Stratigraphy of the Cretaceous Hornbrook 

Formation, Southern Oregon and 

Northern California

U.S. GEOLOaieAk SURVEY PROFESSI0 JJAl FAFER
Stratigraphy of the Cretaceous Hornbrook 

Formation, Southern Oregon and 

Northern California

By TOR H. NILSEN

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1521

A stratigraphic division of the 

Hornbrook Formation into five 

nonmarine and marine members

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1993
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

DALLAS L. PECK, Director

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and does not imply endorsement

by the U.S. Government

Illustrations edited by Dale Russell and prepared by Michelle Coveau

Library of Congress Cataloging in Publication Data

Nilsen, Tor Helge.

Stratigraphy of the Cretaceous Hornbrook Formation, southern Oregon and northern California / by Tor H. 

Nilsen.

p. cm. (U.S. Geological Survey professional paper ; 1521) 

Includes bibliographical references. 

Supt. of Docs, no.: I 19.16:1521

1. Geology, Stratigraphic Cretaceous. 2. Geology California. 3. Geology Oregon. 4. Hornbrook 

Formation (Calif, and Or.) 

I. Title. II. Series. 

QE685.N55 1993

551.7'7'097952 dc20 , 93-3585

CIP

For sale by Book and Open-File Report Sales, U.S. Geological Survey, 

Federal Center, Box 25286, Denver, CO 80225
CONTENTS

Page

Abstract________________________________ __ 1

Introduction 1

Previous work 5

Hornbrook Formation 10

Stratigraphic subdivisions of the Hornbrook Formation 14

Klamath River Conglomerate Member 14

Definition 14

Areal extent 16

Type section 16

Lithology 19

Stratigraphic relations 21 

Thickness - ----- - 22

Depositional environment 22 

Age and correlation 23 

Osburger Gulch Sandstone Member 23 

Definition 23 

Areal extent 24 

Type section 26 

Lithology 28

Stratigraphic relations 30 

Thickness - - ~ ~~~~~ ------------- 31

Depositional environment 32 

Age and correlation 33 

Ditch Creek Siltstone Member 34 

Definition 34 

Areal extent 35 

Type section 36 

Lithology 38

Stratigraphic relations 42 

Thickness - - 42

Depositional environment 43 

Age and correlation 43 

Rocky Gulch Sandstone Member 44 

Definition 44 

Areal extent 45 

Type section 45 

Lithology 47

Stratigraphic relations 49 

Thickness ___________________________ 50

Page

Stratigraphic subdivisions of the Hornbrook Formation 

Continued 

Rocky Gulch Sandstone Member Continued

Depositional environment 50 

Age and correlation 51 

Blue Gulch Mudstone Member 51 

Definition 51 

Areal extent 54 

Type section 55 

Lithology 62

Stratigraphic relations 64 

Thickness 64 

Depositional environment 65 

Age and correlation 65 

Rancheria Gulch Sandstone Beds 67 

Definition 6 7 

Areal extent 68 

Type section 68 

Lithology 69 

Stratigraphic relations 70 

Thickness 71

Depositional environment 71

Age and correlation 71

Hilt Bed 72

Definition 72

Areal extent 72

Type section 72 

Lithology 73

Stratigraphic relations 74 

Thickness 74

Depositional environment 75

Age and correlation 75

Summary and conclusions 75

References cited 77

Appendix I. Megafossils collected from the Hornbrook

Formation 84 

Appendix II. Foraminifers collected from the Hornbrook

ILLUSTRATIONS

PLATE 1. Geologic map of the Hornbrook Formation, Oregon and California

Page 

In pocket

FIGURE 1. Map showing location of outcrops of the Hornbrook Formation, major physiographic provinces, and adjacent outcrop

2. Index map of outcrop area of the Hornbrook Formation 

3. Cross section of Cotton wood Creek valley area 

4. Photograph of view northwest from Paradise Craggy toward Cottonwood Creek valley, showing northeast-dipping

strata of the Hornbrook Formation and related geologic features 

5. Simplified geologic map of the Hornbrook Formation, showing distribution of major Stratigraphic units 

in
IV CONTENTS

6. Charts showing development of stratigraphic nomenclature and correlation of units within the Hornbrook Formation

and showing subdivisions of Late Cretaceous time proposed by various workers in the Hornbrook Formation 8

7. Composite measured section of the Hornbrook Formation 11

8. Geologic map of Cottonwood Creek valley area, showing location of type sections of stratigraphic subunits of the

Hornbrook Fonnfltion ----- ---- ------- .------- ------ -------- - ------------------ --------« «-« -«"«« -«---»«« «»« -" -" « T2

9. Photographs of type section of the Klamath River Conglomerate Member of the Hornbrook Formation 15

10. Type section of the Klamath River Conglomerate Member of the Hornbrook Formation and explanation of symbols

on stratigraphic sections 20

11. Photographs of type section of the Osburger Gulch Sandstone Member of the Hornbrook Formation 25

12. Type section of the Osburger Gulch Sandstone Member of the Hornbrook Formation 29

13. Photographs of type section of the Ditch Creek Siltstone Member of the Hornbrook Formation 35

14. Type section of the Ditch Creek Siltstone Member of the Hornbrook Formation 38

15. Photographs of type section of the Rocky Gulch Sandstone Member of the Hornbrook Formation 44

16. Type section of the Rocky Gulch Sandstone Member of the Hornbrook Formation 48

17. Photographs of type section of the Blue Gulch Mudstone Member of the Hornbrook Formation 53

18. Type section of the Blue Gulch Mudstone Member of the Hornbrook Formation 60

19. Photographs of type section of the Rancheria Gulch Sandstone Beds of the Blue Gulch Mudstone Member of the

HomliToolc Formfltion --------------- ------«-« --«-« --"---«- ---«-- ------« --»«--»-«-«--«»«« -«"-« --«« « »-"- -----« -««" 67

20. Type section of the Rancheria Gulch Sandstone Beds of the Blue Gulch Mudstone Member of the Hornbrook

Formation _ . .... _....._......_....._......___._._.____............................___...........___................. 70

21. Photographs of the Hilt Bed of the Blue Gulch Mudstone Member of the Hornbrook Formation 73

22. Type section and reference section of the Hilt Bed of the Blue Gulch Mudstone Member of the Hornbrook

Formation - 74

TABLES

 Page

TABLE 1. Composition of conglomerate clasts in the type section of the Klamath River Conglomerate Member 22

2. Composition of conglomerate clasts in the type section of the Osburger Gulch Sandstone Member along the

southbound lane of Interstate Highway 5, 0.5 km west of the type section 30

3. Geochemistry of coal sample collected from the Ditch Creek Siltstone Member near the Hagedorn Ranch in the

Shflstfl. Vfl.ll.Gv fli*Gfl ------------------------------- --- - - --- - -««««--»- «»««««-«- -----««-- *-"- « --"-----"« -««' - «-*«- 40

4. Maceral analysis of coal sample collected from the Ditch Creek Siltstone Member near the Hagedorn Ranch in the

Shasta Valley area 41
STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, 

SOUTHERN OREGON AND NORTHERN CALIFORNIA

By TOR H. NILSEN

ABSTRACT

The Cretaceous Hornbrook Formation crops out continuously 

for a distance of about 80 km from the Medford Valley in southwestern Oregon to the Shasta Valley in north-central California; 

it also crops out as an outlier in the Grave Creek area of Oregon, 

about 39 km northwest of the northernmost outcrops in the 

Medford Valley. In its main outcrop belt, which is the area of 

concern in this paper, the Hornbrook Formation forms a northwest-striking and northeast-dipping homoclinal sequence of 

nonmarine and marine sedimentary rocks. The Hornbrook Formation rests unconformably on Paleozoic, Triassic, and Jurassic 

igneous, metasedimentary, and metavolcanic rocks of the core of 

the Klamath Mountains; these rocks form the southwestern margin of the outcrop belt. To the northeast, the Hornbrook Formation is unconformably overlain by Tertiary volcanic and 

sedimentary rocks of the Western Cascade Series. Although no 

continuous section of the Hornbrook Formation has been measured, a composite section made up of measured type sections of 

its various members indicates a total thickness of 1,235.7 meters 

for the formation in its type area located adjacent to Hornbrook, 

California. The Hornbrook Formation contains abundant 

megafossils and microfossils within its marine units that are indicative of an age of late Albian(?) to early Maestrichtian for the 

rocks of its main outcrop belt.

The Hornbrook Formation is subdivided into five members, 

each of which, except for the lowest member, is present over most 

of its outcrop length. The members are, in ascending order, (1) 

the Klamath River Conglomerate Member (36.5 m thick), consisting of basal nonmarine breccia, grus, debris-flow and streamflow 

conglomerate, sandstone, siltstone, and mudstone, with interbedded paleosols; (2) the Osburger Gulch Sandstone Member 

(116.5 m thick), consisting of shallow-marine sandstone with 

some interbedded conglomerate and thin shale partings in its upper part (containing abundant megafossils); (3) the Ditch Creek 

Siltstone Member (61.6 m thick), consisting of massive, nonmarine to deep-marine, bioturbated, muddy siltstone and silty 

mudstone, with locally abundant sandstone interbeds, thin coals, 

and megafossils; (4) the Rocky Gulch Sandstone Member (171.2 

m thick), consisting of deep-marine thin- to thick-bedded turbidite sandstone and siltstone with shale interbeds, and locally 

abundant conglomerate in its upper part; and (5) the Blue Gulch 

Mudstone Member (849.9 m thick), consisting of shallow- to deepmarine mudstone with thinly interbedded turbidite sandstone. 

Within the Blue Gulch Mudstone Member, two additional subunits have been differentiated: (1) the Rancheria Gulch SandManuBcript approved for publication, January 17, 1991.

stone Beds (85.8 m thick), consisting of shallow-marine sandstone, with locally abundant megafossils, that forms a lens in the 

lower part of the Blue Gulch Mudstone Member in the Hornbrook 

area; and (2) the Hilt Bed (as thick as 4.71 m), a compound turbidite sandstone bed about 410 m above the base of the Blue 

Gulch Mudstone that forms a mappable horizon from the Shasta 

Valley area to the Ashland area. The members of the Hornbrook 

Formation appear to form an essentially conformable sequence, 

although previous workers have suggested the presence of an 

unconformity between the Ditch Creek Siltstone Member and the 

Rocky Gulch Sandstone Member.

INTRODUCTION

Lower and Upper Cretaceous sedimentary strata 

that crop out in Siskiyou County, California, and 

Jackson County, Oregon, were named the Hornbrook 

Formation by Peck and others (1956). These rocks 

had previously been assigned by various authors to 

the Chico Formation (Diller, 1906) or "Chico Group" 

(Gabb, 1869), which crops out on the northeastern 

margin of the Sacramento Valley in California. Peck 

and others (1956) measured a composite type section 

of the Hornbrook Formation that is 2,673 ft (815 m) 

thick in the vicinity of Hornbrook, California. On the 

basis of extensive mapping and detailed measurement of stratigraphic sections of the Hornbrook Formation over the entire extent of its outcrop area, 

which ranges far to the south and northwest of the 

Hornbrook area, Nilsen (1984a) subdivided the 

Hornbrook Formation into the following members, in 

ascending stratigraphic order: Klamath River Conglomerate Member, Osburger Gulch Sandstone Member, Ditch Creek Siltstone Member, Rocky Gulch 

Sandstone Member, and Blue Gulch Mudstone Member (including the Rancheria Gulch Sandstone Beds 

and the Hilt Bed). These members are discussed 

herein, and their areal extents, type sections, lithologies, stratigraphic relations, thicknesses, depositional 

environments, ages, and correlations are summarized. Other aspects of the Hornbrook Formation, 

such as sedimentology, petrography, paleogeography,
STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

and tectonic framework, are treated elsewhere 

(Nilsen, 1984a, c, d).

The Hornbrook Formation rests unconformably 

on older Paleozoic, Triassic, and Jurassic accreted 

terranes and plutons that form the core of the Klamath Mountains (fig. 1; Irwin, 1960, 1966, 1981, 

1984, 1985, 1989; Harper and Wright, 1984; Wright 

and Fahan, 1988). It underlies a generally low-lying 

group of valleys on the northeast flank of the Klamath Mountains (fig. 2), where it forms a northweststriking and northeast-dipping homoclinal sequence 

(Elliott, 1984; pi. 1). It is overlain unconformably to 

the northeast by Tertiary sedimentary and volcanic 

rocks of the Cascade Range (figs. 3, 4; McKnight, 

1971, 1984; Hammond, 1983; Vance, 1984; Chesterman and Saucedo, 1984; Bestland, 1987).

The Hornbrook Formation crops out northward 

from the areas southwest and southeast of Yreka,

125° 120° 115°

45°

40°

SUPLEE;AREA _/\I_-X

OCHOCO BASIN

HORNBROOK 

FORMATION

MONTGOMERY 

CREEK AREA

COASTAL BELT 

TERRANE OF 

FRANCISCAN

ASSEMBLAGE

I______

200 KILOMETERS

FIGURE 1. Outcrop locations of the Hornbrook Formation, major physiographic provinces, and adjacent outcrop areas of Cretaceous

strata (shown by dark pattern).
INTRODUCTION

123°00' 122°45' 122°30' 122°15'

42°30' -

42°15' -

42°00' -

41°45' -

\Hiit\U Bailey Hill

Little Pilot

v^Cottonwood Creek valley 

^- ^ . Horn Peak /^

Badger Mountam / 

Shasta Valley overloo

FIGURE 2. Index map of outcrop area of the Hornbrook Formation (modified from Nilsen, 1984c).
STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

SW

KLAMATH MTS

CASCADE RANGE

NEFEET 

Tertiary 4800

strata

Hornbrook Formation

A

Osburger Gulch 

Sandstone Member

Rancheria Gulch 

Sandstone Beds

800 feet = 243.8 meters VERTICAL EXAGGERATION X 2T/ XRocky Qu|ch B|ue Qu|ch

Ditch Creek Sandstone Member Mudstone Member 

Siltstone Member

1 4 KILOMETERS

FIGURE 3. Northeast-southwest cross section in Cottonwood Creek valley area. Topography and stratigraphic boundaries of geologic units shown are modified from Elliott (1971). The basal member of the Hornbrook Formation, the Klamath River Conglomerate Member, is not shown.

FIGURE 4. View northwest from Paradise Craggy toward Cottonwood Creek valley, showing northeast-dipping strata of the Hornbrook 

Formation and related geologic features. Kh, Cretaceous Hornbrook Formation, MA, Mount Ashland; Mzftb, Paleozoic and Mesozoic 

basement complex; Ssf, Siskiyou Summit fault; PR, Pilot Rock; TVS, Tertiary volcanic and sedimentary rocks. Large highway in 

center of valley is Interstate Highway 5; valley is approximately 4 km wide at highway. Contacts are approximately located.
PREVIOUS WORK

adjacent to the Shasta Valley, through the Cottonwood Creek valley to about 6 km north of the 

Oregon-California border. Here the Hornbrook Formation is offset along the Siskiyou Summit fault to 

the Bear Creek valley area. From the Bear Creek valley area, exposures extend northwestward into the 

Medford Valley and to the Dark Hollow and Jacksonville areas (fig. 5). Cretaceous strata in the Grave 

Creek area of southwestern Oregon, located about 35 

km northwest of the northernmost outcrops in the 

Medford Valley, were also included in the Hornbrook 

Formation by Wells (1955) and Peck and others 

(1956); these strata are not discussed in this report 

because a more detailed discussion is being prepared 

for publication elsewhere. The members of the 

Hornbrook Formation, except for the lowermost member, 

the Klamath River Conglomerate Member, are mapped 

almost continuously over its entire outcrop extent.

Upper Cretaceous sedimentary rocks that are, at 

least, partly correlative with the Hornbrook Formation are present in the northern Sacramento Valley 

(Great Valley sequence) of California (Popenoe, 1943; 

Matsumoto, 1960; Trujillo, 1960; Murphy and others, 

1969; Ingersoll, 1979; Haggart and Ward, 1984; 

Haggart, 1986a; Russell and others, 1986; Verosub 

and others, 1989), the Gold Beach area of southwestern Oregon (Diller, 1907; Imlay and others, 1959; 

Dott, 1971; Blake, 1984a; Blake and others, 1985a; 

Bourgeois and Dott, 1987), and the Mitchell and adjacent areas (McKnight, 1964; Dickinson and Vigrass, 

1965; Wilkinson and Oles, 1968; Oles, 1969; Oles and 

Enlows, 1971; Kleinhans and others, 1984; Thompson 

and others, 1984) of central Oregon (fig. 1). In addition, parts of the Franciscan assemblage and related 

rocks of the northern California Coast Ranges include 

strata of Late Cretaceous and younger age (Evitt and 

Pierce, 1975; Blake and Jones, 1981; McLaughlin and 

others, 1982, 1988; Blake, 1984b; Blake and others, 

1985b, 1988, 1989). Nonmarine strata that crop out 

in the Klamath Mountains between the southernmost 

outcrops of the Hornbrook Formation and northernmost outcrops of the Great Valley sequence along the 

northern edge of the Sacramento Valley have been 

assigned to the Montgomery Creek Formation; 

Higinbotham (1987) has suggested that the basal 

conglomerate of this unit is of Late Cretaceous age, 

based partly on analyses of fossil pollen, which would 

make this unit partly correlative with the Hornbrook 

Formation.

PREVIOUS WORK

The name "Hornbrook Formation" was applied to 

outcropping clastic sedimentary rocks in Siskiyou

County, California, and Jackson County, Oregon, by 

Peck and others (1956). They proposed and measured 

a composite type section near Hornbrook, California, 

based on suggestions by F.G. Wells of the U.S. Geo-. 

logical Survey, that included conglomerate, sandstone, siltstone, and mudstone. Their composite type 

section is at least 2,673±50 ft (815±15 m) thick and is 

subdivided into six informal members denoted by Roman numerals I-VI (fig. 6). They inferred, on the basis of collected megafossil assemblages, an age of 

Cenomanian, Turonian, and Campanian for the 

Hornbrook Formation, with an unconformity between 

members II and III that spanned chiefly the 

Coniacian to early Campanian interval of the Late 

Cretaceous.

The strata of the Hornbrook Formation had previously been referred to as the Chico Group, the 

name applied to Upper Cretaceous clastic sedimentary rocks around the northern margins of the Sacramento basin of California, about 140 km south of 

Hornbrook. It was chiefly because of the great distance separating the two outcrop areas that Wells 

proposed that the strata of the northern outcrops be 

named the Hornbrook Formation, a suggestion followed by Peck and others (1956).

Gabb (1869) applied the name "Chico Group" to 

Cretaceous strata in northern California and all of 

Oregon. He established type sections on the east side 

of the Sacramento Valley adjacent to Chico Creek. 

Diller (1893) described strata of "Chico age" near 

Yreka, California, and along Grave Creek in Oregon. 

Dunn (1894) and Turner (1903) described auriferous 

conglomerate at the base of the Chico Group in the 

Cottonwood Creek valley area.

Anderson (1895) described some Cretaceous rocks 

in the Bear Creek valley and subsequently (Anderson, 1902) described the stratigraphy and megafossils 

of the Chico Group in the Cottonwood Creek valley 

and Bear Creek valley. Anderson (1931) described the 

Chico Group near Hornbrook as consisting of about 

2,000 ft (610 m) of lower fossiliferous sandstone with 

a basal conglomerate and 1,000 ft (305 m) of upper 

clay shale; he inferred an age of middle Turonian for 

the lower part of the sandstone unit and an age of 

late Turonian and early Senonian for the upper part 

of the sandstone unit. Anderson (1943) made the 

Chico Group on the Pacific coast a series, dividing it 

into three groups that spanned the Cenomanian to 

Danian time interval, but he did not define a type 

section for it.

Diller (1909) reported the presence of coal in 

shale and shaly sandstone along the east side of Bear 

Creek valley and near Ager, California. Diller and 

Kay (1924) described the thickness and lithology of
Medford' \Qal\ 

ValleX\\ M*H\ i

EXPLANATION

Alluvium (Quaternary) Also includes 

other surficial deposits

Volcanogenic landslide deposits (Quaternary) Arrows indicate direction of movement

Intrusive rocks (Tertiary) Volcanic rocks (Tertiary) Sedimentary rocks (Tertiary)

AREA OF MAP

Hornbrook Formation (Cretaceous) Divided into:

Blue Gulch Mudstone Member 

Rocky Gulch Sandstone Member Ditch Creek Siltstone Member 

Osburger Gulch Sandstone MemberKlamath River Conglomerate 

Member

Basement complex (Mesozoic and 

Paleozoic)

ContactGeneralized strike and dipFault Dashed where concealed

O

183

3i

FIGURE 5. Simplified geologic map of the Hornbrook Formation, showing distribution of major stratigraphic units (modified from Nilsen and others, 1983).
PREVIOUS WORK

the Upper Cretaceous strata along Grave Creek. 

Wells (1939) mapped the Chico Group in the Medford 

30-minute quadrangle, Oregon, and subsequently 

called the Upper Cretaceous rocks the Hornbrook 

Formation when he republished the Medford 30- 

minute quadrangle in 1956. Williams (1949, p. 16-18, 

pi. 4) described Upper Cretaceous strata of the Chico 

Formation in the Yreka area west of the MacDoel 30- 

minute quadrangle in northern California.

Subsequent to the naming of the Hornbrook Formation by Peck and others (1956), F.M. Anderson's 

detailed faunal lists, locations, and regional correlations for the Upper Cretaceous strata on the Pacific 

coast were posthumously published (Anderson, 1958). 

Mack (1959) mapped Cretaceous strata around the 

margins of the Shasta Valley as the Chico Formation 

and indicated a thickness of about 1,100 ft (330 m) 

near Black Mountain and only several hundred feet 

(100 m) east of Yreka, where only the lower part of 

the formation is exposed. Mack (1959) mapped several conglomerate bodies exposed about 16 km to the 

south of Yreka, principally in the Pythian Cave area 

(NW% sec. 7, T. 43 N., R. 6 W., Yreka 15-minute 

quadrangle), as Cretaceous in age and correlative 

with the Hornbrook Formation to the north.

Jones (1959) remapped the Hornbrook area and 

proposed a stratigraphic subdivision of the Hornbrook 

Formation into three mappable units in ascending 

order, units A, B, and C (fig. 6A); in addition, he 

mapped a major fossiliferous sandstone lens in the 

lower part of unit C, equivalent to the Rancheria 

Gulch Sandstone Beds of this paper. Jones determined a total thickness of about 4,880 ft (1,490 m) for 

the Hornbrook Formation. He collected late Turonian 

to early Coniacian megafossils from unit A and 

middle Campanian to Maestrichtian(?) ammonites 

from unit C. Jones (1960a) described Pterotrigonia 

pelecypods, which range in age from Turonian to 

Campanian, from the Hornbrook Formation in the 

Yreka-Hornbrook area and also from the Grave 

Creek area. Jones (1960b) described Albian (Early 

Cretaceous) fossils from the Grave Creek area of Oregon. Popenoe and others (1960) reported the presence of Cenomanian megafossils at Dark Hollow, east 

of Jacksonville, Oregon.

Elliott (1971) completed a Ph.D. thesis on the 

stratigraphy of the Upper Cretaceous rocks near Hilt 

and Hornbrook, California (fig. 6). He divided the 

Cretaceous strata into two units: (1) his lower 

Hornbrook Formation, 1,002 ft (305 m) thick, of late 

Turonian to early Coniacian age, subdivided into an 

unnamed lower arenaceous member (principally conglomerate and sandstone) and his overlying Henley 

mudstone unit (principally siltstone); and (2) his Hilt

unit, 3,059 ft (932 m) thick, of middle Campanian to 

Maestrichtian age, subdivided into an unnamed lower 

arenaceous member (principally sandstone) and his 

overlying Bailey Hill mudstone unit (principally mudstone). M.A. Elliott (unpub. data, 1980) subsequently 

prepared a revised geologic map of the Cottonwood 

Creek valley area between Hilt and Hornbrook; on 

this map, he revised his previously proposed nomenclature, including all the Cretaceous strata within his 

expanded Hornbrook Formation and dropped his formal names in favor of four informal units, A to D, in 

ascending order (fig. 6). He also recognized the presence of a local nonmarine unit at the base of the 

Hornbrook Formation (the Klamath River Conglomerate Member of this paper) and included it within 

the Hornbrook Formation. Elliott (1971; unpub. data, 

1980) embraced the concept of a major unconformity 

within the Hornbrook Formation between his units B 

and C that he thought represented much or all of the 

late Coniacian to middle Campanian interval.

McKnight (1971) mapped and described the 

Hornbrook Formation and overlying Tertiary strata 

in the Medford-Ashland region. He recognized a 

thickness of more than 3,000 ft (910 m) of the 

Hornbrook Formation in the Ashland area and recovered fossils of Turonian, Campanian, and 

Maestrichtian age from it. McKnight subdivided the 

Hornbrook Formation into four units three lower 

sandstone units and an upper mudstone unit (fig. 6). 

The basal sandstone unit, 360 ft (110 m) thick, is fossiliferous, locally conglomeratic and coquinoid, and 

rests unconformably on the basement complex. His 

second sandstone unit, 75 ft (23 m) thick, consists of 

interbedded sandstone, siltstone, mudstone, and 

shale. The uppermost sandstone unit, about 660 ft 

(200 m) thick, consists of thin- to thick-bedded sandstone that is generally unfossiliferous. McKnight's 

(1971) upper mudstone unit, about 1,900 ft (580 m) 

thick, consists of mudstone with locally abundant 

thin interbeds of fine-grained sandstone.

Hotz (1977) mapped the Yreka 15-minute quadrangle and showed the distribution of discontinuous 

remnants of the Hornbrook Formation in the Shasta 

Valley area to the south of the Cottonwood Creek valley. He did not subdivide the Hornbrook Formation of 

this area into subunits and suggested that only the 

lower part of the formation, about 250 m of sandstone with some conglomerate, was exposed. Hotz 

(1977) also estimated a thickness of about 40 m for 

the outlying exposures of the Hornbrook Formation 

on Soap Creek Ridge to the west of the Shasta 

Valley. He suggested an age of late Turonian and 

early Coniacian for the Hornbrook Formation in the 

Yreka area, but he did not present any new fossil
8 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

data. Hotz (1977) mapped several conglomerate units 

in the southern part of the Yreka quadrangle, particularly the extensive beds of conglomerate at 

Pythian Caves (NWK sec. 7, T. 43 N., R. 6 W., Yreka 

15-minute quadrangle), as being of TertiaryC?) age, 

principally because of their abundant suite of volcanic clasts that resembled volcanic rocks of the western Cascade Range.

Page and others (1977) mapped the outcrops of 

Lower Cretaceous strata along Grave Creek in the 

Wimer 7/^-minute quadrangle, referring to them as 

the "Grave Creek strata." Ramp and Peterson (1979) 

mapped a small outlier of the Lower Cretaceous rocks 

of Grave Creek in the Glendale 15-minute quadrangle, Josephine County, Oregon, that had not been 

mapped by Page and others (1978). Beaulieu and 

Hughes (1977) mapped rocks in Jackson County, Oregon, including the Hornbrook Formation and the 

Cretaceous strata at Grave Creek, as an undivided 

unit of Cretaceous sedimentary rocks. Smith and others (1982) mapped the Medford 1° x 2° quadrangle 

and showed the distribution of the Hornbrook Formation in the Grave Creek area. Black and others 

(1983) mapped the Hornbrook Formation in the 

Ashland area but did not divide it into subunits. 

Wagner and Saucedo (1987) mapped the distribution 

of the Hornbrook Formation in the Weed 1:250,000-

scale map sheet covering northwestern California, 

and Wagner (1988) described the geology of the map 

sheet.

Schultz and Levi (1981) reported on paleomagnetic studies of the Hornbrook Formation. They initially reported a post-Late Cretaceous 50° clockwise 

rotation of the Hornbrook Formation in their abstract, but verbally they retracted this conclusion. 

Mankinen and Irwin (1982) and Mankinen and others (1984, 1988) reported the possibility of 

11.5°±15.8° and 14.4°±6.9° of post-Cretaceous clockwise rotation of the Hornbrook Formation, far less 

than that reported for the Oregon Coast Range to the 

north (Beck and Plumley, 1980), but similar to that 

reported from lower Tertiary volcanic rocks of the 

southern Cascade Range to the east (Beck and others, 1986).

Nilsen and others (1983) mapped the entire outcrop length of the Hornbrook Formation, subdividing 

it into nine informal units (fig. 6). Nilsen and Barats 

(1983) discussed the general sedimentology of the 

Hornbrook Formation, and Nilsen (1983) the sedimentology of the "Hilt bed." Bourgeois and Leithold 

(1983) described and measured two sections in the 

lower part of the Hornbrook Formation, interpreting 

these sections as high-energy, storm wave-dominated 

transgressive sequences. Nilsen (1984a) discussed the

Peck and others 

(1956)

Jones (1959) 

and 

unpub. mapping

McKnight(1971) Elliott(1971) Elliott (unpub. 

data, 1980)

Nilsen and 

others (1983)

Nilsen (1984d) 

and this paper

Tertiary volcanic and sedimentary rocks

Member VI

Member V

Member IV

Member III

-\X~\_/Xx-X^"N_^N

Member II

Member I

UnitC

(^Sandstone lens)

UnitB

/\^^ r~^^-^S\S*

Unit A

Mudstone and 

sandstone 

(Khm)

Sandstone 

(Khs3 )

Shale mudstone 

(Khs 2 )

Sandstone 

(KhSl )

'c 

^

I

c: 

o

«

E

o 

u_

_*

8

.Q 

O

Bailey Hill 

mudstone 

unit

Lower 

arenaceous 

member

Henley mudstone unit

Lower 

arenaceous 

member

Hornbrook Formation

UnitD

UnitC 

UnitB

Unit A

Nonmarine 

unit

Marine shale 

(unit d)

Marine turbitite 

sandstone (unit m)

<C]Ss^ lens (unitj]^> 

<^§slens (unjtTJ^

Sandstone & 

conglomerate (unit c)

Marine siltstone 

(unit b)

Marine sandstone 

(unit a)

Marine cgl 

sandstone (unit s)

Nonmarine 

conglomerate (unit r)

^ Hilt Bed

Blue Gulch 

Mudstone Member

y Rancheria 

--=-. 77. "^ TS. Gulch 

C ' / ': '. ' ^Sandstone ' "* "--"' Beds

Rocky Gulch 

Sandstone Member

Ditch Creek 

Siltstone Member

Osburger Gulch 

Sandstone Member

Klamath River 

Conglomerate Member

Basement rocks

FIGURE 6. Stratigraphic and age relations of the Hornbrook Formation. A, Development of stratigraphic nomenclature and correlation 

of units within the Hornbrook Formation. B, Subdivisions of Late Cretaceous time proposed by various workers.
PREVIOUS WORK

tectonic framework of sedimentation of the 

Hornbrook Formation, and Nilsen (1984b) discussed a 

depositional model for member III of the Hornbrook 

Formation of Peck and others (1956; the Rocky Gulch 

Sandstone Member of this paper).

