01 0.02 <0.01 <0.01 0.21quartz-pyrite vein

K01-254 Last Dollar Mine (McCullough Gulch) <20 5 <0.01 0.43 0.11 <0.01 0.35quartz w/ pyrite, other sulfides

01007 Iron-stained sandstone, Morrison Fm. <20 6 <0.01 0.04 0.09 <0.01 0.11

01059A Monte Cristo Mine, mineralized 32 <1 <0.01 0.28 0.18 <0.01 0.04sandstone of Parting Quartzite

01059B Monte Cristo Mine, mineralized <20 <1 <0.01 0.91 0.27 <0.01 0.04sandstone of Parting Quartzite

01064 Near Monte Cristo Mine, sulfide <20 <1 <0.01 0.02 <0.01 <0.01 0.05replacement in Sawatch Quartzite

01080 Vanderbilt Mine area, sulfide <20 <1 <0.01 0.36 0.22 <0.01 0.06replacement in Peerless Shale

01122A Fredonia Mine, replacement in 49 <1 <0.01 0.22 0.38 <0.01 0.05limestone bed, Maroon Fm.

01122B Fredonia Mine, silicified and <20 <1 <0.01 1.28 2.98 <0.01 0.08mineralized limestone, Maroon Fm.

01181 Hunter Boy area copper oxide <20 4 <0.01 0.02 0.58 <0.01 0.04mineralization of Tqpm

SampleNumber Description Sr Y Ga Li Nb Sc Zr

01122C Fredonia Mine, ore grab sample 7 2 <2 <1 <1 <5 3

K01-139 Mine dump sample (North Star area) - <1 <1 2 3 5 <5 5quartz-pyrite vein

K01-179 Solitary Mine (upper Crystal Lake), 25 <1 <2 <1 2 <5 2quartz-pyrite vein

K01-254 Last Dollar Mine (McCullough Gulch) 2 3 3 9 6 <5 3quartz w/ pyrite, other sulfides

01007 Iron-stained sandstone, Morrison Fm. 20 6 <2 <1 2 <5 4

01059A Monte Cristo Mine, mineralized 8 2 <2 <1 9 <5 4sandstone of Parting Quartzite

01059B Monte Cristo Mine, mineralized <1 2 <2 <1 7 <5 4sandstone of Parting Quartzite

01064 Near Monte Cristo Mine, sulfide <1 <1 4 <1 5 <5 6replacement in Sawatch Quartzite

01080 Vanderbilt Mine area, sulfide 7 <1 4 7 <1 <5 9replacement in Peerless Shale

01122A Fredonia Mine, replacement in 13 3 <2 <1 2 <5 3limestone bed, Maroon Fm.

01122B Fredonia Mine, silicified and 15 1 <2 <1 <1 <5 3mineralized limestone, Maroon Fm.

01181 Hunter Boy area copper oxide 75 2 <2 <1 <1 <5 1mineralization of Tqpm

Potential geologic hazards in the Breckenridgequadrangle fall into four categories: (1) landslides,(2) floods and debris flows, (3) abandoned minedlands and placer deposits, and (4) seismicity.

LANDSLIDES AND ROCK SLIDESLandslide deposits occur throughout the Brecken-ridge quadrangle. These landslides are mostlyrock slumps, rock slides, and debris slides accor-ding to the terminology of Cruden and Varnes(1996). Older (pre-Pinedale glaciation) landslidedeposits occur in several cirques of the TenmileRange and consist of large, semi-intact masses ofbedrock derived from the cirque walls. Younger(post-glacial) landslide deposits occur in diversesettings: (1) slumps in Pinedale till; (2) shallowdebris slides in Bull Lake till; (3) slumps anddeep-seated gravitational spreading in Tertiaryintrusives; (4) rock slides and slumps in Paleozoicstrata on the eastern flank of the Tenmile Range;and (5) slumps and rockslides in Proterozoic crys-talline rocks. The latter group of landslides arerestricted to a north-northwest-trending belt onthe eastern flank of the Tenmile Range that coin-cides with a zone of faults and shear planes.

Aside from the landslide deposits mapped,there are also areas of incipient slope failure anddeep-seated gravitational spreading called “sack-ung” structures by Varnes and others (1989). Theseareas are marked by upslope- and downslope-facing scarps and troughs that parallel slopecontours. At some places, such as the north slopeof Red Mountain, sackung landforms are adjacentto young landslide detachment areas, suggestingthat future landslides will detach along thesackung scarps and troughs.

