High-resolution sequencestratigraphy of the UpperOrdovician Montoya Group,southern New Mexico andwestern Texas: Outcrop analogof an unconventional chertand carbonate reservoirMichael C. Pope

ABSTRACT

The Upper Ordovician Montoya Group crops out in southern NewMexico and westernmost Texas and records predominantly subti-dal deposition on a gently dipping carbonate ramp that was sub-sequently almost entirely dolomitized. The Montoya Group is athird-order composite sequence composed of six regionally cor-relative, shallowing-upward, third-order depositional sequences(M0–M5). Sequence M0 has sandstone at its base that is overlainby skeletal packstone-grainstone. Sequence M0 occurs onlylocally and was likely deposited in a topographic low formed dur-ing regional development of the unconformity following El PasoGroup deposition. Sequence M1, marking the initial widespreadtransgression over the Ellenburger unconformity, consists ofsandstone updip that passes downramp into skeletal packstone.The highstand systems tract (HST) of M1 consists of a progradingskeletal grainstone that was subaerially exposed upramp.Sequence M2, which contains the second-order maximum flood-ing surface, has abundant subtidal cherty carbonate at its base,which shallows upward into a widespread, prograding coral pack-stone-grainstone in the HST. Sequence M3 also contains abundantdownramp chert that passes upramp into an aggrading crinoidalshoal and farther upramp into peritidal mudstone. Sequence M4records an extensive basinward shift in facies as peritidal bur-rowed and cryptalgalaminated mudstone prograded over subtidal

Copyright ©2014. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received October 24, 2012; provisional acceptance February 06, 2013; revised manuscriptreceived July 11, 2013; final acceptance October 15, 2013.DOI: 10.1306/10151312177

AUTHOR

Michael C. Pope ∼ Department of Geologyand Geophysics, Texas A&M University,College Station, Texas 77843–3115;mcpope@geos.tamu.edu

Mike Pope earned a B.S. degree in earth andspace science from the University ofCalifornia in Los Angeles (1985), an M.S.degree in geology from the University ofMontana (1989), and a Ph.D. in geology fromthe Virginia Polytechnic Institute and StateUniversity (1995). He was a postdoctoralresearcher at the Massachusetts Institute ofTechnology and taught for 10 years atWashington State University before movingto Texas A&M University in 2009. He teachescourses in stratigraphy, carbonatedepositional systems, sequence and seismicstratigraphy analysis, and field camp. Hiscurrent research projects includestratigraphic and sedimentologic studies ofOrdovician siliciclastic rocks (EurekaQuartzite, St. Peter Sandstones, andequivalents), Mesozoic and Cenozoiccarbonates in Libya and Saudi Arabia, EagleFord Group rocks of west Texas, and theSmackover Formation in Alabama.

ACKNOWLEDGEMENTS

Supported for this research was provided byACS Petroleum Research Fund 35837-G8.White Sands Missile Range geologist BobMyers was very helpful in accessing sectionsin the San Andres range. Jessica Steffen, DanHunter, Bryn Clark, Luke LeMond, SteveTurpin, and John Bengelsdorf all providedinvaluable field assistance. Jessica Steffeninitiated many useful discussions about theMontoya Group chert. Dave Thomas, HuabaoLiu, Doug Kenaley, Pak Wong, Joel Collins,and Jim Weber provided lively discussionsand debate about Montoya Groupstratigraphy, and some disagreed with myinterpretations; any remaining errors aresolely mine. Kate Giles provided manyinteresting and useful discussions about theNew Mexico stratigraphy. Insightful reviewsby Brian Coffey, Maya Elrick, and SteveRuppel greatly improved this manuscript. Thisarticle was approved for public release byWhite Sands Missile Range; distribution is

AAPG Bulletin, v. 98, no. 8 (August 2014), pp. 1577–1597 1577

carbonate. Sequence M5 is only locally developed downramp andconsists of crinoidal grainstone with abundant evidence of subae-rial exposure. A regional unconformity separates the MontoyaGroup from the Silurian Fusselman Dolostone or younger units.Parasequences (meter-scale cycles) recording low- to moderate-amplitude relative sea level fluctuations are ubiquituous featuresat individual outcrops but are difficult to correlate regionally.

The abundance of syn- or early depositional chert in the sub-tidal facies indicates that the Montoya Group was depositedwithin a region of strong regional upwelling along southernLaurentia. This early formed chert was the reservoir facies in asuccessful Upper Ordovician gas play in Ward and ReevesCounties, Texas.

INTRODUCTION

Chert reservoirs commonly are difficult to characterize (Rogersand Longman, 2001) into a single depositional model, with reser-voirs occurring in primary depositional settings and in diageneticsettings along major unconformities (so-called “chat” reservoirs).A gas reservoir was discovered in cherty carbonate (AlemanFormation) of the Upper Ordovician Montoya Group in westTexas (Thomas and Liu, 2003). This play was developed usinghorizontal technology by Mobil, and the first horizontal well wasdrilled and completed in 1999.

Interbedded marine carbonate and chert are common lithofaciesin Upper Ordovcian units of southern Laurentia, occuring withinthe Simpson Group in southern Oklahoma and northeasternTexas, the Maravillas Formation of south-central Texas, and theMontoya Group of western Texas and eastern New Mexico (Pope,2004a, b). These interbedded carbonate and chert units interfingerwith calcareous turbidites in the Maravillas Formation (McBride,1969, 1970, 1989) and with crinoid grainstones in the Montoyaand Simpson Groups (Galvin, 1983; Brown and Sentfle, 1997;Denison, 1997; O‘Brien and Derby, 1997; Pope, 2004b), indicatingthat they formed in shallow to deep subtidal environments. A depo-sitional model for many of these Upper Ordovician chert-rich car-bonate rocks suggested that they formed along the southernmargin of Laurentia during upwelling that provided the phosphatein these rocks and nutrients for the numerous sponges that providedthe abundant sponge spicules that form many of the chert beds(Pope and Steffen, 2003; Pope, 2004a, b). This article provides adetailed outcrop-based sequence-stratigraphic analysis of theUpper Ordovician Montoya Group in southern New Mexico andwestern Texas to better understand Montoya Group depositionand provide an analog for the subsurface reservoir in west Texas.

unlimited. Operations Security review wascompleted on August 19, 2002.The AAPG Editor thanks the followingreviewers for their work on this paper:Brian P. Coffey, Maya Elrick, and StephenC. Ruppel.