A wealth of new information about the Hornbrook 

Formation was presented in a field trip guidebook edited by Nilsen (1984c). This volume includes (1) a description of field trip stops and a roadlog to the 

outcrop areas of the Hornbrook Formation (Nilsen 

and others, 1984); (2) a synthesis of the stratigraphy, 

sedimentology, and tectonic framework of the 

Hornbrook Formation (Nilsen, 1984d); (3) a summary 

of paleontologic data from the Hornbrook Formation 

(Sliter and others, 1984); (4) a summary of the sandstone petrography of the Hornbrook Formation (Golia

and Nilsen, 1984); (5) a summary of the conglomerate 

clast composition of the Hornbrook Formation 

(Barats and others, 1984); (6) a description of coal 

samples from the Hornbrook Formation in the Shasta 

Valley area (Zigler and Nilsen, 1984); (7) results from 

analyses of both the porosity and permeability, as 

well as a discussion of the diagenetic history of surface samples of the Hornbrook Formation (Kreighin 

and Law, 1984); (8) a discussion of the petroleum 

source-rock potential of the Hornbrook Formation 

(Law and others, 1984); and (9) discussions of the 

stratigraphy and sedimentology of the marine 

Osburger Gulch Sandstone Member of the Hornbrook 

Formation (Gaona, 1984; Bourgeois, 1984). In addition to these papers on the outcrop geology of the 

Hornbrook Formation, papers by Fuis and Zucca

Age 

(Ma)

70 -

80 - 

90 - 

 inn 

Harland and others 

(1964)

Maestrichtian

Campanian

Santonian

Coniacian

Turanian

Cenomanian

van Hinte 

(1976)

Maestrichtian

Campanian

Santonian

Coniacian

Turanian

Cenomanian

Harland and others 

(1982)

Maestrichtian

Campanian

Santonian

Coniacian

Turanian

Cenomanian

Albian

Palmer 

(1983)

Danian

Maestrichtian

Campanian

Santonian

Coniacian

Turanian

Cenomanian

Albian

Age 

(Ma)

RP . . ,,.

- 70 

- 80

- 90 

_ mn 

FIGURE 6. Continued.
10 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

(1984) and Erskine and others (1984) discussed the 

subsurface extent of the Hornbrook Formation from 

geophysical studies.

Ages previously presented for the members of the 

Hornbrook Formation by Nilsen (1984a, d) have been 

revised in the present paper. Initial paleontologic age 

determinations from benthic and planktonic foraminifers for the members were incorrect for various reasons; the ages reported herein are more correct, and 

the data are presented in appendices I and II and 

summarized by Sliter and others (1984).

Subsequent work on the conglomeratic bodies of 

the Pythian Cave area to the south of Yreka, previously mapped as Cretaceous by Mack (1959), as Tertiary(?) by Hotz (1977), and not considered to be part 

of the Hornbrook Formation by Nilsen and others 

(1983) and Nilsen (1984a, d), has yielded mid-Cretaceous (Albian and Cenomanian) ages (Jameossanaie 

and others, 1986; Wetzstein and others, 1986; 

Wetzstein, 1986; Nancy Lindsley-Griffin, written 

commun., May 12, 1987). These ages were obtained 

from angiosperm pollen and trilete spores in the 

lower part of the main conglomeratic unit.

HORNBROOK FORMATION

The stratigraphic divisions of the Hornbrook Formation proposed by Nilsen (1984a, d) (figs. 6, 7) were 

based on previous stratigraphic studies by Peck and 

others (1956), Jones (1959), Elliott (1971; unpub. 

data, 1980), McKnight (1971), and Nilsen and others 

(1983). The previous nomenclatural subdivisions utilized Roman-numeral or letter systems and were 

clearly suitable for informal stratigraphic usage. The 

basal nonmaririe member, the Klamath River Conglomerate Member, is the only discontinuous member, cropping out in thicknesses sufficient to be 

mapped in the Shasta Valley, southern Cottonwood 

Creek valley, northern Bear Creek valley, and Jacksonville areas (pi. 1). The other members, which are 

marine except for possibly a coal-rich interval in the 

Ditch Creek Siltstone Member in the Shasta Valley, 

are continuous except for the Rocky Gulch Sandstone 

Member, which is locally absent on the south flank of 

Black Mountain. The Rancheria Gulch Sandstone 

Beds of the Blue Gulch Mudstone Member, distinguished in the measured type section of the 

Hornbrook Formation by Peck and others (1956) and 

mapped by D.L. Jones (unpub. data, 1960), is present 

only in the type area of the Hornbrook Formation 

near the town of Hornbrook. The Hilt Bed of the Blue 

Gulch Mudstone Member was first recognized and 

mapped but not named by Nilsen and others (1983);

it crops out more or less continuously from the northern Shasta Valley to the Ashland area, although locally it has been eroded off by the unconformity at 

the top of the Hornbrook Formation or has not been 

recognized because of poor exposures or Quaternary 

cover.

The Hornbrook Formation was defined by Peck 

and others (1956) for exposures of clastic sedimentary 

rocks near Hornbrook, California, where they measured their composite type section. The type sections 

of Nilsen (1984 a, d) for the stratigraphic units of the 

Hornbrook Formation are nearly all in the Hornbrook 

area, so that they are relatively close to each other 

and within the type area (fig. 8). All stratigraphic 

names were taken from place names in the type area 

except for that of the Hilt Bed, which was named for 

the abandoned town of Hilt, located about 9 km 

northwest of Hornbrook, where the bed is thickest 

and best developed.

The thickness of the Hornbrook Formation varies 

considerably along the length of its outcrop, partly as 

a result of variable depths of erosive downcutting 

along the pre-Tertiary unconformity. Peck and others 

(1956) determined an incomplete thickness of 815±15 

m for the Hornbrook Formation in the type area. 

Jones (1959) found a total thickness of about 1,490 m 

for the Hornbrook Formation in its type area, and 

Elliott (1971) a thickness of 1,237 m in the northern 

part of the Cottonwood Creek valley area. McKnight 

(1971) determined a thickness of more than 910 m for 

the Hornbrook Formation in the Bear Creek valley 

area. The thickness of the Hornbrook Formation in 

the type area presented herein, which is based on 

adding up the separate thicknesses of individual 

measured type sections of the members and subunits 

(rather than the measurement of a continuous section), is 1,236 m.

Previous workers (Peck and others, 1956; Jones, 

1959; Elliott; 1971: unpub. data, 1980) concluded that 

there is a major unconformity within the lower part 

of the Hornbrook Formation. The unconformity was 

thought to be located at the contact between strata 

equivalent to the Ditch Creek Siltstone Member and 

the Rocky Gulch Sandstone Member; an erosive contact can be observed locally in outcrops between these 

members in the Hornbrook area, and Elliott (1971) 

concluded that there is a 3° 4° angular discordance 

between the members. The unconformity was thought 

to span Coniacian and Santonian time; however, no 

megafossils or microfossils had been obtained from 

strata equivalent to the Rocky Gulch Sandstone 

Member or the lower part of the Blue Gulch Mudstone Member below the Rancheria Gulch Sandstone 

Beds. Megafossils from strata equivalent to the
HORNBROOK FORMATION 11

600 -

500 -

400

- 300 -

200 -

100 -

Rancheria Gulch 

Sandstone Beds

Rocky Gulch 

Sandstone Member

(Locally unconformable) 

Ditch Creek 

Siltstone Member

Osburger Gulch 

Sandstone Member

f' Klamath River 

~\ Conglomerate Member

1200 -

Tertiary volcanic rocks 

-^ U n co nf o rm ity-

&

1100 - Covered

GC

o

o 

o

DC 

CD

z

DC

O

X

1000 -

O'.D- gy.

\ \

"^UnconformityBasement complex

Basal 

-breccia

900 -

800

700 -

&

Hilt Bed

&

Igneous and metamorphic rocks

Sedimentary breccia

Conglomerate

Sandstone

Siltstone

EXPLANATION

^^r- Shale and mudstone 

& Foraminiferal fossils 

# Plant fossils 

© Molluscan fossils 

/ Generalized paleocurrent direction

DC 

O

O 

O

DC 

CD

z

QC

O

X

Trough cross strata 

Volcanic breccia

FIGURE 7. Composite measured section of the Hornbrook Formation.
12 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA 

122° 36 122°33'

42°55'

42°5T
HORNBROOK FORMATION 13

Qal

Qf

Qt

TV

underlying Osburger Gulch Sandstone Member and 

Ditch Creek Siltstone Member in the type area had 

been indicative of Turonian to possibly early 

Coniacian ages, and megafossils from strata equivaEXPLANATION

Alluvium (Quaternary)

Alluvial fan deposits (Quaternary)

Alluvial terrace deposits (Quaternary)

Volcanic rocks (Tertiary)

Intrusive rocks (Tertiary)

Hombrook Formation (Cretaceous) Divided into: 

Blue Gulch Mudstone Member Includes: 

Hilt Bed

Rancheria Gulch Sandstone Beds 

Rocky Gulch Sandstone Member 

Ditch Creek Siltstone Member 

Osburger Gulch Sandstone Member 

Klamath River Conglomerate Member

Khdc

Khog

Khkr

Basement complex of Klamath Mountains (Mesozoic and 

Paleozoic)

Contact

------ Fault Dotted where concealed

A A1 

I | Measured section location

C2" TTS> Landslide deposit Arrow indicates direction of movement 

-L!° Strike and dip of beds

FIGURE 8. Geologic map of Cotton wood Creek valley area, showing location of type sections of stratigraphic units of the 

Hombrook Formation. Type sections: (1) A-A\ Klamath River 

Conglomerate Member; (2) B-B', Osburger Gulch Sandstone 

Member; (3) C-C", Ditch Creek Siltstone Member; (4) D-D', 

Rocky Gulch Sandstone Member; (5) E-E', lower part of the 

Blue Gulch Mudstone Member; (6) E'-E", Rancheria Gulch 

Sandstone Beds; (7) F-F', middle part of the Blue Gulch Mudstone Member, including the Hilt Bed; (8) G-G', upper part of 

the Blue Gulch Mudstone Member above the Hilt Bed; and (9) 

H-H', uppermost part of the Blue Gulch Mudstone Member. 

Although the basement complex is shown here as being composed of Paleozoic and Mesozoic rocks, the area shown by this 

map is probably underlain wholly by rocks of Mesozoic age 

(W.P. Irwin, oral commun., 1984). Base map from U.S. Geological Survey l:62,000-scale Hombrook, Calif, quadrangle.

lent to the middle and upper parts of the Blue Gulch 

Mudstone Member were indicative of middle 

Campanian to Maestrichtian(?) ages.

In this paper, new paleontological data, chiefly 

benthic and planktonic foraminiferal collections and 

some megafossil collections, are presented that place 

further constraints on the proposed unconformity. 

Megafossils from the basal part of the Osburger 

Gulch Sandstone Member in the area south of 

Hombrook are indicative of a Turonian age, and one 

collection of megafossils from the Ditch Creek Siltstone Member is indicative of a Turonian age (app. I). 

Two collections (Mf6580 and Mf6581) of benthic foraminifers from the Ditch Creek Siltstone Member in 

the Cottonwood Creek valley area are indicative of a 

late Turonian to Coniacian age. Megafossils from the 

Rancheria Gulch Sandstone Beds are indicative of a 

late Campanian age (Sliter and others, 1984), and 

foraminifers from the middle and upper parts of the 

Blue Gulch Mudstone Member in the Cottonwood 

Creek valley area and its type section are indicative 

of ages that range generally from Campanian to 

Maestrichtian (app. I). Thus, in the type area and in 

the Cottonwood Creek valley area, a local unconformity that represents Santonian time could possibly 

be present between the Ditch Creek Siltstone Member and the Rocky Gulch Sandstone Member; alternatively, and more likely, in my opinion, the Santonian 

interval, which represents only about 2.5 m.y (fig. 

6.B), is present within the unfossiliferous Rocky 

Gulch Sandstone Member and lower part of the Blue 

Gulch Mudstone Member below the Rancheria Gulch 

Sandstone Beds.

To the north in the Bear Creek valley, Medford 

Valley, and Dark Hollow areas, strata mapped herein 

as the Rocky Gulch Sandstone Member and the Blue 

Gulch Mudstone Member are considered to be middle 

and late Turonian in age (app. II). Thus, the proposed unconformity in the type area and in the Cottonwood Creek valley area would be present 

stratigraphically above the youngest strata farther 

north. As a result of these new age determinations, it 

is not clear whether the proposed unconformity can 

be substantiated. In this paper, the contact is considered gradational and conformable, except in the type 

area and in parts of the northern Cottonwood Creek 

valley, where it is erosive and possibly locally unconformable.

The Hombrook Formation undoubtedly extends 

northeastward in the subsurface beneath the Cascade 

Range, as inferred from gravity, magnetic, and seismic data (Leaver and others, 1984; Erskine and others, 1984; Fuis and Zucca, 1984; Blakely and others 

1985; Fuis and others, 1987). However, it has been 

penetrated by only one well drilled for petroleum, the
14 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

Kuck No. 1 (sec. 26, T. 46 N., R. 5 W., Copco 15- 

minute quadrangle) at the northeastern edge of the 

Shasta Valley south of Bogus Mountain (Alldredge 

and Meigs, 1984; Montgomery, 1988, p. 32-35); even 

though about 1,405 m (4,500 ft) of Hornbrook Formation was penetrated in this well, it is difficult to regionally project possible facies changes or changes in 

thickness beneath the volcanic cover.

To the southeast and northwest, the Hornbrook 

Formation probably originally extended as a continuous sheet of marine conglomerate, sandstone, and 

shale with local nonmarine deposits at its base. However, it is covered by Quaternary alluvium in the 

Medford Valley and Shasta Valley. At the southeast 

end of the Shasta Valley, volcanogenic landslide deposits partly overlap it (Chesterman and Saucedo, 

1984; Crandell and others, 1984). In the Pythian 

Cave area, nonmarine conglomeratic bodies along the 

western flank of Shasta Valley and west of the landslide deposits are considered to be mid-Cretaceous in 

age; these conglomerates may be correlative with the 

Klamath River Conglomerate Member of the 

Hornbrook Formation and indicate southward continuation of the basal fluvial deposits. Farther to the 

northwest and southeast, Tertiary volcanic rocks rest 

directly on the Klamath Mountains basement complex, either as a result of complete overlap of the 

Hornbrook Formation by virtue of the angular 

unconformity between the Cretaceous and Tertiary 

strata (Elliott, 1971, p. 17-18) or owing to complete 

erosion of the Cretaceous strata beneath the postCretaceous unconformity. The lateral continuity of 

the members of the Hornbrook Formation over its entire outcrop extent suggests that the Hornbrook Formation originally extended much farther to the 

southeast and northwest. To the southeast, it may 

have connected with Cretaceous strata of the Great 

Valley sequence in the Redding area (Nilsen, 1984a), 

although this idea has been opposed by other workers 

(Haggart, 1986b); the deposits in the Redding area 

also rest unconformably on basement rocks of the 

Klamath Mountains and are at least partly coeval 

with the Hornbrook Formation. To the northwest, in 

the Grave Creek area, Cretaceous strata rest on basement rocks of the Klamath Mountains in two 

downfaulted blocks; here the lowest marine strata 

are of Albian age, older than the Cenomanian strata 

in the Dark Hollow area, and facies-equivalent strata 

of the Rocky Gulch Sandstone Member are absent.

To the west, a downfaulted block of Cretaceous 

strata within the basement rocks of the Klamath 

Mountains near O'Brien, Oregon, contains similar 

nonmarine deposits at its base and overlying transgressive shallow-marine conglomerate, sandstone,

and siltstone (Shenon, 1933). Megafossils from this 

area, however, are indicative of Early Cretaceous 

ages (Shenon, 1933), although the uppermost part of 

the sequence has been eroded off, so that any Upper 

Cretaceous deposits that might have originally been 

present were subsequently removed. Upper Cretaceous (Campanian and Maestrichtian) shallow-marine deposits of the Cape Sebastian Sandstone of Dott 

(1971) and deep-marine deposits of the Hunters Cove 

Formation of Dott (1971) in the Gold Beach area of 

coastal southwestern Oregon (fig. 1) are correlative 

with the upper part of the Hornbrook Formation. 

However, these strata are fault bounded and may 

have accreted to southwestern Oregon in post-middle 

Eocene time (Blake, 1984a; Blake and others, 1985a, 

1989; Bourgeois and Dott, 1987).

STRATIGRAPHIC SUBDIVISIONS OF THE 

HORNBROOK FORMATION

KLAMATH RIVER CONGLOMERATE MEMBER

DEFINITION

The name "Klamath River Conglomerate Member" was applied by Nilsen (1984a) to discontinuous 

nonmarine conglomeratic strata that form the basal 

part of the Hornbrook Formation. The member was 

named for exposures west of the Klamath River, 

about 4 km south of Hornbrook, California (figs. 8, 9). 

The type section was measured in the Blue Gravel 

Mine directly west of the southbound lane of Interstate Highway 5, about 0.4 km west of the Klamath 

River and about 0.7 km southeast of the McCavick 

Mine (fig. 9).

The Klamath River Conglomerate Member is the 

lowest member of the Hornbrook Formation. It had 

previously been mapped by M.A. Elliott (unpub. data, 

1980) as his nonmarine unit and by Nilsen and others (1983) as their unit r (of Hornbrook Formation), a 

nonmarine conglomerate, sandstone, pebbly mudstone, mudstone, and siltstone that contains plant 

fossils, paleosols, and a basal breccia. Peck and others (1956) did not separate this unit from member I 

in their stratigraphic subdivision of the Hornbrook 

Formation. The member rests unconformably on Paleozoic and Mesozoic igneous and metamorphic basement rocks and is overlain conformably by the 

Osburger Gulch Sandstone Member of the Hornbrook 

Formation.

The lower boundary of the Klamath River Conglomerate Member is defined by the unconformable 

depositional contact on the basement rocks. Paleosols 

that are as thick as several meters are locally present
KLAMATH RIVER CONGLOMERATE MEMBER 15

FIGURE 9. Type section of the Klamath River Conglomerate 

Member of the Hornbrook Formation. A, View northwestward 

of Blue Gravel Mine, showing northeast-dipping strata; massive bluffs above underlain by Osburger Gulch Sandstone 

Member (Khog) of the Hornbrook Formation. Qaf, artificial fill; 

KhKr, Klamath River Conglomerate Member (of Hornbrook Formation); Mzftb, basement complex of Klamath Mountains. B-D, 

Interbedded light-colored conglomerate and dark-colored, less 

resistant pebbly mudstone. Rock hammer (circled in B and D) 

is 35 cm long. E and F, Basal breccia consisting of matrix-supported angular clasts of metavolcanic rocks in pebbly mudstone.
16 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

on the top of the basement rocks, but in other places 

the sedimentary strata rest directly on unweathered 

basement rocks. A basal breccia (composed wholly of 

angular blocks of basement rocks that directly underlie the breccia) or a conglomeratic grus (consisting 

mostly of untransported granitic detritus derived 

from underlying granitic rocks) can form the basal 

part of the Klamath River Conglomerate Member.

The upper boundary of the Klamath River Conglomerate Member is placed at the base of the first 

bed of massive, well-sorted, calcareous sandstone or 

conglomeratic sandstone. This massive sandstone, 

which may locally be trough cross-bedded or thinly 

laminated, may contain marine molluscan fossils and 

marine burrows. It is more resistant than the underlying nonmarine deposits of the Klamath River Conglomerate Member because of its well-developed 

carbonate cement. The calcareous sandstone is typically medium gray (N5) where fresh, in contrast to 

the grayish red (5R 4/2) and grayish brown (5YR 3/2) 

colors of the underlying nonmarine rocks. The basal 

beds of sandstone of the Osburger Gulch Sandstone 

Member in the type area and in its type section consist of siltstone and platy, very fine grained sandstone that contains some local marine mollusks and 

evidence of marine bioturbation.

AREAL EXTENT

The Klamath River Conglomerate Member extends discontinuously across almost the entire outcrop extent of the Hornbrook Formation. It is 

commonly present as a veneer of nonmarine conglomeratic strata as thick as several meters between the 

basement rocks and the Osburger Gulch Sandstone 

Member. However, it has been mapped only where it 

is much thicker (pi. 1). The northernmost mapped 

outcrops of the Klamath River Conglomerate Member 

are present about 1 km east of the city of Jacksonville, Oregon, along the southwestern flank of 

Bellinger Hill (sees. 90, 91, 92, and 95, T. 37 S., R. 2 

W., Medford 15-minute quadrangle). In addition, 

there are loose blocks of red-weathering conglomerate 

present near a school on a small hill on the east side 

of Jacksonville (NEM sec. 37, T. 37 S., R. 2 W., 

Medford 15-minute quadrangle). These blocks are 

composed of conglomerate and contain angular, 

poorly sorted, matrix-supported clasts as long as 10 

cm that resemole clasts present in the Klamath River 

Conglomerate Member. The Klamath River Conglomerate Member also forms prominent outcrops in foothills east of Coleman Creek (NA ] sec. 21, T. 38 S., R. 

1 W., Medford 15-minute quadrangle), where conglomerate containing clasts as long as 25 cm is 

present.

In most of the Bear Creek valley and Cottonwood Creek valley areas, the Klamath River Conglomerate Member is either very thin or absent. 

Adjacent to Ashland, fossiliferous conglomerate that 

forms a coquina locally rests directly on basement. 

Near the Siskiyou Summit fault, as much as 5 m of 

the Klamath River Conglomerate Member was measured at the base of a section of the Osburger Gulch 

Sandstone Member along Interstate Highway 5 

(NWM sec. 21, T. 40 S., R. 2 E., Ashland 15-minute 

quadrangle).

The Klamath River Conglomerate Member is 

thickest and best exposed southwest of Hornbrook, 

California, in the following areas: the southeast end 

of the Cottonwood Creek valley, west of the Klamath 

River; along the northwest and southwest margin of 

the Shasta Valley; and in the Soap Creek Ridge outlier of the Hornbrook Formation southwest of Yreka. 

The member extends more or less continuously from 

the Hornbrook area southward to the southernmost 

outcrops of the Hornbrook Formation in sees. 5 and 6, 

T. 44 N., R. 6 W., sec. 1, T. 44 N., R. 7 W., and sec. 1, 

T. 44 N., R. 8 W., Yreka 15-minute quadrangle. The 

conglomeratic bodies at Pythian Cave and adjacent 

areas, located about 10 km south of the southernmost 

outcrops of the Hornbrook Formation as mapped 

herein, have recently yielded Albian and Cenomanian 

pollen and spores (Wetzstein, 1986; Wetzstein and 

other, 1986). These conglomeratic bodies could be 

correlative with the Klamath River Conglomerate 

Member. However, because of their dissimilar clast 

compositions, sandstone compositions, and 

paleocurrent orientations, these bodies will not be 

considered to be part of the Hornbrook Formation until additional data are published. The widespread 

outcrops of the Klamath River Conglomerate Member 

in the Shasta Valley area result from the abundant 

faulting of the Cretaceous section and low angles of 

dip (pi. 1).

TYPE SECTION

The type section of the Klamath River Conglomerate Member of the Hornbrook Formation 

was measured along the northern margin of an 

area of artificial fill within old mine workings west 

of Interstate Highway 5 in the S/zSE^ sec. 32, T. 

47 N., R. 6 W., Hornbrook 15-minute quadrangle 

(fig. 10).

The section is about 36.5 m thick and is almost 

completely exposed.
KLAMATH RIVER CONGLOMERATE MEMBER 17

Thickness 

(in meters)

Hornbrook Formation (lower part):

Osburger Gulch Sandstone Member (lower part):

Sandstone, medium- to coarse-grained; pebble 

stringers, trough cross-stratified; minor 

amounts of bioturbation; calcareous cement; 

medium gray (N5) where fresh, moderate yellowish brown (10YR 5/4) where weathered .....

Covered interval (float of siltstone) ......................

Siltstone, massive to laminated; plant 

fragments .............................................................

Sandstone, fine-grained, massive to laminated; 

plant fragments on bedding surfaces; some 

scattered molluscan fossils ................................

Siltstone, massive to laminated............................

Conformable contact.

Klamath River Conglomerate Member:

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 4 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 1 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 5 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 1 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 2 cm ............

Pebbly siltstone, laminated, carbonaceous with 

coaly laminae; plant fossils in growth position; scattered pebbles of maximum clast size 

0.5 cm ...................................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 5 cm ............

Siltstone, massive to laminated, laterally discontinuous; abundant plant fossils, some possibly in growth position......................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 3 cm ............

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 8 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 2 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 3 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded; matrix-supported; maximum 

clast size 2 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 5 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 1 cm .....................................................

10.00

12.05

3.05

.22 

.22

.35

.80

.30

.10

.12

.42

.42

.10

.18

.60

.90

.15

.95

.12

.30

Klamath River Conglomerate Member Continued:

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 3 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 2 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 7 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 2 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 8 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 2 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 4 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 1 cm; partly laminated ......................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 8 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 3 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 8 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported, partly laminated, conglomeratic at base; maximum clast 

size 2 cm ...............................................................

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported, partly laminated in upper part; maximum clast size 

2 cm ......................................................................

Conglomerate, partly stratified, moderately 

sorted, matrix- supported; slightly imbricated 

clasts, not graded; clasts subangular to subrounded; maximum clast size 17 cm ................

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 0.5 cm ..................................................

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 0.5 cm ..................................................

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 2 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to subrounded; bed thickens laterally to 150 cm; 

maximum clast size 15 cm .................................

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 2 cm .....................................................

Thickness 

(in meters)

.11

.62

.38

.55

.63

.35

.07

.12

.55

.12

.20

.42

.60

.80

.80

.90

.45

.28

.60
18 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

Klamath River Conglomerate Member Continued:

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 8 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 2 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 8 cm ............

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 8 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 0.5 cm ..................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; channelized base; maximum 

clast size 8 cm .....................................................

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 3 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 5 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 2 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 10 cm ..........

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 5 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 2 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 0.5 cm ..................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 2 cm ............

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 2 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 10 cm ..........

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 11 cm ..........

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to subrounded; erosional base; maximum clast 

size 4 cm...............................................................

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 2 cm .....................................................

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 5 cm .....................................................

Thickness 

(in meters)

.10

.42

.24

.18

.08

.30

.38

.10

.30

.45

.80

.07

.05

.12

.10

.52

.58

.11

.38

.27

Klamath River Conglomerate Member Continued:

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 10 cm ..........

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 18 cm ..........

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 1 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to subrounded; 

erosional base; maximum clast size 10 cm ......

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 4 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 11 cm ..........

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 1 cm ............

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; erosional base; maximum clast 

size 3 cm ...............................................................

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 8 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 13 cm ..........

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 3 cm ............

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 3 cm ............

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 18 cm ..........

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 13 cm ..........

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 18 cm ..........

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 15 cm ..........

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 20 cm ..........

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to subrounded; 

erosional base; maximum clast size 15 cm.....

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 2 cm .....................................................

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; erosional base; maximum clast 

size 22 cm.............................................................

Thickness 

(in meters)

.55

.80

.12

.62

.33

.20

.08

.10

.30

.47

.15

.15

.20

.22

.75

.60

1.35

.81

.20

.70
KLAMATH RIVER CONGLOMERATE MEMBER 19

Thickness 

(in meters)

Klamath River Conglomerate Member Continued:

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; laterally dis 

continuous; maximum clast size 1 cm.............. .12

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; erosional base; maximum clast 

size 22 cm............................................................. 1.25

Pebbly mudstone, massive; clasts subangular to 

subrounded, matrix-supported; maximum 

clast size 10 cm ................................................... .90

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 20 cm .......... .65

Siltstone, sandy, massive, laterally 

discontinuous....................................................... .10

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 18 cm .......... .60

Siltstone, sandy, massive, laterally

discontinuous....................................................... .80

Conglomerate, reverse-graded, matrix-supported; 

mudstone matrix; clasts subangular to 

subrounded; maximum clast size 21 cm .......... .67

Conglomerate, massive; appears to be ungraded, 

matrix-supported, poorly exposed; clasts angular to subrounded; maximum clast size 

22 cm .................................................................... 2.30

Breccia and conglomerate, partly residual and 

partly transported; about 95 percent larger angular clasts of underlying metavolcanic basement rocks and about 5 percent smaller 

subrounded clasts of quartzite, chert, and 

quartz, which increase in abundance upward; 

maximum clast size 62 cm ................................. .50

Total thickness of the Klamath River Conglomerate Member...................................... 36.50

Unconformable contact.

Applegate Group (basement complex):

Metavolcanic rocks of Triassic age

The colors of the Klamath River Conglomerate 

Member vary considerably. The basal breccia and 

conglomerate, 0.50 m thick, are dark gray (N3) and 

grayish black (N2) on fresh, unweathered surfaces 

and light brown (5YR 5/6) on weathered surfaces. 

The overlying conglomerate beds are medium dark 

gray (N4) on fresh, unweathered surfaces and pale 

yellowish brown (10YR 6/2) on weathered surfaces. 

The pebbly mudstone beds are generally grayish red 

(5R 4/2) and grayish brown (5YR 3/2) on fresh, unweathered surfaces and very dusky red (10R 2/2) and 

dark reddish brown (10R 3/4) on weathered surfaces. 

In general, the entire unit has a reddish appearance 

from a distance, with the color probably reflecting the 

abundance of hematite and other oxidized iron minerals in the mudstone matrix of the pebbly mudstone, 

siltstone, and conglomerate units.

LITHOLOGY

The Klamath River Conglomerate Member consists mostly of conglomerate. In the type section, it 

consists of alternating beds of conglomerate and pebbly mudstone, with the distinction based chiefly on 

the abundance and size of clasts. Some interbedded 

siltstone with dispersed sand-sized grains is present, 

particularly in the upper part of the section; some of 

these siltstone beds contain plant fossils, a few of 

which appear to be in growth position. No 

bioturbation has been noted in the unit. At the base 

of the type section, the Klamath River Conglomerate 

Member contains a coarse sedimentary breccia that 

grades locally into conglomerate.

