Field Trip Guide to the Upper Cretaceous Hornbrook Formation and Cenozoic Rocks of southern Oregon and northern California

Students in SOU’s field geology course examining sandstones of the Rocky Gulch Member of

the Hornbrook Formation near Hilt, California.

Field Trip Leader: Bill Elliott Department of Geology, Southern Oregon University

Saturday, September 8, 2007

Introduction The Klamath Mountains are an elongated north-trending geological province that

occupies approximately 19,000 km2 in southwestern Oregon and northern California. The Klamath Mountains are made-up of numerous terranes that accreted during the Antler (Devonian), Sonoman (Permian to Late Triassic), and Nevadan (Jurassic to Early Cretaceous) orogenies (Mortimer, 1984). These terranes have been grouped into four metamorphic belts, from oldest (east) to youngest (west): Eastern Klamath Belt; Central Metamorphic; Western Paleozoic and Triassic; and Western Jurassic (Irwin, 1966; Irwin, 1994). In the Late Jurassic to Early Cretaceous, numerous magma bodies intruded the Klamath Mountains, including the Jurassic Mt. Ashland pluton and Early Cretaceous Grants Pass pluton (Hotz 1971; Gribble et al., 1990). During the Late Jurassic to Early Cretaceous, a subduction zone complex and forearc basin developed along the western margin of North America while folding and thrusting of Paleozoic and lower Mesozoic rocks associated with the Sevier orogeny triggered the formation of the Cordilleran foreland basin in the interior of North America (Figs. 1 and 2).

The Hornbrook Formation (Upper Cretaceous) consists of a sequence of dominantly marine clastic sedimentary rocks about 1,200 meters thick exposed along the northeastern margin of the Klamath Mountains in southwestern Oregon to northern California (Fig. 3). These marine sediments are interpreted to have been deposited in a forearc basin similar to the Upper Cretaceous sediments of the Great Valley forearc in central California (Sliter et al., 1984; Nilsen, 1984, 1993). The Hornbrook Formation lies nonconformably on Paleozoic and Mesozoic igneous and metamorphic rocks of the Klamath Mountains, and is overlain disconformably by Tertiary sedimentary rocks and/or volcaniclastic sediments (Fig. 4). Generally, the Hornbrook Formation strikes N30oW to N45oW and dips 20o to 30o to the northeast on the eastern flank of the Klamath Mountains. The Hornbrook Formation is subdivided into five members, in ascending order: Klamath River Conglomerate, Osburger Gulch Sandstone, Ditch Creek Siltstone, Rocky Gulch Sandstone, and Blue Gulch Mudstone (Fig. 5; Nilsen, 1984; Nilsen 1993).

The Hornbook Formation is overlain unconformably by the Payne Cliffs Formation (Eocene) in the Bear Creek Valley of southwestern Oregon or by the Colestin Formation (Late Eocene to Oligocene) in the Cottonwood Creek Valley of northern California. The Payne Cliffs Formation consists of conglomerates and sandstones with subordinate amounts of siltstone and mudrock (McKnight, 1971; 1984). In the Siskiyou Pass area of southwestern Oregon and northern California, the Colestin Formation is dominated by volcaniclastic sediments (Bestland, 1985; 1987). The lateral and vertical facies relationships between the Payne Cliffs and Colestin Formations are poorly constrained. The Colestin Formation and its unnamed equivalents in the Bear Creek Valley are overlain by the Roxy Formation (Late Oligocene to Early Miocene), which is composed of vesicular basalt flows, volcanic breccias, volcaniclastic siltstones, shales, and conglomerates, with subordinate amounts of fine-grained, planar stratified, volcaniclastic sandstones (Vance, 1984). The Colestin and Roxy Formations are approximately 1,550 m thick in southwestern Oregon and northern California. The Roxy Formation is overlain by the Wasson Formation (Early Miocene) which is composed of ash-flow tuffs, with interlayers of lavas and fluvial deposits (Vance, 1984). The Heppsie Andesite (Miocene) sits above the Wasson Formation. The Colestin, Roxy, Wasson, and Heppsie Formations collectively are referred to as the Western Cascades.

1

N 48o

N 44o

N 40o

N 36o

N 36o

N 40o

N 44o

W 120o

W 116o

W 112o

W 108o

W 124o

0100

200300

400NKilometers

MONTANAWYOMING

UTAH

ARIZONA

NEVADA

IDAHO

WASHINGTON

OREGON

CALIFORNIANEWMEXICO

COLORADO PLATEAU

X

X

X

X

XX

X

X

X

X

X X

X

X

X

XX

X

X

X

BASIN & RANGE PROVINCE

Pre-Cretaceoussubduction zone complexes

Late Pz & Mzarc terranes

Pz arcterranes

Cretaceous plutonic belts

Cretaceous forearc sediments

Cretaceous subduction zone complexes

Snake River Plain

Cascade Volcano

XX

Cretaceous Fold & Thrust Belt

W 120 W 116 W 112 W 108W 124

San Andreas Fault

B B’

A

A’

Modified from Dorsey & LaMaskin (in review)

Figure 1: Tectonic map of North America highlighting the major geologic provinces. This map is modified from Dorsey & LaMaskin (in review).