1578 Sequence Stratigraphy Montoya Group, New Mexico and Texas

GEOLOGIC SETTING

The Upper Ordovician Montoya Group (as much as180 m [590 ft] thick) of southeastern New Mexicoand western Texas was deposited during a transitionfrom Early and Middle Ordovician global greenhouseconditions to Late Ordovician global glacial condi-tions that reached their acme during the very latestOrdovician Hirnantian Age (Frakes et al., 1992;Brenchley et al., 1994, 2003; Pope and Read, 1998;Poussart et al., 1999; Finnegan et al., 2011). TheUpper Ordovician Montoya Group was depositedwithin 30° of the paleoequator on a mature passivemargin along the southern margin of Laurentia (MacNiocaill et al., 1997; Scotese, 1997). High-frequency,moderate-amplitude (20–30 m [66–98 ft], every20–100 k.y.) sea level fluctuations are recorded inthe Upper Ordovician Lexington Limestone inKentucky (Pope and Read, 1997a, b; 1998), andan influx of cool oceanic waters occurred over muchof equatorial Laurentia during the Late Ordovician(Brookfield, 1988; Patzkowsky and Holland, 1993;Holland and Patzkowsky, 1997; Pope and Read,1998; Kolata et al., 2001).

The Montoya Group outcrops in mountain rangesthroughout southern New Mexico and westernmostTexas (Figure 1). These mountain ranges were pro-duced by Cenozoic basin-and-range extension andcommonly trend north-northwesterly. Forty-two fulland partial sections (Figure 1) were measured at thebed-by-bed scale for this study.

The biostratigraphic framework (Figure 2) for theMontoya Group outcrops is provided by numerousworkers using a variety of fossils (Flower, 1956,1961, 1969; Hill, 1959; Howe, 1959; Lemone, 1969;Sweet, 1979). These studies indicate that the age ofthe Montoya Group is Late Ordovician (Chatfieldianto Cincinnatian), and that the latest Ordovician(Hirnantian) is missing in this area. These and pre-vious stratigraphic studies of the Montoya Group(Kelley and Silver, 1952; Pray, 1958; Howe, 1959;Pratt and Jones, 1961; Kottlowski, 1963; Hayes,1975; Measures 1985a, b) produced the grossregional lithostratigraphy (Figure 2) that remains inuse today. The duration of Montoya Group deposi-tion (Chatfieldian to Cincinnatian) was 6–7 m.y.(Bergström et al., 2008).

The Montoya Group is subdivided into threedistinct formations (Figure 2) given in ascendingorder: Upham, Aleman, and Cutter. Almost all ofthe Montoya Group outcrops are extensively dolomi-tized with limestone occurring locally only in theUpham Dolomite and basal Aleman Formation.Upper Ordovician Montoya Group strata of southernNew Mexico and western Texas formed a gentlysloping carbonate ramp (sensu Ahr, 1973; Read,1985) because shallow- to deep-water facies pass

Figure 1. Map showing locations of mountain ranges wherethe Upper Ordovician Montoya Group outcrops in southernNew Mexico and westernmost Texas (modified from Pope,2004a). Cross section line BB′, from the northern San AndresRange to the Franklin Mountains and to the Hueco Mountains,is shown in Figure 6 (A) (northern half), (B) (southern half),and (C) (complete, simplified diagram). The location of the mea-sured sections (from the north to the south) are SM = SheepMountain; SW = Sweetwater Canyon; MA = MarkinsonCanyon; WM = Workman Canyon; LM = Lost Man Canyon;MY = Mayberry Canyon; SA = San Andres Canyon; AS = AshCanyon; BP = Bear Peak Canyon; BCN = Bishops Cap North;BCS = Bishops Cap South; AN = Anthony's Nose; NFC = NorthFranklin Mountains C; NFD = North Franklin Mountains D;NFE = North Franklin Mountains E; NFF = North FranklinMountains F; NFG = North Franklin Mountains G; NFH = NorthFranklin Mountains H; MCN = McKelligon Canyon North; EF =East Franklin Mountains; FRT = Franklin Radio Tower; SFB =South Franklin Mountains B; SFA = South Franklin MountainsA; SF = South Franklins; PLC = Pole Line Canyon in HuecosRange. Nakaye Mountain in the Caballos Range is labeled NM.

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laterally into one another (Figure 3) without evidencefor a substantial change in depositional slope (Pope,2004a, b).

GENERALIZED FACIES DESCRIPTION OFTHE MONTOYA GROUP

A brief outline of the facies in the Montoya Group isgiven below and shown graphically in the ramp crosssections in Figure 3. More detailed descriptions of thefacies of the Montoya Group are outlined elsewhere(Pope and Steffen, 2003; Pope, 2004a, b).

Cable Canyon Sandstone

The initial Montoya Group deposit, the CableCanyon Sandstone (0–16 m [0–52 ft] thick)is a mixed carbonate-siliciclastic unit of burrowedto cross-bedded coarse sandstone or granule con-glomerate containing abundant carbonate skeletal

grains that only occurs updip on the ramp (Table 1).The Cable Canyon Sandstone was interpreted as asand-wave complex deposited in an open-marine sub-tidal environment during the transgression over theLower Ordovician El Paso Group (Bruno andChafetz, 1988). The well-rounded nature of the silici-clastic particles suggests that many of these grainswere previously deposited as sand dunes or werereworked multiple times in a fluvial or marine setting.

The Cable Canyon Sandstone is interpreted tohave formed during the initial transgression on theunconformity developed on the El Paso Group. Thesource of the sand was likely exposed Precambrianbasement and older Paleozoic siliciclastics of thetranscontinental arch in northern New Mexico orareas immediately to the north.

Upham Dolomite

Where the Cable Canyon Sandstone is thin or absent,the basal unit of the Montoya Group is burrowedskeletal wackestone-packstone (Table 1) of theUpham Dolomite (13–42 m [43–138 ft] thick) thatdisconformably overlies the Lower Ordovician ElPaso Group. The base of the Upham Dolomite com-monly is rich in quartz sand (as much as 30% by vol-ume), but the sand content decreases within a fewtens of centimeters of the basal contact. Phosphatealso occurs within the Upham Dolomite as a replace-ment of bryozoans and small pellets and as coatingsalong hardgrounds. Coarse-grained crinoidal grain-stone beds commonly occur in the upper part of theUpham Dolomite (Table 1).

The burrowed skeletal wackestone-packstone ofthe Upham Dolomite records subtidal carbonate dep-osition on an open-marine carbonate ramp. The phos-phate in this unit was likely brought into thisdepositional setting by upwelling currents that trans-ported phosphate-rich waters onto the shallowcarbonate ramp (Pope and Steffen, 2003).