The conglomerate clasts in the type section of the 

Klamath River Conglomerate Member consist mostly 

of metavolcanic rocks derived from the underlying 

Applegate Group, with subordinate amounts of resistant quartzose clasts (table 1). Near the top of the 

unit are small amounts of sandstone clasts that may 

possibly represent reworking of eroded fragments of 

the Klamath River Conglomerate Member itself, or of 

fragments of a previously deposited Mesozoic sandstone no longer preserved because of erosion on the 

basal unconformity. The uniformity of clast composition and the relative angularity of the clasts indicate 

that the sedimentary material was derived chiefly 

from the underlying basement, without long distances of transport.

Adjacent to its type section, in roadcuts along the 

northbound lane of Interstate Highway 5, a thinner 

section of the Klamath River Conglomerate Member 

is exposed. Here, the basal unconformity and contact 

with the overlying Osburger Gulch Sandstone Member are well exposed. The Klamath River Conglomerate Member here is lithologically similar to the type 

section, consisting of interbedded matrix-supported 

conglomerate and pebbly mudstone; however, it contains finer conglomerate and thinner beds of conglomerate than in the type section. The base of the 

member here consists of a red, nodular, nonpebbly 

mudstone that grades upward from a whitish-weathering paleosol that caps the underlying metavolcanic 

rocks. This red mudstone, which is about 2 m thick, 

may also partly consist of paleosol materials.

The Klamath River Conglomerate Member south 

of its type section consists of reddish-weathering conglomerate with subordinate amounts of sandstone, 

siltstone, and mudstone. It generally contains a basal 

breccia or a paleosol at its base, overlain by conglomerate. In most areas adjacent to the Shasta Valley 

and Yreka, it consists of clast-supported, wellrounded, imbricated, polymict conglomerate with 

interbedded cross-stratified to parallel-stratified
20 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

medium- and coarse-grained sandstone, ripplemarked fine-grained sandstone and siltstone, and 

massive silty mudstone that contains thin paleosols 

and very thin coal seams. Plant fossils are common in 

these southern exposures, both as larger fragments of 

leaves, branches, and trunks in the coarser-grained 

strata and as roots of plants in growth position in the 

finer-grained strata. Conglomerate clasts in the 

southern exposures include major amounts of metavolcanic rocks, metasedimentary rocks, quartzite, chert, 

and (locally) granitic rocks and possibly peridotite.

The best exposures of the Klamath River Conglomerate Member in the Yreka-Shasta Valley area 

are roadcuts along Oberlin Road east of Yreka, in the 

NE corner of sec. 35, T. 45 N., R. 7 W., Yreka 15- 

minute quadrangle. This section is a good reference 

section, one that could be adopted if the type section 

were ever destroyed. The basal contact is not exposed, but the upper contact with the Osburger 

Gulch Sandstone Member is well exposed. About 75 

m of the Klamath River Conglomerate Member is exposed here. The lowermost 22 m consists of interbedded conglomerate, conglomeratic sandstone, and 

medium- to coarse-grained sandstone. These units 

form lens-shaped beds characterized by abundant 

channeling, medium- to large-scale crossbedding, and 

fining-upward couplets of channel conglomerate and 

sandstone. In this outcrop, all of the conglomerate is 

clast supported, in contrast to the type section. The 

overlying approximately 50 m consists of conglomerate, sandstone, and mudstone that are organized into 

fining-upward cycles with channelized bases. These, 

cycles range from less than 2 m to almost 10 m in thickness. Paleosols, coal seams, and fossil plant roots are 

present in the mudstone strata at the top of the cycles.

North of its type area, the Klamath River Conglomerate Member is commonly not present or consists of a thin unit of basal conglomerate or 

conglomeratic grus at the contact of the Hornbrook 

Formation with basement rocks. In a roadcut exposure adjacent to the southbound lanes of Interstate 

Highway 5 near the Siskiyou Summit (NWXNWK sec. 

21, T. 40 S., R. 2 E., Ashland 15-minute quadrangle), 

the basal layers of the Hornbrook Formation on the 

Mount Ashland plutonic complex of Wells (1956) consist of 5.12 m of conglomeratic grus with some 

interbedded coarse-grained sandstone (Nilsen and 

others, 1984, stop 1). The basal 4.10 m consist of a 

massive to poorly stratified mixture of granitic or 

quartz dioritic grus, derived from the underlying plutonic rocks, and scattered angular to well-rounded 

clasts of quartzite, chert, and metavolcanic rocks that 

are as long as 10 cm. The overlying 1.02 m consist of 

two massive beds of conglomeratic, very coarsegrained sandstone that grade reversely upward into 

conglomerate that contains clasts as long as 10 cm. 

The sandstone and conglomerate matrix consists almost wholly of grus-like sediment derived from the 

underlying plutonic rocks. No molluscan or plant fossils or paleosols are present in this exposure of the 

Klamath River Conglomerate Member.

Farther north, in the Dark Hollow and Jacksonville 

areas, the Klamath River Conglomerate Member consists

40 -

\

Osburger Gulch Sandstone 

Member of Hornbrook Formation

Klamath River

Conglomerate

Member

0 -

breccia

Basement complex

GRAIN SIZE

sh

vc 

ss

32 50 

mm cm

FIGURE 10. Type section of the Klamath River Conglomerate 

Member (including basal breccia) of the Hornbrook Formation, 

sh, shale; vc, very coarse grained; ss, sandstone.
KLAMATH RIVER CONGLOMERATE MEMBER 21

mostly of polymict conglomerate resting directly on the 

basement complex. The best exposure of the unit is in a 

quarry west of Talent (SW^NE^ sec. 21, T. 38 S., R. 1 

W., Medford 15-minute quadrangle), where conglomerate 

consisting of metavolcanic rocks, metasedimentary rocks, 

granitic rocks, quartzite, and chert is exposed. This conglomerate contains clasts that are as long as 25 cm and 

are well rounded to angular. The conglomerate beds are 

reverse graded and matrix supported, similar to those in 

the type section. Other reverse-graded, matrix-supported, 

poorly sorted conglomeratic beds of the Klamath River 

Conglomerate Member crop out in the Jacksonville area.

STRATIGRAPHIC RELATIONS

The Klamath River Conglomerate Member rests 

directly on either the pre-Cretaceous basement rocks 

of the Klamath Mountains or a paleosol developed on 

those rocks. It generally rests with profound angular 

unconformity on metasedimentary or metavolcanic

rocks, but locally it rests nonconformably on Jurassic 

plutonic rocks. Where it is not present, the Osburger 

Gulch Sandstone Member rests directly on the various basement rocks.

The Klamath River Conglomerate Member is conformably overlain everywhere by the Osburger Gulch 

Sandstone Member. This contact is generally marked by 

the abrupt upward lithologic change from conglomeratic 

beds with some interbedded sandstone and mudstone to 

resistant, cross-stratified, massive bluffs of calcareous 

sandstone that contain some fine conglomerate in crossbed troughs. The lower beds of sandstone of the 

Osburger Gulch Sandstone Member typically form steep 

bluffs capped by large outcrops of sandstone that contrast strongly with the less resistant beds of the Klamath 

River Conglomerate Member.

In the Hornbrook area, the basal part of the 

Osburger Gulch Sandstone Member consists of a less 

resistant siltstone unit that is 3-15 m thick (see description of the type section of the Klamath River 

Conglomerate Member). This siltstone contains marine

EXPLANATION OF SYMBOLS ON STRATIGRAPHIC SECTIONS

Igneous and metamorphic rocks

Sedimentary breccia 

. Conglomerate

S^i^t Shale and mudstone

& Foraminiferal fossils

© Molluscan fossils

&> Plant fossils

/} Bioturbation

>->^. Trough cross strata 

^S/S Tabular cross strata

HCS Hummocky cross stratification 

 ~~ Parallel stratification

Contorted stratification 

Current ripple markings 

Convolute lamination 

\_ Sedimentary flaps 

; * ^ . Rip-up clasts 

OV^ Dish structure

© Carbonate concretions

\ <\ Thinning-upward and 

\ ^ fining-upward cycles

/* Paleocurrent direction

FIGUHE 10. Continued.
22 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

TABLE 1. Composition of conglomerate clasts in the type section of the Klamath 

River Conglomerate Member

[Data are from Barats and others (1984, table 1) and are based on three random samples of 

100 clasts larger than 2 cm in longest dimension]

Location of pebble count

At contact 

with basement

Sandstone 

TV.* ol _________ ______

91 

0 

1 

8 

0

inn

3.8 m above 

basal contact

95 

1 

0 

4 

0

inn

35.3 m above 

basal contact

93

1

3 

3

inn

megafossils and marine burrows, which are not 

present in the underlying Klamath River Conglomerate Member. This siltstone unit was recognized by 

Peck and others (1956, p. 1978) as unit 1 of their 

member I. They described it as 8.7 m thick, sandy, 

soft, poorly sorted, and nonbedded, with minor 

amounts of granules and pebbles. At the type section 

of the Osburger Gulch Sandstone Member along 

Osburger Gulch east of the Klamath River (SWK sec. 

33, T. 47 N., R. 6 W., Hornbrook 15-minute quadrangle), it underlies most of the covered interval between the first massive sandstone beds of the 

Osburger Gulch Sandstone Member and the metavolcanic basement rocks.

The upper contact of the Klamath River Conglomerate Member with the Osburger Gulch Sandstone 

Member thus is generally abrupt, apparently marking the change from nonmarine to marine conditions 

of sedimentation. However, locally there is some intermixing of the units over a thickness of a few 

meters, or the presence of sedimentary rocks that are 

transitional between the two units.

Lateral contacts of the Klamath River Conglomerate Member cannot be seen in outcrop. Prior to the 

stripping away of much of the Hornbrook Formation 

from the Klamath Mountains to the southwest, it 

probably persisted in all directions as a laterally discontinuous, thin conglomeratic unit resting directly 

on the basement and forming the basal part of the 

Hornbrook Formation.

THICKNESS

The Klamath River Conglomerate Member 

ranges in thickness from 0 to possibly as much as 

90 m. At its type section, it is 36.50 m thick (fig. 

10), but several hundred meters to the east it is 

about 11 m thick, and several hundred meters farther east, near Osburger Gulch, it may be only a 

few meters thick. At the reference section along 

Oberlin Road east of Yreka, it is about 49 m thick, 

but because its lower part is not exposed, it might 

be 5-10 m thicker. Along the southbound lane of 

Interstate Highway 5 near the Siskiyou Summit, 

where it consists mostly of grus and conglomerate 

resting directly on the Mount Ashland plutonic 

complex, it is 5.12 m thick. In many areas along 

the northern Cottonwood Creek valley and southern Bear Creek valley areas, it appears to be either 

absent or very thin, because marine sandstone of 

the Osburger Gulch Sandstone Member rests directly on basement. In the Dark Hollow area, it is 

generally 0-15 m thick.

DEPOSITIONAL ENVIRONMENT

The Klamath River Conglomerate Member, as defined herein, is wholly nonmarine in origin and was 

deposited in at least three different depositional environments: (1) debris-flow and mudflow-dominated alluvial fans, (2) braided to meandering fluvial 

systems, and (3) residuum in the form of either 

weathered blocks of basement rocks derived from the 

directly underlying basement or granitic grus, mixed 

with some rounded and transported pebbles. In addition, paleosols developed within the Klamath River 

Conglomerate Member form an important part of 

some sections.

At its type section, the Klamath River Conglomerate Member was probably deposited as a small alluvial fan (Nilsen and others, 1984, stop 2). It 

consists almost wholly of matrix-supported debrisflow conglomerate and pebbly mudstone; these deposits were probably formed by subaerial debris flows 

and mudflows, most likely on a small alluvial fan 

within the uplifted Klamath Mountains block. The
OSBURGER GULCH SANDSTONE MEMBER 23

fan was subsequently overlapped by the transgressing Cretaceous sea. Bourgeois and Leithold (1983, p. 

11) and Bourgeois (1984), who also infer an alluvialfan origin for these red beds in the type section, note 

the presence of thin beds of tuffaceous silty sandstone 

in the red beds; I have not observed any tuffaceous 

material in the Klamath River Conglomerate Member, despite a careful search for it.

At its reference section east of Yreka, the massive 

conglomerate and cross-stratified conglomeratic sandstone in the lower part of the Klamath River Conglomerate Member were probably deposited by 

braided streams (Nilsen and others, 1984, stop 3). 

The fining-upward cycles of conglomerate, sandstone, 

and mudstone in the upper part of the section were 

probably deposited by more sinuous, possibly meandering streams. The mudstone units, which contain 

plants in growth position and interstratified 

paleosols, were probably deposited on flood plains.

At the base of most sections of the Klamath River 

Conglomerate Member are weathered angular blocks 

of the basement complex that probably formed 

largely by weathering of the basement with little or 

no transport. Where plutonic rocks form the basement, a conglomeratic grus forms the base of the Klamath River Conglomerate Member. Paleosols are 

locally thick and well developed on the basement directly beneath the conglomerate.

AGE AND CORRELATION

No fossil or radiometric ages have been determined from the Klamath River Conglomerate Member. The youngest rocks that it rests on are felsic 

plutons in the northern Klamath Mountains. The 

Mount Ashland pluton has yielded potassium-argon 

ages of 146, 147, 151, 152, 164, and 167 Ma, and the 

Jacksonville pluton has yielded potassium-argon ages 

of 137±4.1 and 141±4.2 Ma (Lanphere and others, 

1968; Hotz, 1971; Fiebelkorn and others, 1983; M. A. 

Lanphere, oral commun., September 2, 1983; Irwin, 

1984, and oral commun., February 11, 1984). Since 

the Jurassic-Cretaceous boundary is generally placed 

at 144 Ma (Harland and others, 1982; Palmer, 1983), 

these ages suggest that the Klamath River Conglomerate Member must be post-Jurassic in age.

The youngest possible age of the member is constrained by paleontologic dating of the overlying marine Osburger Gulch Sandstone Member, which is 

locally fossiliferous in its basal parts. The oldest ages 

reported from basal marine strata of the Hornbrook 

Formation (listed from northwest to southeast) are 

Albian in the Grave Creek area (Jones, 1960b); late

Albian(?) to early Cenomanian in the Dark Hollow 

area (Peck and others, 1956); middle Turonian in the 

Bear Creek valley area, the type area, parts of the 

Shasta Valley area, and the outlier west of Yreka 

along Soap Creek Ridge (app. I); and possibly early 

Coniacian in part of the Black Mountain area (Sliter 

and others, 1984). From these ages, the Osburger 

Gulch Sandstone Member is clearly time-transgressive from northwest to southeast.

Thus, although the Klamath River Conglomerate 

Member could possibly range in age from earliest 

Cretaceous (or even latest Jurassic, locally) to early 

Coniacian, it is most likely Albian, Cenomanian, and 

Turonian in age because it conformably underlies the 

basal marine strata of the Hornbrook Formation. 

There is no way to determine (because of the lack of 

age-diagnostic fossils within it) if the Klamath River 

Conglomerate Member is also time-transgressive or if 

it has significant variations areally in its age.

The Klamath River Conglomerate Member is 

probably correlative with marine sedimentary rocks 

of the Hudspeth and Gable Creek Formations in central Oregon near Mitchell (Wilkinson and Oles, 1968; 

Kleinhans and others, 1984), with the Bernard Formation (Dickinson and Vigrass, 1965) in the Suplee 

area, and with rocks in the subsurface Ochoco basin 

(Thompson and others, 1984). Because the oldest possible age of the Klamath River Conglomerate Member is so poorly constrained, it is possible that the 

member is partly correlative with Lower Cretaceous 

marine sedimentary rocks of the Days Creek Formation in the Days Creek and Riddle areas (Imlay and 

others, 1959; Popenoe and others, 1960). It could possibly partly correlate with Lower Cretaceous sedimentary rocks along the coast of southwestern 

Oregon, with the Humbug Mountain Conglomerate 

and the Rocky Point Formation of Koch (1966), and 

possibly with some other poorly dated Cretaceous 

units discussed by Dott (1971, p. 26). To the south, 

in California, it is correlative with parts of the Great 

Valley sequence on the west side of the Sacramento 

Valley (Ingersoll, 1979), the Budden Canyon Formation of Murphy and others (1969) at the northwestern 

edge of the Sacramento Valley, and possibly older 

parts of the Chico Formation on the east side of the 

Sacramento Valley.

OSBURGER GULCH SANDSTONE MEMBER

DEFINITION

The name "Osburger Gulch Sandstone Member" 

was applied by Nilsen (1984a) to continuous outcrops 

of marine sandstone strata in the lower part of the
24 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

Hornbrook Formation. The member was named for 

exposures in Osburger Gulch, located about 3.5 km 

south of Hornbrook, California (figs. 8, 11). The type 

section was measured near the mouth of and along 

the north flank of Osburger Gulch, which is located 

east of and across the Klamath River from the type 

section of the Klamath River Conglomerate Member 

(fig. 8).

The Osburger Gulch Sandstone Member is, in ascending stratigraphic order, the second lowest member of the Hornbrook Formation. It is equivalent to 

the upper part of member I (units 2-6) and the lower 

part of member II (unit 7) (of Hornbrook Formation) 

as mapped by Peck and others (1956) (fig. 6). The 

Osburger Gulch Sandstone Member is also equivalent 

to the lower part of unit A (of Hornbrook Formation) 

of Jones (1959), the upper part of the unnamed lower 

arenaceous member (of Hornbrook Formation) of 

Elliott (1971), the upper part of unit Khst (of 

Hornbrook Formation) of McKnight (1971), unit A (of 

Hornbrook Formation) of M.A. Elliott (unpub. data, 

1980), and units a and s (of Hornbrook Formation) as 

mapped by Nilsen and others (1983).

The Osburger Gulch Sandstone Member rests 

either conformably on the Klamath River Conglomerate Member of the Hornbrook Formation or unconformably on Paleozoic and Mesozoic igneous and 

metamorphic rocks where the Klamath River Conglomerate Member is absent. It is overlain conformably by the Ditch Creek Siltstone Member of the 

Hornbrook Formation.

The lower boundary of the Osburger Gulch Sandstone Member is defined either by the first fossiliferous beds of sandstone, siltstone, or conglomerate, or 

by the unconformable depositional contact on the 

basement rocks. Several meters of paleosol, thin residual breccia, or conglomeratic grus derived from 

weathering of the basement rocks may be present locally on the top of the basement rocks; the breccia 

and conglomeratic grus form part of the underlying 

Klamath River Conglomerate Member, but they are 

commonly too thin to map at a scale of 1:62,500. The 

lower boundary is generally marked by massive beds 

of trough cross-stratified, well-sorted, conglomeratic 

sandstone with rare molluscan megafossils and trace 

fossils. The massive beds underlie ridge crests, particularly in the Shasta Valley area.

The lower boundary of the Osburger Gulch Sandstone Member in its type area, however, is marked by 

beds of siltstone and very fine-grained sandstone that 

contain sparse molluscan megafossils and rest directly on the subaerial debris-flow and mudflow deposits of the Klamath River Conglomerate Member.

In most of the outcrop area of the Hornbrook Formation, this basal siltstone unit is absent.

The upper boundary of the Osburger Gulch Sandstone Member is marked by an abrupt to gradational 

change from dominantly resistant sandstone to less 

resistant siltstone of the Ditch Creek Siltstone Member. This contact, like the lower one, is typically 

marked by a major topographic bench. The upper 

boundary is arbitrarily placed at the top of the highest bed of well-sorted, resistant fine-grained sandstone.

AREAL EXTENT

The Osburger Gulch Sandstone Member extends 

continuously along the entire outcrop extent of the 

Hornbrook Formation as a ridge-forming unit (fig. 5; 

pi. 1). It appears to always be present in the lower 

part of the Hornbrook Formation. It is also present in 

each of the outliers of the Hornbrook Formation west 

of the main outcrop belt, such as those west of Jacksonville and west of Yreka.

The northernmost outcrops of the Osburger Gulch 

Sandstone Member are almost 3 km north-northwest 

of Jacksonville in sees. 19, 20, 29, 38, and 39, T. 37 

S., R. 2 W., Medford 15-minute quadrangle. Wells 

(1956), Beaulieu and Hughes (1977), and Smith and 

others (1982) show the Hornbrook Formation, which 

in this area probably consists of the Osburger Gulch 

Sandstone Member, extending farther to the north of 

Jacksonville. On the map of Beaulieu and Hughes 

(1977) of Jackson County, Oregon, it is shown as far 

north as sees. 61, 62, and 63, T. 36 S., R. 2 W., 

Medford 15-minute quadrangle, about 4.5 km south 

of the Rogue River. However, none of these maps 

show any strikes and dips measured from the 

Hornbrook Formation. Detailed examination of this 

area failed to reveal any outcrops or rubble of the 

Osburger Gulch Sandstone Member (Nilsen and others, 1983), although it probably does extend farther 

north in subsurface beneath the Bear Creek valley, 

resting on either basement rocks or on the Klamath 

River Conglomerate Member.

The southernmost outcrops of the Osburger Gulch 

Sandstone Member are located southeast and southwest of Yreka, at the southern limit of the Hornbrook 

Formation, in sees. 5 and 6, T. 44 N., R. 6 W., sec. 1, 

T. 44 N., R. 7 W., and sec. 1, T. 44 N., R. 8 W., Yreka 

15-minute quadrangle. The outcrops of the Osburger 

Gulch Sandstone Member are discontinuous in the 

Shasta Valley-Yreka area as a result of extensive 

faulting.
OSBURGER GULCH SANDSTONE MEMBER 25

FIGURE 11. Type section of the Osburger Gulch Sandstone Member of the Hornbrook Formation. A and B, View northeast 

across Klamath River of type section adjacent to Osburger 

Gulch. Mzftb, Paleozoic and Mesozoic basement complex; Khkr, 

Cretaceous Klamath River Conglomerate Member (of 

Hornbrook Formation); Khog, Cretaceous Osburger Gulch 

Sandstone Member (of Hornbrook Formation); Khdc, Cretaceous Ditch Creek Siltstone Member (of Hornbrook Formation); Khrg, Cretaceous Rocky Gulch Sandstone Member (of 

Hornbrook Formation); Ti, Tertiary intrusive rocks. Northbound lanes of Interstate Highway 5 in foreground. C and D, 

Trough cross-stratified sandstone, Osburger Gulch. E and F, 

Parallel-laminated and hummocky cross-stratified sandstone, 

Osburger Gulch. Note megafossils in carbonate concretions below hammer (F). Pen (in C and D) is 14 cm long; rock hammer 

(in E and F) is 35 cm long.
26 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

TYPE SECTION

The type section of the Osburger Gulch Sandstone Member of the Hornbrook Formation was measured along the northern margin of Osburger Gulch 

near its confluence with the Klamath River in SW/4 

sec. 33, T. 47 N., R. 6 W., Hornbrook 15-minute quadrangle. The section is about 116.5 m thick (fig. 12). It 

is almost completely exposed except for the basal siltstone and very fine grained platy sandstone unit at 

its base, which is covered, as is the basal contact 

with the Klamath River Conglomerate Member. However, the basal siltstone unit was measured at the 

type section of the Klamath River Conglomerate 

Member, and the thickness of that unit (15.5 m) is 

used in this type section.

Hornbrook Formation (lower part): 

Ditch Creek Siltstone Member (lower part): 

Siltstone, massive, bioturbated, poorly 

exposed ....................................................

Conformable contact.

Osburger Gulch Sandstone Member:

Sandstone, fine-grained, massive .........................

Sandstone, very fine grained, silty, thoroughly 

bioturbated...........................................................

Covered interval (float of sandstone, very fine 

grained, silty, bioturbated) ................................

Sandstone, fine-grained, laminated to trough 

cross-stratified.....................................................

Covered interval (float of sandstone, very fine 

grained, silty, bioturbated) ................................

Sandstone, fine-grained, laminated to massive ..

Sandstone, very fine grained, silty, bioturbated, 

partly covered......................................................

Sandstone, fine-grained, massive, thoroughly 

bioturbated; some molluscan fossils .................

Sandstone, fine- to medium-grained, massive, 

thoroughly bioturbated; some molluscan 

fossils....................................................................

Covered interval (float of sandstone, very fine 

grained, silty, bioturbated) ................................

Sandstone, fine-grained, massive; scattered 

molluscan fossils, locally bioturbated ...............

Sandstone, fine-grained, laminated......................

Sandstone, fine- to medium-grained, massive to 

trough cross-stratified, locally parallel-stratified and hummocky cross-stratified; scattered 

molluscan fossils; some pebbles at 

base; maximum clast size 2 cm .........................

Conglomerate and conglomeratic sandstone; 

some lenses of sandstone, medium- to coarsegrained; maximum clast size 7 cm; abundant 

broken molluscan shells .....................................

Sandstone, fine-grained; wavy stratification .......

Conglomerate; maximum clast size 2 cm; scattered molluscan fossils; thin lenses of 

sandstone .............................................................

Thickness 

(in meters)

5.00

1.00 

.70 

.50 

.70

.70 

1.10

1.60

1.20

.60 

.50

2.90 

.30

4.50

1.05 

.25

.60

Thickness 

(in meters)

Osburger Gulch Sandstone Member Continued: 

Sandstone, fine-grained, silty; low-angle trough

cross-strata and parallel stratification; some

scattered lenses with molluscan fossils,

bioturbated, concretionary................................. 6.90

Sandstone, fine- to medium-grained, massive;

scattered molluscan fossils, concretionary....... .70

Sandstone, fine- to medium-grained; low-angle

trough cross-strata.............................................. 1.00

Sandstone, fine- to medium-grained; scattered

molluscan fossils, concretionary....................... .25

Sandstone, fine- to medium-grained; trough

cross-strata and crude parallel-strata,

bioturbated........................................................... 1.50

Sandstone, fine- to medium-grained; scattered

molluscan fossils, concretionary........................ .22

Sandstone, fine- to medium-grained; trough

cross-strata, bioturbated .................................... 2.60

Sandstone, fine-grained; trough cross-strata and

parallel-strata; hummocky cross-strata at

top ......................................................................... 1.00

Sandstone, very fine grained to fine-grained,

thoroughly bioturbated....................................... .80

Sandstone, fine- to medium-grained, massive at

base to parallel-stratified at top; erosive

base....................................................................... .60

Sandstone, fine-grained; hummocky

cross-strata .......................................................... .50

Conglomerate; maximum clast size 8 cm;

scattered molluscan fossils ................................ .10

Sandstone, fine-grained, bioturbated ................... .10

Sandstone, fine- to medium-grained; scattered

pebbles; maximum clast size 3 cm; hummocky

cross-strata .......................................................... 1.50

Sandstone, medium- to coarse-grained, conglomeratic; maximum clast size 3 cm; abundant

molluscan fossils; erosive base .......................... .60

Sandstone, fine-grained, thoroughly

bioturbated........................................................... .10

Sandstone, fine- to medium-grained; hummocky

cross-strata .......................................................... .60

Sandstone, fine-grained, thoroughly

bioturbated........................................................... .40

Sandstone, fine- to medium-grained; low-angle

trough cross-strata at base and hummocky

cross-strata at top ............................................... 1.20

Sandstone, fine-grained, thoroughly

bioturbated........................................................... 1.30

Sandstone, fine- to medium-grained, parallelstratified and hummocky cross-stratified ........ 2.40

Sandstone, very fine grained, thoroughly

bioturbated........................................................... 1.00

Sandstone, fine- to medium-grained, conglomeratic; maximum clast size 6 cm; parallel-strata

and hummocky cross-strata; scattered plant

fossils.................................................................... 1.50

Conglomerate; maximum clast size 13 cm;

lens-shaped .......................................................... .30

Sandstone, very fine grained to fine-grained;

trough cross-strata at base; parallel-strata

and hummocky cross-strata at top,

bioturbated........................................................... 1.10

Conglomerate; maximum clast size 10 cm .......... .15
OSBURGER GULCH SANDSTONE MEMBER 27

Thickness 

(in meters)

Osburger Gulch Sandstone Member Continued: 

Sandstone, fine-grained; trough cross-strata in

lower part; parallel-strata and hummocky

cross-strata in upper part.................................. 2.00

Conglomerate; maximum clast size 10 cm .......... .10

Sandstone, fine-grained; parallel-strata and

hummocky cross-strata....................................... .50

Conglomerate; maximum clast size 5 cm;

scattered molluscan fossils ................................ .07

Sandstone, fine-grained; parallel-strata and

hummocky cross-strata....................................... 1.00

Conglomerate; maximum clast size 10 cm;

erosive base.......................................................... .23

Sandstone, very fine grained to fine-grained;

parallel-strata and hummocky cross-strata..... .40

Sandstone, fine- to medium-grained, massive .... 1.40

Conglomerate; maximum clast size 15 cm;

channeled base .................................................... 1.40

Sandstone, medium- to coarse-grained,

lens-shaped.......................................................... .10

Conglomerate; maximum clast size 32 cm;

channeled base .................................................... .65

Sandstone, coarse- to very coarse grained; scattered pebbles; maximum clast size 5 cm;

trough and tabular cross-strata; channeled

base....................................................................... 2.40

Sandstone, fine-grained, thoroughly

bioturbated........................................................... .50

Sandstone, fine-grained; scattered granules and

pebbles; maximum clast size 0.5 cm; parallel

strata and hummocky cross-strata ................... .80

Sandstone, fine-grained; scattered pebbles; maximum clast size 6 cm; massive to lowangle cross-strata, bioturbated.......................... .50

Conglomerate; maximum clast size 6 cm ............ .20

Conglomeratic sandstone; maximum clast size

3 cm; trough cross-strata; erosive base ............ 2.80

Sandstone, medium- to coarse-grained at base,

fine-grained at top; scattered pebbles in lower

part; maximum clast size 2 cm; massive at

base to trough cross-stratified at top ............... .45

Conglomeratic sandstone; maximum clast size 10

cm; massive; molluscan fossil debris at

base....................................................................... .40

Sandstone, medium- to coarse-grained; scattered

pebbles; maximum clast size 2 cm;

massive to trough cross-stratified..................... 1.20

Conglomerate; maximum clast size 4 cm ............ .03

Sandstone, fine-grained to very coarse

grained; massive, bioturbated ........................... .25

Conglomeratic sandstone; maximum clast size

7 cm; massive ...................................................... .80

Conglomerate; maximum clast size 7 cm;

massive................................................................. .30

Conglomerate; maximum clast size 1 cm;

massive................................................................. .12

Sandstone, medium-grained, massive .................. .10

Conglomerate; maximum clast size 7 cm;

massive................................................................. .65

Sandstone, fine-grained; scattered pebbles;

maximum clast size 5 cm; parallelstrata and trough cross-strata,

bioturbated........................................................... .90

Thickness 

(in meters)

Osburger Gulch Sandstone Member Continued: 

Conglomerate; maximum clast size 3 cm; 

massive; erosive base.......................................... .03

Sandstone, very coarse grained; low-angle 

trough cross-strata.............................................. .03

Conglomerate; maximum clast size 2 cm;

reverse graded..................................................... .12

Sandstone, very fine grained to fine-grained, 

thoroughly bioturbated....................................... .45

Conglomeratic sandstone; maximum clast size

3 cm; trough cross-strata ................................... .60

Sandstone, fine- to medium-grained; scattered

pebbles; maximum clast size 1 cm; massive at

base; tabular cross-strata at top ....................... 1.55

Conglomeratic sandstone; maximum clast size

0.5 cm; massive, bioturbated ............................. .95

Conglomerate; maximum clast size 3 cm;

massive................................................................. .14

Sandstone, medium-grained, massive .................. .05

Conglomerate; maximum clast size 2 cm;

massive................................................................. .20

Sandstone, medium-grained, massive .................. .05

Conglomerate; maximum clast size 6 cm;

massive................................................................. .10

Sandstone, very coarse grained, massive ............ .12

Conglomerate; maximum clast size 9 cm;

massive................................................................. .15

Sandstone, very coarse grained, low-angle

cross-strata .......................................................... .20

Conglomerate; maximum clast size 3 cm;

massive................................................................. .03

Conglomeratic sandstone; maximum clast size

4 cm; massive ...................................................... .40

Conglomeratic sandstone; maximum clast size

0.5 cm; trough cross-strata ................................ .10

Conglomerate; maximum clast size 8 cm; 

massive................................................................. .50

Granule conglomerate; tabular cross-strata; 

lens-shaped .......................................................... .08

Conglomerate; maximum clast size 7 cm;

massive................................................................. .14

Granule conglomerate; tabular cross-strata; 

lens-shaped.......................................................... .12

Conglomerate; maximum clast size 5 cm; 

massive................................................................. .05

Sandstone, medium- to coarse-grained; smallscale trough cross-strata .................................... .55

Sandstone, fine-grained, massive, bioturbated ... .05

Sandstone, medium-grained; lens of conglomerate near top with maximum clast size 2 cm; 

small-scale trough cross-strata ......................... 3.20

Sandstone, medium- to coarse-grained; smallscale trough cross-strata; mudstone rip-up 

clasts..................................................................... 2.55

Conglomerate; maximum clast size 7 cm;

massive................................................................. .40

Sandstone, fine-grained, massive ......................... .23

Covered interval (float of sandstone) ................... 9.00

Sandstone, fine-grained, massive ......................... .60

Sandstone, very fine grained to fine-grained, 

massive, bioturbated........................................... .10

Sandstone, fine- to medium-grained, massive to 

parallel-stratified................................................ .80
28 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

Thickness 

(in meters)

Osburger Gulch Sandstone Member Continued: 

Sandstone, very fine grained to fine-grained,

massive, bioturbated........................................... .70

Sandstone, fine- to medium-grained, massive to

parallel-stratified; scattered fragments of

plant fossils.......................................................... .80

Sandstone, fine-grained, massive, bioturbated ... .08

Sandstone, medium-grained, massive to

parallel-stratified ................................................ .18

Sandstone, fine- to medium-grained, massive;

molluscan fossils, bioturbated ........................... .35

Sandstone, medium-grained, massive; scattered

fragments of plant fossils................................... .18

Sandstone, very fine grained to fine-grained;

abundant molluscan fossils in growth

positions............................................................... 3.00

Covered interval (float of sandstone) ................... 1.20

Sandstone, medium- to coarse-grained; trough

cross-stratified..................................................... 1.00

Covered interval (float of sandstone) ................... 1.00

Conglomeratic sandstone, coarse-grained; maximum clast size 3 cm; trough cross-stratified... 2.00 

Covered interval (float of siltstone of lower part

of Osburger Gulch Sandstone Member)........... 15.50

Total thickness of the Osburger Gulch 

Sandstone Member ...................................... 116.50

Conformable contact. 