The linear belt of large bedrock landslides inthe Tenmile Range is associated with two bedrockcontrols. North of Spruce Creek, landslides coin-cide with rock units weakened by faulting,shearing, or by hydrothermal activity associatedwith Tertiary intrusions. A landslide complex (Qlsc)occurs on the northwestern flank of Mount Argen-tine, where the landslide is coincident with theintrusive contact of Tertiary porphyry against rocksof the Maroon Formation. South of Spruce Creek,

large landslide complexes coincide with the thinoutcrop band of Paleozoic sedimentary rocks,particularly the incompetent shale within theMinturn Formation. For landslides in Quaternarydeposits, such as glacial till, the main contributorycauses are slope oversteepening from glacial orfluvial erosion and local saturation of slopes due toconcentrated spring or seep areas. However, at thesouthern margin of the quadrangle along AqueductRoad, it appears that the shallow groundwatercausing slumps in Pinedale till occurs only wherethe till overlies shaley Paleozoic units, such as inthe Minturn Formation.

FLOODS AND DEBRIS FLOWSIntense summer rainstorms or rapid melting ofdeep snowpack during unusually warm springthaws may cause localized flooding and debris-flow activity. The 100- and 500-year flood plains,as mapped in the Town of Breckenridge by theFederal Emergency Management Agency (FEMA),are restricted to Holocene alluvium (Qal) andyounger Pinedale outwash (Qop2) in the Blue RiverValley. In tributary drainages these flood areashave not been defined by FEMA but should alsogenerally coincide with the limits of Holocenealluvium.

However, a more widespread hazard is that ofdebris flows in ephemeral and intermittent streamsand on alluvial fans. Holocene alluvial fans (mapunits Qf, Qfy) are potentially subject to debris-flowdeposition anywhere on their surfaces. Fans withthe highest hazard are those whose drainage basinscontain steep tributaries affected by debris slides,such as Pennsylvania Creek.

ABANDONED MINES AND PLACER DEPOSITS

Collapse of abandoned mine shafts and tunnels,many of which may be covered by thin surficialmaterial, pose a potential hazard, especially in thenortheastern part of the quadrangle. Mine wateralso tends to be acidic and commonly containstoxic levels of metals, such as zinc, cadmium,lead, and arsenic. Soil contaminated by acid minedrainage may be corrosive to metal and concrete

39

GEOLOGIC HAZARDS

and pose a constraint to building foundations. Inareas mined by dredging or hydraulic methods,surface soil may ravel or wash downward intosubsurface void spaces in the underlying, coarse,matrix-free gravel. This process, known as piping,can create cylindrical cavities, holes, and smallsinkholes at the surface. The vertical bluffs inunconsolidated alluvium left by hydraulic mining,some of which are up to 10 m high, can collapse ifsaturated.

SEISMICITYThe Breckenridge quadrangle lies near the north-ern terminus of the Rio Grande rift, an active zoneof crustal extension. As in other parts of the rift,the level of historic seismicity is very low. Asearch of the USGS/NEIC catalog of earthquakes(NEIC, 1992) in the eastern, central, and mountainstates of the U.S. (1534–1986 A.D.) reveals only asingle earthquake within a 20 km radius of thecenter of the Breckenridge quadrangle. This earth-quake occurred on 23 May 1964, in the CopperMountain quadrangle west of Breckenridge, butthe magnitude and depth were not recorded.

Despite the low level of historic seismicity,geologic evidence suggests that some faults mayhave been active as recently as the lower to middlePleistocene. Kellogg and others (2002) mapped aseries of down-to-the-east normal faults on theeastern flank of the Tenmile Range in the Friscoquadrangle, which is directly north of the Brecken-ridge quadrangle. These faults occur at a majorbreak in slope between the steep, bedrock-coredslopes of the Tenmile Range, and the erodedremnants of an old erosion surface underlain byearly Quaternary gravels (map unit QTgg). Thisbreak in slope continues southward into the Breck-enridge quadrangle, through the Breckenridge SkiArea, and as far south as Carter Gulch, beyondwhich it is buried by glacial deposits. However, noyoung fault scarps are preserved in Quaternarydeposits along this escarpment, so the fault hasprobably not been active since at least Pinedaletime (16–23 ka). Recurrence intervals for damagingearthquakes in and near the Breckenridge quad-rangle are largely unknown.