Aleman Formation

The Aleman Formation (16–85 m [52–279 ft] thick)is a complex subtidal carbonate unit whose chertabundance is quite variable, ranging from 0% to70%, and averaging between 20% and 40%. The

Figure 2. Stratigraphic chart for the Montoya Group with ageconstraints from Bergström et al. (2008) and Gradstein et al.(2012). Chat = Chatfieldian Stage; Eden = Edenian Stage;May = Maysvillian Stage; Rich = Richmondian Stage; Hirnant =Hirnantian Stage; Gamach = Gamachian Stage; Sand =Sandbian Stage; Cyn = Canyon; and Ss = sandstone. P indicatesthe location of abundant (1–5 wt. %) phosphate. The gray shad-ing in the Aleman Formation indicates abundant chert (average10%–40% by volume). The Aleman Formation contains themain reservoir facies in the subsurface Montoya Group in thePermian Basin of west Texas.

1580 Sequence Stratigraphy Montoya Group, New Mexico and Texas

Aleman Formation can commonly be subdivided intoupper and lower cherty units that are separated by awidespread medial packstone-grainstone marker unit.The depositional environments represented by theAleman Formation range from shallow, high-energyshoals to deep-water settings, below the stormwave base.

Even-Bedded Laminated Calcisiltite or Mudstone andSpiculitic ChertEven-bedded laminated calcisiltite and spiculiticchert (Table 1) is the basal unit of the AlemanFormation in the north-central part of the field areaoccurring primarily within an approximately east–west trend that includes the Cooks Range, Silver

City, Nakaye Mountain, and southern San AndresRange localities.

The even-bedded calcisiltite or lime mudstoneinterbedded with spiculitic chert is interpreted to re-present deposition in deep subtidal waters because itcontains no mechanically produced laminations. Thecalcisiltite or lime mudstone represents depositionbelow the storm wave base on this ramp, whereasthe spiculitic chert formed as the disarticulatedsponge spicules moved downslope on the ramp bybed-load processes and accumulated in beds. The rarehummocky beds (e.g., Cooks Range, southern SanAndres Range) within this facies indicate that thestorm wave base rarely impinged upon the sea floorduring deposition of this facies.

Figure 3. Depositional profiles showing the relationships of Montoya Group facies along depositional dip. The upper figure during thecomposite third-order HST (= middle and upper Aleman Formation to Cutter Formation), the lower figure during the composite third-order TST (= Cable Canyon Sandstone through lower Aleman Formation). Ms = mudstone; Ws = wackestone; Ps = packstone; Gs =grainstone.

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Table 1. Depositional Facies of the Montoya Group, Southern New Mexico and Western Texas

Rock Type Fossils Other GrainsChert Type andAbundance

SedimentaryStructures

DepositionalEnvironment

Stratigraphic andGeographic Distribution

Coarse sandstoneto granuleconglomerate

Crinoids, gastropods – None Skolithos burrows,local cross-bedding

Open-marinesand-wavecomplex or beach

Initial deposit ofMontoya, base ofsequences M0 and M1

Skeletal wackestoneto packstone

Crinoids, corals, brachiopods,bryozoans, gastropods,stromatoporoids,Receptaularid algae

Phosphate as pellets,hardground coatings,replacement of fossils

Rare (<5 vol.%)as replacementof fossils

Massive bedding,burrows

Open-marineshallow carbonateshelf

Most of UphamDolomite upper part ofsequence M1

Skeletal grainstone Crinoids Phosphate replacingfossils

Rare Locally cross-beddedor massive

High-energyskeletal shoals

Upper part of UphamDolomite (sequence M1)and top of Cutter Form-ation (sequence M5)

Spiculitic calcisiltite Sponge spicules Rare phosphatereplacing spicules

Abundant, even bedsof sponge spicules

Rare hummocky orplanar laminations incalcisiltite; rare cross-bedded spiculite

Open-marine deepramp with localstorm andturbidite beds

Aleman Formation, basalpart of sequences M2and M3

Cherty skeletalwackestone-packstone

Brachiopods, bryozoans None Common as nodulesand as replacement ofburrows and fossils

Massive bedding, raregraded beds

Open-marinemidramp

Aleman Formation,middle part ofsequences M2 and M3

Massive chertybreccia

Brachiopods None Common as angularclasts

Possible water-escapestructures

Debris flows orseismicallyshaken beds

Throughout the AlemanFormation; sequencesM2 and M3

Skeletal packstone-grainstone

Crinoids, corals,stromatoporoids

Phosphate as pellets,and replacement offossils

Rare, replaces corals Cross-bedding, uprightand overturnedcorals

Open-marine high-energy shoal

Is a marker horizon inmiddle of AlemanFormation, separatessequences 2 and 3

Skeletal packstoneand limemudstone

Bryozoans, brachiopods,gastropods

None None Massive bedding,biosturbated

Open-marine ramp As thin flooding eventswithin the CutterFormation

Burrowedmudstone

Ostracods, gastropods None None Abundant burrows Lagoon, or low-energy ramp

Cutter Formation,sequence M4

Laminateddolomudstone orfenestraldolomudstone

None Intraclasts of mudstone Rare chert afterevaporites

Cryptalgalaminites,mudcracks, teepeestructures, fenestrae,intraclast

Inner-ramp tidalflats

Cutter Formation,sequences 3 and 4

1582Sequence

StratigraphyMontoya

Group,NewMexico

andTexas

Skeletal Wackestone to Packstone with Irregular,Discontinuous Bedded to Nodular ChertFragmental to whole skeletal wackestone to pack-stone containing irregular and discontinuouslybedded chert (Table 1) up to a few meters wide anda few centimeters thick grades laterally and verticallyinto fragmental and whole skeletal wackestone-pack-stone with nodular bedded chert (Figure 4). Thisfacies occurs within both the lower and upperAleman Formation, in front of the grainstone shoalcomplex and above the interbedded calcisiltite ormudstone and spiculitic chert (Figure 3). The abun-dance of chert in this facies ranges from 5% to 60%.The chert nodules range from a few to tens of centi-meters in diameter. The chert margins vary fromsmooth to sharp and irregular. Some chert nodules

contain carbonate within their centers, giving them ahollow appearance (see also descriptions byHowe, 1959).

These discontinuously bedded to nodular beddedcherty skeletal carbonate facies formed on an open-marine ramp. The lack of bedding and nodularappearance of carbonate and chert suggests that thisfacies was intensely bioturbated, similar to nodularbedded subtidal carbonate from other Ordovician car-bonate ramps (e.g., Harris and Sheehan, 1996, 1997;Pope and Read, 1997a; Holland and Patzkowsky,2012). The variety of chert abundance and morpholo-gies reflects both original depositional features andsubsequent early diagenetic silica enrichment, priorto or coeval with bioturbation.