Klamath River Conglomerate Member: 

Covered interval (float of conglomerate and 

pebbly mudstone)................................................ 8.00

Conglomerate; maximum clast size 6 cm; matrix 

supported; mudstone matrix; clasts subangular to subrounded; reddish weathering ............ 4.50

Unconformable contact. 

Covered interval (float of basement complex)..... 11.00

Applegate Group (basement complex): 

Metasedimentary and metavolcanic rocks of 

Triassic age

The color of sandstone from the Osburger Gulch 

Sandstone Member is generally medium gray (N5) on 

fresh, unweathered surfaces, and moderate yellowish 

brown (10YR 5/4) on weathered surfaces. The 

interbedded conglomerate may have a darker color 

because of its abundant dark clasts.

LITHOLOGY

The Osburger Gulch Sandstone Member consists 

mostly of sandstone, but it also includes some conglomerate, siltstone, and shale. In the type section, it 

consists almost wholly of sandstone, conglomeratic 

sandstone, and conglomerate, except for the poorly 

exposed basal siltstone unit that is about 15.5 m 

thick. Molluscan fossils are very common locally, both

in growth position and as transported clasts. Plant 

fossil fragments are present locally on some bedding 

surfaces. Bioturbation is common and generally increases in abundance upward within the member; in 

the upper part of the member, beds of fine-grained 

sandstone are locally so thoroughly bioturbated that 

no remnants of any original stratification remain.

Conglomeratic clasts in the type section of the 

Osburger Gulch Sandstone Member consist principally of a mixture of metavolcanic and 

metasedimentary rocks derived from the underlying 

Applegate Group (56 percent), and quartzite and 

chert (41 percent). A small number of granitic or dioritic clasts is also present (table 2). Along the 

southbound lane of Interstate Highway 5, about 0.5 

km west of the type section, conglomerate clasts in 

the Osburger Gulch Sandstone Member consist of 

metavolcanic and metasedimentary rocks from the 

underlying Applegate Group (63 percent), quartzite 

and chert (33 percent), and granitic or dioritic clasts 

(4 percent). Conglomerate clasts in the Osburger 

Gulch Sandstone Member to the north and south of 

the type area are more variable in composition but 

generally consist of metavolcanic or metasedimentary 

rocks, quartzite, chert, and granitic or dioritic clasts.

The basal part of the Osburger Gulch Sandstone 

Member in its type area consists of about 15.5 m of 

nonresistant siltstone that is best exposed along the 

northbound lane of Interstate Highway 5, about 0.5 

km west of the type section. This siltstone contains 

scattered but locally abundant molluscan fossils and 

abundant plant fossil fragments. The siltstone is micaceous and carbonaceous, commonly with platy or 

scaly structure. The siltstone is generally thoroughly 

bioturbated, but locally it is laminated and ripple 

laminated. Very fine grained sandstone is present as 

both laminae and as grains scattered in the siltstone. 

This basal siltstone unit is present only in the type 

area and, in particular, in the type section; it is apparently a thin lens that pinches out within a few 

kilometers to the northwest and southeast.

Overlying the basal siltstone in the type area is a 

very resistant sandstone unit that forms rugged 

sandy bluffs. This sandstone unit is about 5 m thick 

in the type section, although much of it is covered 

there, and is as much as 50 m thick in other areas. 

This sandstone unit consists chiefly of thick beds of 

large-scale trough cross-stratified, conglomeratic, medium-grained to very coarse grained sandstone. Conglomerate is generally randomly distributed in the 

sandstone or is concentrated as thin lenses in the 

axes of trough cross-strata. This resistant sandstone 

unit was mapped separately by Nilsen and others 

(1983) as unit s (of Hornbrook Formation), which
OSBURGER GULCH SANDSTONE MEMBER 29

40 -

30 -

- 20 -

10-

=^^v

(Float of 

siltstone)

0--- _ _

(Float of conglomerate

and 

pebbly mudstone)

Klamath River

Conglomerate

 Member

(of Hornbrook

Formation)

(Float derived

from

basement 

complex)

\~"y /) Basement 

~ \~^j """complex

Base of Ditch Creek Siltstone 

Member (of Hornbrook Formation)

110

100

90-

70 -

60 -

A 

= A

GRAIN SIZE

vc 32 50 

sh ss mm mm

HCS

HCS

HCS

HCS

HCS

FIGURE 12. Type section of the Osburger Gulch Sandstone Member of the Hornbrook Formation. See figure 10 for 

explanation of symbols. Abbreviations: sh, shale; vc, very coarse grained; ss, sandstone. Gaps in lithology represent covered intervals.
30 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

TABLE 2. Composition of conglomerate clasts in the type section of 

the Osburger Gulch Sandstone Member and along the 

southbound lane of Interstate Highway 5, 0.5 km west of the 

type section

[Type section located in SE^NE% sec. 32, T. 47 N., R. 6 W., Hornbrook 15- 

minute quadrangle. Data are from Barats and others (1984, table 1) 

and are based on two random samples of 100 clasts larger than 2 cm 

in longest dimension]

Type 

section

Along 

Interstate 5

Metavolcanic rocks 

Metasedimentary rocks 

Metamorphic rocks (unspecified) 

Quartzite - 

Chert (including jasper) 

Granite-diorite 

55

1

0

39

2

3

52

4

7

19

14

4

Total 100 100

forms a mappable unit in the southern part of the 

Cottonwood Creek valley area, in the Shasta ValleyYreka area, and in the Soap Creek Ridge outlier west 

of Yreka. Most of the prominent bluffs east of Yreka 

consist of this unit. This sandstone is marine because 

it locally contains scattered molluscan fossils. In this 

report, unit s is incorporated within the lowermost 

part of the Osburger Gulch Sandstone Member. In 

the northwestern Cottonwood Creek valley area, the 

Bear Creek valley area, and the Jacksonville area, 

this basal unit could not be differentiated at the mapping scale of 1:62,500, but it is commonly present in 

these areas as a thin lithologic unit that is less resistant than it is in the southern Cottonwood Creek valley and adjacent areas.

The middle part of the Osburger Gulch Sandstone 

Member in the type section consists generally of 

sandstone that is finer grained than that in the lower 

part and that contains some thin beds of conglomerate. This middle part is about 45 m thick in the type 

section and is as much as 60 m thick in other areas. 

The sandstone is parallel-stratified, trough crossstratified, or massive, and it is commonly bioturbated 

to some extent. The conglomeratic interbeds form layers that are more laterally continuous than in the 

lower part, commonly with erosive bases. Molluscan 

fossils are both scattered within the sandstone or 

concentrated as clasts within some of the conglomeratic beds.

The upper part of the Osburger Gulch Sandstone 

Member in the type section is generally finer grained 

than the lower and middle parts and is characterized 

by more abundant hummocky cross-strata and bioturbation. The upper part is about 51 m thick in the

type section and is as much as 100 m thick in other 

areas. It contains some thick conglomerate interbeds 

and scattered molluscan fossils.

Over its entire outcrop region, the Osburger 

Gulch Sandstone Member varies considerably in lithology, although it is almost always predominantly 

composed of sandstone and contains at least some 

molluscan fossils. Locally, it is very conglomeratic 

and in some places displays at its base large rounded 

boulders of basement rocks in a matrix of sandstone 

containing molluscan fossils. Such marine boulder 

conglomerate is common at the base of the member 

in the Jacksonville area, for instance, where it has 

been mined for gold. At a mine about 5 km southwest 

of Jacksonville (NE corner of sec. 11, T. 38 S., R. 3 

W., Gold Hill 15-minute quadrangle), boulders as 

long as 200 cm that are subrounded to well rounded 

rest directly on red, fine-grained paleosol formed on 

the underlying basement complex. Within the sandstone matrix of the marine boulder conglomerate are 

large gastropods and other molluscan fossils. More 

typical fine- to coarse-grained sandstone that is massive to parallel-stratified and bioturbated overlies the 

boulder conglomerate. In much of the Bear Creek valley area, the Osburger Gulch Sandstone Member is 

generally less than 50 m thick and forms a thin veneer of coarse sediment that lies directly on the 

basement complex because the Klamath River Conglomerate Member is absent. Near the town of 

Ashland (NWK sec. 5, T. 39 S., R. 1 E., Ashland 15- 

minute quadrangle), the basal part of the Osburger 

Gulch Sandstone Member consists of a well-sorted coquina of broken shell fragments, generally 0.5-1.0 cm 

long, mixed with terrigenous detritus. This coquina is 

several meters thick and is massive to crudely 

parallel-stratified.

Thus, it can be seen that the lithology of the 

basal part of the Osburger Gulch Sandstone Member 

is variable. It ranges from siltstone in the type area 

to boulder conglomerate, coquina, large-scale trough 

cross-stratified conglomeratic sandstone, and massive 

to parallel-stratified sandstone or conglomeratic 

sandstone in other areas. However, the major part of 

the member consists of medium- to thick-bedded marine sandstone, with subordinate amounts of conglomerate and siltstone.

STRATIGRAPHIC RELATIONS

Where the Klamath River Conglomerate Member 

is absent, the Osburger Gulch Sandstone Member 

rests either unconformably on the pre-Upper Cretaceous basement rocks or on a paleosol developed on
OSBURGER GULCH SANDSTONE MEMBER 31

those rocks; locally, it rests nonconformably on Jurassic plutonic rocks. Where the Klamath River Conglomerate Member is present, the Osburger Gulch 

Sandstone Member rests with apparent conformity on 

it. The contact between the Klamath River Conglomerate Member and the Osburger Gulch Sandstone 

Member in the type area is marked by the following 

upward changes: (1) Change from brown, red, and 

reddish-brown colors on weathered surfaces to a moderately yellowish-brown color; (2) change in conglomerate composition from clasts that are almost wholly 

derived from the underlying basement to a more 

polymict composition; (3) change in conglomerate 

clast texture from angular and subrounded to well 

rounded; (4) change in the conglomerate fabric from 

matrix-supported and poorly oriented clasts to clastsupported and well-oriented clasts; (5) change in fossil content from an absence of fossils or (locally) some 

broken plant fossils to molluscan fossils; (6) change in 

lithology from dominantly conglomerate to dominantly sandstone; (7) change in stratification from 

massive and very poorly stratified to well stratified, 

marked by cross-strata, parallel-strata, and hummocky strata; (8) change from no bioturbation to 

abundant bioturbated intervals; (9) change from 

poorly sorted sandstone to well-sorted sandstone; and 

(10) change from compositionally immature sedimentary rock to compositionally mature sedimentary 

rock. Most of these changes reflect the differences in 

depositional process and depositional environment 

between the two members.

In other areas, the contact between the Klamath 

River Conglomerate Member and the Osburger Gulch 

Sandstone Member may be more difficult to ascertain, particularly where the former is very thin. However, some of the criteria listed above are generally 

applicable in most areas, such as differences in color, 

dominant lithology, texture and composition, fossil 

content, and types of stratification. The nonmarine 

Klamath River Conglomerate Member is locally organized into fining-upward cycles of conglomerate, 

sandstone, and mudstone that reflect fluvial conditions of sedimentation; these cycles are not present in 

the Osburger Gulch Sandstone Member. Mudstone 

deposits in the Klamath River Conglomerate Member 

locally contain paleosols and root structures of plants 

in growth position, whereas no paleosols and only 

transported fossil plant fragments are found in the 

Osburger Gulch Sandstone Member.

The upper contact between the Osburger Gulch 

Sandstone Member and the Ditch Creek Siltstone 

Member in the type area is gradational and marked 

by the gradual upward change from well-stratified 

and well-sorted very fine grained to medium-grained

sandstone with some conglomerate below to poorly 

stratified and poorly sorted siltstone and silty very 

fine grained sandstone above. The contact is generally placed at the lithologic change from predominantly sandstone to predominantly siltstone or silty 

very fine grained sandstone. The Ditch Creek Siltstone Member is generally much less resistant than 

the Osburger Gulch Sandstone Member, so the contact between them is commonly marked by a topographic bench or step and lack of good exposures 

above the bench. The Ditch Creek Siltstone Member 

is commonly darker on fresh and weathered surfaces 

than is the Osburger Gulch Sandstone Member, a 

difference that reflects its greater content of clay 

minerals and textural immaturity. The Ditch Creek 

Siltstone Member is also more thoroughly bioturbated and locally contains abundant calcareous 

concretions that may follow bedding surfaces or form 

subspherical bodies.

Locally, however, the contact between the 

Osburger Gulch Sandstone Member and the Ditch 

Creek Siltstone Member is so gradational that it is 

very difficult to determine in a measured section. In 

such areas, I place the boundary at the highest bed of 

fine-grained sandstone.

Lateral contacts and stratigraphic relations of the 

Osburger Gulch Sandstone Member cannot be observed in outcrop. It is present in all outcrop areas as 

a laterally continuous thin sheet of marine sandstone 

at or near the base of the Hornbrook Formation. Initially, it was probably deposited in all directions from 

the present outcrops and is undoubtedly present in 

the subsurface to the northeast.

THICKNESS

The Osburger Gulch Sandstone Member is generally 75-150 m thick but can be either thinner or 

thicker locally; in one area near Ashland, it appears 

to be completely missing. At its type section, south of 

Hornbrook, it is 116.5 m thick. The equivalent unit of 

Peck and others (1956), measured west of Hornbrook, 

which includes the upper part of their member I and 

the lower part of their member II (of Hornbrook Formation), is about 132 m thick (fig. 6). Jones (1959) 

measured a thickness of 213-243 m for his unit A (of 

Hornbrook Formation), which includes, however, the 

Klamath River Conglomerate Member, the Osburger 

Gulch Sandstone Member, and the Ditch Creek Siltstone Member. A thickness of 247 m, which includes 

the thickness of the underlying Klamath River Conglomerate Member, was determined for the lower 

arenaceous member of the Hornbrook Formation of
32 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

Elliott (1971) and for unit A (of Hornbrook Formation) of M.A. Elliott (unpub. data, 1980), both of 

which are partly equivalent to the Osburger Gulch 

Sandstone Member (fig. 6), at the north end of the 

Cottonwood Creek valley area. In the Bear Creek valley area, McRnight (1971) determined a thickness of 

110 m for deposits equivalent to both the Osburger 

Gulch Sandstone Member and the underlying Klamath River Conglomerate Member. Bourgeois and 

Leithold (1983) and Bourgeois (1984) cited a measured thickness of 105-110 m for the equivalent of 

the Osburger Gulch Sandstone Member in roadcuts 

along the north- and southbound lanes of Interstate 

Highway 5 south of Hornbrook; this section, from 

which I have subtracted the thickness of the Klamath 

River Conglomerate Member shown on their figure 8, 

is located directly west of the type section. Bourgeois 

and Leithold (1983) and Bourgeois (1984) also measured a thickness of what appears to be about 95 m 

for the Osburger Gulch Sandstone Member near the 

Siskiyou Summit between Hilt and Ashland. I subsequently measured this section, exposed in roadcuts 

along the west side of the southbound lane of Interstate Highway 5 (NWA 1 sec. 21, T. 40 S., R. 2 E., 

Ashland 15-minute quadrangle) and obtained a thickness of 127.5 m.

The Osburger Gulch Sandstone Member appears 

to be thinnest in parts of the Bear Creek valley area, 

particularly in the areas 0-5 km northwest of and 

10-15 km southeast of downtown Ashland. In the 

area about 5 km northwest of Ashland (southeast corner sec. 31, T. 38 S., R. 1 E., Ashland 15-minute 

quadrangle), the Osburger Gulch Sandstone Member 

is locally missing, and siltstone of the Ditch Creek 

Siltstone Member rests directly on basement rocks. 

The Osburger Gulch Sandstone Member is also relatively thin along the southwest flank of Black Mountain, between the Cottonwood Creek valley area and 

the Shasta Valley area.

The Osburger Gulch Sandstone Member appears to 

be thickest at the northwest end of the Cottonwood 

Creek valley area. It is also relatively thick in the Shasta 

Valley area, where it forms a widespread cover.

In summary, the Osburger Gulch Sandstone 

Member alternately thickens and thins in a northwest-southeast direction, parallel to the outcrop belt. 

The member has an observed minimum thickness of 

0 m and a maximum thickness of possibly 250 m; it 

probably averages about 100 m in thickness.

DEPOSITIONAL ENVIRONMENT

Sedimentary structures, textures, trace fossils, 

and molluscan fossils suggest that the Osburger

Gulch Sandstone Member was deposited on a marine 

shelf. It overlies either basement rocks unconformably or nonmarine strata conformably and thus 

records transgression of the late Early(?) and early 

Late Cretaceous sea over the Klamath Mountains 

area. The conformably overlying Ditch Creek Siltstone Member appears to have been deposited generally in somewhat deeper, or at least lower energy, 

marine conditions, evidence supporting the idea that 

the Osburger Gulch Sandstone Member represents 

the basal deposits of a transgressive sea.

The general vertical trends in measured sections 

of the Osburger Gulch Sandstone Member suggest an 

upward change generally from very high energy, 

nearshore conditions to lower energy, offshore conditions. The lower part of the Osburger Gulch Sandstone Member characteristically consists of mediumto large-scale trough cross-stratified conglomeratic 

sandstone (unit s of Nilsen and others, 1983). This 

unit contains sparse trace fossils, scattered, highly 

abraded shell debris, and abundant conglomerate, 

and it consists of mostly medium-grained to very 

coarse grained sandstone. This lower part suggests 

deposition in high-energy nearshore conditions. The 

local presence of coquinoid conglomerate, planar stratification with heavy mineral concentrations, and marine 

boulder conglomerate supports this suggestion.

In its type area, the fossiliferous siltstone unit at 

the base of the Osburger Gulch Sandstone Member 

represents a different depositional environment. The 

siltstone contains molluscan megafossils in growth 

position, abundant plant debris locally concentrated 

into coaly laminae, abundant and variable bioturbation, and abundant ripple cross-lamination and 

disrupted planar laminae. These deposits probably 

accumulated in lagoonal conditions along a part of 

the transgressive Cretaceous shoreline that had a 

more complex pattern.

The middle and upper parts of the Osburger 

Gulch Sandstone Member are characterized generally 

by sandstone that is finer grained than that of the 

lower part, hummocky cross-stratification, planar 

stratification, erosion surfaces, amalgamation of 

sandstone beds, abundant bioturbation, molluscan 

fossils in growth position, sheets of pebble to cobble 

conglomerate locally containing abundant molluscan 

shell fragments, and (toward the top of the member) 

some interbeds of siltstone. Trough cross-strata are 

locally present in the middle and upper parts of the 

member, but they are generally of much smaller amplitude, are developed in finer grained sandstone, and 

are less abundant than in the lower part of the member.

The middle and upper parts of the Osburger 

Gulch Sandstone Member appear to have most of the
OSBURGER GULCH SANDSTONE MEMBER 33

characteristics of deposits on wave-dominated 

shelves. The abundant hummocky cross-stratified 

units probably reflect wave reworking of the bottom, 

with bioturbated very fine grained to fine-grained 

sandstone and siltstone deposited during periods of 

low wave activity. Conglomeratic sheets, both those 

containing lithic pebbles and those containing shell 

debris, were probably transported offshore by stormgenerated currents.

The molluscan fauna of the Hornbrook Formation 

is also generally indicative of high-energy shallowmarine conditions. Oysters, Turritellas, and other 

gastropods and pelecypods generally prefer well-agitated, sandy, or pebbly substrates. Trace fossils include Ophiomorpha and other deposit-feeding forms 

suggestive generally of shallow-marine conditions.

Bourgeois and Leithold (1983) and Bourgeois 

(1984) concluded that the two sections of the 

Osburger Gulch Sandstone Member that they measured were typical of storm- and wave-dominated 

shelf sedimentation. Gaona (1984) inferred deposition 

in lagoonal, nearshore, and offshore environments. 

Elliott (1971) suggested shallow-marine to nearshore 

deposition for all of the Hornbrook Formation, including the strata herein assigned to the Osburger Gulch 

Sandstone Member. McKnight (1971) concluded that 

his unit Kh^ in the Bear Creek valley area, which is 

equivalent, in part, to the Osburger Gulch Sandstone 

Member, was deposited in a neritic environment 

characterized by low- to high-energy conditions.

AGE AND CORRELATION

The age of the Osburger Gulch Sandstone Member has been determined in most areas from megafossil collections. Megafossils reported herein from at 

or near its base at three localities in the Hornbrook 

and Yreka areas (app. I) are indicative of middle 

Turonian age.

Peck and others (1956) summarized age data for 

the lower part of the Hornbrook Formation from 

megafossil collections by previous workers. They concluded that except for one well-preserved ammonite 

and one fragment of an ammonite from the Jacksonville area, all previously reported megafossils from 

the basal part of the Hornbrook Formation are suggestive of Turonian age. A well-preserved specimen of 

the ammonite Mantelliceras, obtained from a quarry 

about 1.5 km northwest of Jacksonville (SW/4 sec. 29, 

T. 37 S., R. 2 W., Medford 15-minute quadrangle) indicated to Peck and others (1956) a Cenomanian age 

for the lower part of the Osburger Gulch Sandstone 

Member in this area. An ammonite fragment from

the Dark Hollow area (sec. 13, T. 38 S., R. 2 W., 

Medford 15-minute quadrangle) southeast of Jacksonville, identified by Peck and others (1956) as 

Anagaudryceras sacya (Forbes), generally ranges in 

age from Albian to Cenomanian. Peck and others 

(1956) inferred the age of strata equivalent to the uppermost part of the Osburger Gulch Sandstone Member in the southern Cottonwood Creek valley area to 

be late Turonian.

Jones (1959) indicated a late Turonian and early 

Coniacian age for strata equivalent to the Osburger 

Gulch Sandstone Member based on ammonite collections from the unit in the Hornbrook-Yreka area. 

D.L. Jones (in Popenoe and others, 1960) concluded 

that no early or middle Turonian megafossils are 

present in the Yreka-Hornbrook area and that the 

lower part of the Hornbrook Formation is of late 

Turonian and early Coniacian age in that area.

For the northern part of the outcrop area of the 

Hornbrook Formation, D.L. Jones (in Popenoe and 

others, 1960) concluded that there are no megafossils 

in the lower part of the Hornbrook Formation south 

of the Dark Hollow area that were older than 

Turonian. McKnight (1971, p. 11) reported several 

fossil localities from the lower part of the Hornbrook 

Formation in the Ashland area; identifications by D.L. 

Jones indicated Turonian or late Turonian ages from ammonites and molluscan fossils in these collections.

Jones (1960b) found the basal marine Cretaceous 

strata of the Grave Creek area to be of Albian age. 

Sliter and others (1984, fig. 2), on the basis of all previous megafossil age determinations, concluded that 

the age of the Osburger Gulch Sandstone Member 

and strata correlative with it ranges (from northwest 

to southeast) from middle Albian in the Grave Creek 

area to late Albian(?) to middle Cenomanian in the 

Dark Hollow area, early to middle Turonian in the 

Ashland area, middle Turonian to early Coniacian in 

the Hornbrook area, to early Coniacian in the Black 

Mountain area. Thus, the age of the Osburger Gulch 

Sandstone Member within the main outcrop area of 

the Hornbrook Formation, excluding the Grave Creek 

area, is here considered to be late Albian(?) to early 

Coniacian.

The Osburger Gulch Sandstone Member is thus 

for the most part correlative with strata of Albian to 

Coniacian age in surrounding areas. The upper parts 

of Albian and possibly Cenomanian marine sedimentary strata of the Hudspeth and Gable Creek Formations that crop out near Mitchell, Oregon (Wilkinson 

and Oles, 1968; Kleinhans and others, 1984), the Bernard Formation in the Suplee area, and subsurface 

marine strata in the adjacent Ochoco basin (Thompson and others, 1984) probably correlate with the
34 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

Osburger Gulch Sandstone Member. Cretaceous 

strata of the Days Creek Formation at Days Creek 

and Cretaceous strata at O'Brien, Oregon, are 

probably mostly older than the Osburger Gulch 

Sandstone Member. Cretaceous strata along the 

southwestern coast of Oregon also appear to be either 

older or younger than the Osburger Gulch Sandstone 

Member (Dott, 1971).

To the south, in the northern Sacramento Valley 

area, the Osburger Gulch Sandstone Member is probably correlative with the lower part of Turonian to 

Senonian strata of the Great Valley sequence that 

crop out near Redding, California (Popenoe, 1943; 

Matsumoto, 1960). It is also partly correlative with 

the upper members (the Bald Hills Member and Gas 

Point Member) of the Budden Canyon Formation of 

Murphy and others (1969), which crop out about 30 

km southwest of Redding. The Bald Hills Member 

consists chiefly of beds and lenses of sandstone, conglomerate, and pebbly mudstone in mudstone and 

siltstone; it is about 570 m thick and is late Albian 

through Cenomanian and possibly early Turonian in 

age (Murphy and Rodda, 1960). The Gas Point Member consists chiefly of siltstone and mudstone with 

some interbeds of sandstone; it is about 1,150 m 

thick and ranges from Cenomanian to late Turonian 

or possibly early Coniacian in age (Murphy and others, 1969, p. 23).

The Osburger Gulch Sandstone Member appears 

to be older than most of the Chico Formation to the 

south in the Chico area of the Sacramento Valley 

(Haggart and Ward, 1984). It does correlate, however, 

with much of the middle part of the Great Valley sequence on the west side of the Sacramento Valley. It 

correlates with strata assigned to the Boxer and 

Cortina Formations of Dickinson and Rich (1972), 

which include the previously defined Venado and 

Yolo Formations of Bowen (1962) of the subsurface 

part of the Sacramento Valley. The Osburger Gulch 

Sandstone Member is also correlative with parts of 

the Franciscan assemblage, chiefly the Coastal belt 

terrane, which includes strata of Late Cretaceous and 

early Tertiary age.

DITCH CREEK SILTSTONE MEMBER

DEFINITION

The name "Ditch Creek Siltstone Member" was 

applied by Nilsen (1984a) to almost continuous outcrops of dominantly marine strata in the lower part 

of the Hornbrook Formation that are predominantly 

composed of siltstone. The member was named for exposures adjacent to Ditch Creek, about 3.2 km west 

of Hornbrook, Calif. The type section, however, was 

measured in roadcuts along the northbound lane of 

Interstate Highway 5 adjacent to and west of the 

Klamath River, about 3 km south of the town of 

Hornbrook (figs. 8, 13).