Fold & Thrust Belt

Cordilleran Foreland Basin

Magmatic Arc

Forearc Basin

Trench

Precambrian Basement

Lith

osph

ere

SubductionComplex

B’(EAST)B (WEST)

SLS SLS

Precambrian Rocks

Proterozoic to Paleozoic Crust with Mesozoic shortening

Paleozoic Rocks

Late Paleozoic & Mesozoic arc terranesPre-Creatceous subduction zone complexes

Paloezoic arc terranes

Jurassic & Early Cret.Intrusive rocks

Mid-Cretaceous intrusive& volcanic rocks

Cretaceous subductionzone complexes

Cretaceous sandstone

X X

XX

X

X XX

LEGEND

TECTONIC RECONSTRUCTIONS FOR THE CRETACEOUS

Fold & Thrust Belt

Cordilleran Foreland Basin

Magmatic Arc

Forearc Basin

Trench

Precambrian Basement

Lith

osph

ere

SubductionComplex

A’(EAST)A (WEST)

SLS SLSX X

X X

X

KLAMATH MOUNTAINS

SIERRA NEVADA MOUNTAINS

Figure 2: Reconstructed tectonic cross-sections for the mid-Cretaceous of western North America. Refer to Figure 1 for the location of these cross-sections.

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0 10 20

N

OREGONCALIFORNIA

Medford

Ashland

Hilt

YrekaKilometers

Western Cascades (Paleogene)Hornbrook Formation(Late Cretaceous)Diorite & granodiorite plutons (Late Jurassic)

IndexMap

Hayfork & Applegate Terranes (Permian to Early Jurassic)

Rattlesnake Creek Terrane(Late Triassic to Early Jurassic)

Condrey Mountain Terrane (Late Jurassic)

Mt Ashland Pluton

X

X

X X

X

X X

X X

XX

X

X

X

X

XX

X

X

X

X X

X

X

X

X

X

X

X

XX

X

X

X

X

X

X

XXX

X

X X

X X

XX

X

X

XX

X

X

X XX

XXX

Vesa Bluffs Pluton

N42 15’o

N42o

Hornbrook

5

5

5

W123 45’o

W123 30’o

W123o

30

Modified from Mortimer & Coleman (1984).

Central Metamorphic Belt (Devonian)

Figure 3: Generalized geologic map of southwestern Oregon and northern California showing the extent of exposures of the Hornbrook Formation. Generally, the Hornbrook Formation strikes to the northwest along the northeast flank of the Klamath Mountains. In addition, the Hornbrook Formation is dissected by numerous northeast-trending normal faults.

Figure 4: Southwest to northeast geologic cross-section near Hornbrook California showing the homoclinal package of sedimentary rocks that make-up the Hornbrook Formation. Figure from Nilsen (1993).

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600

500

400

300

200

100

0

700

800

900

1,000

Shale & mudstone

Siltstone

Sandstone

Conglomerate

Igneous & metamorphic rocks

Foraminiferal fossils

Molluscan fossils

Plant fossils (leaf & wood)

LEGEND

meters

Klamath River Conglomerate Member

Osburger Gulch Sandstone Member

Ditch CreekSiltstone Member

Rocky Gulch Sandstone Member

Blue GulchMudstone Member

Hilt Bed

Rancheria GulchSandstone Beds

HO

RN

BRO

OK

FOR

MAT

ION

Stratigraphic column of the Upper Cretaceous

Hornbrook Formationnear Yreka, California

LOW

ER

CA

MPA

NIA

NC

ON

IAC

IAN

TUR

ON

IAN

UP

PE

R C

AM

PAN

IAN

STA

GE

Trough Cross-stratification

Figure 5: Idealized stratigraphic column of the Upper Cretaceous Hornbrook Formation. The basal Klamath River Conglomerate is only present in the southernmost portion of the study area. In the Hilt and Bear Creek areas, the Osburger Gulch Sandstone Member sits unconformably on igneous and metamorphic rocks of the Klamath Mountain province. Stratigraphic section modified from Nilsen (1984).

