Download Field Trip Guide to the Upper Cretaceous Hornbrook Formation and

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
W 124
W 116
W 120
WAS
HING
TON
Cascade Volcano
Snake River Plain
X
W 112
X
Cretaceous
o
Fold & Thrust N 48
Belt
X
Cretaceous subduction
zone complexes
o
N 44
X
X
X
X
Cretaceous
forearc sediments
Cretaceous
plutonic belts
OREGON
X
A
o
N 36
IDAHO
X
NEVAD
A UTAH
X
o
N 44
A’
BASIN &
RA
PROVIN NGE
CE
o
N 40
X
B’
X
X
as
dre
An
N
X
COLORADO
PLATEAU
X
X
X
o
N 36
ult
Fa
Kilometers
200 400
0
300
100
MONTANA
WYOMING
X
X
n
Sa
B
o
X
X
X
Late Pz & Mz
arc terranes
Pre-Cretaceous
subduction zone
complexes
Pz arc
terranes
N 40
W 108
CALIFORNIA
o
o
W 124
o
o
W 120
NEW
MEXICO
ARIZONA
Modified from Dorsey & LaMaskin (in review)
o
W 112
W 116
W 108
Figure 1: Tectonic map of North America highlighting the major geologic provinces. This map
is modified from Dorsey & LaMaskin (in review).
TECTONIC RECONSTRUCTIONS FOR THE CRETACEOUS
Subduction
Magmatic
Complex
Arc
Forearc
Trench
Basin
X
X
LEGEND
Fold &
Thrust Belt
Cordilleran
Foreland Basin
X
X
X
B (WEST)
A’(EAST)
Precambrian
Basement
SIERRA NEVADA MOUNTAINS
Subduction Magmatic
Complex
Arc
Trench Forearc
Basin
SL
X
B’(EAST)
X
X
X
X
X
Cretaceous subduction
zone complexes
Mid-Cretaceous intrusive
& volcanic rocks
Jurassic & Early Cret.
Intrusive rocks
Late Paleozoic &
Mesozoic arc terranes
Pre-Creatceous
subduction zone
complexes
Paloezoic arc terranes
Fold &
Thrust Belt
Cordilleran
Foreland Basin
X
X
Cretaceous sandstone
SL
Lithosphere
SL
KLAMATH MOUNTAINS
Precambrian
Basement
Paleozoic Rocks
SL
Lithosphere
A (WEST)
Proterozoic to
Paleozoic Crust with
Mesozoic shortening
Precambrian Rocks
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.
2
o
W123
o
5
o
W123 45’
W123 30’
Western Cascades (Paleogene)
Medford
o
X
X
X
N42 15’
X
Ashland
X
X
X
X
X
X
X
X
X
X
X
Mt Ashland
Pluton
X
X
X
X
X
X
5
X
X
o
X
N42
X
X
Hilt
X
X
X
X
X
Hornbrook Formation
(Late Cretaceous)
Diorite & granodiorite plutons
(Late Jurassic)
Condrey Mountain Terrane
(Late Jurassic)
Rattlesnake Creek Terrane
(Late Triassic to Early Jurassic)
Hayfork & Applegate Terranes
(Permian to Early Jurassic)
Central Metamorphic Belt
(Devonian)
OREGON
CALIFORNIA
X
Index
Map
N
Hornbrook
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Vesa
Bluffs
Pluton
X
X
X
X
X
5
0
X
X
X
X
Yreka
10
20
30
Kilometers
Modified from Mortimer & Coleman (1984).
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).
3
STAGE
meters
1,000
UPPER CAMPANIAN
900
Stratigraphic column
of the Upper Cretaceous
Hornbrook Formation
near Yreka, California
Hilt Bed
800
LEGEND
700
Blue Gulch
Mudstone Member
Trough Cross-stratification
Foraminiferal fossils
600
TURONIAN
CONIACIAN
LOWER CAMPANIAN
500
Rancheria Gulch
Sandstone Beds
400
300
200
100
HORNBROOK FORMATION
Molluscan fossils
Rocky Gulch
Sandstone Member
Plant fossils (leaf & wood)
Shale & mudstone
Siltstone
Sandstone
Conglomerate
Igneous & metamorphic rocks
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).