The Ditch Creek Siltstone Member is, in ascending stratigraphic order, the third (or middle) member 

of the Hornbrook Formation. It is equivalent to the 

upper part of member II (units 8 and 9) (of Hornbrook Formation) as mapped by Peck and others 

(1956) (fig. 6). The Ditch Creek Siltstone Member is 

also equivalent to the finer grained upper part of unit 

A (of Hornbrook Formation) of Jones (1959), unit 

Khs2 (of Hornbrook Formation) of McKnight (1971), 

unit B (of Hornbrook Formation) of M.A. Elliott 

(unpub. data, 1980), and unit b (of Hornbrook Formation) as mapped by Nilsen and others (1983).

The Ditch Creek Siltstone Member rests conformably on the Osburger Gulch Sandstone Member. I believe that it is generally overlain conformably, 

although locally possibly unconformably, by the 

Rocky Gulch Sandstone Member of the Hornbrook 

Formation. Other workers, however, believe that the 

upper contact is unconformable in the Hornbrook and 

Black Mountain areas (fig. 6; Sliter and others, 

1984). Locally, in the Ashland area, the Ditch Creek 

Siltstone Member is overlain unconformably by Tertiary nonmarine sedimentary rocks.

The lower boundary of the Ditch Creek Siltstone 

Member is marked by the abrupt to gradational upward change from dominantly resistant, cross-stratified to hummocky cross-stratified, fine-grained 

sandstone of the upper part of the Osburger Gulch 

Sandstone Member to less resistant, massive and 

highly bioturbated, poorly sorted, very fine grained 

silty sandstone and siltstone of the Ditch Creek Siltstone Member. The lower boundary is arbitrarily 

placed at the top of the highest bed of well-sorted, 

fine-grained sandstone. The boundary in most outcrops is typically marked by a pronounced topographic bench that separates the less resistant Ditch 

Creek Siltstone Member, which commonly underlies 

low-lying valleys and ravines, from the ridge crests 

and higher topography underlain by the more resistant Osburger Gulch Sandstone Member.

The upper boundary of the Ditch Creek Siltstone 

Member is marked by the gradational to abrupt upward change from massive, concretionary siltstone of 

the Ditch Creek Siltstone Member to massive, very 

thick bedded sandstone of the Rocky Gulch Sandstone Member. The contact is arbitrarily placed at 

the base of the first bed of sandstone that is more 

than 1 m thick. This contact is erosional in the
DITCH CREEK SILTSTONE MEMBER 35

Hornbrook area, as can easily be seen from the 

roadcuts that expose the contact along Interstate 

Highway 5 at the type section of the Ditch Creek 

Siltstone Member (Nilsen and others, 1984, stop 4).

Based on the erosional nature of the contact in 

this area and the apparent lack of fossils representing some stages of the Late Cretaceous, Peck and 

others (1956), Jones (1959), Peterson (1967), Elliott 

(1971; unpub. data, 1980), and Sliter and others 

(1984) inferred the presence of a major intraformational unconformity between the Ditch Creek 

Siltstone Member and the overlying Rocky Gulch 

Sandstone Member. The hiatus represented by the 

proposed unconformity was thought to include late 

Coniacian, Santonian, and early Campanian time. 

However, at many other localities the same contact is 

very gradational and nonerosional. For example, 

McKnight (1971) did not recognize an unconformity 

in exposures of the Hornbrook Formation in the Bear

Creek valley area. In mapping the Hornbrook Formation between Jacksonville to the north and Yreka to 

the south, Nilsen and others (1983) recognized no 

physical evidence for a major unconformity at the top 

of the Ditch Creek Siltstone Member. The erosional 

surface appears to be areally restricted to the 

Hornbrook area.

AREAL EXTENT

The Ditch Creek Siltstone Member extends almost continuously along the entire outcrop extent of 

the Hornbrook Formation as a poorly exposed unit 

that underlies a series of narrow, discontinuous valleys. It appears to be present everywhere, albeit locally very thin, as a fine-grained member between 

coarser and more resistant strata of the underlying 

Osburger Gulch Sandstone Member and of the overlying Rocky Gulch Sandstone Member.

FIGURE 13. Upper part of type section of the Ditch Creek Siltstone Member of the Hornbrook Formation. A, View westward 

across Klamath River showing contact, marked by dashed 

line, between the Ditch Creek Siltstone Member to left and 

the Rocky Gulch Sandstone Member of the Hornbrook Formation to right. B and C, Contact between two members, showing irregularity and erosive character, concretionary nature of 

the Ditch Creek Siltstone Member, and massive character of 

the Rocky Gulch Sandstone Member. D, Abundant concretions 

in uppermost part of the Ditch Creek Siltstone Member; staff 

is 1.5 m long and is divided into 10-cm intervals.
36 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

The northernmost outcrops of the Ditch Creek 

Siltstone Member are located in the western part of 

the Dark Hollow area, about 6 km southeast of Jacksonville and 6 km south-southwest of Medford (S/^ 

sec. 11, T. 38 S.f R. 2 W., Medford 15-minute quadrangle). In this area, the Ditch Creek Siltstone Member underlies a number of interconnected narrow 

valleys and is generally poorly exposed except at a 

few roadcuts.

The Ditch Creek Siltstone Member is covered by 

Quaternary alluvial fans marginal to Bear Creek valley between the eastern edge of the Dark Hollow area 

southwest of Phoenix, Oregon (NWKNWM sec. 2, T. 

38 S., R. 1 W., Medford 15-minute quadrangle), and 

the Bear Creek valley area about 2 km southeast of 

Talent, Oregon (S/s sec. 70 and WA sec. 72, T. 38 S., 

R. 1 W., Talent 15-minute quadrangle). It crops out 

discontinuously through the rest of the Bear Creek 

valley to the Siskiyou Summit fault, except for the 

region between downtown Ashland and Clayton 

Creek, where it is covered by Quaternary alluvial 

fans derived from the Klamath Mountains to the 

southeast.

In the Cottonwood Creek valley area, the Ditch 

Creek Siltstone Member crops out discontinuously 

from the Siskiyou Summit fault to the southwest 

flank of Black Mountain. It is locally covered in the 

central part of the Cottonwood Creek valley area by 

Klamath-derived Quaternary alluvial fans, Quaternary alluvial terrace deposits, and Quaternary alluvium (pi. 1). Some of the best exposures of the 

member are in the mountainous areas west and 

south of Hilt, California, at the north end of the Cottonwood Creek valley and along roadcuts of Interstate Highway 5 adjacent to and west of the Klamath 

River at the south end of the Cottonwood Creek valley area.

In the northern Shasta Valley area, the Ditch 

Creek Siltstone Member crops out south of Black 

Mountain and along the southeast flank of Paradise 

Craggy, east of the Ager fault. The southernmost exposures of the member are located in roadcuts 

through a small hill where Tertiary igneous rocks 

have intruded along the contact between the Ditch 

Creek Siltstone Member and the Rocky Gulch Sandstone Member (NW corner of sec. 25, T. 46 N., R. 6 

W., Copco 15-minute quadrangle). Farther south in 

the Shasta Valley and in the Yreka area, the Ditch 

Creek Siltstone Member does not crop out; here it appears to be covered by Quaternary alluvium, alluvialfan deposits, and debris-avalanche deposits. The 

member does not crop out in the outlier along Soap 

Creek Ridge west of Yreka; here, as in other outliers 

west of Jacksonville, Oregon, it has been removed by

erosion, and only a thin sedimentary veneer consisting of the Klamath River Conglomerate Member and 

Osburger Gulch Sandstone Member remains.

TYPE SECTION

The type section of the Ditch Creek Siltstone 

Member was measured in roadcuts on the west side 

of the northbound lane of Interstate Highway 5 in 

the NE^NEM sec. 32, T. 47 N., R. 6 W., Hornbrook 

15-minute quadrangle (fig. 14). The member is almost completely exposed along the roadcuts, with few 

covered intervals.

Hornbrook Formation:

Rocky Gulch Sandstone Member (lower part): 

Conglomeratic medium-grained sandstone, massive; maximum clast size 2 cm; abundant ripup clasts of mudstone .........................................

Thickness 

(in meters)

0.70

Unconformable(?) contact. 

Ditch Creek Siltstone Member:

Siltstone and silty very fine grained sandstone, 

massive, thoroughly bioturbated; very abundant spherical carbonate concretions of 10-20 

cm diameter; abundant molluscan fossils; 

tubular fossil-like calcareous burrows.............. 2.20

Siltstone and very fine grained sandstone, massive, thoroughly bioturbated; abundant spherical carbonate concretions of 3-5 cm 

diameter; fossil plant fragments ....................... 3.90

Sandstone, very fine grained, massive................. .20

Siltstone and very fine grained sandstone massive, thoroughly bioturbated, concretionary .... .60

Sandstone, very fine grained, massive................. .22

Siltstone and very fine grained sandstone, 

massive, thoroughly bioturbated....................... .75

Sandstone, very fine grained, massive................. .22

Siltstone and very fine grained sandstone,

massive, thoroughly bioturbated....................... .20

Sandstone, very fine grained, massive,

concretionary ....................................................... .06

Siltstone and very fine grained sandstone,

massive, thoroughly bioturbated....................... .30

Sandstone, very fine grained, massive,

concretionary ....................................................... .07

Sandstone, very fine grained, silty, concretionary, thoroughly bioturbated ............................... 3.90

Sandstone, very fine grained, massive, 

concretionary, bioturbated................................. .10

Siltstone and very fine grained sandstone, massive, concretionary, thoroughly bioturbated .... 2.05

Sandstone, very fine grained, massive, 

bioturbated........................................................... .20

Siltstone and very fine grained sandstone, massive; fossil plant fragments, thoroughly 

bioturbated........................................................... .24

Sandstone, very fine grained, massive, concretionary, bioturbated............................................ .28
DITCH CREEK SILTSTONE MEMBER 37

Thickness 

(in meters) 

Ditch Creek Siltstone Member Continued:

Siltstone and very fine grained sandstone,

massive, thoroughly bioturbated....................... .06

Sandstone, very fine grained, massive,

concretionary ....................................................... .11

Siltstone and very fine grained sandstone, 

massive, thoroughly bioturbated....................... .55

Sandstone, very fine grained, silty, massive,

concretionary ....................................................... .10

Siltstone and very fine grained sandstone, massive; scattered molluscan fragments, 

concretionary, thoroughly bioturbated ............. 1.60

Sandstone, very fine grained, massive, 

bioturbated........................................................... .11

Siltstone and very fine grained sandstone, massive; fossil plant fragments, thoroughly 

bioturbated........................................................... .16

Sandstone, very fine grained, massive, 

bioturbated........................................................... .12

Siltstone and very fine grained sandstone, massive; fossil plant fragments, thoroughly 

bioturbated........................................................... .11

Sandstone, very fine grained, massive,

concretionary, bioturbated ................................. .08

Siltstone and very fine grained sandstone, 

massive, thoroughly bioturbated....................... .18

Sandstone, very fine grained, laminated ............. .12

Siltstone and very fine grained sandstone, massive; fossil plant fragments, bioturbated.......... .10

Sandstone, very fine grained, massive,

concretionary, bioturbated ................................. .10

Siltstone and very fine grained sandstone, massive; fossil plant fragments, thoroughly 

bioturbated........................................................... .92

Sandstone, very fine grained, laminated ............. .10

Sandstone, very fine grained, and siltstone; 

laminated fossil plant fragments, 

bioturbated........................................................... .32

Sandstone, very fine grained, massive,

concretionary, bioturbated ................................. .17

Sandstone, very fine grained, silty; molluscan

fossils; fossil plant fragments, bioturbated...... .70

Sandstone, very fine grained, laminated,

concretionary ....................................................... .10

Siltstone and very fine grained sandstone, laminated; fossil plant fragments............................. .22

Sandstone, very fine grained, massive, bioturbated at top.......................................................... .20

Sandstone, very fine grained, silty, massive, 

bioturbated........................................................... .87

Sandstone, very fine grained, massive................. .22

Sandstone, very fine grained, silty, massive;

fossil plant fragments......................................... .52

Sandstone, very fine grained to fine-grained, 

massive, concretionary ....................................... .15

Siltstone and sandstone, very fine grained; 

fossil plant fragments, bioturbated .................. 2.75

Sandstone, very fine grained, laminated ............. .22

Siltstone and very fine grained sandstone, 

massive, thoroughly bioturbated....................... .20

Sandstone, very fine grained, laminated ............. .15

Siltstone and very fine grained sandstone, 

massive, thoroughly bioturbated....................... .60

Thickness 

(in meters)

Ditch Creek Siltstone Member Continued: 

Sandstone, very fine grained, massive to

laminated............................................................. .15

Siltstone and very fine grained sandstone,

massive, bioturbated, mostly covered............... 2.25

Siltstone and very fine grained sandstone,

massive, thoroughly bioturbated....................... .50

Sandstone, very fine grained, massive,

bioturbated........................................................... .30

Covered interval (float of siltstone and very

fine-grained sandstone)...................................... 3.60

Sandstone, very fine grained to fine-grained,

massive, concretionary ....................................... .45

Sandstone, very fine grained to fine-grained,

massive, concretionary, bioturbated ................. .50

Siltstone and very fine grained sandstone,

partly laminated; fossil plant fragments ......... 1.00

Sandstone, very fine grained, massive,

bioturbated........................................................... .40

Siltstone and very fine grained sandstone, massive; fossil plant fragments, thoroughly

bioturbated........................................................... .45

Sandstone, fine-grained, massive; fossil plant

fragments, bioturbate......................................... .50

Siltstone and very fine grained sandstone, massive; fossil plant fragments, thoroughly

bioturbated........................................................... .55

Sandstone, very fine grained to fine-grained,

massive, bioturbated........................................... .23

Siltstone and very fine grained sandstone,

massive, thoroughly bioturbated....................... 1.32

Sandstone, very fine grained to fine-grained,

massive to partly laminated .............................. 1.05

Siltstone and very fine grained sandstone,

massive, thoroughly bioturbated....................... 1.25

Sandstone, fine-grained, silty, massive,

thoroughly bioturbate......................................... .70

Siltstone and very fine grained sandstone,

massive, thoroughly bioturbate......................... 1.12

Sandstone, very fine grained, massive to partly

laminate............................................................... .33

Sandstone, very fine grained, massive,

thoroughly bioturbated....................................... .18

Sandstone, very fine grained, laminated in

lower part, bioturbated in upper part.............. .08

Siltstone and very fine grained sandstone,

massive, bioturbated........................................... .07

Sandstone, very fine grained, massive in lower

part, laminated at top ........................................ .40

Siltstone and very fine grained sandstone,

massive; fossil plant fragments, thoroughly

bioturbated........................................................... 1.32

Sandstone, very fine grained to fine-grained;

fossil plant fragments; laminated in lower

part, bioturbated in upper part......................... .30

Siltstone and very fine grained sandstone,

massive; fossil plant fragments, thoroughly

bioturbated........................................................... 1.95

Sandstone, very fine grained, massive; fossil

plant fragments, thoroughly bioturbated......... .40

Siltstone and very fine grained sandstone,

laminated; fossil plant fragments,

bioturbated........................................................... 1.70
38 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

Ditch Creek Siltstone Member Continued:

Sandstone, very fine grained, massive; fossil 

plant fragments, thoroughly bioturbated.........

Siltstone and very fine grained sandstone, partly 

laminated; fossil plant fragments, 

bioturbated...........................................................

Sandstone, very fine grained, massive to laminated; fossil plant fragments.............................

Siltstone and very fine grained sandstone, massive; fossil plant fragments, thoroughly 

bioturbated ...........................................................

Sandstone, very fine grained, laminated; fossil 

plant fragments, bioturbated.............................

Siltstone, sandy, massive to laminated; fossil 

plant fragments; ammonite fossils, 

bioturbated...........................................................

Covered interval (float mostly of very fine 

grained sandstone, silty, laminated) ................

Total thickness of the Ditch Creek Siltstone 

Member .........................................................

Conformable contact. 

Osburger Gulch Sandstone Member: 

Covered interval (float mostly of very fine 

grained, massive sandstone)..............................

Sandstone, very fine grained, massive; molluscan fossils, bioturbated.......................................

Thickness 

(in meters)

.90

5.70

.06

1.40

.08

2.30

.75

61.64

.75 

1.40

The colors of the Ditch Creek Siltstone Member 

are, for siltstone, medium dark gray (N4) on fresh 

surfaces and moderate yellowish brown (10YR 5/4) on 

weathered surfaces; for sandstone, medium gray (N5) 

on fresh surfaces and moderately yellowish brown 

(10YR 5/4) on weathered surfaces; and for the abundant concretions, medium dark gray (N4) on fresh 

surfaces and dark yellowish brown (10YR 6/6) and 

light brown (5YR 5/6) on weathered surfaces.

LITHOLOGY

The Ditch Creek Siltstone Member consists 

mostly of siltstone and silty very fine grained sandstone. In some areas it contains fine-grained sandstone and mudstone in its lower part, where it grades 

upward from the Osburger Gulch Sandstone Member. 

Where it is gradational upward into the overlying 

Rocky Gulch Sandstone Member in many exposures 

outside of the Hornbrook area, it locally contains 

some interbedded sandstone and mudstone in its upper part. Conglomerate is absent from the Ditch 

Creek Siltstone Member, in contrast to its common 

presence in the overlying and underlying sandstone 

members.

Molluscan fossils are locally abundant both in 

growth position and as transported fragments. 

Bioturbation is extremely abundant and dense; it has 

destroyed most of the original stratification and sediBase of Rocky Gulch

Sandstone Member (of Hornbrook Formation)

METERS

,

CO O

HICKNESS

2>/t

Top of Osburger Gulch

Sandstone Member (of Hornbrook Formation)

FIGURE 14. Type section of the Ditch Creek Siltstone Member 

of the Hornbrook Formation. See figure 10 for explanation 

of symbols and caption of figure 12 for explanation of abbreviations. Gaps in lithology represent covered intervals.
DITCH CREEK SILTSTONE MEMBER 39

mentary structures in the siltstone and sandstone. 

The intense bioturbation has resulted in the member 

commonly having a monotonous and homogeneous 

character, with a minimal amount of bedding or layering. Plant debris is also very abundant and present 

in almost every sample; it is typically finely comminuted and thus has not been helpful for dating the 

member.

In the Shasta Valley area, a coal bed is present 

in what has been mapped (Nilsen and others, 1983) 

as the Ditch Creek Siltstone Member near the 

Hagedorn Ranch adjacent to the Yreka-Ager Road, 

about 4.5 km southwest of Willow Creek School (SW/4 

SWK sec. 24, T. 46 N., R. 6 W., Copco 15-minute 

quadrangle). This coal was previously thought to be 

of Tertiary age (Averill, 1935). The coal bed was 

mined in the past, as indicated by two dump piles of 

mine tailings; O'Brien (1947) reported some production figures for the coal mine in the early 1900's. 

Averill (1935) concluded that the coal bed was an average of 0.6 m thick and was very irregular in thickness. The apparent lateral discontinuity of the coal 

bed could result from minor fault offsets.

The coal is a high-volatile B bituminous coal 

whose geochemistry has been summarized by Zigler 

and Nilsen (1984). Many of the geochemical characteristics of the coal are summarized in tables 3 and 4. 

The coal has a dry-base density of 1.40 gm/cm3 , an 

ash content of 28.2 percent, and a vitrinite reflectance value (based on 100 measurements) of 0.63, 

which is characteristic of high-volatile B bituminous 

coal. The maceral content, based on 1,000 analyses, 

was 87 percent vitrinite (cell wall and material in solution), 1 percent sporinite (plant spores), 3 percent 

cutinite (plant leaf material), 2 percent resinite (plant 

resins), 5 percent semifusinite and 1 percent fusinite 

(carbonized plant remains), 1 percent macrinite and 

inertodetrinite (other carbonized materials), and 1 

percent detrinite. The coal is clearly derived from terrestrial plants, based on the amounts of sulfur ash 

and volatile matter; a nonmarine origin is probable 

(R. Stanton, oral commun., November 1983), although deposition could have been close to a marine 

shoreline.

In its type section, the Ditch Creek Siltstone 

Member consists almost entirely of massive, bioturbated siltstone and silty to very fine grained 

sandstone with scattered marine megafossils and 

abundant fossil plant debris. The unit is thin bedded 

to very thickly bedded, with beds ranging from 6 cm 

to 5.70 m in thickness. The member is relatively uniform lithologically from base to top in its type section, 

except for an increasing number of calcareous concretions upward and a great abundance of concretions at 

the very top of the section. Some beds of sandstone

are laminated or partly laminated; this lamination 

may originally have been characteristic of the sediment at the time of deposition in the vicinity of the 

type section, before postdepositional burrowing destroyed most of the primary sedimentary structures. 

In one bed of laminated, very fine grained to finegrained sandstone in the lower part of the section, 

about 15 m above the base, primary current lineation 

with an east-southeast or west-northwest trend was 

observed. The beds of silty sandstone are of four 

types: (1) massive throughout, with some distinct 

burrows present; (2) massive at the base grading upward into laminated sandstone, with the entire bed 

containing some distinct burrows; (3) massive or 

laminated at the base and thoroughly bioturbated, 

with no distinct burrows, at the top; and (4) thoroughly bioturbated throughout, with no laminations 

and few distinct burrows. No cycles are apparent in 

the type section, although the alternation of finer and 

coarser grained beds is characteristic of the member.

The Ditch Creek Siltstone Member southeast of 

its type section near Hornbrook is poorly and incompletely exposed. It is chiefly covered along the southern and western flanks of Black Mountain by 

landslides, although two prominent outcrops on the 

south flank of the mountain (NW/4 sec. 14, T. 46 N., 

R. 6 W., Hornbrook 15-minute quadrangle) appear 

beneath the landslide cover (pi. 1). Here the Ditch 

Creek Siltstone Member consists, at least in part, of 

massive, thoroughly bioturbated, silty fine-grained 

sandstone. This sandstone rests directly on more resistant pebbly sandstone of the Osburger Gulch 

Sandstone Member that is trough cross-stratified and 

contains some marine molluscan fossils. The bioturbated sandstone is overlain directly by mudstone 

and thinly interbedded sandstone of the Blue Gulch 

Mudstone Member, with the Rocky Gulch Sandstone 

Member missing.

In the Shasta Valley area, at the south end of its 

outcrop belt, the Ditch Creek Siltstone Member crops 

out in some hilly areas and in several roadcuts along 

the Yreka-Ager Road. In addition to the coal that 

forms part of it, the member here appears to consist 

mostly of siltstone and very fine grained sandstone 

that contains marine molluscan fossils and is commonly bioturbated. The molluscan fossils, which include pelecypods and oysters, are present both in 

growth position and as transported fragments.

North of its type section, in the Cottonwood 

Creek valley area, the Ditch Creek Siltstone Member 

is generally poorly exposed and appears to consist 

mostly of concretionary siltstone and silty very fine 

grained sandstone. Scattered small exposures of the 

member along the bottom of Cottonwood Creek between Hilt and Hornbrook (sec. 1, T. 47 N., R. 7 W.,
40 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

TABLE 3. Geochemistry of coal sample collected from the Ditch Creek Siltstone Member near the Hagedorn Ranch

in the Shasta Valley area

[Data from Zigler and Nilsen (1984, table 1); SW^SWK sec. 24, T. 46 N., R. 6 W., Copco 15-minute quadrangle. Analysis by 

U.S. Geological Survey Analytical Laboratory in Lakewood, Colorado]

Element Percent Parts per million

Major elements

Ca- ----- 0.84/0.86

Na - - - .16/<.03

K 1 QQ/^ Qn

_______ _ _ 94 9970

P20fi ----- .15

Qi 9^*7 nnn

Al 15.80/16.00

J.1 .U<J/.U^

Rare earth elements

Element Percent Parts per million

CV 1 30

Pr .. , . ____ ^Kf)

\TH ,.,., __ 20

Q-m ___ .. ^fl

EU --- - - <10

rjj ... _ ... .-.en

Th X U ___ «»»« « » ._ ^cn ^tlv/

Dv . .-----, ..... ^20

Ho <20

T31 ** ^OA

rrv__

Yh __ - ^

Lu ______

Hornbrook 15-minute quadrangle; not shown on pi. 1) 

consist of siltstone and mudstone that contain abundant interbeds of graded fine-grained sandstone. The 

beds of graded sandstone, which appear to be most 

abundant in the lower part of the member, have 

prominent sole marks (chiefly large flute casts) and 

are organized into Bouma sequences. The member is 

here cut by numerous subvertical cross-faults that 

trend northerly, northeasterly, and easterly. Some of 

these faults are filled with sandstone dikes that are 

offset by successive periods of faulting. 

Farther north in the Cottonwood Creek valley 

area, the Ditch Creek Siltstone Member is generally 

coarser grained than it is at the type section. In a 

second measured section at the north end of the Cottonwood Creek valley west of Hilt (NEi4 sec. 26, T. 48 

N., R. 7 W., Hornbrook 15-minute quadrangle), the

Element Percent Parts per million

Minor elements

Ag <10

Ac -. ............... tf'lfl

An J _________ ^/10

B .

Ba 1,100

Be <5

Bi <50

Ct\ <10/20

f!n SI

rv 120

Cu - 200

F Olfi

Ga - 30 

Ge -- <50

Hg ------- _ .12

T La 60f*f\

Li 120

Mn 70

Mo <10

Nb <20

Ni - 110

Pb 30/30

G,, OL» -« -«« ---- _______ u\j KQ

Sn 

qr 1 100

Ta INAA(23/1) <200 

Th INAA(4.50/4.25) <20

U <500

V QCf\

w 

V fiO

Zn 200

7-r

lower and middle parts of the member contain abundant beds of fine-, medium-, and coarse-grained sandstone as thick as 2.5 m. These beds are distinctly 

graded and locally contain both granule-size detritus 

in their basal parts and large rip-up clasts of shale 

and fine-grained sandstone suspended in the sandstone. These graded beds of sandstone are interbedded with shale, silty shale, and siltstone, and they 

generally have erosive contacts with the shale. Flute 

casts, groove casts, and flame structures are present 

on the soles of the sandstone beds; dish structure, pillar structure, parallel stratification, convolute lamination, current ripple markings, and fossil plant 

fragments are present within the sandstone beds. 

The upper part of the second section consists of 

massive, concretionary, thoroughly bioturbated shale 

with very thin interbeds of massive to laminated,
DITCH CREEK SILTSTONE MEMBER 41

TABLE 4. Maceral analysis of coal sample collected from the Ditch Creek Siltstone Member near 

the Hagedorn Ranch in the Shasta Valley area

[Data from Zigler and Nilsen (1984, table 2). Analysis by U.S. Geological Survey Analytical Laboratory in 

Lakewood, Colorado, on dry mineral-free basis from 1,000 point counts, using technique described by 

Crelling (1980, p. 25)3

Maceral group

Vitrinite

Exinite-liptinite

Maceral Description

 Plant cell wall 

material.

Waxy and resinous

Volume 

percent

87

sporinite 

cutinite - 

resinite -

parts of plants: 

plant spores 

plant leaf material 

plant resins 

Inertinite Plant material strongly 

altered during 

coalification: 

semifusinite carbonized plant remains

detrinite do. 

____.__ ~.___ Q

__ . 1

1

. - i

locally current ripple-marked, concretionary siltstone. 

Plant fragments are very abundant in this part of the 

section. The section is abruptly overlain by very thick 

beds of conglomeratic sandstone of the Rocky Gulch 

Sandstone Member. Thus, the Ditch Creek Siltstone 

Member near Hilt, California, forms a very distinct 

fining-upward unit, changing from sandstone at its 

base to shale at its top.

In the Bear Creek valley area, the Ditch Creek 

Siltstone Member is not well exposed. McKnight 

(1971) described it as mudstone, shale, siltstone, and 

sandstone, with concretions locally abundant at its 

top. The base of the Ditch Creek Siltstone Member is 

exposed at the top of a measured section of the 

Osburger Gulch Sandstone Member along the southbound lane of Interstate Highway 5 near the Siskiyou 

Summit (NWK sec. 21, T. 40 S., R. 2 E., Ashland 15- 

minute quadrangle). The lowest 5 m consist chiefly of 

siltstone with some interbeds of silty very fine 

grained sandstone. The siltstone is generally very 

concretionary and bioturbated, and it contains 

megafossils consisting of ammonites and locally abundant pelecypods and gastropods in growth position. 

The interbedded sandstone is massive to laminated 

and locally graded.

Between Ashland and the Siskiyou Summit fault, 

there are few outcrops of the Ditch Creek Siltstone 

Member. Massive siltstone and very fine grained 

sandstone containing abundant molluscan and ammonite fossils crop out along the bed of the Southern 

Pacific Railroad about 12 km southeast of Ashland 

(NWKSWK sec. 5, T. 40 S., R. 2 E., Ashland 15- 

minute quadrangle). An outcrop of the Ditch Creek 

Siltstone Member adjacent to Interstate Highway 5 

about 9 km southeast of Ashland (SW corner of sec. 

30, T. 39 S., R. 2 E., Ashland 15-minute quadrangle) 

consists mostly of mudstone with thin interbeds of 

bioturbated silty very fine grained sandstone. Along 

State Highway 99 northeast of its crossing with the 

Southern Pacific Railroad, about 2 km northwest of 

downtown Ashland (NEKNWK sec. 5, T. 39 S., R. 1 

E., Ashland 15-minute quadrangle), the Ditch Creek 

Siltstone Member consists almost wholly of silty 

mudstone with a few thin interbeds of concretionary 

siltstone.

The northwesternmost exposures of the Ditch 

Creek Siltstone Member are in several good readouts 

in the Dark Hollow area between Talent and Jacksonville along Dark Hollow Road. The chief lithologies include siltstone, silty mudstone, and silty very 

fine grained sandstone. The siltstone and silty mudstone typically are massive, highly bioturbated, and 

locally fossiliferous (mollusks and ammonites). The 

interbeds of silty very fine grained sandstone typically are laterally discontinuous, concretionary, 

partly bioturbated, and massive. However, the sandstone interbeds are locally laminated, current ripplemarked, and graded; they locally also have sharp,
42 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

partly erosional contacts with the underlying siltstone or silty mudstone.

In summary, the lithology of the Ditch Creek Siltstone Member consists mostly of siltstone, silty mudstone, and silty very fine grained sandstone, partly to 

wholly bioturbated. It locally contains massive to 

laminated and ripple-marked beds of sandstone, particularly near its basal contact with the Osburger 

Gulch Sandstone Member. It forms a generally thin 

and nonresistant unit that in many areas is extensively covered by Quaternary deposits.