Massive Cherty BrecciaMassive cherty breccia (Table 1) occurs rarely withinthe Aleman Formation (Figure 5). The massive chertybreccia ranges from matrix to clast supported andcomprises angular fragments of chert, carbonate, andcherty carbonate in a muddy carbonate matrix. Thethree-dimensional geometry of these units is unclear,but they do not crosscut bedding and appear to be len-soidal units tens to hundreds of meters wide and a fewmeters thick.

Figure 4. Photograph of skeletal wackestone-packstone withsmooth chert nodules (∼30% by volume), upper AlemanFormation, northern Franklin Mountains, Texas.

Figure 5. Photograph of chert breccia, Aleman Formation,Caballos Range, New Mexico. The chert in this photograph islight colored, whereas the dark material is carbonate mudstoneor skeletal wackestone. The chert in this outcrop is identical withthe nodular chert except for the angular characteristic of itsexternal morphology. This facies is interpreted to have formedthrough violent shaking on the sea floor (e.g., tectonism, debrisflow, etc.) after formation of the silica but prior to lithification,as it occurs within bedding.

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The massive cherty breccia in the AlemanFormation formed early on the sea floor because itdoes not crosscut bedding and it contains chert clasts.These breccias likely formed by gravitational insta-bilities such as downslope slumping, water escape,or tectonic shaking (e.g., Grimm and Orange, 1997)after the chert was lithified on the sea floor.

Skeletal Packstone-GrainstoneSkeletal packstone-grainstone with an open-marinebiota and colonial coral bafflestone (Table 1) was

deposited basinward of burrowed mudstone or periti-dal facies (Figure 3) and occurs as a widespreadmarker unit in the middle of the Aleman Formation(Figure 6). The small amount of chert in this faciesoccurs most commonly as a replacement of colonialcorals.

The skeletal packstone-grainstone and coral baf-flestone is interpreted to represent a widespread high-energy skeletal shoal or coral bioherm. Cross-beddingin the skeletal packstone-grainstone indicates high-energy currents during deposition of this unit.

Figure 6. (A) Detailed measured sections of Montoya Group, San Andres Range to northern Franklin Mountains (measured sections fromSM to NFC) and its interpreted sequence stratigraphy. The symbols and colors are from Figure 3; the colors represent facies types along thedip profiles. Red lines mark the composite third-order sequence boundaries, whereas blue-gray lines delineate the high-frequency third-order sequence boundaries. The section is hung on the M3–M4 sequence boundary. (B) Detailed measured sections ofMontoya Group, northern Franklin Mountains to the Hueco Mountains (measured sections NFD to PLC). Same conventions as (A).(C) Generalized composite third-order sequence stratigraphy of Montoya Group along cross section BB′. The composite third-order sequenceis composed of up to six high-frequency third-order sequences (M0–M5). High-frequency third-order sequences M1–M4 are regionallycorrelative, whereas sequences M0 and M5 are only locally developed or preserved. See Figure 1 for definitions of abbreviated names.

1584 Sequence Stratigraphy Montoya Group, New Mexico and Texas

Cutter Formation

The Cutter Formation is the uppermost unit of theMontoya Group, and its thickness ranges from 0 to

60 m (0 to 1976 ft) in the study area. The CutterFormation consists of three facies: (1) skeletal packstoneinterbedded with lime mudstone, (2) bioturbated dolo-mudstone, and (3) laminated or fenestral dolomudstone.

Figure 6. Continued.

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Skeletal Packstone Interbedded with MudstoneThe Cutter Formation contains occasional thin (<5 m[<16 ft] thick), burrowed skeletal packstone bedsinterbedded with dolomudstone (Table 1). Thesepackstone beds contain abundant bryozoans, brachio-pods, and crinoids.

Burrowed DolomudstoneMost of the Cutter Formation consists of brown, bur-rowed, or massive dolomudstone with gastropods andostracods. This burrowed dolomudstone is interpretedto represent shallow sudtidal deposition in a restrictedenvironment because of its restricted fauna. Thisfacies formed landward of skeletal shoals or passeddirectly into more basinward shallow subtidal rocks(Figure 6).

Laminated Dolomudstone and Massive FenestralDolomudstoneLight-colored laminated dolomudstone and massivefenestral dolomudstone occur as thin, discontinuousunits within the burrowed dolomudstone. The lami-nated dolomudstone contains abundant mudcracks,small burrows, and rare intraclasts.

The laminations in this facies were likelyproduced by microbial mats and are interpreted ascryptalgalaminites. The fenestrae in the dolomud-stone formed as gas structures within shallowwell-oxygenated mudflats. The laminated dolomud-stone formed on semiarid tidal flats, whereas the fen-estral dolomudstone formed on more humid tidal flats(Read and Grover, 1977; Grover and Read, 1978).

SEQUENCE STRATIGRAPHY

The sequence-stratigraphic terminology in this articlefollows the hierarchy set forward by Weber et al.(1995) and Sarg et al. (1999) for deciphering deposi-tional units. The measured sections were correlatedonto four regional cross sections (Figure 1) bydirectly walking sequence boundaries between sec-tions in closely spaced sections, correlating parase-quence stacking patterns and unconformitiesbetween farther spaced sections (as outlined inHarris et al., 1999), and integrating the available bio-stratigraphy. Although the following discussion of

sequence stratigraphy is based on all of the cross sec-tions, it relies most heavily on line BB′ (Figure 6A,6B) because this line contains the most closely spaceddata across the entire ramp, and it provides the mostdetailed dip profile. This is considered a dip profilebecause the siiliciclastics decrease to the south, theamount of cherty subtidal facies increases to thesouth, tidal flats prograde from north to south, andthe unconformities downcut farther to the north.A generalized outline of the composite third-ordersequence stratigraphy of the BB′ cross section isprovided in Figure 6C.

Composite Third-Order Sequence

The length of time of deposition of the UpperOrdovician Montoya Group (6–7 m.y.) suggests thatthis unit is a composite third-order sequence (sensuWeber et al., 1995; Sarg et al., 1999). The sequenceis bounded by regional unconformities, and inter-nally, its regional facies stacking pattern indicates awidespread transgression followed by a regression(Figure 6C).

