Provenance of cobbles in the Rocky Gulch Sandstone Member of the Hornbrook Formation (Upper Cretaceous) in Siskiyou County, California

Maxwell P. Dahlquist and William S. Elliott, Jr. Department of Geology and Physics, University of Southern Indiana, 8600 University Blvd., Evansville, IN, 47712; mpdahlquist@gmail.com

Introduction

The Upper Cretaceous Hornbrook Formation is mostly a marine sequence conglomerate, sandstone,

and mudrock deposited in a forearc basin. It is exposed on the northeastern flank of the Klamath

Mountains in northern California and southern Oregon. In the type section described by Nilsen

(1993), the Hornbrook is subdivided into five members, in ascending stratigraphic order: the Klamath

River Conglomerate, the Osburger Gulch Sandstone, the Ditch Creek Siltstone, the Rocky Gulch

Sandstone, and the Blue Gulch Mudstone. The objective of this study is to link cobbles collected

from conglomerates of the Rocky Gulch Member of the Hornbrook Formation to their potential parent

rocks in the Klamath Mountains.

The Rocky Gulch Sandstone is generally organized into a series of fining- and thinning-upward cycles

of Turonian to Campanian age (Nilsen 1993). Conglomerate beds that are laterally continuous are

present both near the bottom and top of the unit. Flute casts on the bottom of sandstone beds give

generalized northeastern paleocurrents. The clasts are dominantly composed of felsic metavolcanic

rocks, volcanic breccias, quartzite, chert, and granodiorite. Nilsen (1984) suggested the Klamath

Mountains as the likely source for Hornbrook sediments, based on petrography of framework grains

in sandstone and conglomerates, facies associations, and paleocurrent data.

Data Analysis and Petrographic Examination

MethodsCobbles were collected in the field by William S. Elliott, Jr. from several exposures of the Rocky Gulch

Member of the Hornbrook Formation in Siskiyou County, California. Billets of the cobbles were cut

using a water lubricated rock saw, and surfaces were prepared using 120-, 220-, 320-, 400-, and 600-

grit of silicon carbide on a flat lap wheel. Thin sections of the billets were prepared for each cobble

using standard techniques. Photomicrographs were taken of diagnostic features from selected thin

sections using an Olympus camera and petrographic microscope. Each cobble was also examined using

a hand specimen microscope and digital photographs of the cobbles were captured using a Nikon digital

camera.

0 10 20

N

OREGON

CALIFORNIA

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

XX

X

X

X X

X X

XX

X

X

X

X

X

X

X X

X

XX

X

Vesa Bluffs Pluton

N42 15’o

N42o

Hornbrook

5

5

5

W123 45’o

W123 30’o

W123o

GENERALIZED GEOLOGIC MAP OF THE STUDY AREA

30

Modified from Mortimer & Coleman (1984).

Central Metamorphic Belt (Devonian)

Results

The Eastern Klamath and Hayfork Terranes are the most

probable sources for the metavolcanic clasts that make

up the bulk of the cobbles examined in this study. Most

volcanic and metavolcanic units in the other terranes are

mafic to intermediate in composition, while these

terranes contain felsic metavolcanics. Furthermore, the

Eastern Klamath rocks are also a parent rock possibility

given the northeast paleocurrent, though the Hayfork

Terrane is more directly aligned southwest-northeast

with the Hornbrook Formation.

The Hayfork Terrane (and its correlative rocks in the

central Klamath Mountains) contains felsic to

intermediate metavolcanic rocks which lie in a long (~10

km wide, ~80 km long) exposure aligned southwest-

northeast. These rocks are in depositional contact with

the Hornbrook at the northeastern end of their exposure.

The felsic metavolcanics from the Hayfork Terrane have

a greater area of exposure than do those of the Eastern

Klamath. Chert and metasedimentary rocks are common

in many of the terranes. The granodiorite clasts have a

composition comparable to the Mt. Ashland Pluton

(Barats, Nilsen, Golia, 1984).

Though the Eastern Klamath and Hayfork Terranes

would both be capable of supplying the metavolcanic

cobbles found in the Rocky Gulch, the Hayfork terrane

seems to be the most likely source, fitting with the

northeastern transport direction to the forearc basin in

which the Hornbrook Formation was deposited.

Regional geologic map of the

study area. The Hornbrook

Formation nonconformably

overlies metamorphic and

igneous rocks of the Klamath

Mountains and is disconformably

overlain by the Western Cascades

Group. Note the northeast strike

of contacts as a result of tilting in

the Neogene.