STRATIGRAPHIC RELATIONS

The Ditch Creek Siltstone Member everywhere 

rests gradationally and conformably on the Osburger 

Gulch Sandstone Member. The contact is mapped 

where the predominant lithology changes from sandstone to siltstone, silty very fine grained sandstone, 

interbedded sandstone, and siltstone. It can be placed 

in measured sections at the top of the highest bed of 

fine-grained sandstone.

The Ditch Creek Siltstone Member is overlain almost everywhere by the Rocky Gulch Sandstone 

Member. In the Hornbrook area, this upper contact 

has previously been interpreted as a regional unconformity because it is clearly erosional. However, 

north and south of the Hornbrook area, the contact is 

generally gradational, although it may locally be 

marked by a minor erosional surface. In the Dark 

Hollow area along Dark Hollow Road (SWK sec. 18, 

T. 38 S., R. 2 W., Medford 15-minute quadrangle), 

continuous roadcuts across the contact clearly show 

the gradational nature of the contact, with increasing 

amounts of sandstone present upward in the Ditch 

Creek Siltstone Member. In this report, the upper 

contact is considered generally conformable, although 

locally, as in the Hornbrook area, it is possibly unconformable. However, additional age data from the 

Ditch Creek Siltstone Member and the Rocky Gulch 

Sandstone Member throughout the region would 

clearly be helpful in better defining the possible presence of a hiatus.

The upper contact of the Ditch Creek Siltstone 

Member with the Rocky Gulch Sandstone Member is 

generally placed at the upward lithologic change from 

siltstone or silty mudstone to sandstone. The contact 

in the Hornbrook area and in other parts of the Cottonwood Creek valley area is typically very sharp and 

easily determined. To the north, particularly in the 

Dark Hollow area, where the contact is gradational, I 

place the contact at the base of the first bed of fineto-medium- or medium-grained sandstone.

On the south flank of Black Mountain, directly 

north of the Shasta Valley, I interpret field relations 

to indicate that the overlying Rocky Gulch Sandstone 

Member is missing, which leaves mudstone of the 

Blue Gulch Mudstone Member resting directly on the 

Ditch Creek Siltstone Member (Nilsen and others, 

1983; pi. 1). The contact here appears to be conformable, although it is possible that there is a significant 

hiatus present; this area of generally poor outcrops 

has been interpreted differently by Sliter and others 

(1984). The contact is easily determined here, however, because the unit mapped as the Ditch Creek 

Siltstone Member consists almost wholly of massive, 

bioturbated, fine-grained sandstone and is more resistant than the overlying mudstone.

Lateral contacts and gradations of the Ditch 

Creek Siltstone Member cannot be observed in outcrop. The member is present in all outcrop areas as a 

laterally continuous thin sheet of siltstone and silty 

fine-grained sandstone above the equally continuous 

Osburger Gulch Sandstone Member. The Ditch Creek 

Siltstone Member probably initially persisted in all 

directions from the present outcrop belt and is undoubtedly present in the subsurface to the northeast.

THICKNESS

The Ditch Creek Siltstone Member is generally 

20-80 m thick, although its thickness has not been 

determined in very many areas because of the lack of 

completely exposed sections. At its type section, south 

of Hornbrook, it is 61.64 m thick. The equivalent unit 

of Peck and others (1956), measured west of Hornbrook, which includes the upper part of their member 

II (fig. 6), is about 80.5 m thick. Jones (1959) measured a thickness of 213-243 m for his unit A, which 

includes of the Klamath River Conglomerate Member, the Osburger Gulch Sandstone Member, and the 

Ditch Creek Siltstone Member.

Elliott (1971) determined a thickness of 58.2 m 

for his Henley mudstone unit of the Hornbrook Formation and a similar thickness for his unit B of the 

Hornbrook Formation (M.A. Elliott, unpub. data, 

1980) at the north end of the Cottonwood Creek valley, both of which are equivalent to the Ditch Creek 

Siltstone Member. In the Bear Creek valley area, 

McKnight (1971) obtained a thickness of 23 m for his 

unit Khs2, which is equivalent to the Ditch Creek 

Siltstone Member.

I measured another section of the Ditch Creek 

Siltstone Member along Cottonwood Creek at the 

north end of the Cottonwood Creek valley, about 2 

km south of Hilt (NEKSWK sec. 26, T. 48 N., R. 7 W.,
DITCH CREEK SILTSTONE MEMBER 43

Hornbrook 15-minute quadrangle). At this locality, 

the Ditch Creek Siltstone Member is 77.41 m thick.

No sections of the Ditch Creek Siltstone Member 

have been measured in the Dark Hollow area or in 

the Shasta Valley area, and thus its thicknesses near 

the northwestern and southeastern limits of its outcrop area are not known. It appears in general to be 

as much as 4 or 5 times thicker in the Cottonwood 

Creek valley area than in the Bear Creek valley area. 

However, it is also possible that it irregularly thickens and thins in different areas.

DEPOSITIONAL ENVIRONMENT

The Ditch Creek Siltstone Member appears to 

have been deposited chiefly in a low-energy outershelf environment. It consists mostly of bioturbated 

siltstone and silty very fine grained sandstone; the 

lack of preservation of planar stratification, trough 

cross-stratification, or hummocky cross-stratification 

indicates that sedimentation rates were generally 

slow, which allowed ample time for thorough bioturbation and mixing of sediments. Sandstone interbeds may have been deposited by storm-generated rip 

currents or turbidity currents; however, these beds 

are generally so thoroughly bioturbated that the lack 

of original bed boundaries and internal structures 

prevents a clear interpretation of their process of 

deposition.

Lateral changes in facies within the Ditch Creek 

Siltstone Member suggest that it may have been deposited in deeper marine conditions to the north and 

in shallow-marine to locally nonmarine conditions to 

the south. Deposits to the north between the Hilt and 

Dark Hollow areas form at least locally well-bedded 

sequences, with graded beds of sandstone interbedded with massive siltstone. Many of the sandstone 

beds are well graded and have elements of the 

Bouma sequence; sole markings are locally preserved 

at the bases of these beds, and bioturbation is less 

abundant. The coarsest grained beds, some containing abundant granule-size detritus near their bases, 

are present in the lower part of the member near 

Cottonwood Creek, west and southwest of Hilt. Dish 

structures, beds of massive sandstone, and sandstone 

dikes are present locally in the member in this area.

In the type section south of Hornbrook and in the 

southern Cottonwood Creek valley area in general, the 

Ditch Creek Siltstone Member consists of thoroughly 

bioturbated massive siltstone and silty very fine grained 

and fine-grained sandstone. This unit was probably deposited in more shallow-marine conditions, as is indicated by more abundant molluscan fossils.

Farther south, along the south flank of Black 

Mountain, the Ditch Creek Siltstone Member crops 

out in isolated patches as a bioturbated, crossstratified fine-grained sandstone. Here it appears to 

be a nearshore or possibly lagoonal deposit.

In the southernmost outcrops of the Ditch Creek 

Siltstone Member located in the northern Shasta Valley area, coal of probable nonmarine origin is 

interbedded with fossiliferous marine siltstone. The 

coal suggests at least local subaerial exposure and accumulation of organic matter in a probable freshwater 

swamp. Exposures of the Ditch Creek Siltstone Member 

in the northern Shasta Valley area are generally poor, 

but marine megafossils suggest a shallow-marine, locally 

lagoonal depositional environment.

AGE AND CORRELATION

The Ditch Creek Siltstone Member contains 

abundant megafossils (chiefly pelecypods, gastropods, 

and ammonites) that provide age control. I have also 

collected foraminiferal samples that provide some additional ages. The largest collection of megafossils 

that I obtained from the Ditch Creek Siltstone Member, from the Dark Hollow area (USGS Mesozoic locality M7686), indicates a Turonian age for its upper 

part (app. I). The foraminiferal collections (app. II) 

are indicative of a middle Turonian age in the 

Ashland area (USGS Mesozoic localities Mf6590 and 

Mf6591), a middle Turonian age in the Dark Hollow 

area (USGS Mesozoic locality Mf6592), a late 

Turonian to Coniacian age for a sample taken in the 

northern Cottonwood Creek valley area near Bailey 

Hill from about 2 m below the contact with the Rocky 

Gulch Sandstone Member (USGS Mesozoic locality 

Mf6581), and a late Turonian to Coniacian age for a 

sample taken about halfway between Hilt and 

Hornbrook from the middle part of the Ditch Creek 

Siltstone Member (USGS Mesozoic locality Mf6580). 

Based on these foraminiferal and megafossil ages, the 

Ditch Creek Siltstone Member appears to be younger 

to the south, from middle Turonian in the Dark Hollow and Bear Creek valley areas to late Turonian to 

Coniacian in the Cottonwood Creek valley area. No 

Santonian ages have been obtained to date from either megafossils or foraminifers.

Peck and others (1956) concluded, on the basis of 

molluscan and ammonite collections, that the upper 

part of their member II, equivalent to the Ditch 

Creek Siltstone Member, was of Turonian age. On 

the basis of ammonite and other molluscan faunas, 

Jones (1959) considered his unit A, which includes 

the Klamath River Conglomerate Member and the
44 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

Osburger Gulch Sandstone Member, as well as the 

Ditch Creek Siltstone Member, to be of Turonian and 

early Coniacian age. Sliter and others (1984) suggested ages of early Turonian in the Dark Hollow 

area, middle Turonian in the Ashland area, and early 

Coniacian in the Hornbrook area for strata mapped 

herein as the Ditch Creek Siltstone Member. Partly 

correlative strata in the Grave Creek and Black 

Mountain areas may be of Cenomanian and early 

Coniacian ages, respectively (Sliter and others, 1984). 

Thus, the Ditch Creek Siltstone Member in its 

main outcrop area ranges in age from early Turonian 

to early Coniacian and is correlative with strata of 

Turonian to Coniacian age in surrounding areas. Part 

of the sequence to the north in the subsurface Ochoco 

basin near Mitchell, Oregon, probably correlates with 

the Ditch Creek Siltstone Member (Thompson and 

others, 1984). To the south, the Ditch Creek Siltstone 

Member is probably correlative with parts of the 

Great Valley sequence near Redding (Popenoe, 1943; 

Matsumoto, 1960), the Gas Point Member of the 

Budden Canyon Formation of Murphy and others 

(1969) southwest of Redding, the Great Valley sequence along the west side of the Sacramento Valley 

(Ingersoll, 1979), and the Coastal belt terrane of the 

Franciscan assemblage. It appears to be mostly older 

than the Coniacian to Campanian Great Valley sequence in the Chico area on the east side of the Sacramento Valley (Haggart and Ward, 1984).

ROCKY GULCH SANDSTONE MEMBER

DEFINITION

The name "Rocky Gulch Sandstone Member" was 

applied by Nilsen (1984a) to almost continuous outcrops of marine sandstone strata in the lower part of 

the Hornbrook Formation. The member was named 

for exposures in Rocky Gulch, located about 2 km 

southwest of Hornbrook, Calif. The type section, however, was measured in roadcuts along the northbound 

lane of Interstate Highway 5 adjacent to and west of 

the Klamath River, about 3 km south of the town of 

Hornbrook (figs. 8, 15).

The Rocky Gulch Sandstone Member is the fourth 

member, in ascending stratigraphic order, of the 

Hornbrook Formation (fig. 6). It is equivalent to 

member III and the lower part of member IV (units 

10 and 11) (of Hornbrook Formation) as mapped by 

Peck and others (1956), unit B (of Hornbrook Formation) of Jones (1959), unit Khs3 (of Hornbrook Formation) of McKnight (1971), unit C (of Hornbrook 

Formation) of M.A. Elliott (unpub. data, 1980), and

unit c (of Hornbrook Formation) as mapped by Nilsen 

and others (1983).

FIGURE 15. Type section of the Rocky Gulch Sandstone Member 

of the Hornbrook Formation. A, Massive beds of sandstone in 

lower part of the Rocky Gulch Sandstone Member containing 

prominent shale interval. Staff is 1.5 m long. B, Massive sandstone cut by vertical fault; person circled for scale. C, Conglomerate composed of shale rip-up clasts. Rock hammer is 35 

cm long.
ROCKY GULCH SANDSTONE MEMBER 45

The Rocky Gulch Sandstone Member was thought 

by most previous workers, despite the lack of fossil 

collections from it, to rest unconformably on underlying strata equivalent to the Ditch Creek Siltstone 

Member of the Hornbrook Formation. Although this 

contact is erosional in some exposures in the type 

area of the Hornbrook Formation, it appears to be 

gradational in most other areas to the northwest 

and southeast. As a result, I provisionally herein consider the contact to be generally conformable, pending accumulation of additional fossil age data, 

although it is locally erosive and may possibly be unconformable in the type area. The Rocky Gulch Sandstone Member is overlain conformably by the Blue 

Gulch Mudstone Member of the Hornbrook Formation and is typically a resistant unit that underlies a 

single ridge crest; in most areas, this ridge crest is 

flanked by a narrow valley to the southwest underlain by the less resistant underlying Ditch Creek Siltstone Member and a broad valley to the northeast 

underlain by the less resistant overlying Blue Gulch 

Mudstone Member (fig. 3).

The lower boundary of the Rocky Gulch Sandstone Member is defined by the abrupt to gradational 

and locally erosional upward change from predominantly massive bioturbated siltstone of the Ditch 

Creek Siltstone Member to thick beds of massive to 

graded sandstone that is locally conglomeratic. The 

lower contact, in sections that are gradational, is arbitrarily placed at the base of the first bed of sandstone that is more than 1 m thick.

The upper boundary of the Rocky Gulch Sandstone Member is typically gradational and marked by 

the gradual upward decrease in thickness and coarseness of sandstone beds and the increasing thickness 

of siltstone and mudstone. Because the Rocky Gulch 

Sandstone Member is typically organized into finingupward and thinning-upward cycles, the boundary 

can generally be placed at the top of the highest 

cycle. In detailed measured sections, the top of the 

uppermost bed of fine- or medium-grained sandstone 

that is thicker than 30 cm was used to define the top 

of the Rocky Gulch Sandstone Member.

AREAL EXTENT

The Rocky Gulch Sandstone Member extends almost continuously in a northwest-southeast direction 

for nearly the entire outcrop extent of the Hornbrook 

Formation (pi. 1). It is generally well exposed, forming a low, generally narrow ridge between the higher 

elevations of the Klamath Mountains and the Cascade Range. It appears to be missing only locally,

along the southern flank of Black Mountain, in parts 

of sees. 10, 11, 14, and 15, T. 46 N., R. 6 W., 

Hornbrook 15-minute quadrangle, where it appears 

that the Blue Gulch Mudstone Member rests directly 

on the Ditch Creek Siltstone Member (pi. 1).

The northernmost outcrops of the Rocky Gulch 

Sandstone Member are located about 3 km southeast 

of Medford, in roadcuts adjacent to Interstate Highway 5 (N1A sec. 51, SE corner of sec. 41, and NE corner of sec. 32, T. 37 S., R. 1 W., Medford 15-minute 

quadrangle). The best exposures in this northern 

area, however, are in roadcuts along the northeast 

side of Interstate Highway 5 (SA 1 sec. 51, EK sec. 48, 

and NE corner of sec. 38, T. 37 S., R. 1 W., Medford 

15-minute quadrangle) about 1 km to the southeast 

of the northernmost outcrops (Nilsen and others, 

1984, stop 10). The freeway roadcuts expose a thickness of about 70 m of the Rocky Gulch Sandstone 

Member; at the southeastern end of the roadcut, the 

Rocky Gulch is juxtaposed by faulting with Tertiary 

nonmarine conglomerate of the Payne Cliffs Formation of McKnight (1971, 1984). These strata of the 

Rocky Gulch Sandstone Member were mapped by 

Wells (1956) as the Eocene Umpqua Formation, by 

McKnight (1971) as the Payne Cliffs Formation, and 

by Smith and others (1982) as Tertiary nonmarine 

strata. However, there is little doubt, on the basis of 

foraminiferal faunas, petrography, and character of 

the sedimentary sequences, that these rocks are properly assigned to the Rocky Gulch Sandstone Member 

of the Hornbrook Formation.

The Rocky Gulch Sandstone Member also crops 

out extensively in the Dark Hollow area, where it underlies a prominent northwest-trending ridge. The 

member does not crop out farther west or north in 

the Jacksonville or Medford Valley areas, however.

The southernmost outcrops of the Rocky Gulch 

Sandstone Member are in some low ridges along the 

northern margin of the Shasta Valley. The southernmost outcrops form an outlier along a low ridge about 

2 km southwest of Mary's Peak (central part of sec. 

25, T. 46 N., R. 6 W., Copco 15-minute quadrangle). 

Other good exposures are present a few kilometers to 

the northwest (WA sec. 24, EA 1 sec. 23, T. 46 N., R. 6 

W., Hornbrook and Copco 15-minute quadrangles). 

The Rocky Gulch Sandstone Member is not exposed 

in the Yreka or Soap Creek Ridge areas.

TYPE SECTION

The type section of the Rocky Gulch Sandstone 

Member of the Hornbrook Formation was measured 

from roadcuts along the northbound lane of Inter-
46 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

state Highway 5 in the NE%NE^ sec. 32, T. 47 N., R. 

6 W., Hornbrook 15-minute quadrangle (fig. 16). The 

member is almost completely exposed along the highway and begins directly above the top of the type section of the Ditch Creek Siltstone Member (Nilsen and 

others, 1984, stop 4).

Hornbrook Formation: 

Blue Gulch Mudstone Member (lower part): 

Mudstone, massive

Conformable contact.

Rocky Gulch Sandstone Member:

Sandstone, fine- to medium-grained, massive ....

Shale, silty, massive...............................................

Sandstone, fine- to medium-grained, massive ....

Siltstone, massive ...................................................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Sandstone, graded, fine- to medium-grained, 

massive; scattered mudstone rip-up clasts ......

Sandstone, graded, fine- to medium-grained, 

massive; scattered mudstone rip-up clasts ......

Sandstone, graded, fine- to medium-grained and 

massive in lower part, very fine grained and 

laminated in upper 5 cm ....................................

Sandstone, graded, fine- to medium-grained and 

massive in lower part, very fine grained and 

laminated in upper 5 cm ....................................

Siltstone, massive...................................................

Sandstone, slightly graded, fine- to mediumgrained, massive; scattered mudstone rip-up 

clasts; erosive base..............................................

Shale, massive.........................................................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Sandstone, fine- to medium-grained, massive ....

Shale, massive.........................................................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Sandstone, graded, fine-grained, massive; 

erosive base..........................................................

Sandstone, graded, fine- to medium-grained, 

massive.................................................................

Shale, silty, massive...............................................

Covered interval .....................................................

Sandstone, graded, fine-grained, partly covered, 

mostly massive but partly parallel-stratified, 

bioturbated...........................................................

Covered interval (float mostly of shale)...............

Shale, silty, massive, bioturbated.........................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Sandstone, graded, fine-grained, massive ...........

Siltstone and shale, laminated .............................

Sandstone, graded, fine-grained, massive ...........

Sandstone, very fine grained, laminated .............

Sandstone, graded, fine- to medium-grained, 

massive; erosive base..........................................

Sandstone, very fine grained to fine-grained, 

laminated .............................................................

Thickness 

(in meters)

0.70 

.55 

.40 

.15

.20

1.40

1.50

1.50

1.90 

.10

.85 

.10

.10 

.07 

.30

.30 

4.50

4.50

4.00

2.00

2.80

7.20

1.25

.25 

.52 

.15 

.75 

.40

3.00 

.40

Rocky Gulch Sandstone Member Continued: 

Sandstone, graded, fine- to medium-grained,

massive; erosive base ..........................................

Shale, massive .........................................................

Sandstone, graded, fine- to medium-grained,

massive .................................................................

Covered interval (float of shale) ...........................

Sandstone, graded, fine- to medium-grained,

massive.................................................................

Covered interval (float of shale) ...........................

Sandstone, graded, fine- to medium-grained,

massive .................................................................

Covered interval (float of shale) ...........................

Sandstone, graded, medium-grained at base and

fine-grained at top, massive; erosive base .......

Sandstone, graded, fine-grained, massive in

lower part, laminated at top ..............................

Covered interval (float of sandstone) ...................

Sandstone, graded, fine- to medium-grained,

massive; erosive base ..........................................

Covered interval (float mostly of shale) ...............

Shale, silty, massive ...............................................

Sandstone, very fine grained; hummocky crosslaminae .................................................................

Siltstone, massive to laminated, bioturbated ......

Sandstone, graded, fine- to medium-grained,

massive .................................................................

Sandstone, coarse-grained, conglomeratic,

graded, massive; maximum clast size 1 cm.....

Sandstone, graded, medium-grained....................

Sandstone, coarse-grained, conglomeratic,

graded, massive; maximum clast size 0.5 cm;

mudstone rip-up clasts; erosive base ................

Sandstone, fine- to medium-grained, massive ....

Covered interval (float of sandstone) ...................

Sandstone, graded, medium-grained, massive....

Sandstone, coarse-grained, conglomeratic,

graded, massive; maximum clast size 0.5 cm;

mudstone rip-up clasts; erosive base ................

Sandstone, graded, fine- to medium-grained,

massive.................................................................

Covered interval .....................................................

Sandstone, coarse-grained, conglomeratic,

graded, massive; maximum clast size 0.5 cm;

mudstone rip-up clasts; erosive base ................

Sandstone, graded, medium-grained, massive ....

Sandstone, coarse-grained, conglomeratic,

graded, massive; maximum clast size 1 cm;

mudstone rip-up clasts; erosive base ................

Shale, silty, massive ...............................................

Sandstone, fine- to medium-grained, massive,

lenticular ..............................................................

Shale, silty, massive ...............................................

Sandstone, graded, fine-grained, laminated........

Sandstone, graded, medium-grained, massive....

Sandstone, medium- to coarse-grained, conglomeratic, graded, massive; maximum clast size

1 cm; erosive base ...............................................

Shale, silty, massive...............................................

Sandstone, very fine grained to fine-grained;

current ripple markings .....................................

Sandstone, graded, fine- to medium-grained,

parallel-stratified ................................................

Thickness 

(in meters)

1.60 

.30

.20 

.30

.25 

.70

3.00

2.50

5.70

.25 

5.60

5.00

2.70

.70

.10 

1.10

5.70

.50 

2.60

.50

5.20

.20

1.10

.40

3.10

4.90

1.00

4.30

2.00 

.05

.05

.10

.30

2.50

2.50

.15

.15 

.20
ROCKY GULCH SANDSTONE MEMBER 47

Thickness 

(in meters)

10.50

2.85

.80

10.50

7.60 

.20

4.65

4.20

7.50

Rocky Gulch Sandstone Member Continued:

Sandstone, graded, medium-grained, massive ....

Sandstone, coarse-grained, conglomeratic, 

graded, massive; maximum clast size 2 cm; 

erosive base..........................................................

Covered interval (float of very fine grained 

sandstone) ............................................................

Sandstone, graded, medium-grained, massive, 

concretionary .......................................................

Sandstone, medium-grained, conglomeratic, 

graded, massive; maximum clast size 1 cm; 

mudstone rip-up clasts; erosive base ................

Shale, massive.........................................................

Sandstone, medium- to coarse-grained, conglomeratic, graded, massive; maximum clast size 

2 cm; scattered mudstone rip-up clasts............

Conglomerate, consisting of more than 80 percent mudstone rip-up clasts as large as 100 

cm, less than 20 percent lithic pebbles as large 

as 8 cm, and some clasts of carbonate concretions; matrix of medium- to coarse-grained 

sandstone; erosive base ......................................

Sandstone, medium- to coarse-grained, graded, 

massive, conglomeratic in lower 100 cm; 

maximum clast size 2 cm ...................................

Conglomerate, matrix-supported; dominantly 

lithic clasts in lower part, maximum clast size 

15 cm; dominantly mudstone rip-up clasts in 

upper part, maximum clast size 40 cm; massive; rare molluscan fossil fragments; erosive 

base.......................................................................

Sandstone, graded, medium- to coarse-grained, 

conglomeratic; scattered lithic pebbles less 

than 1 cm and mudstone rip-up clasts less 

than 50 cm in length; some swirly stratification and subparallel stratification ....................

Conglomerate, massive; maximum clast size 

2 cm; lens-shaped; erosive base ........................

Sandstone, graded, medium-grained, massive, 

conglomeratic in lower 70 cm; lithic pebbles 

less than 2 cm in length; abundant mudstone 

rip-up clasts and clasts of carbonate concretions; erosive base...............................................

Total thickness of the Rocky Gulch Sandstone Member...............................................

Erosive contact, possibly unconformable.

Ditch Creek Siltstone Member:

Siltstone and silty very fine grained sandstone,

massive, thoroughly bioturbated; abundant

spherical carbonate concretions

The colors of the Rocky Gulch Sandstone Member 

are light brownish gray (5YR 6/1) for fresh sandstone 

and light brown (SYR 6/4), pale yellow brown (10YR 

6/2), and moderate yellow brown (10YR 5/4) for 

weathered sandstone. The interbedded shale is medium dark gray (N4) or medium gray (N5) on fresh 

surfaces and moderate yellowish brown (10YR 5/4) on 

weathered surfaces. The mudstone rip-up clasts, 

abundant at the base of the member, are medium

l.OO

9.50

1.05

1.50

171.19

dark gray (N4) on fresh surfaces and pale yellowish 

brown (10YR 6/2) on weathered surfaces.

LITHOLOGY

The Rocky Gulch Sandstone Member consists predominantly of fine- to medium-grained sandstone. It 

is generally well bedded and evenly bedded. The 

sandstone beds, particularly the thinner beds in the 

upper part of the member, are generally separated by 

thin shale intervals; however, in the lower part of the 

member, and in the lower parts of thinning- and fining-upward cycles, the sandstone beds are commonly 

amalgamated. Conglomerate and siltstone are generally present in most sections. Conglomerate forms the 

dominant lithology in the Ditch Creek area, about 5 

km northwest of the type section. Siltstone is present 

in the upper part of many sections as a current 

ripple-marked interval that rests on sandstone beds. 

However, sandstone probably forms 80-90 percent of 

the thickness of most sections.

The sandstone beds are generally of two types: (1) 

very thick to massive, internally structureless beds, 

and (2) thin to medium beds that are organized into 

Bouma sequences. The beds are typically organized 

into cycles of 5-10 beds that have a thinning- and 

fining-upward character. The lower parts of the 

cycles consist of very thick to massive beds that are 

generally amalgamated, as thick as 10 m or so, and 

are coarse-tail graded (that is, pebbles and granules 

are concentrated near the bases of the beds even 

though most of the sandstone does not appear to be 

graded). The basal surfaces of these beds are clearly 

erosive into underlying beds of shale or sandstone, 

but they do not form channels at outcrop scale. Flute 

casts, groove casts, and load casts are common sole 

markings on these basal surfaces. Rip-up clasts of 

shale and flaps of shale from the underlying bed that 

are partly torn up and embedded in the overlying bed 

of sandstone are locally common (Boggs and 

Swanson, 1970). Some beds contain a thick lower interval of rip-up-clast conglomerate where the contact 

with the underlying shale is conspicuously erosive. 

Dish structure and irregular, swirly lamination have 

been noted at several localities. The thicker beds are 

characteristically composed of medium-grained to 

very coarse grained sandstone. The tops of the beds, 

which are typically sharp and planar, are overlain 

abruptly by shale, except where amalgamation of 

sandstone beds is present These sandstone beds resemble turbidite facies B of Mutti and Ricci Lucchi 

(1972) except that they are laterally persistent and 

not clearly channelized.
48 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

80

70 

60 

50 

- 40 H

o

X

30 -

20

10 

170 -

160 -

150 -~

140 -

130

120 -

110 -

100 -

Base of Blue Gulch 

Mudstone Member 

(of Hornbrook Formation)

Top of Ditch Creek 

Siltstone Member 

(of Hornbrook Formation)

FIGURE 16. Type section of the Rocky Gulch Sandstone Member of the Hornbrook Formation. See figure 10 for explanation of symbols and caption of 

figure 12 for explanation of abbreviations. Gaps in lithology represent covered intervals.
ROCKY GULCH SANDSTONE MEMBER 49

The upper parts of the fining- and thinning-upward cycles, as well as most of the upper part of the 

Rocky Gulch Sandstone Member, consist of beds of 

very fine grained to medium-grained sandstone. 

These beds typically range from 10 cm to 5 m in 

thickness and are laterally continuous. They are 

graded and separated by shale beds; amalgamation is 

uncommon. The basal surfaces are planar but erosive 

into the underlying shale beds. Flute casts, groove 

casts, prod marks and other tool marks, and minor 

load casts are present as sole markings. The upper 

surfaces are also planar but are gradational into 

overlying shale except where intervening siltstone 

deposits are present. These beds are massive or, 

more commonly, organized into Bouma sequences of 

the Ta_e or Tb_e type. They resemble turbidite facies 

C and D of Mutti and Ricci Lucchi (1972).

Beds of conglomerate that appear to be laterally 

continuous are present in many sections of the Rocky 

Gulch Sandstone Member. They appear to be most 

common in the lower part of the member, but conglomerate beds are also present in the upper part of 

the member, especially in the Bear Creek valley area. 

These conglomerate beds are generally graded with 

erosive basal surfaces and are composed of a variety 

of clasts. The beds generally range from 15 cm to 10 

m in thickness. However, very coarse conglomerate 

units at least as thick as 60 m and possibly as thick 

as 95 m are present in the upper part of the Rocky 

Gulch Sandstone Member in the Ditch Creek area. 

The conglomerate beds generally consist of wellrounded clasts of pebble size, but in the Ditch Creek 

area, boulders as long as 32 cm are present. The conglomerate is typically well bedded, imbricated, and 

clast supported; it has a matrix of smaller clasts and 

sandstone and contains in a few places medium- to 

large-scale tabular cross-strata. Although no pebble 

counts were made in the type section of the Rocky 

Gulch Sandstone Member, a pebble count from the 

Ditch Creek area yielded an average composition of 

53 percent quartzite, 23 percent metavolcanic rocks, 

13 percent chert, \ percent granitic rocks, and 4 percent metasedimenta^y rocks. The conglomerate beds 

appear to be similar,to the facies A turbidites of 

Mutti and Ricci Lucchi (}972).