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Alaric, S., 1952, General geology and ore deposits ofNorth Star Mountain and the Ling Mine,Summit and Park Counties, Colorado: Golden,Colo., Colorado School of Mines, unpublishedM.Sc. thesis, 79 p.

Ashley, G.H., Cheney, M.G., Galloway, J.J., Gould, C.N.,Hares, C.J., Howell, B.F., Levorsen, A.I., Miser,H.D., Moore, R.C., Reeside, J.B., Rubey, W.W.,Stanton, T.W., Stose, G.W., and Twenhofel,W.H., 1933, Classification and nomenclature ofrock units: Geological Society of AmericaBulletin, v. 44, p. 423–459.

Ball, S.H., 1906, Precambrian rocks of the Georgetownquadrangle, Colorado: American Journal ofScience, 4th ser., v. 21, p. 371–389.

Barker, Fred, and Brock, M.R., 1965, Denny CreekGranodiorite Gneiss, Browns Pass QuartzMonzonite, and Kroenke Granodiorite, MountHarvard quadrangle, Colorado, in Changes instratigraphic nomenclature by the U.S. Geolog-ical Survey, 1964: U.S. Geological SurveyBulletin 1224-A, p. A23–A26.

Bergendahl, M.H., 1963, Geology of the northern partof the Tenmile Range, Summit County, Colo-rado: U.S. Geological Survey Bulletin 1162-D,19 p., map, scale 1:24,000.

_______ 1969, Geologic map and sections of the south-west quarter of the Dillon quadrangle, Eagleand Summit Counties, Colorado: U.S. Geolog-ical Survey Miscellaneous Investigations MapI-563, scale 1:24,000.

Bergendahl, M.H., and Koschmann, A.H., 1971, Oredeposits of the Kokomo-Tenmile District,Colorado: U.S. Geological Survey ProfessionalPaper 652, 53 p., map scale 1:24,000.

Berman, A.E., Poleschook, Jr., and Dimelow, T.E., 1980,Jurassic and Cretaceous Systems of Colorado,in Kent, H.C., and Porter, K.W., eds., ColoradoGeology: Rocky Mountain Association of Geol-ogists Symposium Proceedings, Denver, p.111–128.

Best, M.G., 1982, Igneous and metamorphic petrology:San Francisco, W.H. Freeman and Co., 630 p.

Birkeland, P.W., Miller, D.C., Patterson, P.E., Price, A.B.,and Shroba, R.R., 1996, Soil-geomorphic rela-tionships near Rocky Flats, Boulder andGolden, Colorado area, with a stop at the pre-Fountain Formation paleosol of Whalstrom(1948) in Thompson, R.A., Hudson, M.R., andPillmore, C.L., eds., Geologic excursions to theRocky Mountains and beyond; Field trip guide-book for the 1996 Annual Meeting, GeologicalSociety of America: Colorado GeologicalSurvey, Special Publication 44, CD-Rom.

Brennan, W.J., 1969, Structural and superficial geologyof the west flank of the Gore Range, Colorado:Boulder, Colo., University of Colorado, unpub-lished Ph.D thesis, 102 p.

Butler, B.S., 1941, Ore deposits in the vicinity of theLondon fault of Colorado: U.S. GeologicalSurvey Bulletin 911, p. 74 p.

Brill, K.G., Jr., 1944, Late Paleozoic stratigraphy, west-central and northwestern Colorado: GeologicalSociety of America Bulletin, v. 55, no. 5, p.621–656.

Chadwick, O.A., Hall, R.D., and Phillips, F.M., 1997,Chronology of Pleistocene glacial advances inthe central Rocky Mountains: GeologicalSociety of America Bulletin, v. 109, no. 7, p.1443–1452.

Cruden, D.M., and Varnes, D.J., 1996, Landslide typesand processes, in Turner, A.K., and Schuster,R.L., eds., Landslides—Investigation and miti-gation: Washington, DC, National AcademyPress, Transportation Research Board SpecialReport 247, p.36–75.

DeVoto, R.H., 1965, Facies relationship between GaroSandstone and Maroon Formation, South Park,Colorado: American Association of PetroleumGeologists Bulletin, v. 49, no. 4, p 460–462.

_______ 1971, Geologic history of South Park andgeology of the Antero Reservoir quadrangle,Colorado: Quarterly of the Colorado School ofMines, v. 66, no. 3, scale 1:62,500, 90 p.