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Geologic Mapping in southern Oregon and northern California Wells (1956) published a geologic map of the Medford 15-minute quadrangle highlighting the sedimentary deposits. Smith and others (1982) produced a 1:250,000 scale geologic map of the 1o by 2o Medford quadrangle. In recent years, the focus has shifted from the compilation of regional geologic maps to conducting detailed geologic mapping of 7.5-minute quadrangles at a scale of 1:24,000. The most recent geologic mapping at this scale in southern Oregon was completed by Wiley and Smith (1993) of the Medford East, Medford West, Eagle Point, and Sams Valley quadrangles in Jackson County, Oregon. Within the next decade, geologic mapping at 1:24,000 will hopefully be initiated by the Oregon Department of Geology and Mineral Industries, followed by compilation of 1:100,000 geologic maps of southwestern Oregon. Geologic mapping in the Klamath Mountains is difficult due to the ruggedness of the terrain and limited access. Regardless, there have been numerous geologic maps and reports published summarizing the complex geology of the Klamath Mountains. In particular, Hotz published geologic maps of the Condrey Mountain 15-minute quadrangle (1967) and Yreka 15-minute quadrangle (1977) in northern California. Irwin (1994) published the first geologic map of the entire Klamath Mountains at a scale of 1:500,000. Future geologic mapping at a scale of 1:24,000 will undoubtedly provide new insights into understanding the formation of the Klamath Mountains. Structural Geology The Hornbrook Formation and the lower part of the Western Cascades Group are dissected by a series of northeast-southwest trending normal faults. One of the most significant of these structures, the Siskiyou Summit Fault, offsets stratigraphic units about 10 km. Recent structural studies along several of these faults suggest Cenozoic reactivation with oblique-slip to strike-slip motion. Between these major bounding faults, homoclinal sequence of sedimentary and volcaniclastic rocks of the Hornbrook and Colestin Formations respectively are tilted 20o to 30o to the northeast. Rocks of the upper part of the Western Cascades Group are not offset by these faults, and are only tilted 5o to 10o to the northeast. The most recent tilting is attributed to Miocene doming of the Klamath Mountains (Mortimer and Coleman, 1984; Mortimer and Coleman, 1985). The timing of this doming is constrained by horizontal lava flows of Table Rocks that are approximately 7 million years old (Hladky, 1998). Hornbrook Formation Cretaceous sediments are abundant along the western margin of North America (Fig. 6). Originally, Cretaceous rocks exposed in northern California and southwestern Oregon were assigned to the Chico Formation (Diller, 1906, 1907). Later work by Peck and others (1956) designated the Cretaceous sedimentary rocks exposed along the northeast flank of the Klamath Mountains as the Hornbrook Formation. Peterson (1967) proposed the occurrence of an Upper Cretaceous discontinuity within the Hornbrook Formation. Elliott (1971) used this discontinuity to subdivide the Upper Cretaceous rocks in the Cottonwood Creek Valley into the Hornbrook Formation (below the discontinuity) and the Hilt unit (above the discontinuity). Nilsen (1984) renamed the informal units designated by Elliott (1971) into five named members of the Hornbrook Formation, in ascending order, Klamath River Conglomerate, Osburger Gulch Sandstone, Ditch Creek Siltstone, Rocky Gulch Sandstone, and Blue Gulch Mudstone (Fig. 5). Nilsen (1993) also proposed that the discontinuity between the Ditch Creek Siltstone and Rocky

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Gulch Sandstone Members was localized to the Cottonwood Creek Valley and Hornbrook areas, and that the contact between these members was conformable elsewhere in the field area. Further work is necessary to resolve the extent of this unconformity, especially biostratigraphic and age control of the Rocky Gulch Sandstone Member of the Hornbrook Formation.

Figure 6: Exposures of the Hornbrook Formation, major physiographic provinces, and adjacent outcrop areas of Cretaceous strata (shaded area). Figure from Nilsen (1993).

Paleocurrent data from trough cross-stratification in the Klamath River Conglomerate Member indicates a north to northeast flow direction. Additional paleocurrent data from flute casts, traction marks, and ripple cross-lamination from the marine members of the Hornbrook Formation also indicate a northeast flow direction. Nilsen (1984) used this paleocurrent data to reconstruct a paleogeographic map of the Hornbrook Formation and its relationship to surrounding geologic provinces (Fig.7). In contrast, Haggart (1986) concluded that the

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Hornbrook Formation was restricted to only a small area, and that the Hornbrook Basin probably was not connected to the Great Valley Basin or the Ochoco Basin.

Figure 7: Paleogeographic map showing inferred setting of the Late Cretaceous sedimentation in northern California and Oregon. This map is from Nilsen (1984).

Dickinson and Suczek (1979) provided insights into sandstone compositions and their

relationship to tectonics (Fig. 8). The sandstone petrofacies of a forearc basin is unique, with a mixture of lithic, feldspar, and quartz grains. In addition, as a forearc basin fills with sediment, it records the unroofing of the adjacent magmatic arc. The Great Valley sequence in California is a classic example of an unroofing sequence, recording the denudation of the Sierra Nevadan Arc from the Late Jurassic through Late Cretaceous. The Hornbrook Formation is also interpreted to have been deposited in a forearc basin (Nilsen, 1984; Nilsen, 1993), and therefore should have similar sandstone petrofacies. Golia and Nilsen (1984) conducted detailed petrographic studies on the sandstones of the Hornbrook Formation, indicating a mixed provenance source (Fig, 9). This petrographic data did not conform to the tectonic petrofacies established by Dickinson and

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Suczek (1979) for a forearc basin. Instead, Golia and Nilsen (1984) concluded the Hornbrook sandstones were derived from a mixed provenance source rather than an unroofing sequence typical of a forearc basin because most of the sediment was derived from the Klamath Mountain geological province, consistent with paleocurrent studies.