The Montoya Group was deposited on an irregu-lar unconformity surface that records a prolonged(about 30 m.y. duration) period of erosion or nonde-position that developed between the El Paso Groupand the Montoya Group. The character of this uncon-formity is quite variable regionally. In the FranklinMountains of westernmost Texas and southern NewMexico, the unconformity is a pronounced karsticsurface with as much as 20 m (66 ft) relief beneathwhich an extensive paleocave system is well devel-oped (Lucia, 1988). In the Mud Spring Mountains,the contact is a karst surface where sandstone hasinfilled voids between blocks of the El Paso Group(Figure 7A) that were subaerially exposed prior toMontoya Group deposition. Elsewhere, the El PasoGroup and Montoya Group contact is sharp with littleor no relief (Figure 7B). However, the contact is gen-erally easy to identify in the field because it is com-monly marked by a sandy bioturbated carbonate ofthe lower Upham Dolomite, commonly containingrip-up clasts of the underlying El Paso Group, occur-ring above well-laminated dolostone of the El PasoGroup. A lowstand systems tract is not discerniblein the study area.

1586 Sequence Stratigraphy Montoya Group, New Mexico and Texas

Updip, the lower part of the composite third-order transgressive systems tract (TST) consists ofthe carbonate-cemented, burrowed, coarse CableCanyon granule sandstone that passes conformably

basinward and upward into massive, bioturbated skel-etal wackestone-packstone of the Upham Dolomiteand the basal part of the Aleman Formation.

The base of the Aleman Formation is a very sharptransition in the field between mostly noncherty rocksbelow (Upham Dolomite) and cherty rocks (AlemanFormation) above (Figures 6, 8). A third-order maxi-mum flooding surface (MFS) is not recognized inthese deep-water facies; instead, a maximum floodingzone (MFZ) characterized by an approximately10–20-m (33–66-ft)-thick interval of dark-gray calci-siltite or lime mudstone interbedded with dark,evenly bedded spiculitic chert deposited immediatelyabove skeletal packstone-grainstone (Figure 8).Above this zone, facies generally shallow upwardfrom deep subtidal carbonate into shallow subtidal

Figure 7. (A) Photograph of karsted El Paso Group (light col-ored) and Cable Canyon Sandstone, basal Montoya Group (darkcolored), Mud Spring Mountains, New Mexico. (B) Photographof sharp surface between the underlying Lower Ordovician ElPaso Group and the overlying Upper Ordovician UphamDolomite, southern Franklin Mountains, Texas. The UphamDolomite is a burrow-mottled skeletal wackestone-packstone.

Figure 8. Photograph of composite third-order maximumflooding zone (MFZ) in basal Aleman Formation, Cooks Range,New Mexico. The lower part of the figure is crinoidal grainstonethat formed at the top of the Upham Dolomite that is overlain byinterbedded spiculitic chert and lime mudstone of the basalAleman Formation. Hammer in foreground for scale is 30 cm(12 in.) in length.

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or peritidal carbonate. The Montoya Groupcomposite third-order highstand systems tract (HST)consists of interbedded carbonate and chert of theAleman Formation that passes upramp into massive,burrowed peritidal carbonate and cryptalgalaminiteor fenestral dolomudstone of the progradationalCutter Formation. The Cutter Formation records shal-low subtidal and peritidal deposition that occurredupramp from a skeletal shoal during Aleman deposi-tion and then prograded across the Montoya Groupramp during a long-term relative sea level fall.

The Montoya Group is everywhere unconform-ably overlain by younger Paleozoic rocks. In southernNew Mexico and western Texas, the Montoya Groupis unconformably overlain by the Lower to middleSilurian Fusselman Dolostone, but to the north,the Montoya Group is unconformably overlain by theMiddle Devonian Percha Shale or younger units. Thisunconformity varies from a karstic surface with asmuch as a few meters relief, to a low-angle angularunconformity, to a sharp surface (Figure 9), with littleor no evidence of an unconformity. Karst sinkholes,as much as 5 m (16 ft) across and a few meters deep,containing Fusselman Dolostone clasts, occur in dolo-mitic Montoya Group rocks in the Franklin Mountains.

High-Frequency Third-Order Sequences

The Montoya Group composite third-order sequenceis composed of six high-frequency third-order

sequences with likely average durations of 1–2 m.y.by dividing the duration (6–7 m.y.) by the numberof sequences (4 to 6). The Montoya Group high-frequency third-order sequences are numberedsequentially from the base M0–M5 as outlined online BB′ (Figure 6A, B).

Sequence M0

Sequence M0 (Figure 6A) is the lowest sequence ofthe Montoya Group and occurs only in topographiclows that developed on the El Paso Group karst sur-face in the far northern part of the field area. Thebasal unit of this sequence is a coarse, carbonate-cemented sandstone formed during the initial TSTthat grades upward into bioturbated skeletal wacke-stone and packstone of the HST. The MFZ of thissequence is poorly defined, likely occurring lowwithin the skeletal wackestone-to-packstone succes-sion. The sequence boundary between sequencesM0 and M1 is a sharp surface directly overlain byanother, much more widespread, carbonate-cementedbioturbated sandstone.

Sequence M1

Sequence M1 consists of the transgressive CableCanyon Sandstone in the north grading upward andbasinward into burrow-mottled skeletal wackestone topackstone of the Upham Dolomite (Figure 6A, B).Sequence M1 ranges in thickness from less than10 m (33 ft) to greater than 40 m (131 ft), with thethickest accumulation near the middle of the fieldarea. The Cable Canyon Sandstone is overlain andgrades laterally into skeletal wackestone and pack-stone of the Upham Dolomite. The MFZ in this zonelikely occurs low in the skeletal wackestone or pack-stone above the Cable Canyon Sandstone. The skel-etal wackestone and packstone of the basal HST arecapped by the uppermost unit of sequence M1 thatis a cross-bedded crinoid grainstone at or near thetop of the HST. Small karstic voids filled with cherty,sandstone-rich carbonate occurring at the top of theskeletal grainstone in the Mud Spring Mountainslikely formed from short-lived subaerial exposure.Elsewhere, the boundary between sequences M1 and

Figure 9. Photograph of irregular karsted surface atop theCutter Formation unconformably overlain by the SilurianFusselman Dolostone, Arrow Canyon, San Andres Range, NewMexico.

1588 Sequence Stratigraphy Montoya Group, New Mexico and Texas

M2 is a sharp transgressive surface without clear evi-dence for subaerial exposure.