_______ 1972, Pennsylvanian and Permian stratigraphyand tectonism in central Colorado, in DeVoto,R.H., ed., Paleozoic stratigraphy and structuralevolution of Colorado: Quarterly of theColorado School of Mines, v. 67, no. 4, p.139–185.

41

REFERENCES

_______ 1980, Pennsylvanian stratigraphy and historyof Colorado; in Kent, H.S., and Porter, K.W.,eds., Colorado Geology: Denver Colo., RockyMountain Association of Geologists, p. 71–101.

DeVoto, R.H., and Peel, F.A., 1972, Pennsylvanian andPermian stratigraphy and structural history,northern Sangre de Cristo Range, in DeVoto,R.H., ed., Paleozoic stratigraphy and structuralevolution of Colorado: Quarterly of theColorado School of Mines, v. 67, no. 4, p.283–320.

Ehlers, E.G., and Blatt, Harvey, 1982, Petrology;Igneous, sedimentary, and metamorphic: SanFrancisco, W.H. Freeman and Company, 732 p.

Emmons, S.F., 1886, Geology and mining industry ofLeadville, Colorado: U.S. Geological SurveyMonograph 12, 770 p.

Erslev, E.A., Kellogg, K.S., Bryant, B., Ehrlich, T.K.,Holdaway, S.M., and C.W, Naeser, 1999,Laramide to Holocene structural developmentof the northern Colorado Front Range, inLageson, D.R., Lester, A.P., and Trudgill, B.D.,eds., Colorado and adjacent areas: Boulder,Colo., Geological Society of America FieldGuide 1, p. 21–40.

Gutiérrez, F., and Matthews, V., 2002, Tectonic defor-mation of a Holocene rock glacier in thesouthern Rocky Mountains, Tenmile Range,Colorado, USA: Estudios recientes (2000–2002)en geomorfología; Patrimonio, montaña ydinámica territorial 2002, Dpto. Geografía-UVA, Valladolid, p. 193–203.

Holt, H.E., 1961, Geology of the lower Blue River area,Summit and Grand Counties, Colorado:Boulder, Colo., University of Colorado, Ph.D.thesis, 107 p.

Kellogg, K.S., 1999, Neogene basins of the northern RioGrande rift; partitioning and asymmetry inher-ited from Laramide and older uplifts: Tectono-physics, v. 305, p. 141–152.

_______ 2002, Geologic map of the Dillon quadrangle,Summit and Grand Counties, Colorado: U.S.Geological Survey Miscellaneous Field StudiesMap MF-2390, scale 1:24,000.

Kellogg, K.S., Bartos, P.J., and Williams, C.L., 2002,Geologic map of the Frisco quadrangle,Summit County, Colorado: U.S. GeologicalSurvey Miscellaneous Field Studies Map MF-2340, scale 1:24,000.

Kellogg, K.S., Bryant, Bruce, and Redsteer, M.H., inpreparation, Geologic map of the Vail Eastquadrangle, Eagle County, Colorado: U.S.Geological Survey Miscellaneous Field StudiesMap MF-2375, scale 1:24,000.

Lindsey, D. A., Clark, R. F., and Soulliere, S. J., 1985,Reference section for the Minturn Formation(Middle Pennsylvanian), northern Sangre deCristo Range, Custer County, Colorado, U.S.Geological Survey Miscellaneous Field StudiesMap MF-1622-C.

Lovering, T.S., 1934, Geology and ore deposits of theBreckenridge Mining District, Colorado: U.S.Geological Survey Professional Paper 176, 64 p.

_______ 1935, Geology and ore deposits of theMontezuma quadrangle, Colorado: U.S.Geological Survey Professional Paper 178, 119 p., plates.

Lovering, T.S., and Goddard, E.N., 1950, Geology andore deposits of the Front Range, Colorado: U.S.Geological Survey Professional Paper 223, 319 p.

Mach, C.J., and Thompson, T.B., 1998, Geology andgeochemistry of the Kokomo mining district,Colorado: Economic Geology, v. 93, p. 617–638.

Marvin, R.F., Young, E.J., Mehnert, H.H., and Naeser,C.W., 1974, Summary of radiometric age deter-minations on Mesozoic and Cenozoic igneousrocks and uranium and base metal deposits inColorado: Isochron/West, no. 11, 41 p.