Qt

F L F

Qm

Lt

Recycled OregenProvenance

Continental BlockProvenance

Plutonic>Volcanic

Volcanic>Plutonic

Plutonic>Volcanic

Volcanic>Plutonic

Increasing Ratio of Chert to Quartz[MIXED]

Qt = total quartz (including chert)

Qm = monocrystalline quartzF = plagioclase & potassium feldspar

L = aphanitic rock fragmentsLt = chert & aphanitic rock fragments

Magmatic ArcProvenance

Crat

on

Base

men

tModified from Dickinson (1988).

Fold & Thrust Belt Foreland

Basin

Magmatic Arc

Forearc Basin

Lith

osph

ere

SubductionComplex

SLS SLSX

XX

XX

RiftBasins

Continental CrustX

X

XX

XX

XX

X

XX

X

XX

XX

XX

XX

X

XX

XX

X

X

XX

X

XX

X

X

X

X

XX

X

X

VV

VVV

Figure 8: Model relating sandstone petrofacies to tectonic setting proposed by Dickinson and Suzcek (1979) and later modified by Dickinson (1988).

Qt

F L F

Qm

Lt

Recycled OregenProvenance

Continental BlockProvenance

Qt = total quartz (including chert)Qm = monocrystalline quartzF = plagioclase & potassium feldsparL = aphanitic rock fragmentsLt = chert & aphanitic rock fragments

Magmatic ArcProvenance

Mesozoic Great Valley Group data from Ingersoll (1983) & Dickinson (1988).

Blue Gulch Mudstone MemberRocky Gulch Sandstone MemberDitch Creek Siltstone MemberOsburger Gulch Sandstone MemberKlamath River Conglomerate Member

HO

RN

BR

OO

K F

M.

Upper Cret.

Jur.-Cret.

mid-Cret.

Hornbrook Data from Golia & Nilsen (1984)

Figure 9: Petrographic data of sandstones from the Great Valley Group of California and Upper Cretaceous Hornbrook Formation of southwest Oregon and northern California. The absence of volcanic rock fragments and the mixed petrofacies observed in sandstones

of the Hornbrook Formation may be the result of (1) compaction and/or obliteration of volcanic

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rock fragments during diagenesis, e.g. formation of pseudomatrix, (2) alteration of volcanic detritus to clay minerals by circulating diagenetic fluids, or (3) absence of volcanic detritus during deposition of the Hornbrook Formation. Bywater and Elliott (2007) conducted detailed petrographic studies of sandstone concretions from the various members of the Hornbrook Formation. The early cementation associated with these concretions reduced the effects of compaction and/or alteration by diagenetic fluids. The petrofacies from the sandstone concretions in this study contain 10 to 20 percent more feldspar (Fig. 10) as compared to the petrographic work of Golia and Nilsen (1984). This difference in abundance of feldspar may be the result of selective preservation within the calcite-cemented concretions.

Qt

F L F

Qm

Lt

Recycled OregenProvenance

Continental BlockProvenance

Qt = total quartz (including chert)Qm = monocrystalline quartzF = plagioclase & potassium feldsparL = aphanitic rock fragmentsLt = chert & aphanitic rock fragments

Magmatic ArcProvenance

Data from Golia & Nilsen (1984)

Blue Gulch Mudstone MemberRocky Gulch Sandstone MemberDitch Creek Siltstone MemberOsburger Gulch Sandstone Member

HO

RN

BRO

OK

FM

.

Figure 10: Petrographic data of sandstone concretions from the Upper Cretaceous Hornbrook Formation of northern California collected near Hilt, California.

Field Trip

This field trip guide will focus on the Upper Cretaceous Hornbrook Formation and Cenozoic rocks of the northern part of Cottonwood Creek Valley and Siskiyou Summit. The field trip stops are located on Figure 11 and correspond to the detailed descriptions provided in the travel log.

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N

Stops 9 & 10

Stops 1 & 2 Stop 8

Stop 11 Stop 7

Stop 3 Stops 4, 5, & 6

Figure 11: Map of the major roads, rivers, physiographic features, and communities along the Interstate-5 corridor in southwestern Oregon and northern California. Field trip stops for this field guide are located on this map. Map modified from Nilsen (1993).

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Travel Log Below is the mileage and geological summary of the field trip stops. Refer to Figure 11 for the location of roads, waterways, and locations for each locality. The mileages provided are cumulative.

0 miles Leave parking lot from the front of the Science building at Southern Oregon University. Follow University Way north to the intersection with Siskiyou Boulevard. Turn right onto Siskiyou Boulevard and proceed to Interstate-5. Follow Interstate-5 to the south for 8 miles.