Siltstone beds locally overlie the graded beds of 

fine- to medium-grained sandstone in the Bear Creek 

valley and Cottonwood Creek valley areas (Nilsen 

and others, 1984, p. 24-25, 33-36; Nilsen, 1984d). 

These siltstone beds commonly truncate the upper 

parts of Bouma sequences in the sandstone beds and 

are overlain abruptly to gradationally by shale. The 

siltstone is typically wavy bedded or parallel laminated and locally contains current ripple markings

and convolute laminations. These beds appear to be 

contourite deposits (Bouma and Hollister, 1973).

Interbedded shale is generally less than 1 m 

thick and is commonly covered by colluvium. In the 

lower parts of the Rocky Gulch Sandstone Member, 

the shale is commonly massive and contains evenly 

distributed silt grains, whereas in the upper parts, 

the shale locally contains thin beds or laminae of siltstone that have sharp, erosive bases and are graded. 

Bioturbation is not abundant, but Nereites-facies 

grazing burrows have been observed on a few bedding surfaces.

STRATIGRAPHIC RELATIONS

The Rocky Gulch Sandstone Member rests locally 

with erosional contact but generally conformably on 

the Ditch Creek Siltstone Member and is conformably 

overlain by the Blue Gulch Mudstone Member. The 

lower contact is placed where the predominant lithology changes upward from siltstone to sandstone, and 

the upper contact is set at the upward change from 

sandstone to mudstone.

The lower contact is clearly erosional in the 

Hornbrook area and has previously been interpreted 

as an unconformity. However, to the northwest and 

southeast of the type area, the contact appears to be 

gradational, and it is considered herein to be generally conformable. Better age control would be helpful 

in completely defining the contact. I place the contact 

at the base of the first bed of fine- to medium-grained 

or medium-grained sandstone that is more than 1 m 

thick.

The upper contact is clearly gradational, although 

locally abrupt, and is placed at the top of the highest 

fining- and thinning-upward cycle of sandstone beds 

in the Rocky Gulch Sandstone Member. The boundary is placed at the top of the highest bed of finegrained or medium-grained sandstone that is thicker 

than 30 cm.

Lateral stratigraphic relations of the Rocky Gulch 

Sandstone Member are poorly defined. The member 

undoubtedly extends northeastward beneath the Cascade Range, but the distance is not known because 

the subsurface structure is poorly known. To the 

southeast and northwest, it is covered and overlapped 

by Tertiary volcanic rocks and Quaternary alluvium 

in the Medford Valley and Shasta Valley areas. 

Stratigraphic equivalents of the Rocky Gulch Sandstone Member do not appear to be present in the 

Grave Creek area, so the member probably pinches 

out between the Dark Hollow area and the Grave 

Creek area. The Rocky Gulch Sandstone Member
50 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

probably also originally extended southwestward over 

part of the Klamath Mountains, but the distance is 

impossible to determine.

THICKNESS

The Rocky Gulch Sandstone Member is relatively 

constant in thickness over its outcrop area, generally 

ranging from 115 to 225 m. Peck and others (1956) 

determined a thickness of 62.8 m and Jones (1959) a 

thickness of 40 m for strata equivalent to the Rocky 

Gulch Sandstone Member in the type area of the 

Hornbrook Formation. Elliott (1971) determined a 

thickness of 137 m in the northern Cottonwood Creek 

valley area and McRnight (1971) a thickness of 220 

m in the Bear Creek valley for strata equivalent to 

the Rocky Gulch Sandstone Member.

I have measured a number of additional sections 

of the Rocky Gulch Sandstone Member north of the 

type section described herein, which is 171.2 m thick. 

A section adjacent to Ditch Creek (WM sec. 18, T. 47 

N., R. 6 W., Hornbrook 15-minute quadrangle), about 

5 km northwest of the type section, yielded a thickness of 221.4 m. Two additional sections were measured south of Hilt in the northern Cottonwood Creek 

valley. The first, northeast of Cottonwood Creek 

(NWKNEK sec. 1, T. 47 N., R. 7 W., Hornbrook 15- 

minute quadrangle), is 115 m thick and the second, 

along the west fork of Cottonwood Creek (NEJ4SWK 

sec. 26, T. 48 N., R. 7 W., Hornbrook 15-minute quadrangle), is 126.4 m thick. Three partial sections of the 

Rocky Gulch Sandstone Member were also measured: 

(1) a thickness of 46 m for a section about 1 km 

northwest of the type section, along some roadcuts 

southwest of Interstate Highway 5 (center, sec. 29, T. 

47 N., R. 6 W., Hornbrook 15-minute quadrangle); (2) 

a thickness of about 74 m for a section about 1 km 

northeast of downtown Ashland in roadcuts along 

Eagle Mill Road, adjacent to its intersection with Oak 

Street (SWtf sec. 3, T. 38 S., R. 1 E., Ashland 15- 

minute quadrangle); and (3) a thickness of 75 m for a 

section 4 km southeast of Medford (SW1/ sec. 32, T. 

37 S., R. 1 W., Medford 15-minute quadrangle).

The reasons for the low thicknesses obtained for 

the Rocky Gulch Sandstone Member by Peck and others (1956) and Jones (1959) in the type area are not 

certain, but they probably reflect differences in placement of the upper boundary of the member. To the 

southeast, the Rocky Gulch Sandstone Member appears to thin, and it is absent on the south flank of 

Black Mountain. To the northwest, the Rocky Gulch 

Sandstone Member appears to extend continuously to 

the Dark Hollow area with minor fluctuations in 

thickness.

DEPOSITIONAL ENVIRONMENT

The depositional environment of the Rocky Gulch 

Sandstone Member has been the subject of varied 

speculation by previous workers. Although they found 

no fossils in strata equivalent to the Rocky Gulch 

Sandstone Member, Peck and others (1956) apparently felt that the strata nevertheless were marine. 

Jones (1959), possibly influenced by his interpretation of an unconformity at the base of strata equivalent to the Rocky Gulch Sandstone Member, 

concluded that the strata were partly nonmarine in 

origin. Elliott (1971) inferred a moderate- to high-energy shallow-marine depositional environment for 

strata equivalent to the Rocky Gulch Sandstone 

Member chiefly on the basis of what he considered to 

be the dominant cross-bedded character of the unit; 

however, he considered at least one outcrop, about 

2.5 km south of Hilt, to be of fluvial origin (Elliott, 

1971, p. 37). McKnight (1971) concluded on the basis 

of sedimentary features that strata equivalent to the 

Rocky Gulch Sandstone Member in the Bear Creek 

valley area were deposited in marine, marginal marine, and nonmarine depositional environments.

On the basis of my detailed work on the Rocky 

Gulch Sandstone Member, which includes measurement of four complete and three partly complete sections and abundant sampling of shale interbeds for 

microfossils, I infer that the member is chiefly of 

deep-marine origin. There is no evidence for subaerial 

deposition by rivers or for high-energy or low-energy 

marine shelf sedimentation, as suggested by others. 

Medium- and large-scale cross-bedding is very rare in 

the Rocky Gulch Sandstone Member; complex patterns of jointing and curviplanar near-surface fracturing have been misinterpreted by previous workers to 

be cross-bedding. When examined in detail, these 

fractures were observed to cut across stratification.

Most beds of sandstone are normally graded, are 

separated by thin shale intervals or are amalgamated, and have sole markings at their base. Only 

fragments of molluscan shells have been observed, 

and these have been broken and abraded, clearly carried by currents that transported sand and gravel. 

Megafossils (including pelecypods and ammonites) reported by McKnight (1971) from sandstone in the 

lower part of his subunit Khs3a suggest that this subunit may actually lie within the upper part of the 

Ditch Creek Siltstone Member.

The thick-bedded to massive sandstone beds that 

lack any internal stratification and dominate the 

lower part of the Rocky Gulch Sandstone Member 

were probably deposited by sediment gravity flows, 

most likely high-concentration turbidity currents, 

sandy debris flows, grain flows, or fluidized sediment
BLUE GULCH MUDSTONE MEMBER 51

flows. These beds, some as thick as 10 m, lack the 

internal sedimentary structures such as cross-stratification or thorough bioturbation that typify shallowmarine or fluvial sandstones. Their erosive bases, 

grading, and amalgamation suggest deposition by repeated sediment gravity flows of very sand-rich suspensions.

The thinner beds of sandstone that are graded 

and organized into Ta_e and Tb_e Bouma sequences 

were probably deposited by low-concentration turbidity currents. These beds are laterally continuous and 

most abundant in the upper part of the Rocky Gulch 

Sandstone Member. Reworking of the tops of many of 

these beds suggests the presence of contour currents.

The stratigraphic position of the Rocky Gulch 

Sandstone Member between outer-shelf deposits of 

the underlying Ditch Creek Siltstone Member and 

basin-plain deposits of the overlying Blue Gulch Mudstone Member, the organization of the Rocky Gulch 

Sandstone Member into laterally continuous fmingand thinning-upward cycles, the preponderance of 

sediment gravity-flow deposits, the lateral continuity 

of the member over a northwest-southeast distance of 

about 80 km despite a thickness of only 115-225 m, 

and the presence of reworked tops of some turbidite 

sandstones all suggest that the Rocky Gulch Sandstone Member was deposited as a turbidite apron 

mantling a regional submarine slope of the ancestral 

Klamath Mountains (Nilsen, 1984b). There is no evidence in the sedimentary record of the Hornbrook 

Formation for major deep-sea fan sedimentation.

AGE AND CORRELATION

No fossil ages had been previously determined 

from strata equivalent to the Rocky Gulch Sandstone 

Member. The unconformity postulated at the base of 

the Rocky Gulch Sandstone Member was thought by 

Peck and others (1956) and most subsequent workers 

to range from late Coniacian to early Campanian in 

age. The missing Santonian Stage is generally 

thought to be about 3 m.y. in length (Palmer, 1983; 

pl. 1).

Although I collected many samples of shale from 

the Rocky Gulch Sandstone Member for foraminiferal 

analysis, only one sample, from exposures along Interstate Highway 5 east of Medford (USGS Mesozoic 

locality Mf6587), yielded foraminifers (Nilsen and 

others, 1984, stop 10). These foraminifers are indicative of a middle Turonian age. Unfortunately, the 

sample is from a measured section in which the base 

is covered and the top is faulted, so it is not certain 

from which part of the Rocky Gulch Sandstone Member the sample was taken. However, the unit is virtually identical to other outcrops of the Rocky Gulch 

Sandstone Member and has confidently been mapped 

as such (Nilsen and others, 1983, pl. 1).

Samples from the type section of the Rocky Gulch 

Sandstone Member were barren of microfossils. A 

sample from the uppermost part of the underlying 

Ditch Creek Siltstone Member, taken from a section 

measured near Bailey Hill in the northern Cottonwood Creek valley (USGS Mesozoic locality Mfl6581), 

is considered to be late Turonian to Coniacian in age 

based on benthic foraminifers; at this locality, the 

contact between the Ditch Creek Siltstone Member 

and the Rocky Gulch Sandstone Member appears to 

be gradational and conformable.

Thus the age of the Rocky Gulch Sandstone 

Member appears to be middle Turonian to 

Campanian(?). However, the age data are very 

sparse, and more fossiliferous samples from the member are needed to better define its age.

The Rocky Gulch Sandstone Member is, at least 

partly, correlative with a number of other geologic 

units in Oregon and northern California. It may correlate with part of the Cretaceous section at Grave 

Creek. It appears to be younger than exposed Cretaceous strata to the north in the Days Creek, Riddle, 

and Mitchell areas and to the west in the O'Brien 

area. It may correlate, however, with part of the subsurface sequence of the Ochoco basin (Thompson and 

others, 1984). It probably correlates in part with 

strata along the southwestern coast of Oregon that 

are at least partly of Campanian age [the Cape 

Sebastian Sandstone and Hunters Cove Formation of 

Dott (1971)].

To the south, the Rocky Gulch Sandstone Member may possibly be correlative with the uppermost 

part of the Budden Canyon Formation of Murphy and 

others (1969) southwest of Redding and with the 

Great Valley sequence near Redding (Popenoe, 1943; 

Matsumoto, 1960); however, these units are more 

probably at least partly older than the Rocky Gulch 

Sandstone Member. Farther south, the Rocky Gulch 

Sandstone Member is probably at least partly correlative with the Great Valley sequence on both the 

east and west flanks of the Sacramento Valley 

(Ingersoll, 1979; Haggart and Ward, 1984), and with 

parts of the Coastal belt terrane of the Franciscan 

assemblage.

BLUE GULCH MUDSTONE MEMBER

DEFINITION

The name "Blue Gulch Mudstone Member" was 

applied by Nilsen (1984a) to continuous outcrops of
52 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

marine strata that are predominantly composed of 

mudstone; these strata form the uppermost member 

of the Hornbrook Formation. The member was named 

for exposures adjacent to Blue Gulch about 4 km 

south of Hornbrook, California (fig. 17). However, because the mudstone does not generally crop out very 

well, the type section is a composite one that was 

measured in four different places south and west of 

Hornbrook (fig. 8). The main part of the section, however, was measured across Blue Gulch.

The Blue Gulch Mudstone Member contains 

within it two prominent mappable lithologic units. 

The first is a sandstone lens that crops out only in 

the Hornbrook area; it is named the Rancheria Gulch 

Sandstone Beds (Nilsen, 1984a, b, c, d) for outcrops 

adjacent to Rancheria Gulch northwest of Hornbrook 

(figs. 6, 7). The lens is present in the lower part of 

the Blue Gulch Mudstone Member; it is underlain by 

about 80 m of strata that consist predominantly of 

mudstone and is overlain by about 690 m of strata 

that also consist predominantly of mudstone. The 

lens extends laterally for about 8 km before pinching 

out into mudstone to the northwest and southeast.

Stratigraphically higher within the Blue Gulch 

Mudstone Member is the Hilt Bed (Nilsen, 1984a, b, 

c, d) (figs. 6, 7), a thick single bed of sandstone that 

has been mapped almost continuously from the 

northern Shasta Valley to the Ashland area, a distance of about 45 km (pi. 1; Nilsen, 1983). The Hilt 

Bed forms a mappable bed overlain and underlain by 

thick sequences of strata that consist predominantly 

of mudstone. The lateral extent and persistence of 

the Hilt Bed along the outcrop belt of the Hornbrook 

Formation have facilitated mapping of the northeasttrending cross-faults that separate the outcrop belt 

into numerous separate fault-bounded blocks (pi. 1).

The Blue Gulch Mudstone Member is the fifth or 

uppermost member of the Hornbrook Formation. It is 

equivalent to the upper part of member IV and members V and VI (of Hornbrook Formation) as mapped 

by Peck and others (1956) (fig. 6). Their member V is 

equivalent to the Rancheria Gulch Sandstone Beds. 

The Blue Gulch Mudstone Member is also equivalent 

to unit C (of Hornbrook Formation) of Jones (1959), 

who correctly mapped a sandstone lens in the lower 

part of the unit in the Hornbrook area that is equivalent to the Rancheria Gulch Sandstone Beds. The 

Blue Gulch Mudstone Member is also equivalent to 

(1) McKnight's (1971) unit Khm in the Bear Creek 

valley area, which he considered to be the uppermost 

unit of his Hornbrook Formation; and (2) M.A. 

Elliott's (unpub. data, 1980) unit D, which he considered to be the uppermost unit of his Hornbrook Formation in the northern Cottonwood Creek valley

area. Nilsen and others (1983) mapped the Blue 

Gulch Mudstone Member as their unit d of the 

Hornbrook Formation; they mapped the Rancheria 

Gulch Sandstone Beds as separate subunits lj and 12 

and the Hilt Bed as subunit m within unit d (fig. 6).

The Blue Gulch Mudstone Member rests conformably on the Rocky Gulch Sandstone Member throughout the outcrop area of the Hornbrook Formation 

except along the south flank of Black Mountain 

(parts of sees. 10, 11, 14, and 15, T. 46 N., R. 6 W., 

Hornbrook 15-minute quadrangle), where the Rocky 

Gulch Sandstone Member appears to be missing. In 

this area, the lithologies of patchy outcrops beneath 

large landslides derived from the intrusive rocks of 

Black Mountain suggest that the Blue Gulch Mudstone Member rests directly on massive, thoroughly 

bioturbated, silty fine-grained sandstone thought to 

be the Ditch Creek Siltstone Member. However, this 

contact was not directly observed in the field.

The Blue Gulch Mudstone Member is overlain 

unconformably by either conglomeratic Tertiary 

nonmarine strata or by Tertiary volcanic rocks. The 

Tertiary sedimentary rocks, thought to be Eocene in 

age, were mapped by Wells (1939, 1956) as the 

Eocene Umpqua Formation, by McKnight (1971, 

1984) as the Payne Cliffs Formation, and by Beaulieu 

and Hughes (1977) as unnamed Eocene sedimentary 

rocks; these strata overlie the Blue Gulch Mudstone 

Member in most of the Bear Creek valley area but 

appear to be missing in the Cottonwood Creek valley 

area. Similar but much thinner conglomeratic 

nonmarine strata rest unconformably on the Blue 

Gulch Mudstone Member in the northeastern Shasta 

Valley; these unnamed Tertiary sedimentary rocks 

were mapped by Nilsen and others (1983) in this area 

and are shown on plate 1.

Tertiary volcanic rocks of the western Cascade 

Range unconformably overlie the Blue Gulch Mudstone Member in the Cottonwood Creek valley area, 

between the Siskiyou Summit fault and the northern 

margin of the Shasta Valley. These volcanic rocks 

were mapped by Wells (1956), in ascending order, as 

the Eocene Colestin Formation and the Oligocene 

Roxy Formation. In northern California, the volcanic 

rocks form part of the Western Cascade Series of Williams (1949) and have been subdivided by Hammond 

(1983) and discussed by Vance (1984).

The lower boundary of the Blue Gulch Mudstone 

Member is marked by the abrupt to gradual upward 

change from graded beds of fine- to medium-grained 

sandstone with some thin interbeds of mudstone that 

characterize the upper part of the Rocky Gulch Sandstone Member to bioturbated mudstone with a few 

thin interbeds of siltstone and very fine grained sand-
BLUE GULCH MUDSTONE MEMBER 53

stone that characterizes the lower part of the Blue 

Gulch Mudstone Member. The Rocky Gulch Sandstone Member consists chiefly of sandstone and contains beds as thick as several meters; mudstone or 

shale is typically subordinate and forms thin but 

laterally continuous interbeds. The Rocky Gulch 

Sandstone Member is typically organized into finingupward cycles, and the top of the uppermost cycle 

marks the stratigraphic boundary between the members. Because the Rocky Gulch Sandstone Member is 

a resistant unit that underlies a series of ridges and 

the Blue Gulch Mudstone Member is a much less resistant unit that underlies the main parts of the Bear 

Creek, Cottonwood Creek, and Shasta valleys, the

FIGURE 17. Blue Gulch Mudstone Member of the Hornbrook Formation. Khbh, Cretaceous Hilt Bed (of Blue Gulch Mudstone 

Member); Khb, Cretaceous Blue Gulch Mudstone Member; 

Khbr, Cretaceous Rancheria Gulch Sandstone Beds (of Blue 

Gulch Mudstone Member); Khrg, Cretaceous Rocky Gulch 

Sandstone Member (of Hornbrook Formation); Qt, Quaternary 

terrace deposits; Ti, Tertiary intrusive rocks; TVS, Tertiary volcanic and sedimentary rocks. Contact, dashed line, approximately located. A, View southeast of type section on north 

flank of Black Mountain. B, View east of type section across 

Blue Gulch from ridge crest underlain by Rocky Gulch Sandstone Member located in WA sec. 33, T. 47 N., R. 6 W., 

Hornbrook 15-minute quadrangle. C, View northeast of upper 

part of type section of the Blue Gulch Mudstone Member, east 

of Hornbrook, California, in sec. 21, T. 47 N., R. 6 W., 

Hornbrook 15-minute quadrangle. D and E, Outcrops of siltstone and interbedded mudstone in type section east of Blue 

Gulch. Resistant bed is approximately 40 cm thick.
54 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

boundary is also generally marked by a pronounced 

topographic change. I suggest that the base of the 

Blue Gulch Mudstone Member in detailed measured 

sections be placed at the top of the uppermost bed of 

fine- or medium-grained sandstone that is thicker 

than 30 cm.

The upper boundary of the Blue Gulch Mudstone Member is typically well marked by the 

unconformity at the base of the overlying Tertiary 

sequence and by an abrupt change in lithology 

from bioturbated marine mudstone that is locally 

fairly resistant as a result of baking by overlying 

or locally intrusive volcanic rocks to more poorly 

lithified, unbioturbated, nonmarine conglomerate 

and sandstone or volcanic rocks. I have not observed a paleosol at the top of the member at any 

locality. The Tertiary sedimentary rocks consist of 

polymict, imbricated conglomerate interbedded 

with medium-grained to granular arkosic sandstone; these sedimentary rocks were probably deposited by braided streams. Bedded mudstone or 

shale is generally missing from the lower part of 

the Tertiary section, which is dominated by conglomerate and sandstone. Directly above the 

unconformity, however, angular to poorly rounded 

blocks of the Blue Gulch Mudstone Member are incorporated in the lower part of the Tertiary section. 

These blocks have clearly been eroded from the underlying mudstone.

The surface of the unconformity locally has been 

eroded deeply into the Blue Gulch Mudstone Member. In the Ashland area, the Blue Gulch Mudstone 

Member has been completely removed in some locations by erosion, so that the Tertiary nonmarine 

rocks rest directly on the Ditch Creek Siltstone Member and Rocky Gulch Sandstone Member. In this 

area, the contact between the Hornbrook Formation 

and the younger rocks is clearly an angular 

unconformity, whereas in most other parts of the outcrop area, the contact is essentially disconformable. 

The downcutting has probably caused the erosion of 

the Hilt Bed in much of the Bear Creek valley area 

north of Ashland.

Where the upper contact of the Blue Gulch Mudstone Member is marked by volcanic rocks resting on 

marine mudstone, there is commonly abundant deformation in the upper part of the underlying mudstone. 

Slump folding, faulting, shearing, veining, and extensional fracturing affect the mudstone just below the 

unconformable contact with the volcanic rocks. These 

features probably reflect surface processes (chiefly 

subaerial landsliding) caused by the outpouring of 

thick flows of lava on relatively unstable slopes underlain by the mudstone.

AREAL EXTENT

The Blue Gulch Mudstone Member extends essentially continuously for about 73 km, almost the 

entire outcrop extent of the Hornbrook Formation. 

However, it is generally poorly exposed and underlies 

grassy slopes and valleys. Along the northeastern 

margins of the Bear Creek and Cottonwood Creek 

valleys, it underlies moderate to steep slopes held up 

by resistant Tertiary volcanic rocks. The best exposures of the Blue Gulch Mudstone Member are along 

the northwest side of Black Mountain in the Blue 

Gulch area, where major downcutting has exposed 

the mudstone in the floor of Blue Gulch and its tributary drainage network. Other good exposures are 

generally more areally restricted; they include outcrops on the floors of Bear Creek and Cottonwood 

Creek and some of their tributaries, outcrops adjacent to resistant Tertiary intrusive rocks, and exposures made by roadcuts, especially large freeway 

cuts, along Interstate Highway 5 and by railroad cuts 

in both the Bear Creek and Cottonwood Creek valley 

areas. However, very large areas that are probably 

underlain at depth by the Blue Gulch Mudstone 

Member in the Medford and Shasta Valleys are covered by Quaternary alluvium.

Because the Blue Gulch Mudstone Member is the 

thickest member of the Hornbrook Formation, it generally has the widest outcrop width. However, in part 

of the Bear Creek valley between Ashland and Talent, it is very thin because of major erosion at the 

base of the Tertiary sedimentary rocks.

The northernmost outcrops of the Blue Gulch 

Mudstone Member are located at Hanley Hill in the 

Medford Valley, about 4.5 km northeast of Jacksonville and 5 km west-northwest of Medford (sec. 22, 

SE part of sec. 68, and NW corner of sec. 70, T. 37 S., 

R. 2 W., Medford 15-minute quadrangle). This hill is 

underlain by resistant Tertiary intrusive rocks and 

by strata of the Blue Gulch Mudstone Member that 

are also resistant because they were baked and contact metamorphosed by the heat of the intrusive 

rocks. The presence of the Blue Gulch Mudstone 

Member at Hanley Hill, in the northeastern parts of 

the Dark Hollow area, and in a broad low hill about 3 

km south-southeast of Medford (parts of sees. 31, 37, 

43, 45, 46, 47, and 54, T. 37 S. and T. 38 S., R. 1 W., 

Medford 15-minute quadrangle) suggests that it underlies much of the Medford Valley but is buried beneath thick Quaternary alluvial and alluvial-fan 

deposits.

Beaulieu and Hughes (1977), in their map of central Jackson County, Oregon, showed additional and 

more widespread outcrop areas of mid-Cretaceous
BLUE GULCH MUDSTONE MEMBER 55

and Upper Cretaceous strata in the Medford Valley 

area, including outcrops as far north as the area 

northwest of Central Point (sees. 4, 46, 47, 61, 62, 

and 63, T. 36 S., R. 2 W., Medford 15-minute quadrangle). However, they did not indicate the rock types 

present in these areas and appear to have included 

some lower Tertiary strata within their mapped Cretaceous unit, which makes it impossible to determine 

if any of these areas are underlain by the Blue Gulch 

Mudstone Member. In addition, the lack of measured 

strikes and dips in these areas makes stratigraphic 

assignment difficult. My own mapping in these areas 

(pi. 1) failed to reveal any outcrops of the Blue Gulch 

Mudstone Member north of Hanley Hill, nor did I 

find any field indications that other members of the 

Hornbrook Formation crop out in the areas mapped 

by Beaulieu and Hughes (1977) in the northern part 

of the Medford 15-minute quadrangle.

Smith and others (1982) also show large areas of 

outcrop of Cretaceous sedimentary strata in the 

Medford Valley, particularly north of the Jacksonville 

and Dark Hollow areas. Some of these outcrop areas 

appear to be north of the Hanley Hill area. However, 

they did not divide the Cretaceous strata into subunits. My own mapping in the Medford Valley has 

failed to reveal any outcrops of the Blue Gulch Mudstone Member in the areas shown by Smith and others (1982), other than those mapped outcrops shown 

in plate 1 of this report.

Farther southeastward, the Blue Gulch Mudstone 

Member does not crop out between the Dark Hollow 

area and Talent. In this area, it was apparently overlain unconformably, covered by Quaternary alluvial 

and alluvial-fan deposits, and partly eroded prior to 

deposition of the Tertiary nonmarine strata. Southeastward from Talent, however, it crops out more 

continuously to the northern Shasta Valley.

Between Talent and Ashland in the northern 

Bear Creek valley, the outcrop width of the Blue 

Gulch Mudstone Member is narrow and the member 

is poorly exposed. Between Ashland and the Siskiyou 

Summit fault in the Bear Creek valley, its outcrop 

width is wider and it is better exposed because it has 

been uplifted adjacent to numerous resistant Tertiary 

intrusive rocks.

In the Cottonwood Creek valley area, the Blue 

Gulch Mudstone Member crops out extensively, with 

the best exposures of its lower part located south of 

the Klamath River near Blue Gulch, and the best exposures of its upper part located on the hillsides east 

of Hornbrook and in the Bailey Hill area southeast of 

Hilt.

In the northern Shasta Valley area, the outcrop 

width of the Blue Gulch Mudstone Member reaches

its maximum of more than 5 km. The southernmost 

exposures of the Blue Gulch Mudstone Member are 

on two low ridges about 1.5 km south of Marys Peak 

(NE% sec. 25, T. 46 N., R. 6 W., and NWK sec. 30, T. 

46 N., R. 5 W., Copco 15-minute quadrangle) and on 

a flat plateau about 2.5 km to the east (NE/4 sec. 29, 

T. 46 N., R. 5 W., Copco 15-minute quadrangle). The 

Blue Gulch Mudstone Member crops out extensively 

in the northern Shasta Valley, where it is upheld by 

numerous Tertiary intrusive rocks, but it is entirely 

missing from the central and southern parts of the 

Shasta Valley. In these areas, it appears to be concealed beneath unconformably overlying Quaternary 

alluvial deposits, alluvial-fan deposits, and 

volcanogenic debris-avalanche deposits. It probably 

extends for some distance in the subsurface southeast 

of the Shasta Valley but is completely concealed in 

that direction by Tertiary volcanic rocks.

TYPE SECTION

The type section of the Blue Gulch Mudstone 

Member (fig. 18) was measured in four areas (fig. 8): 

(1) the lower part, 164.42 m thick, from the top of the 

underlying Rocky Gulch Sandstone Member to the 

top of the Rancheria Gulch Sandstone Beds, was 

measured near Rancheria Gulch in the SW/4 sec. 20, 

T. 47 N., R. 6 W., Hornbrook 15-minute quadrangle, 

directly west of the Henley exit on Interstate Highway 5; (2) the interval from the top of the Rancheria 

Gulch Sandstone Beds to the top of the Hilt Bed 

(297.2 m thick) was measured across Blue Gulch, 

southeast of the Klamath River, in the NE/4 sec. 33 

and NW% sec. 34, T. 47 N., R. 6 W., Hornbrook 15- 

minute quadrangle; (3) an interval of mudstone and 

thin-bedded turbidites above the Hilt Bed (a total of 

133.27 m) was measured in the SWKSEK sec. 21, T. 

47 N., R. 6 W., Hornbrook 15-minute quadrangle, on 

the hillside about 1 km southeast of the town of 

Hornbrook, north of the Klamath River; and (4) the 

upper 255.00 m was measured in the hills northeast 

of Blue Gulch in the NEK sec. 34 and the SEA 1 sec. 

27, T. 47 N., R. 6 W., Hornbrook 15-minute quadrangle, south of the Klamath River. This last interval 

includes a covered interval of 186.27 m across a 

prominent Quaternary alluvial terrace north of 

Klamathon Spring (fig. 8). Thus, the total thickness 

of the Blue Gulch Mudstone Member in its composite 

type section is 849.87 m (fig. 18); this total does not 

include the thickness of two Tertiary intrusions in 

the upper part of the member, but it does include the 

thickness of the Rancheria Gulch Sandstone Beds in 

its type section and the thickness of the Hilt Bed in 

this section rather than in its type section.
56 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

Volcanic rocks and volcaniclastic sedimentary

rocks of Tertiary age Gower part). 

Unconformable contact. 

Hornbrook Formation: 

Blue Gulch Mudstone Member:

Siltstone and sandstone, very fine grained, interbedded, bioturbated; scattered plant fossils ....