Marvin, R.F., Mehnert, H.H., Naeser, C.W., andZartman, R.E., 1989, U.S. Geological Surveyradiometric dates, compilation “C”; part 5,Colorado, Montana, Utah, and Wyoming:Isochron/West, no. 53, p. 14–19.

Mulienberg, G.A., 1925, Geology of the TarryallDistrict, Park County, Colorado: ColoradoGeological Survey Bulletin 31, 64 p.

National Earthquake Information Center (NEIC), 1992,Global Hypocenter Database CD-Rom, v. 2.0:Denver, Colo., U.S. Geological Survey, June,1992.

Navas, J., 1966, Geology of the Como area, South Park,Park County, Colorado: Golden, Colo.,Colorado School of Mines, Ph.D. thesis, 145 p.

North American Commission on Stratigraphic Nomen-clature, 1983, North American StratigraphicCode: American Association of PetroleumGeologists Bulletin, v. 67, no. 5, p. 841–875.

42

Parker, B.H., Jr., 1974, Gold placers of Colorado (Book1): Quarterly of the Colorado School of Mines,v. 69, no. 3, 268 p.

Patton, H.B., Hoskin, A.J., and Butler, G.M., 1912,Geology and ore deposits of the Alma district,Park County, Colorado: Colorado State Geolog-ical Survey Bulletin 3, 284 p.

Pearson, R.C., Tweto, Ogden, Stern, T.W., and Thomas,H.H., 1962, Age of Laramide porphyries nearLeadville, Colorado, in Geological SurveyResearch 1962: U.S. Geological Survey Profes-sional Paper 450-C, p. C78–C80.

Pettijohn, F.J., 1957, Sedimentary rocks (2nd ed.): NewYork, Harper and Row, 718 p.

Porter, S.C., Pierce, K.L., and Hamilton, T.D., 1983, LateWisconsin mountain glaciation in the westernUnited States, in Wright, H.E., Jr., ed., Late-Quaternary environments of the United States;v.1, The Late Pleistocene: Minneapolis, Minn.,University of Minnesota Press, p.71–111.

Ransome, F.L., 1911, Geology and ore deposits of theBreckenridge district, Colorado: U.S. Geolog-ical Survey Professional Paper 75, 187 p.

Reed, J.C., Bickford, M.E., Premo, W.R., Aleinkoff, J.N.,and Pallister, J.S., 1987, Evolution of the EarlyProterozoic Colorado province; Constraintsfrom U-Pb geochronology: Geology, v. 15, p.861–865.

Scott, R.B., Lidke, D.J., and Grunwald, D.J., 2002,Geologic map of the Vail West quadrangle,Eagle County, Colorado: U.S. GeologicalSurvey Miscellaneous Field Studies Map MF-2369, scale 1:24000.

Shaw, C.A., Karlstrom, K.E., Williams, M.L., Jercinovic,M.J., and McCoy, A.M., 2001, Electron-micro-probe monazite dating of ca. 1.71–1.63 Ga andca. 1.45–1.38 Ga deformation in the Homestakeshear zone, Colorado; Origin and early evolu-tion of a persistent intracontinental tectoniczone: Geology, v. 29, no. 8, p. 739–742.

Sheridan, D.M., and Marsh, S.P., 1976, Geologic map ofthe Squaw Pass quadrangle, Clear Creek,Jefferson, and Gilpin Counties, Colorado: U.S.Geological Survey Geologic Quadrangle MapGQ-1337, scale 1:24,000.

Simmons, E.C., and Hedge, C.E., 1978, Minor-elementand Sr-isotope geochemistry of Tertiary stocks,Colorado mineral belt: Contributions to Miner-alogy and Petrology, v. 67, p. 379–396.

Singewald, Q.D., 1942, Stratigraphy, structure, andmineralization in the Beaver-Tarryall area, ParkCounty, Colorado, a reconnaissance report: U.S.Geological Survey Bulletin 928-A, p. 1–44.

_______ 1951, Geology and ore deposits of the upperBlue River area, Summit County, Colorado:U.S. Geological Survey Bulletin 970, 74 p.

Singewald, Q.D., and Butler, B.S., 1930, Preliminarygeologic map of the Alma mining district,Colorado: Colorado Scientific Society Proceed-ings, v. 12, no. 9, p. 295–308.

_______ 1941, Ore deposits in the vicinity of theLondon Fault of Colorado: U.S. GeologicalSurvey Bulletin 911, 74 p., map scale 1:24,000.