10.2 miles Stop 1 – Osburger Gulch Sandstone Member of the Hornbrook Formation At this locality, the Osburger Gulch Sandstone Member of the Hornbrook

Formation is 125 m thick consisting of fine- to medium-grained sandstones. Along the exposure, several near-vertical normal faults offset marker beds by a few meters. Sandstones in this exposure contain abundant molluscan megafossils and trace fossils; megafossils yield an age of Cenomanian to early Coniacian (Sliter et al, 1984). Sandstones also exhibit planar stratification, hummocky cross-stratification, and most beds are normally graded.

The Osburger Sandstone Member of the Hornbrook Formation is interpreted to have been deposited in a high-energy, storm dominated shelf. Near the top of the member, siltstones are interbedded with fine-grained sandstones consistent with a lower energy offshore environment. Fossils in the Osburger Gulch Sandstone Member of the Hornbrook indicate that the unit is time-transgressive from north to south (Fig. 12), suggesting deposition as a result of a southward to southeastward transgression of the Late Cretaceous seaway over the Klamath Mountains (Nilsen, 1984).

ALBIAN

CENOMANIAN

TURONIANCONIACIANSANTONIAN

CAMPANIAN

MAASTRICHTIAN

100

90

80

70

110

AGE(Ma)

STAGE

EAR

LYLA

TEC

RET

AC

EOU

SPE

RIO

D

EPO

CH

POLA

RIT

Y

Hornbrook Formation in northern California

& southwestern Oregon

Osburger Gulch Sandstone

NorthAshland, OR

SouthHornbrook, CA

Ditch Creek Mbr.

Rocky Gulch Sandstone

Blue Gulch Mudstone

Nonconformity

Unconformity

? ? ?

Klamath River Conglomerate

Figure 12: Correlation chart of the members of the Hornbrook Formation in northern California and southwestern Oregon. The age relationships of the members of the Hornbrook Formation were determined from macrofossil and microfossil studies summarized in Elliott (1971) and Sliter et al. (1984). Magnetic polarity and time scale from Gradstein et al. (1995)

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From Stop 1, continue along the shoulder of Interstate-5 approximately 0.3 miles. 10.5 miles Stop 2 – Klamath River Conglomerate of the Hornbrook Formation

At this locality, the nonconformity separating the Klamath River Conglomerate of the Hornbrook Formation from the underlying granodiorite to diorite of the Mt. Ashland Pluton is exposed. The basal part of the Klamath River Conglomerate consists of thick-bedded conglomerates composed primarily of granitic material. In addition, there are clasts of phyllite, schist, quartzite, and metavolcanics consistent with a provenance from the Klamath Mountains. Above the basal conglomerate, there are several beds of trough cross-stratified arkose with pebbly lags. Gaona (1984) included these trough cross-stratified sandstones in the Osburger Gulch Sandstone Member because of the presence of trace fossils. Paleocurrent data from these trough cross-stratified sandstones indicates a north to northeast flow direction for sediment dispersal (Fig. 13). Overlying these trough cross-stratified units are thick-bedded fine- to medium-grained sandstones exhibiting hummocky cross-stratification and planar stratification that are partially or completely obliterated by bioturbation. The bioturbation includes subhorizontal tubes on bedding surfaces and vertical tubes identified as Ophiomorpha.

Figure 13: Rose diagrams of trough cross-stratification (axis) paleocurrent data from the basal part of the Osburger Gulch Sandstone Member of the Hornbrook Formation. Figure from Gaona (1984). Near the contact between the Klamath River Conglomerate and Osburger

Gulch Sandstone Members of the Hornbrook Formation, thick-bedded, medium-grained sandstones are cemented with chlorite. The presence of chlorite cements

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in these sandstones suggests burial depths of 4 to 6 km for the Hornbrook Formation.

The Klamath River Conglomerate Member is interpreted to represent continental deposition, most likely by rivers. The overlying hummocky cross-stratified sandstones of the Osburger Gulch Sandstone Member attest to a transgression of a Late Cretaceous seaway in southern Oregon.

From Stop 2, continue on Interstate-5 to the south for about 7 miles to the Hilt Exit. Turn right off of the exit ramp onto Hilt Road and drive to the west. This road will descend into Cottonwood Creek Valley; note exposures of the Hilt Bed along the road. At the junction with the railroad tracks in Hilt, turn south onto Hilt-Hungry Road. Continue on this road to the quarry just beyond the abandoned water tower.

20.6 miles Stop 3 – Ditch Creek Siltstone Member of the Hornbrook Formation

This property is owned by the Fruit Growers Supply Company of Hilt California and is within the boundaries of the first geologic mapping exercise of SOU’s summer geology field course. Students enrolled in this course conduct geologic mapping in this area and identify faults by resolving offsets in the members of the Hornbrook Formation. Just to the northwest of the Hungry-Hilt Road is a major northeast-southwest trending normal fault that juxtaposes the Blue Gulch Mudstone Member (northwest) against the Ditch Creek Siltstone Member (southeast).