Sequence M2

The TST of sequence M2 (Figure 6A, B) consists oflaterally discontinuous skeletal packstone above theuppermost Upham Dolomite grainstone gradingupward into interbedded calcisiltite and spiculitic chertor cherty mudstone within the MFZ. The MFZ of M2coincides with the composite third-order MFZ. Thelower HST consists of interbedded calcisiltite or mud-stone and spiculitic chert that grades upward into skel-etal wackestone or packstone with nodular chert. Theupper HST of sequence M2 is characterized by a pro-nounced progradation of skeletal grainstone with amore open-marine biota. This skeletal grainstone com-monly is cross-bedded and contains abundant corals.Sequence M2 ranges from 20 to 40 m (66 to 131 ft)thick, with thickest accumulations to the north andwithin central areas (Pope, 2004a), likely caused byincreased accommodation space created by syndeposi-tional faulting. The sequence boundary betweensequences M2 and M3 commonly is a subtidal surfacepicked as the turnaround from high-energy, cross-bedded grainstone below to a deeper water successionof burrowed skeletal packstone above, without anyevidence of exposure.

Sequence M3

The TST of sequence M3 downdip (Figure 6B) con-sists of a thin skeletal packstone overlain by a region-ally developed hardground (Figure 10) that is sharplyoverlain by interbedded calcisiltite or mudstone andspiculitic chert deposited in a deeper ramp setting.Updip, it is difficult to determine the sequence boun-dary between sequences M2 and M3 because facieschanges in the peritidal and shallow subtidal facies arenot easy to correlate (Figure 6A). The peritidal faciesare separated from open-marine cherty carbonates bya skeletal grainstone shoal or coral bafflestone thataggrades during the early HST then progrades duringthe late HST (Figure 6). South of the grainstone shoal,the MFZ commonly is marked by a widespread marinehardground above a thin unit of skeletal packstone and

grainstone; north of the grainstone shoal, the MFZoccurs within widespread peritidal facies. The HST ofsequence M3 south of the aggrading grainstoneshoal or bafflestone is marked by bioturbated skeletalwackestone-packstone interbedded with calcisiltiteand lime mudstone. To the north of the grainstone shoalor bafflestone, the HST of sequence M3 consistswholly of peritidal carbonates containing little or nochert. The sequence boundary between sequences M3and M4 in the south is the base of a pronounced basin-ward shift of peritidal facies (Figure 6B). In northernexposures, this surface is more difficult to pick becauseit occurs within a more monotonous succession of shal-low subtidal or peritidal carbonate mudstone. SequenceM3 is eroded in the far northern part of the field area(Figure 6A) and is locally greater than 40 m (131 ft)thick to the south (Figure 6B).

Sequence M4

The TST of sequence M4 in the south (Figure 6B)consists of a thin unit of open-marine burrowed skel-etal packstone. The MFZ of sequence M4 is poorlydefined, likely occurring low in the burrowed skeletalpackstone. The HST of sequence M4 is characterizedby bioturbated skeletal packestone capped by lami-nited and fenestral dolomudstone of the CutterFormation. Sequence M4 records a pronouncedbasinward shift in facies as peritidal carbonate, bur-rowed and fenestral dolomudstone, or mudcracked

Figure 10. Photograph of hardground (arrows) marking theMFS of sequence M3, northern Franklin Mountains, Texas. Thehardground truncates several burrows below and is encrustedby iron and phosphate.

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cryptalgalaminites, which prior to this time onlyoccurred landward of the coral-rich skeletalgrainstone-packstone that prograded south and south-west across the entire carbonate ramp at least 100 km(62 mi). The sequence boundary between sequencesM4 and M5 is a pronounced flooding surface that isonly locally developed, such as at section FRT(Franklin Radio Tower) (Figure 6B). Sequence M4commonly is eroded in the northern part of the fieldarea and is greater than 40 m (131 ft) thick in thesouth-central part of the field area (Figure 6B).

Sequence M5

In some downramp settings (northern FranklinMountains, Hueco Mountains, Big HatchetMountains, Florida Mountains), the Montoya Groupcontains another depositional sequence—sequenceM5 (see Figure 6). The TST of sequence M5 consistsof a thin basal skeletal packstone. The MFZ for thissequence is marked as the transition from subtidalskeletal packstone to overlying peritidal facies. TheHST of this sequence consists of bioturbated skeletalwackestone and cryptalgal laminites, similar to thosein the HST of sequence M4. Additionally, severalthick, coarse, skeletal grainstone units in the northernFranklin Mountains and Hueco Mountains(Figure 6B) may be a part of sequence M5. However,the lack of age constraints on the skeletal grainstoneunit also allows this unit to be part of the overlyingLower–middle Silurian Fusselman Dolostone.

Fourth- to Fifth-Order Parasequences

Meter-scale (1–10 m [3.3–33 ft] thick) shallowing-upward parasequences in the Montoya Group are dis-cernible at individual outcrops (Figure 11), but theydo not appear to correlate regionally. Subtidal parase-quences occur in the composite third-order TST andHST basinward of the skeletal grainstone shoal(Figures 6, 11). Subtidal parasequences commonlyhave chert-rich skeletal wackestone or mudstonebases that shallow upward into noncherty skeletalpackstone or grainstone caps. The subtidal parase-quences have an upward increase in skeletal grainsand a concomitant decrease in chert abundance, andthey are bounded by sharp flooding surfaces.

The lower part of the Montoya Group is predomi-nantly a subtidal ramp composed of few subtidal,shallowing-upward parasequences. Sequence M1(Upham Dolomite) is locally composed of two para-sequences, each approximately 5–10 m (16–33 ft)thick (Figure 11). The lower parasequence commonlyhas a quartzose sand base that grades upward intoskeletal wackestone that is capped by skeletal pack-stone or grainstone (Figure 11). The upper parase-quence commonly has a skeletal wackestone baseand a skeletal grainstone cap. The subtidal parase-quences in the Aleman Formation (sequences M2and M3) commonly are chert-rich, fine-grained, cal-cisiltite or skeletal wackestone-packstone bases withchert-poor coarser grained skeletal packstone-grainstone caps (Figure 11).

Peritidal parasequences in sequences M3 andM4 (Cutter Formation) have thick basal units of bio-turbated mudstone overlain by thin caps of cryptal-galaminite or fenestral mudstone (Figure 11).These peritidal parasequences may also have athin transgressive unit of coarse skeletal packstonewith an open-marine biota that indicates an incur-sion of open-marine conditions within this predomi-nantly restricted shallow subtidal or peritidalunit that grades upward into the bioturbatedmudstone.