Ditch Creek
Siltstone Member
Osburger Gulch
Sandstone Member
Klamath River
Conglomerate Member
0
4
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 15minute 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
5
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
6
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
7
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
Qm
Ba
sem
en
t
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
SL
X
X
X
X
X
X
X
Lithosphere
V
V
V
V
X
X
rtz
Rift
Basins
Continental Crust
X
X
X
X
a
Qu
X
X
X
X
X
to
X
X
X
X
V
X
Lt
F
Fold &
Thrust Belt Foreland
Basin
X
X
rt
he
Volcanic>
Plutonic
L
Forearc
Basin
fC
oo
Volcanic>
Plutonic
Magmatic
Arc
SL
[MIXED]
Plutonic>
Volcanic
ati
Subduction
Complex
gR
Magmatic Arc
Provenance
F
s in
Plutonic>
Volcanic
re a
Inc
Cra
to
n
Recycled Oregen
Provenance
Continental Block
Provenance
Qt = total quartz
(including chert)
Qm = monocrystalline
quartz
F = plagioclase &
potassium feldspar
L = aphanitic rock
fragments
Lt = chert & aphanitic
rock fragments
Modified from Dickinson (1988).
Figure 8: Model relating sandstone petrofacies to tectonic setting proposed by
Dickinson and Suzcek (1979) and later modified by Dickinson (1988).
Qt
Continental Block
Provenance
Qm
Recycled Oregen
Provenance
Upper Cret.
mid-Cret.
Magmatic Arc
Provenance
F
F
Lt
Mesozoic Great Valley Group data from
Ingersoll (1983) & Dickinson (1988).
HORNBROOK FM.
Qt = total quartz
(including chert)
Qm = monocrystalline
quartz
F = plagioclase &
potassium feldspar
L = aphanitic rock
fragments
Lt = chert & aphanitic
rock fragments
Jur.-Cret.
L
Blue Gulch Mudstone Member
Rocky Gulch Sandstone Member
Ditch Creek Siltstone Member
Osburger Gulch Sandstone Member
Klamath River Conglomerate Member
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
8
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
Continental Block
Provenance
Qm
Data from Golia &
Nilsen (1984)
Recycled Oregen
Provenance
Magmatic Arc
Provenance
F
HORNBROOK FM.
Qt = total quartz
(including chert)
Qm = monocrystalline
quartz
F = plagioclase &
potassium feldspar
L = aphanitic rock
fragments
Lt = chert & aphanitic
rock fragments
L
Lt
F
Blue Gulch Mudstone Member
Rocky Gulch Sandstone Member
Ditch Creek Siltstone Member
Osburger Gulch Sandstone Member
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.
9
N
Stop 8
Stops 1 & 2
Stops 9 & 10
Stop 11
Stop 7
Stops 4, 5, & 6
Stop 3
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).
10
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.
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 timetransgressive 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).
90
100
110
Hornbrook Formation
in northern California
& southwestern Oregon
North
Ashland, OR
?
?
South
Hornbrook, CA
?
Blue Gulch Mudstone
CAMPANIAN
LATE
80
STAGE
MAASTRICHTIAN
Rocky Gulch Sandstone
SANTONIAN
CONIACIAN
TURONIAN
CENOMANIAN
EARLY
70
CRETACEOUS
AGE
(Ma)
EPOCH
10.2 miles
PERIOD
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.
POLARITY
0 miles
ALBIAN
Unconformity
Ditch Creek Mbr.
tone
h Sands
er Gulc
rg
u
sb
lomerate
O
er Cong
iv
R
th
a
Klam
Nonconformity
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)
11
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, mediumgrained sandstones are cemented with chlorite. The presence of chlorite cements
12
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 crossstratified 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).
13
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.
14
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 thickbedded 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 20to 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 shallowmarine 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.
15
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 wavedominated, 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
16
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)
Epoch
Coos Bay
Orr & Orr
(2000)
Sardine
Fm.
Middle
15
Tarheel Fm.
35
OLIGOCENE
30
Heppsie
Andesite
Columbia River
Basalt Grp.
Wasson Fm.
Lower
20
25
Siskiyou Summit
Bestland (1987)
& Elliott (2003)
Troutdale Fm.
Upper
10
Central Cascades
Peck and others
(1964)
Eugene
Fm.
Lower
Upper
Roxy Fm.
Little Butte
Volcanic Series
Upper
Tunnel Pt. Sandstone Fisher
Fm.
Bastendorff Shale
?
? ?
Colestin ?
Fm.
Spencer Fm.
Colestin
Fm.
?
Payne
Cliffs Fm.
?
?
Coaledo
Formation
40
Middle
45
Bateman Fm.
Tyee Fm.(Elkton Mbr.)
50
Lower
55
Flournoy Fm.
Lookingglass Fm.
Roseburg
Fm.
Tyee Fm.
Umpqua Grp.
Figure 15: Correlation chart of Cenozoic rocks in southern
Oregon to the central Cascades and Coos Bay.
17
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
18
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.
19
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23
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24