Exposure of thin- to medium bedded, medium-grained sandstone interbedded with medium-bedded siltstones and mudrocks. Several sandstone beds exhibit soft-sediment deformation features, such as convoluted bedding, flame structures, and ball-and-pillow structures. Trace fossils are abundant in the Ditch Creek Siltstone Member of the Hornbrook Formation; Chondrites and Planolites have been identified at this locality. This unit contains megafossils of pelecypods, gastropods, and ammonites and microfossil assemblages of foraminifera. The Ditch Creek Siltstone is middle Turonian in age in the Bear Creek Valley, but in the northern part of Cottonwood Creek Valley biostratigraphic collections provide an age of late Turonian to Coniacian (Nilsen, 1993).

The Ditch Creek Siltstone Member is interpreted to have been deposited in a low-energy, outer shelf environment (Nilsen et al., 1984; Nilsen, 1993). Interpretations by Gaona (1984) suggested that the Klamath River Conglomerate, Osburger Gulch Sandstone, and Ditch Creek Siltstone Members of the Hornbrook Formation are laterally equivalent depositional facies that are time-transgressive from the north to south within the Hornbrook Basin (Fig. 14).

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Figure 14: Depositional model for the Klamath River Conglomerate, Osburger Gulch Sandstone, and Ditch Creek Siltstone Members of the Hornbrook Formation. Figure from Gaona (1984).

Continue on Hungry Hilt Road for ~2 miles to the intersection with Geology Lane. Turn left onto Geology Lane and continue north for about 1 mile to where the road crosses Cottonwood Creek.

23.1 miles Stop 4 – Ditch Creek Siltstone Member of the Hornbrook Formation

From the bridge over Cottonwood Creek, the contact between the Ditch Creek Siltstone Member and the overlying Rocky Gulch Sandstone Member is visible on the hillside to the west. At road level, the middle part of the Ditch Creek Siltstone Member is exposed along Cottonwood Creek on the far side of the bridge. The thick- to very thick-bedded, fine- to medium-grained sandstones interbedded with medium-bedded siltstones at this locality are equivalent to the thin-bedded sandstones and siltstones examined at the last stop. These lateral variations in lithofacies within the Ditch Creek Siltstone Member of the Hornbrook Formation may be attributed to shoaling of offshore bars in a shelf environment, and/or progradation of sands associated with a localized sediment source, i.e. delta. In addition, there are numerous concretions (up to several meters in diameter) present within sandstone beds of the Ditch Creek Siltstone Member of the Hornbrook Formation. The concretions are produced by localized areas of calcite cement that formed by nucleation around a fossil fragment, and/or localized precipitation of calcite by groundwater movement in the early diagenetic environment. Follow Geology Lane to the north for about 0.5 miles. Park along the roadway.

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23.6 miles Stop 5 – Rocky Gulch Sandstone Member of the Hornbrook Formation The Rocky Gulch Sandstone Member is well exposed along the hillsides

on either side of Geology Lane. Note the abundant concretions that are weathering out of the sandstone exposures. Generally, the Rocky Gulch Sandstone Member consists of sandstones interbedded with thick- to very thick-bedded conglomerates. These conglomerates are laterally continuous for 100s of meters and contain clasts of metavolcanics (50 to 70 percent), quartzite (20 to 30 percent), metasedimentary rocks (5 to 10 percent), chert (5 to 10 percent), and granodiorite (less than 5 percent).

Several conglomeratic intervals occur in the middle and upper parts of the Rocky Gulch Sandstone Member of the Hornbrook Formation. In the exposure along Geology Lane, fine-grained sandstones are truncated by a sandy conglomerate in the upper part of the Rocky Gulch Sandstone Member. This contact is undulatory with several gutter casts present at the far end of the exposure. This sandy conglomerate also exhibits normal grading, with clasts 20- to 30-cm diameter at its base. These sedimentological features suggest a turbulent flow mechanism for this sandy conglomerate. Above the conglomerate, there are several beds of pebbly sandstone.

Elliott (1971) and McKnight (1971) interpreted the Rocky Gulch Sandstone Member to have been deposited in a moderate- to high-energy shallow-marine environment. Alternatively, Nilsen (1993) interpreted the Rocky Gulch Sandstone Member to have been deposited as a turbidite apron mantling a regional submarine slope of the ancestral Klamath Mountains. Specifically, amalgamated sandstone beds in the lower part of this unit were deposited by sediment gravity flows and graded sandstones and conglomerates in the middle and upper parts of this unit by turbidity currents (Nilsen, 1993).