DISCUSSION

Depositional Setting

Montoya Group facies record predominantly subtidalcarbonate deposition on a gently dipping ramp(Measures, 1985a, b; Pope and Steffen, 2003; Pope,2004a, b) because no pronounced break in slope,marked by a well-established reef system, and down-slope breccias or conglomerates are discernablewithin the study area. The ramp sloped gently to thesouth because the sandstone and peritidal facies wereoriginally restricted to the north, and most ofthe deeper water cherty carbonate predominates tothe south (Figures 3, 6). During the sea level rise thatdeposited the Cable Canyon Sandstone, UphamDolomite and lower Aleman Formation marine sand-stone passed basinward into bioturbated skeletal

1590 Sequence Stratigraphy Montoya Group, New Mexico and Texas

carbonate that graded seaward into chert-rich carbon-ate (Figure 3). During the initial sea level fall thatdeposited the middle Aleman Formation skeletalgrainstone marker (top of sequence M2), the upperAleman Formation (sequence M3) peritidal carbonatedeveloped landward of bioturbated mudstone thatpassed downramp into a skeletal shoal containingcorals, then into chert-rich subtidal carbonate(Figures 3, 6). During deposition of the uppermostCutter Formation, little evidence exists for a skeletalshoal, and bioturbated mudstone apparently passeddirectly into offshore skeletal packstone interbeddedwith mudstone (Figure 6B).

Montoya Group deposition is interpreted to haveformed on a thermohaline stratified ramp (e.g.,James, 1997). The tidal flats and lagoons on the

Montoya Group ramp contain sedimentary structures(cryptalgalaminites, evaporites, fenestrae, and abun-dant burrrows) and a variety of skeletal fossils, espe-cially corals and receptacularid algae (LeMone,1988), which indicate that shallow waters on thisramp were warm. Shoal deposits separating the periti-dal from subtidal environments (Figures 3, 6) are pre-dominantly crinoid grainstone, but they also containsome corals, indicating shallow subtidal waters werewarm. However, the reduction of biota and prolifera-tion of brachiopods and siliceous sponges, in con-junction with an increase in the phosphate content inthe deeper subtidal facies, suggests the deeper subti-dal waters were much cooler. Cool-water carbonatesare a common feature of Upper Ordovician rocksthroughout the United States mid-continent and

Figure 11. Typical parasequences ofthe Montoya Group; measured sectionlocations are in Figure 1. The parase-quence boundaries are marked by hori-zontal black lines, and the colors andsymbols are from Figure 3. The UphamDolomite shallow subtidal parasequen-ces have sandstone-rich bases andskeletal-rich caps. The AlemanFormation deep subtidal parasequencesare predominantly subtidal with calcisiltand mudstone with abundant chert inthe bases with skeletal packstone withmuch lesser chert in the caps. CutterFormation peritidal and shallow subtidalparasequences may locally have skeletalpackstone bases, but predominantly,they have burrowed mudstone baseswith cryptalalminite or fenestral mud-stone caps.

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Appalachian Basin (e.g., Brookfield, 1988;Patzkowsky and Holland, 1993, 1996, 1999; Lavoie,1995; Holland and Patzkowsky, 1996; Pope andRead, 1997a). Similarities between the MontoyaGroup brachiopod fauna (Howe, 1959) and UnitedStates mid-continent brachiopod faunas (Holland,1993; Patzkowsky and Holland, 1993; Holland andPatzkowsky, 1996) also suggest that the subtidal sedi-ments of the Montoya Group were deposited in coolwaters (Pope, 2004a). The cool-water carbonates,which commonly have little or no reservoir potentialbecause of early cementation, were not the reservoirtarget in the subsurface of west Texas; instead, thecoeval or early diagenetic porous chert that formedwithin the cool-water carbonates is the reservoirwithin the subsurface Montoya Group (Thomas andLiu, 2003).

The abundance of chert and phosphate in the sub-tidal facies of the Montoya Group indicates that equa-torial upwelling occurred over a broad region alongthe southern margin of Laurentia (Pope and Steffen,2003; Pope, 2004a). This chert commonly is primary(spiculitic) or formed during very early diagenesisprior to compaction (Pope, 2004 a, b). The porosityin this chert provides the bulk of the reservoir forthe Montoya Group gas play in west Texas (Thomasand Liu, 2003) and is similar to cherty reservoirs inthe Devonian Thirtyone Formation of the southernPermian Basin in Texas (Ruppel and Hovorka,1995). Locally significant amounts of chert replacingevaporites occur updip in the peritidal facies on theMontoya Group ramp (e.g., Geeslin and Chafetz,1982); however, the bulk of the subtidal chert in theMontoya Group is early diagenetic associated within-situ remobilization of dissolved sponge spiculesor silica brought onto the ramp during upwelling.

Sequence Stratigraphy

The composite third-order sequence of the MontoyaGroup is correlative throughout southern NewMexico and western Texas and is directly correlatedwith Upper Ordovician sequences throughout NorthAmerica (Pope, 2004b). The high-frequencey third-order sequences likely were produced by long-term(1–3 m.y. duration) relative rises and falls of sea levelthat affected much of Laurentia (Pope et al., 2001).

Parasequences (meter-scale cycles) in the MontoyaGroup are locally developed, but they are not regionallycorrelative (Figure 12). The lack of laterally continuousoutcrops makes correlating the parasequences greatdistances (a few to tens of kilometers) difficult. Therounded nodules of chert with carbonate interiors are adistinctive chert facies (see discussion of hollow chertin text; Howe, 1959; Pope, 2004a) whose origin isunknown. The cherty facies of sequences M2 and M3are analogous to the reservoir in the subsurface, and atthis scale, they appear to be well-connected units. It isless clear if the well-cemented skeletal grainstone-packstone unit at the top of sequence M2 or the baseof sequence M3 would be a barrier to flow. Althoughindividual parasequences are difficult to trace region-ally, the facies relationships within the sequences thatare correlative over short distances (Figure 12), at thescale of most drilling patterns (20-, 40-, or 80-ac[8-, 16, or 32-ha] spacing), suggest that exploiting thisramp-type chert reservoir may be fairly straightforwardin local areas. Thus, Montoya Group parasequences inthe TSTs commonly are thinner and finer grained andcontain more chert, whereas parasequences in theHSTs are thicker and have more skeletal grains and lesschert, except in sequence M4 which is predominantlyperitidal facies.