Streckeisen, A.L., 1973, Classification and nomenclaturerecommended by the IUGS Subcommission ofthe systematics of igneous rocks: Geotimes, v.18, no. 10, p. 26–30.

_______ 1978, Classification and nomenclature ofvolcanic rocks, lamphrophyres, carbonatites,and melilitic rocks: Neues Jahrbuch fur Miner-alogie, Abhandlungen 134, p. 1–14.

Taranik, J.V., 1974, Stratigraphic and structural evolu-tion of Breckenridge area, central Colorado:Golden, Colo., Colorado School of Mines, Ph.D.thesis, 222 p.

Taylor, R.B., Scott, G.R., and Wobus, R.A., 1975, Recon-naissance geologic map of the Howard quad-rangle, central Colorado: U.S. GeologicalSurvey Miscellaneous Investigations SeriesMap I-892, scale 1:62,500.

Tillman, R.W., 1971, Petrology and paleoenvironments,Robinson Member, Minturn Formation(Desmoinesian), Eagle basin, Colorado: Amer-ican Association of Petroleum GeologistsBulletin, v. 55, no. 4, p. 593–620.

Tweto, Ogden, 1949, Stratigraphy of the Pando area,Eagle County, Colorado: Colorado ScientificSociety Proceedings, v. 15, no. 4, p. 149–235.

_______ 1974, Geologic map of the Mount Lincoln 15’quadrangle, Colorado: U.S. Geological SurveyMiscellaneous Field Studies Map MF-556, scale1:62,500.

_______ 1979, The Rio Grande rift system in Colorado,in Riecker, R.E., ed., Rio Grande Rift; Tectonicsand Magmatism: Washington, D.C., AmericanGeophysical Union, p 33–56.

_______ 1987, Rock units of the Precambrian basementin Colorado: U.S. Geological Survey Profes-sional Paper 1321-A, 54 p., 1 plate.

43

Tweto, Ogden, and Lovering, T.S., 1977, Geology of theMinturn 15-minute quadrangle, Eagle andSummit Counties, Colorado: U.S. GeologicalSurvey Professional Paper 956, 96 p., 1 plate,scale 1:62,500.

Tweto, Ogden, Moench, R.H., and Reed, J.C., 1978,Geologic map of the Leadville 1o by 2o quad-rangle, northwestern Colorado: U.S. GeologicalSurvey Miscellaneous Investigations Series I-999, scale 1:250,000.

Tweto, Ogden, and Sims, P.K., 1963, Precambrianancestry of the Colorado Mineral Belt: Geolog-ical Society of America Bulletin, v. 74, p.991–1014, 1 pl.

Varnes, D.J., Radbruch-Hall, D.H., and Savage, W.Z.,1989, Topographic and structural conditions inareas of gravitational spreading of ridges in theWestern United States: U.S. Geological SurveyProfessional Paper 1496, 28 p.

Wahlstrom, E.E., and Hornback, V.Q., 1962, Geology ofthe Harold D. Roberts Tunnel, Colorado-westportal to station 468–49: Geological Society ofAmerica Bulletin, v. 73, p. 1477–1498.

Wallace, C.A., Cappa, J.A., and Lawson, A.D., 1997,Geologic map of the Salida East quadrangle,

Chaffee and Fremont Counties, Colorado:Colorado Geological Survey Open-File Report97-6, 1:24,000 scale, 23 p.

Wallace, C.A., and Keller, J.W., 2001, Geologic map ofthe Castle Rock Gulch quadrangle, Chaffee andPark Counties, Colorado: Colorado GeologicalSurvey Open-File Report 01-1, scale 1:24,000, 31 p.

Widmann, B.L., Kirkham, R.M., and Rogers, W.P., 1998,Preliminary Quaternary fault and fold map anddatabase of Colorado: Colorado GeologicalSurvey Open-File report 98-8, scale 1:500,000,331 p.

Widmann, B.L., and Miersemann, U., 2001, Geologicmap of the Georgetown quadrangle, ClearCreek, Colorado: Colorado Geological SurveyOpen-File Report 01-5, scale 1:24,000.

Widmann, B.L., Morgan, M.L., Bartos, P.J., Shaver, K.C.,Gutiérrez, Francisco, and Lockman, Andrew,2002, Geologic map of the Keystone quad-rangle, Summit County, Colorado: ColoradoGeological Survey Open-File Report 02-03,scale 1:24,000.

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