The age of the Rocky Gulch Sandstone Member is poorly constrained. Megafossils are extremely rare and only one microfossil locality near Medford has yielded an age of middle Turonian (Nilsen, 1993). Unfortunately the stratigraphic position of this sample is poorly constrained, and may be from the upper part of the Ditch Creek Siltstone Member. In the Cottonwood Creek Valley, Elliott (1971) following Peterson (1967) identified an unconformity separating the Ditch Creek Siltstone and Rocky Gulch Sandstone Members that may span the Coniacian to Early Campanian. Again, there is little fossil evidence to constrain the missing time associated with this unconformity; hopefully future sampling will provide better biostratigraphic control for the Rocky Gulch Sandstone Member. Continue on Geology Lane to the north for about 0.4 miles. Park vehicles on the right side of the road in the large pull-out. Cross the drainage ditch on the west side of the road and hike 200 meters west to mudrock exposures of the Blue Gulch Mudstone Member.

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24.0 miles Stop 6 – Blue Gulch Mudstone Member of the Hornbrook Formation (Hilt Landfill Site)

The Blue Gulch Mudstone Member of the Hornbrook Formation is over 800 m thick, consisting of mudrock with sparse sandstone beds. At this locality, a thin marker bed referred to informally as the “Geology Lane Bed” may be traced for several kilometers. On the bottom of this sandstone bed are abundant sole marks including flute casts, load casts, and trace fossils. The mudrocks above and below this sandstone bed contain sparse calcite cemented concretions. Often, the concretions are cored by fossil fragments of inocermid clams or ammonites.

The Blue Gulch Mudstone Member is middle Campanian to Maestrichtian in the Cottonwood Creek Valley determined by microfossil assemblages of foraminifera (Sliter et al., 1984; Nilsen, 1993). Nilsen (1993) interpreted the Blue Gulch Mudstone Member of the Hornbrook Formation, especially the middle and upper parts, to have been deposited in a deep-marine, basin-floor environment. The basal part of this unit in the vicinity of its type locality to the south is subdivided into the Rancheria Beds, interpreted to represent deposition in a wave-dominated, high-energy, middle- to outer-shelf environment (Nilsen, 1993).

Continue north on Geology Lane to the intersection with the railroad track. Double back on Hilt Road to Interstate-5 to the Texaco Station for lunch and Field Trip Stop 7.

27.1 miles Stop 7 – Lunch Stop & Hilt Bed (Texaco Station)

Across Interstate-5 are exposures of bedded mudrocks of the Blue Gulch Mudstone Member of the Hornbrook Formation. On the north side of this exposure, the beds are cut by a steeply dipping normal fault. Beds along the south side of the fault show drag. These features may be admired from the picnic area just north of the Texaco Station. After lunch, we will drive a short distance to the opposite side of the Highway to observe an exposure of the Hilt Bed. The Hilt Bed is actually made up of several amalgamated sandstone beds that show erosional truncation at their base. The bottom of the Hilt Bed also exhibits traction marks and flute casts. The Hilt Bed is interpreted as a seismoturbidite deposited on the basin floor of the Hornbrook Basin (Nilsen, 2000). The Hilt bed may be traced laterally into the Bear Creek Valley and is a significant marker bed in correlating the Blue Gulch Mudstone Member of the Hornbrook Formation. Turn onto Interstate-5 to the north and follow for about 5 miles to the Siskiyou Summit. This concludes our tour of the Upper Cretaceous Hornbrook Formation; the remainder of the field trip will focus on Cenozoic units.

32.8 miles Stop 8 – Oligocene Colestin Formation (Siskiyou Pass)

At this locality, there are excellent exposures of volcanic sandstones, conglomerates, breccias, and tuffs of the Oligocene Colestin Formation. The greenish color of these volcaniclastic rocks is caused by a diagenetic mineral

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called celadonite. Bestland (1985, 1987) conducted a detailed sedimentologic study of the Colsetin Formation and interpreted numerous transport mechanisms for the transport and deposition of these volcaniclastic sediments, such as volcanic mudflow (Lahar), hyperconcentrated flood flow, and fluid flow. In addition, Bestland (1987) identified several vitric-crystal tuffs that may be used as marker beds to resolve offsets along faults and other structural features. The ash flow tuff exposed is 29.9 Ma, determined by K-Ar radiometric dating of plagioclase (Fiebelkorn et al., 1983). At the base of the ash flow tuff at this locality, there are numerous carbonized lenses. These carbonized lenses are interpreted to represent trees that may have been blown down by a volcanic eruption and then buried by subsequent volcanic ash. In the middle of the exposure, several of the poorly sorted, volcanic conglomerates/breccias are channelized, suggesting that the volcanic mudflows may have been confined by pre-existing valleys. In addition to volcaniclastic deposits, the Colestin Formation also includes several volcanic necks. One of the most prominent of these volcanic necks is Pilot Rock, which may be seen from both Cottonwood Creek and Bear Creek Valleys. Pilot Rock is composed of hornblende andesite and yielded an age of 25.59 + 0.21 Ma determined by 40Ar/39Ar radiometric dating of plagioclase (D’Allura, 2007, personal communication).

Age(Ma)

EpochCentral CascadesPeck and others

(1964)

Coos BayOrr & Orr

(2000)

55

50

45

40

35

30

25

OL

IGO

CE

NE

Lower

Middle

Upper

Upper

Lower

Roseburg Fm.