Broad carbonate ramps that formed during globalgreenhouse conditions were only subjected to small-scale sea level fluctuations (<10 m [<33 ft]), andautocyclic processes were likely important in parase-quence formation (Kozar et al., 1990; Wright, 1992;Goldhammer et al., 1993; Read, 1998). Carbonateparasequences formed under such conditions likelywill show well-developed peritidal successions withextensive tidal-flat caps, relatively minor disconform-ities cap parasequences, and intercalation of shallow-water and deep-water facies is rare (Koerschner andRead, 1989; Borer and Harris, 1991; Wright, 1992).Cryptalgalaminites and fenestral mudstone are tidal-flat facies in the Montoya Group that track higher fre-quency (fourth- and fifth-order) sea level fluctuationsin the composite third-order HST but may also recordautocyclic processes.

At the other extreme, parasequences that formduring times of continental glaciation as in thePleistocene and the Pennsylvanian–Early Permianare subjected to large (>50–100 m [>164–330 ft])

1592 Sequence Stratigraphy Montoya Group, New Mexico and Texas

high-frequency sea level fluctuations (Heckel,1980, 1983, 1985; Wright, 1992; Read, 1998).Parasequences formed under these conditions showonly limited tidal-flat caps on parasequences;parasequence-capping disconformities are developedover much of the shallow ramp; and on the outer ramp,intercalation of deeper water and shallow-waterfacies occurs within individual parasequences.Parasequences with this type of facies architectureand distribution do not occur in the Montoya Group.

The Upper Ordovician rocks of Kentucky andVirginia showed a transition in development of parase-quences from wholly peritidal parasequences in thelower Upper Ordovician to parasequences with inter-calated subtidal and peritidal facies in the upper

Upper Ordovician (Pope and Read, 1998). This transi-tion in parasequence development, along with faunaland geochemical data (Elrick et al., 2013), is inter-preted as evidence for the onset of widespreadmoderate-amplitude (>15 m [>49 ft], possibly asmuch as 30 m [98 ft]) glacioeustasy that persisted untilthe end of the Ordovician (Pope and Read, 1997a,1998). The juxtaposition of open-marine subtidalfacies in shallow subtidal to peritidal parasequencesof the composite third-order HST (Figure 12, CutterFormation) indicates that some moderate-amplitudeeustatic sea level fluctuations were recorded in theMontoya Group. Additionally, facies changes withinsubtidal parasequences (Figure 12, AlemanFormation) indicate relative sea level changes from

Figure 12. Seven closely spaced measured sections in the northern end of the Franklin Mountains (also in Figure 6A, B) demonstratethe lateral continuity of facies, and third-order sequences are easy to correlate across these closely spaced sections; however, parase-quences are very difficult to trace regionally between the sections. See Figure 1 for definitions of abbreviated names.

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below the storm wave base to possibly within thetidal range, suggesting moderate-amplitude (>10-m[>33-ft]) relative sea level changes. Thus, the faciesdistribution within the parasequences of the MontoyaGroup also indicates that they were deposited duringa transitional climate moving toward Late Ordovicianglobal icehouse climate conditions (Frakes et al.,1992; Brenchley et al., 1994, 2003).

Tectonic Control on Facies and SequenceDevelopment

Isopachs of the third-order sequences and the distri-bution of distinctive facies (e.g., sandstone, basinallimestone, and chert) in these sequences suggest thata pre-existing or active structural feature influencedthe deposition of the Cable Canyon Sandstone andsequences M0, M2, and M3 (Pope, 2004a).Sequence M0 only occurs in limited exposures ofthe San Andres Mountains and SacramentoMountains, within a topographic low area on the ElPaso karst surface or possibly within an incised val-ley. The Cable Canyon Sandstone thickens to thesouth within the San Andres Mountains and west-ward toward the Cooks Range (Figure 6A).However, south of the present position of theCenozoic Organ Mountain intrusion, the CableCanyon Sandstone thins greatly or is absent(Figure 6A). Deep-water calcisiltite interbedded withspiculitic chert of the lower Aleman Formation onlyoccurs in the southern San Andres Mountains,Nakaye Mountain, Cooks Range, Lone Mountain,and Silver City. Associated with these deep-waterunits are thin beds of chert breccia and thickspiculitic-rich (as much as 70% by volume) beds inter-preted to have formed during slumping or deep water.

The distribution of these facies is delineated byan approximately east–west area that stretches fromthe Organ Mountains to south of the Cooks Rangeand Silver City. The facies distributions suggest thatthe area north of the east–west line was downdroppedrelative to the southern block prior to or during initialdeposition of the Montoya Group and was sub-sequently reactivated in a normal sense during thedeposition of the Montoya Group. Immediately southof the area with deep- water sediments is a thick inter-val of peritidal facies (sections BCN and BCS of

Figure 6A) surrounded by slightly deeper burroweddolomudstone interbedded with thin peritidal units.The abundance of peritidal facies in this area suggeststhat this southern area was uplifted and stayed highuntil the deposition of the Montoya Group was com-plete. The deep-water facies occur within a structuralfeature defined by modern faults (Cather andHarrison, 2002), suggesting that the faults that boundthis depression were long-lived structural lineamentsthat were later reactivated. These subtle tectonicstructures may greatly influence the stratigraphy oframps and may make delineating and developing areservoir play in this setting much more difficult.

CONCLUSIONS

The Upper Ordovician Montoya Group of southernNew Mexico and west Texas records deposition ona gently dipping carbonate ramp. The abundance ofprimary and early diagenetic spiculitic chert on thisramp, as well as 2–5 wt. % phosphate, indicates thatthe ramp formed in a regional area of upwelling alongthe southern margin of Laurentia. The chert is themain reservoir facies in the Montoya Group petro-leum system of nearby west Texas.

The Montoya Group is a composite third-ordersequence containing parts of six third-order depositio-nal sequences (M0–M5). Sequence M0 was onlylocally preserved in a topographic low on the uncon-formity that developed following Lower OrdovicianEl Paso Group deposition. Sequences M1 throughM4 are widespread regionally correlative rock succes-sions. Sequence M5 was only deposited downramp asrelative sea level fell. Parasequences within theMontoya Group indicate small- to moderate-amplitude, high-frequency relative sea level fluctua-tions, are not regionally correlative, and likely formedduring a transition to Late Ordovician icehouse cli-mate conditions. The correlation of high-frequencythird-order sequences and facies of the MontoyaGroup over short distances (less than 3 km [1.8 mi]),well within the spacing of most conventional well-drilling patterns, suggests that exploiting this ramp-type chert reservoir may be fairly straightforward inlocal areas. However, subtle tectonic structures maygreatly influence the stratigraphy of these types of

1594 Sequence Stratigraphy Montoya Group, New Mexico and Texas

ramps, and these may make delineating and develop-ing a reservoir play in this setting much more difficult.

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