Photomicrographs of thin-sections collected from the Rocky Gulch conglomerate, all in cross-polarized light: (A) Quartzite, sample HB-1; (B)

Volcanic breccia, sample HB-2. Note plagioclase inclusion within larger crystal; (C) Meta-argillite, sample HB-4; (D) Meta-dacite, sample HB-11;

and (E) Granodiorite sample HB-19. The following are photographs of cobbles exhibiting diagnostic textures that permit identification: (F) Metatuff,

sample HB-32, note fiamme; (G) Volcanic breccia, sample HB-36; (H) Meta-sandstone, sample HB-44 note cross-bedding; and (I) Amygdaloidal

basalt, sample HB-37.

Study Area

AcknowledgmentsWe would like to acknowledge the Department of Geology and Physics in the Pott College of Science,

Engineering, and Education at USI for providing the equipment and materials needed in preparing polished

rock surfaces and thin-sections. We also would like to thank Fruit Growers and Supply for access to field

sites near Hilt, California. Finally, Max Dahlquist would like to acknowledge the Endeavor Program at USI

for providing travel funds to attend the annual meeting of the Geological Society of America in Charlotte,

North Carolina.

500 mm 500 mm500 mm

500 mm

500 mm

A. B. C.

D. E. F.

G. H. I.

Cross section of Hornbrook

Formation in Cottonwood

Creek Valley showing

stratigraphic relationships

between members of the

Formation. From Nilsen

(1993).

References CitedBarats G.M., Nilsen, T.H., and Golia, R.T., 1984, Conglomerate clast composition of the Upper Cretaceous Hornbrook

Formation, Oregon and California, in Nilsen, T.H., ed., Geology of the Upper Cretaceous Hornbrook Formation,

Oregon and California: Pacific Section, Society of Economic Paleontologists and Mineralogists, v. 42, p. 111-122

Irwin, W. P., 1994, Geologic map of the Klamath Mountains, California and Oregon: U.S. Geological Survey, Miscellaneous

Investigations Series Map I-2148, scale 1:500000.

Nilsen, T., 1993, Stratigraphy of the Cretaceous Hornbrook Formation, Southern Oregon and Northern California: U. S.

Geological Survey Professional Paper 1521.

Nilsen, T., 1984, Tectonics and sedimentation of the Upper Cretaceous Hornbrook Formation, Oregon and California, in

Crouch, J.K., and Bachman, S.B., eds., Tectonics and Sedimentation Along the California Margin: Pacific Section,

Society of Economic Paleontologists and Mineralogists, v. 38, p. 101-117

Interpretations & Conclusions

Approximately 100 pebbles and cobbles collected from an exposure of the Rocky Gulch Sandstone were used in this study. These pebbles

and cobbles are composed primarily of quartzite, felsic to intermediate metavolcanic rocks, and chert with subordinate amounts of

metasedimentary rocks and granodiorite. Based on the cobbles available, the composition is 64 percent metavolcanic rocks, 15 percent

chert, 11 percent metasedimentary rocks, and 10 percent granodiorite. Pebbles and cobbles in these conglomerates are rounded to well

rounded and typically range in size from 0.5 to 25 cm. The metasedimentary clasts consist of quartzite, metasandstone, metasiltstone, and

meta-argillite, and contained primary sedimentary structures such as cross-stratification and lamination. Jasper and black chert are also all

present. Most of the metavolcanics examined using a petrographic microscope contained sanidine, plagioclase, and quartz, consistent with

compositions of rhyolite to dacite. The plagioclase in these metavolcanic cobbles also exhibited zoning. In addition, there were several

samples of metatuff that contained fiamme and several cobbles of volcanic breccia. Five of the cobbles studied were granodiorite, in which

the mafic constituents are dominated by biotite. Finally, one of the cobbles pictured above is a metamorphosed amygdaloidal basalt.

Paleocurrent indicators from flute casts on the bottom of sandstone beds in the Rocky Gulch Member indicate a northeast paleoflow

(Nilsen 1993), suggesting the Klamath Mountains as a potential source for these sediments.

Map of exotic terranes within the Klamath Mountains.

The Hornbrook Formation is located on northeast flank

of the Klamaths. Cobbles in the Rocky Gulch

Conglomerate are interpreted to have been derived from

the Hayfork Terrane and Mt. Ashland Pluton. Map from

Irwin (1994).

Annual Meeting of the Geological Society of America, Charlotte, North Carolina, 7 Nov. 2012