Lookingglass Fm.Flournoy Fm.

Bateman Fm.Tyee Fm.(Elkton Mbr.)

Coaledo Formation

Bastendorff ShaleTunnel Pt. Sandstone

Spencer Fm.

Fisher Fm.

Eugene Fm.

Little ButteVolcanic Series

20

15

10

Siskiyou SummitBestland (1987)& Elliott (2003)

Upper

Lower

Middle

Colestin Fm.

Roxy Fm.

Colestin Fm.

Columbia River Basalt Grp.

Troutdale Fm.

Sardine Fm.

Tyee Fm.

Umpqua Grp.

Tarheel Fm.

???

Payne Cliffs Fm.

Wasson Fm.

?? ?

?

Heppsie Andesite

Figure 15: Correlation chart of Cenozoic rocks in southern Oregon to the central Cascades and Coos Bay.

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The correlation of the Colestin Formation to Cenozoic units in the Bear Creek Valley is problematic (Fig. 15). The Colestin Formation has been dated by Ficke and others (2007) at 33.13 + 0.34 using 40Ar/39Ar of a plagioclase from an ash flow tuff near the base of the unit, above the unconformity with the Hornbrook Formation. In the Bear Creek Valley, the Hornbrook Formation is unconformably overlain the Eocene Payne Cliffs Formation. The age of the Payne Cliffs Formation has been determined by identification of fossil leaves in mudrocks near the base of the unit. The lower part of the Payne Cliffs Formation does not contain volcanic detritus, but the upper part of the Payne Cliffs Formation does contain volcanics. Detailed studies need to be undertaken to resolve the age relationships of the Payne Cliffs Formation in the Bear Creek Valley to the Colestin Formation near Siskiyou Pass.

On the far side of this exposure, there are several basaltic andesite dikes that cut across the layered strata of the Colestin Formation. Note the large blocks volcanic sandstone and ash flow tuff that are included in the intrusive dike. Continue north on Interstate-5 for about 0.7 miles to the Mt. Ashland Exit. Turn onto Highway 99 south and follow for about 3.2 miles

34.7 miles Stop 9 – View of Cottonwood Creek Valley, Siskiyou Summit Fault, & Mt. Ashland

From this view point, you can see the northern extent of Cottonwood Creek Valley, which is truncated to the north by the Siskiyou Summit Fault. Mt. Ashland is visible to the west, capped by granodiorite of the Mt. Ashland Pluton. Also visible is the Siskiyou Summit Fault, demarcated by the change in relief of the granodiorite versus the juxtaposed Hornbrook and Colestin Formations making up the northern extent of Cottonwood Creek Valley.

Also at this locality, notice the jog in Highway 99 around an active landslide. This landslide has been active since the New Year’s Day flood of 1997. Note fragments of asphalt on the hillside below and the projected pathway of the old road service. The Highway Department is continuously working to maintain this stretch of Highway 99. Numerous landslides are active in this area, especially within the volcaniclastic units of the Colestin Formation. Continue south on Highway 99 for about two miles.

36.7 miles Stop 10 – Roxy Formation (Late Oligocene to Early Miocene)

The contact between the Colestin and Roxy Formations is identified by the first occurrence of a widespread lava flow (Bestland, 1987). At this locality, there are several basaltic andesite lava flows separated by paleosols (red layers). The abundance of vesicles in an individual lava flow increases toward the top of each flow; most vesicles are filled with zeolites. The paleosols developed on topography at the top of these lava flows; flow breccias occur at the base of each flow. Detailed geologic mapping of the lava flows in the Roxy Formation indicate that some were restricted to paleovalleys, consistent with topographic relief in the Late Oligocene to Early Miocene of southern Oregon and northern

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California. The basal paleosol at this locality is well developed containing root traces, clay cutans, ped structures, and pseudoslickensides. Minimally, there are five lava flows that may be identified at this locality bounded by reddish colored paleosols. Continue on Highway 99 for about 3 miles to the Interstate-5 onramp.

40.2 miles Stop 11 – Colestin Formation (Oligocene) Mudrock of the lower part of the Colestin Formation exposed along

Interstate-5 and along the onramp. At this locality, there are several thin beds of whitish silty mudrock encased in laminated, brown to black mudrocks; the thin beds of whitish silty mudrock contain abundant fossil leaves, including Metasequoia. These sediments are interpreted to have been deposited in wetlands that existed adjacent to floodplains and/or lake environments. Further sedimentological work is necessary to fully understand the complexity of lithofacies within the Colestin Formation and more age dating is necessary to resolve correlation of this unit to other Oligocene sediments of Oregon and California.

Again, there are spectacular views of the Cottonwood Creek Valley, Mt. Ashland, and Pilot Rock from this locality. Most of the topography that may be viewed from this locality is controlled by the underlying geology!

Follow Interstate-5 to the Hilt Exit and then turn onto Interstate-5 heading north

to Ashland. This concludes the field trip log.

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