Sandstone, very fine grained to fine-grained, 

graded, bioturbated; Tabcde Bouma sequence ..

Siltstone, massive; plant fossil fragments...........

Sandstone, very fine grained to fine-grained, 

graded, bioturbated; T^ Bouma sequence.......

Mudstone and siltstone, thoroughly bioturbated; 

scattered plant fossils.........................................

Basalt, Tertiary, intrusive.....................................

Sandstone, very fine grained to fine-grained; 

tightly folded tectonically adjacent to overlying intrusive rock................................................

Siltstone and mudstone, thoroughly bioturbated; 

scattered plant fossils.........................................

Sandstone, very fine grained, graded, bioturbated; 

Tabcde Bouma sequence ......................................

Mudstone with thinly interbedded siltstone 

turbidites, bioturbated........................................

Sandstone, very fine grained, graded, 

bioturbated, concretionary.................................

Mudstone with thinly interbedded siltstone 

turbidites, bioturbated........................................

Sandstone, very fine grained to fine-grained, 

graded, bioturbated; Tabce Bouma sequence....

Mudstone with thinly interbedded siltstone 

turbidites, bioturbated........................................

Sandstone, very fine grained, bioturbated ..........

Mudstone with thinly interbedded siltstone 

turbidites, bioturbated........................................

Sandstone, very fine grained to fine-grained;

Thickness 

(in meters)

Tace Bouma sequence,

Mudstone with thinly interbedded siltstone

turbidites, bioturbated..................................

Basalt, Tertiary, intrusive...............................

Sandstone, very fine grained; Tabcde Bouma

sequence.........................................................

Mudstone with thinly interbedded siltstone

turbidites........................................................

Sandstone, very fine grained; Tabcde Bouma

sequence.........................................................

Mudstone with thinly interbedded siltstone

turbidites........................................................

Covered interval (float of mudstone)..............

Mudstone with thinly interbedded siltstone

turbidites........................................................

Covered interval (float of mudstone)..............

Mudstone, massive ...........................................

Sandstone, very fine grained; Tj^ Bouma

sequence.........................................................

Mudstone, massive ...........................................

Sandstone, very fine grained; Tbcde Bouma

sequence .........................................................

Mudstone, massive ...........................................

Sandstone, very fine grained; Tbcde Bouma

sequence.........................................................

Mudstone, massive ...........................................

Sandstone, very fine grained; T^g Bouma

sequence.........................................................

4.25

.40 

.05

.38

3.30 

(2.25)

.67

1.50

.14

.75

.10

4.30

.16

7.50 

.05

10.50

.12

8.85 

(1.10)

.15 

.60 

.90

9.00

7.50

7.50

186.27

16.50

.05 

9.00

.05 

6.30

.08 

4.40

.15

Blue Gulch Mudstone Member Continued: 

Mudstone, massive ..............................................

Sandstone, very fine grained to fine-grained;

^bcde Bouma sequence.....................................

Mudstone, massive ..............................................

Sandstone, very fine grained; T^g Bouma

sequence ............................................................

Mudstone, massive ..............................................

Sandstone, very fine grained; Tbcde Bouma

sequence............................................................

Mudstone, massive ..............................................

Sandstone, very fine grained; Tcde Bouma

sequence ............................................................

Mudstone, massive ..............................................

Sandstone, very fine grained; Tcde Bouma

sequence ............................................................

Mudstone, massive ..............................................

Sandstone, very fine grained to fine-grained;

Bouma sequence with convolute lamination 

Mudstone, massive ..............................................

Sandstone, very fine grained to fine-grained;

^bcde Bouma sequence.....................................

Mudstone, massive ..............................................

Sandstone, very fine grained; T^g Bouma

sequence............................................................

Mudstone, massive ..............................................

 Sandstone, fine-grained; Tabcde Bouma

sequence ............................................................

Mudstone, massive ..............................................

Sandstone, fine-grained; Tabcde Bouma

sequence ............................................................

Mudstone, massive ..............................................

Sandstone, fine-grained; T^g Bouma

sequence ............................................................

Mudstone, massive ..............................................

Sandstone, very fine grained; T^ Bouma

sequence............................................................

Mudstone, massive ..............................................

Sandstone, very fine grained; T^ Bouma

sequence ............................................................

Mudstone, massive ..............................................

Sandstone, very fine grained; T^ Bouma

sequence ............................................................

Mudstone, massive ..............................................

Sandstone, fine-grained; Tabcde Bouma

sequence ............................................................

Mudstone, massive ..............................................

Sandstone, very fine grained to fine-grained; 

Tabcde Bouma sequence .......................................

Mudstone, massive ..............................................

Sandstone, very fine grained; T^g Bouma

sequence ............................................................

Mudstone, massive ..............................................

Sandstone, very fine grained; T^g Bouma

sequence ............................................................

Mudstone, massive ..............................................

Sandstone, very fine grained; T^xAe Bouma

sequence ............................................................

Mudstone, massive ..............................................

Sandstone, very fine grained; TbcAe Bouma

sequence............................................................

Mudstone, massive ..............................................

Sandstone, very fine grained; T^g Bouma

sequence ............................................................

Thickness 

(in meters)

2.50

.12 

2.35

.18 

8.95

.05 

8.40

.05 

5.85

.08 

4.50

.45 

3.15

.55 

3.20

.10 

1.73

.40 

4.25

.43 

1.52

.15 

.22

.05 

.55

.05 

1.12

.05 

1.15

.22 

1.10

.20 

.55

.15 

1.35

.10 

2.40

.18 

9.05

.10 

2.25

.15
BLUE GULCH MUDSTONE MEMBER 57

Blue Gulch Mudstone Member Continued:

Mudstone with a few thinly interbedded siltstone turbidites ...................................................

Sandstone, very fine grained; TbcAe Bouma 

sequence ...............................................................

Mudstone, massive, mostly covered .....................

Hilt Bed; chiefly sandstone, fine- to very fine 

grained, graded; Tabababcde Bouma sequence ..

Mudstone, massive, partly covered ......................

Sandstone, very fine grained; TcAe Bouma 

sequence ...............................................................

Mudstone, massive .................................................

Sandstone, very fine grained to fine-grained; 

Tfacde Bouma sequence........................................

Mudstone, massive .................................................

Sandstone, very fine grained to fine-grained; 

Tbcde Bouma sequence ........................................

Mudstone, massive .................................................

Sandstone, fine-grained; Tabcde Bouma sequence

Mudstone, massive .................................................

Siltstone; Tde Bouma sequence, concretionary....

Mudstone, massive .................................................

Sandstone, fine-grained; TbcAe Bouma 

sequence ...............................................................

Mudstone, massive .................................................

Sandstone, very fine grained; T^,, Bouma 

sequence ...............................................................

Mudstone, massive .................................................

Sandstone, very fine grained; T^g Bouma 

sequence ...............................................................

Mudstone with some thinly interbedded siltstone 

turbidites..............................................................

Sandstone, fine-grained; T^g Bouma 

sequence ...............................................................

Mudstone, silty, massive........................................

Sandstone, very fine grained; TcAe Bouma 

sequence ...............................................................

Mudstone, silty, massive........................................

Sandstone, very fine grained; TcAe Bouma 

sequence...............................................................

Mudstone, silty, carbonaceous, concretionary, 

massive.................................................................

Covered interval (float of mudstone)....................

Siltstone, partly laminated....................................

Covered interval (float of siltstone) ......................

Siltstone, massive; fossil plant fragments, 

concretionary .......................................................

Covered interval (float of siltstone) ......................

Siltstone, massive, carbonaceous; fossil plant 

fragments, bioturbated.......................................

Siltstone and very fine grained sandstone, massive, concretionary, thoroughly bioturbated ....

Siltstone, massive; fossil plant fragments, 

thoroughly bioturbated .......................................

Sandstone, very fine grained; TcAe Bouma 

sequence, bioturbated at top .............................

Siltstone; massive fossil plant fragments, 

thoroughly bioturbated .......................................

Sandstone, very fine grained, massive; fossil 

plant fragments, thoroughly bioturbated, 

concretionary .......................................................

Siltstone and very fine grained sandstone; massive fossil plant fragments, thoroughly 

bioturbated...........................................................

Thickness 

(in meters)

4.35

.20

22.30

3.31

28.50

.06 

1.80

.18 

4.95

.10

1.90

.33

49.50

.15

13.50

.40

12.00

.30

18.00

.25

14.50

.23

22.00

.20 

1.20

.12

56.80

1.40

3.00

9.00

4.90

10.77

3.80 

.30 

.80 

.03 

.35

.75

4.15

Thickness 

(in meters) 

Blue Gulch Mudstone Member Continued:

Sandstone, fine- to medium-grained; Tabcde 

Bouma sequence, bioturbated, concretionary .. .20

Siltstone and very fine grained sandstone, massive; fossil plant fragments, thoroughly 

bioturbated........................................................... 1.80

Siltstone and very fine grained sandstone, massive, thoroughly bioturbated, concretionary .... .25

Siltstone and very fine grained sandstone, massive, carbonaceous, bioturbated ........................ 1.55

Siltstone and very fine grained sandstone, massive to laminated; fossil plant fragments, bioturbated, concretionary ...................................... .65

Siltstone and very fine grained sandstone, massive; fossil plant fragments, thoroughly 

bioturbated........................................................... 9.05

Covered interval (float of siltstone and very fine 

grained sandstone).............................................. 4.20

Siltstone and very fine grained sandstone, massive, thoroughly bioturbated.............................. 3.40

Siltstone and very fine grained sandstone, massive to laminated, bioturbated........................... .20

Siltstone and very fine grained sandstone, massive, thoroughly bioturbated.............................. 1.10

Siltstone and very fine grained sandstone, massive to laminated, bioturbated........................... .45

Sandstone, very fine grained, and siltstone, 

massive, thoroughly bioturbated....................... 1.20

Sandstone, very fine grained, and siltstone, 

laminated............................................................. .30

Sandstone, very fine grained, and siltstone, 

massive, thoroughly bioturbated....................... 3.30

Rancheria Gulch Sandstone Beds; chiefly sandstone, fine-grained, massive and hummocky 

cross-stratified; molluscan fossils, partly 

bioturbated........................................................... 85.77

Mudstone, massive, bioturbated (partly covered) ... 8.00

Sandstone, very fine grained; hummocky crosslaminae and convolute laminations .................. .10

Siltstone, massive, bioturbated............................. .80

Sandstone, very fine grained; hummocky crosslaminae; convolute laminations; current ripple 

markings .............................................................. .10

Siltstone, massive, bioturbated............................. .20

Sandstone, very fine grained; silty; hummocky 

cross-laminae....................................................... .05

Siltstone, massive, bioturbated............................. .10

Sandstone, very fine grained; hummocky crosslaminae................................................................. .05

Siltstone, massive, bioturbated............................. .20

Sandstone, very fine grained; hummocky crosslaminae, bioturbated........................................... .15

Siltstone, massive, bioturbated............................. .15

Sandstone, very fine grained; hummocky crosslaminae, bioturbated........................................... .03

Siltstone, massive, bioturbated............................. .10

Sandstone, very fine grained; hummocky crosslaminae................................................................. .10

Siltstone, massive................................................... .02

Siltstone, very fine grained; convolute laminae 

and hummocky cross-laminae ........................... .06

Siltstone, massive................................................... .30

Sandstone, very fine grained; hummocky crosslaminae................................................................. .04
58 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

Blue Gulch Mudstone Member Continued:

Siltstone, massive...................................................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Siltstone, massive ...................................................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Siltstone, massive...................................................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated ...........................................

Siltstone, massive, bioturbated .............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae and convolute laminae, bioturbated ..

Siltstone, massive...................................................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated .............................

Sandstone, very fine grained; hummocky crosslaminae .................................................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Siltstone, massive to partly laminated, 

bioturbated...........................................................

Sandstone, very fine grained; laminated .............

Siltstone, massive, bioturbated .............................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Siltstone, very fine grained; hummocky crosslaminae.................................................................

Sandstone, very fine grained; hummocky crosslaminae .................................................................

Siltstone, massive ...................................................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated .............................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Siltstone, massive...................................................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Siltstone, massive, bioturbated .............................

Sandstone, very fine grained, laminated .............

Siltstone, massive...................................................

Sandstone, very fine grained, laminated .............

Siltstone, massive, bioturbated .............................

Sandstone, very fine grained, laminated .............

Siltstone, massive ...................................................

Sandstone, very fine grained, laminated .............

Siltstone, massive, bioturbated .............................

Sandstone, very fine grained, laminated .............

Siltstone, massive...................................................

Sandstone, very fine grained; hummocky crosslaminae............;....................................................

Thickness 

(in meters)

.50

.05 

.50

.05 

.30

.05 

.50

.05 

.30

.05 

.30

.05 

.30

.05 

.20

.20 

.20

.05 

.05

.05

1.30 

.20 

.05

.05 

.02

.05 

.02

.05 

.03

.02 

.05

.03 

.15 

.05 

.70 

.05 

.10 

.05 

.20 

.06 

.02 

.03 

.15

.02

Blue Gulch Mudstone Member Continued:

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae .................................................................

Siltstone, massive ...................................................

Sandstone, very fine grained; hummocky crosslaminae .................................................................

Siltstone, massive, bioturbated .............................

Siltstone and very fine grained sandstone; 

hummocky cross-laminae...................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained, laminated .............

Siltstone, massive ...................................................

Sandstone, very fine grained, laminated .............

Siltstone and very fine grained sandstone, 

laminated .............................................................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Siltstone, massive ...................................................

Sandstone, very fine grained; hummocky crosslaminae .................................................................

Siltstone, massive ...................................................

Sandstone, fine-grained, ungraded, parallellaminated at base, convolute-laminated in 

middle, and ripple-marked at top .....................

Siltstone, massive, bioturbated .............................

Sandstone, fine-grained, laminated......................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae .................................................................

Siltstone, massive, bioturbated .............................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Siltstone, massive, bioturbated, partly covered ..

Sandstone, fine-grained; hummocky crosslaminae .................................................................

Siltstone, massive ...................................................

Sandstone, very fine grained; hummocky crosslaminae .................................................................

Siltstone, massive, bioturbated .............................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated .............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated .............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated ...........................................

Siltstone, massive...................................................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Siltstone, massive ...................................................

Sandstone, very fine grained; hummocky crosslaminae .................................................................

Siltstone, massive ...................................................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Thickness 

(in meters)

.02

.04 

.05

.03 

.30

.40 

.20 

.05 

.20 

.10

.20

.05 

.15

.05 

1.30

.10

3.60

.05

1.60

.10 

.05

.10 

4.60

.08 

.20

.03 

.50

.02 

.60

.05 

.50

.05 

3.40

.05 

.30

.15 

.40

.05 

.40

.03 

.20

.02
BLUE GULCH MUDSTONE MEMBER 59

Blue Gulch Mudstone Member Continued:

Siltstone, massive ...................................................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Siltstone, massive ...................................................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Siltstone, massive; fossil plant fragments, 

thoroughly bioturbated, concretionary .............

Sandstone, very fine grained, laminated, 

bioturbated...........................................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated .............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated ...........................................

Siltstone, massive, bioturbated .............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated .............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, bioturbated.............................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Siltstone, massive, thoroughly bioturbated .........

Mudstone, massive, partly bioturbated ...............

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Mudstone, massive .................................................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Mudstone, massive .................................................

Sandstone, very fine grained; hummocky crosslaminae.................................................................

Thickness 

(in meters)

.30

.03 

.15

.10

14.50

.05 

1.80

.05

.15

.10 

.70

.03 

.40

.05 

.50

.01 

.10

.04 

.20

.15 

.50

.10 

.40

.03 

.50

.04 

.70

.05 

.45

.06 

.50

.07 

.90

.04

2.30

2.10

.02

.15

.02 

.20

.02

Blue Gulch Mudstone Member Continued: 

Mudstone, massive, bioturbated ...........................

Sandstone, very fine grained; hummocky crosslaminae, bioturbated...........................................

Mudstone, massive, bioturbated ...........................

Sandstone, very fine grained; T^ Bouma

sequence ...............................................................

Mudstone, silty, massive........................................

Covered interval (float of mudstone)....................

Sandstone, very fine grained; T^, Bouma

sequence ...............................................................

Mudstone, silty, massive........................................

Sandstone, very fine grained; T^ Bouma

sequence ...............................................................

Covered interval (float of mudstone)....................

Sandstone, very fine grained, and siltstone;

Tgg Bouma sequence ...........................................

Covered interval (float of mudstone) ....................

Sandstone, very fine grained, ripple-marked ......

Mudstone, massive .................................................

Sandstone, very fine grained; T^ Bouma

sequence ...............................................................

Mudstone, silty, massive........................................

Sandstone, very fine grained; T^ Bouma

sequence ...............................................................

Mudstone, silty, massive........................................

Sandstone, very fine grained; T^ Bouma

sequence ...............................................................

Mudstone, massive .................................................

Sandstone, very fine grained; T^ Bouma

sequence ...............................................................

Mudstone, silty, massive........................................

Sandstone, very fine grained, laminated .............

Mudstone, silty, massive........................................

Sandstone, very fine grained, ripple-marked ......

Mudstone, massive .................................................

Sandstone, very fine grained; T^ Bouma

sequence ...............................................................

Mudstone, silty, massive........................................

Sandstone, fine-grained; Tcde Bouma sequence .. 

Mudstone, silty, massive........................................

Sandstone, very fine grained; T^ Bouma

sequence ...............................................................

Mudstone, silty, massive........................................

Sandstone, medium-grained, laminated,

concretionary .......................................................

Mudstone, silty, massive........................................

Sandstone, very fine grained; Tcde Bouma

sequence ...............................................................

Mudstone, silty, massive........................................

Sandstone, very fine grained, micaceous; T^

Bouma sequence..................................................

Shale, silty, massive, micaceous ...........................

Total thickness of the Blue Gulch Mudstone 

Member (including the Rancheria Gulch 

Sandstone Beds and Hilt Bed) ...................

Conformable contact.

Rocky Gulch Sandstone Member (upper part): 

Sandstone, medium- to coarse-grained, graded, 

massive; rip-up clasts .........................................

Thickness 

(in meters)

1.70

.04 

.20

.02

.50

1.50

.03 

.20

.03 

1.20

.02

1.90

.01

.20

.01 

.10

.02 

.15

.03 

.20

.03 

.35 

.02 

.20 

.04 

.45

.20 

.10 

.12 

.25

.02 

.55

.16 

.60

.04 

.30

.03 

.40

849.87

1.0
60 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

METERS THICKNESS, IN 

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FIGURE 18. Type section of the Blue Gulch Mudstone Member (including its Rancheria Gulch Sandstone Beds and Hilt Bed) of the 

Hornbrook Formation. See figure 10 for explanation of symbols and caption of figure 12 for explanation of abbreviations (pbls,
600 -i

550 - -_

500 J

& Mf 6589

BLUE GULCH MUDSTONE MEMBER

700 -i » 800-1-1

61

650 -

& Mf6588

600 J

750 -

700 J

GRAIN SIZE 

sh ss

850 -

800 -

Tertiary

^ volcanic

rocks

Mf 6586

in 3m)

pebbles). Numbers designated with Mf indicate USGS Mesozoic microfossil localities that have yielded foraminifers. 

Gaps in lithology represent covered intervals.
62 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

The colors of the Blue Gulch Mudstone Member 

vary slightly. In its lower part, below the Rancheria 

Gulch Sandstone Beds, mudstone and siltstone are 

medium dark gray (N4) or medium gray (N5) on 

fresh surfaces and moderate yellowish brown (10YR 

5/4) on weathered surfaces, whereas sandstone is medium gray (N5) on fresh surfaces and pale yellowish 

brown (10YR 6/2) on weathered surfaces. In its 

middle and upper parts, above the Rancheria Gulch 

Sandstone Beds, mudstone is medium gray (N5), medium dark gray (N4), olive gray (5Y 4/1), and brownish gray (SYR 4/1) on fresh surfaces and pale 

yellowish brown (10YR 6/2) or grayish green (10GY 5/ 

2) on weathered surfaces. The sandstone interbeds 

are medium gray (N5), light olive gray (5Y 5/2), or 

yellowish gray (5Y 7/2) on fresh surfaces and moderate yellowish brown (10YR 5/4) on weathered surfaces.

LITHOLOGY

The Blue Gulch Mudstone Member consists 

chiefly of mudstone, which is commonly silty and 

dominantly nonfissile. Areally it can be divided into 

two distinct lithologic assemblages. In the type area, 

adjacent to the town of Hornbrook, where the 

Rancheria Gulch Sandstone Beds form part of the 

Blue Gulch Mudstone Member, it contains more 

interbeds and irregularly distributed bioturbated 

siltstone and very fine grained sandstone, especially 

in its lower part, directly above and below the 

Rancheria Gulch Sandstone Beds. In other areas to 

the northwest and southeast of the Hornbrook area, 

where the Rancheria Gulch Sandstone Beds are absent, the Blue Gulch Mudstone Member consists 

more uniformly of mudstone with thin interbeds of 

very fine grained sandstone.

In the type area, I informally divide the Blue 

Gulch Mudstone Member into lower, middle, and upper parts (fig. 18). The lower part of the Blue Gulch 

Mudstone Member (from the top of the Rocky Gulch 

Sandstone Member to the top of the Rancheria Gulch 

Sandstone Beds), 164.42 m thick, consists chiefly of 

massive, bioturbated siltstone with abundant but 

thin interbeds of very fine grained sandstone and the 

overlying Rancheria Gulch Sandstone Beds. However, 

the lowermost 10 m, directly above the Rocky Gulch 

Sandstone Member, consist of nonbioturbated mudstone with thin interbeds of graded, very fine grained 

sandstone. The sandstone interbeds are as thick as 

20 cm and are similar lithologically to some of the 

coarser and thicker beds of sandstone in the upper 

part of the underlying Rocky Gulch Sandstone Member. The beds are graded, nonbioturbated, and laterally continuous; in addition, they have erosive bases, 

generally become thinner upward within the 10-m interval, and are organized into Bouma (1962) sequences of the Tce type. This lowest interval clearly 

reflects the upward continuation of depositional environments similar to those that characterized the 

Rocky Gulch Sandstone Member. The lithology of the 

Rancheria Gulch Sandstone Beds is discussed in a 

later section.

The middle part of the Blue Gulch Mudstone 

Member (from the top of the Rancheria Gulch Sandstone Beds to the top of the Hilt Bed), 297.18 m 

thick, consists of several different lithologies. The 

lowermost 70 m consist chiefly of massive bioturbated 

siltstone with thin interbeds of massive to laminated 

very fine grained sandstone. This part of the Blue 

Gulch Mudstone Member is lithologically similar to 

the section below the Rancheria Gulch Sandstone 

Beds and is generally coarser than the main part of 

the member. This sequence changes gradually upward in the following ways: (1) the massive siltstone 

changes to silty mudstone and finally to mudstone; 

 (2) the interbeds of sandstone become more distinct, 

well-bedded, and graded, and toward the top of the 

70-m-thick interval, they are internally organized 

into Bouma sequences; (3) the amount of bioturbation 

in both the siltstone to mudstone and the sandstone 

interbeds decreases progressively; and (4) the abundance of plant fragments and carbonate concretions 

also decreases.

The uppermost 227 m of the middle part of the 

Blue Gulch Mudstone Member in the Hornbrook area 

consist of massive mudstone with thin to medium, 

graded interbeds of laterally continuous siltstone and 

very fine grained sandstone. This is the most common lithology in the Blue Gulch Mudstone Member, 

and it is observed from 270 m below the top of the 

Hilt Bed within the middle part to about 345 m 

above the top of the Hilt Bed within the upper part of 

the member. Thus, a thickness of about 572 m of the 

Blue Gulch Mudstone Member contains the dominant 

mudstone lithology. The mudstone forms massive intervals as thick as 50 m that do not contain interbeds 

of sandstone or siltstone that are as thick as 5 cm. 

The mudstone can locally be divided into two types: 

medium-gray (N5) to medium-dark-gray (N4) mudstone that characteristically forms thick layers between the sandstone interbeds; and olive-gray (5Y 4/ 

1) to brownish-gray (SYR 4/1) mudstone that characteristically forms thinner layers that rest directly on 

the top of the graded interbeds of sandstone. The 

mudstone locally contains tests of large foraminifers 

that can be seen with the naked eye or with a hand
BLUE GULCH MUDSTONE MEMBER 63

lens. Bioturbation is locally present in this mudstone 

interval of the member, but it is not as common as it 

is in underlying and overlying intervals and generally 

is present as solitary burrows rather than as massively bioturbated beds.

The abundant sandstone and siltstone interbeds 

within this mudstone interval are graded, have sharp 

erosive bases, and are organized into Bouma sequences (generally Tbcde, Tcde, and Tde). These beds 

are typically laterally continuous at outcrop scale and 

rarely exhibit any prominent lateral changes in thickness, grain size, or Bouma sequence. The beds of 

sandstone generally range in thickness from less 

than 1 cm to 55 cm, although one 90-cm-thick bed 

was observed in the type section near the top of this 

572-m-thick interval. In the type section, only 

interbeds of sandstone or siltstone that are thicker 

than 5 cm were measured and are plotted on the section (fig. 18).

The upper part of the Blue Gulch Mudstone 

Member, 388.47 m thick, can be subdivided into two 

subunits: a lower subunit of about 345 m of mudstone with graded interbeds of siltstone and very fine 

grained sandstone, and an upper subunit of about 43 

m of bioturbated siltstone with abundant interbeds of 

sandstone (fig. 18). The latter subunit in the 

Hornbrook area and in the type section resembles the 

70-m-thick interval above the Rancheria Gulch Sandstone Beds within the middle part of the member. It 

consists dominantly of massive bioturbated siltstone 

with abundant interbeds of very fine grained to finegrained sandstone. Plant fossils are present in the 

upper part of this subunit, and bioturbation has locally completely destroyed the bedding. Basalt intrusions have caused local deformation of the 

interbedded sandstone and siltstone small folds, 

faults, and other structural disturbances can be attributed to forceful intrusion of the basalt. In outcrops without intrusive rocks but with deformation, 

the deformation locally resembles synsedimentary 

slumping rather than deformation related to intrusion of the basalt. This sandstone-rich uppermost interval of the Hornbrook Formation in its type area is 

commonly missing in other areas because of erosive 

downcutting of the surface of unconformity at the top 

of the Hornbrook Formation.

In outcrops to the northwest and southeast of the 

type area of the Hornbrook Formation, the Blue 

Gulch Mudstone Member is lithologically more uniform. It generally consists of mudstone with 

interbeds of very fine grained sandstone and is similar to the 572-m-thick interval of mudstone and 

interbedded sandstone within the middle part of the 

type section. The mudstone is typically nonbioturbated or poorly bioturbated, containing only 

solitary burrows. The sandstone interbeds are more 

abundant near the base of the member, where they 

resemble the sandstone beds of the upper part of the 

Rocky Gulch Sandstone Member. The sandstone beds 

are always characterized by normal grading, erosive 

bases, lateral continuity, few lateral changes in either 

thickness or grain size, and Bouma T^ to Tc_e sequences. Abundant sole markings, including flute 

casts, groove casts, and prod casts, permit the determination of paleocurrent directions. The Hilt Bed is 

present everywhere between Ashland and the northern Shasta Valley except where it has been eroded off 

along the pre-Tertiary unconformity. Thus, the Blue 

Gulch Mudstone Member is lithologically relatively 

uniform over most of its outcrop area and is easily 

mapped on the basis of its dominant lithology.

The only other lithologies of local significance in 

the Blue Gulch Mudstone Member are coarse-grained 

sandstone and conglomerate that form graded beds 

and that contain large fossil fragments. In the Cottonwood Creek valley area (SWXSEX sec. 7, T. 47 N., 

R. 6 W., Hornbrook 15-minute quadrangle), for example, a thick bed of poorly graded granular sandstone crops out in one location. The bed contains 

abundant broken molluscan shell debris throughout 

and rip-up clasts of shale within its lower part, and it 

has flute casts on its basal surface. The bed is 4.2 m 

thick and is intercalated within typical mudstone of 

the Blue Gulch Mudstone Member. The contact with 

overlying mudstone is abrupt rather than gradational. The bed is massive and contains no sedimentary structures or divisions of the Bouma sequence. 

The bed appears to be laterally discontinuous, for it 

has not been found in any other places. This bed is 

clearly distinct from the Hilt Bed, which crops out 

nearby at a lower stratigraphic position within the 

Blue Gulch Mudstone Member.

In the Dark Hollow area, conglomeratic beds with 

abundant broken megafossil debris are locally abundant within the upper part of the Blue Gulch Mudstone Member above the Hilt Bed. Conglomeratic 

beds have been observed in roadcuts on the west side 

of Dark Hollow Road (NWKNWX sec. 12, T. 38 S., R. 

2 W., Medford 15-minute quadrangle), on the east 

side of Dark Hollow Road (SEX sec. 50, T. 38 S., R. 2 

W., Medford 15-minute quadrangle), and on the north 

side of Pioneer Road (SWKNEK sec. 51, T. 38 S., R. 1 

W., Medford 15-minute quadrangle). These beds of 

conglomerate are interbedded with typical mudstone 

of the Blue Gulch Mudstone Member that contains 

abundant thin interbeds of normally graded siltstone 

and very fine grained sandstone that are organized 

into T Bouma sequences. The conglomeratic beds
64 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

are as thick as 0.5 in and contain clasts of subangular to angular chert and quartzite as large as 3 

cm. These beds also contain angular, broken molluscan shell debris. Some of the conglomerate beds are 

reverse graded, in contrast to the interbeds of sandstone and siltstone. The conglomerate beds are also 

massive and contain no internal sedimentary structures or divisions of the Bouma sequence. The tops of 

the beds have a sharp, rather than gradational, contact with the overlying mudstone. The thickness of 

some of the beds changes laterally, and marked lateral changes in grain size take place over distances of 

a few meters. Synsedimentary folding is associated 

with some of the conglomerate beds.

Outcrops of boulder and cobble conglomerate in the 

Dark Hollow area contain both rounded and angular 

clasts of carbonate concretions and fewer clasts of 

shale, quartzite, fossiliferous sandstone, siltstone, 

and granitic and metamorphic rocks. The clasts are 

poorly sorted to locally well sorted, are as large as 

2 m, and are supported by a mudstone matrix. The 

conglomerate crops out as small patches in roadcuts 

along Pioneer Road (near the NE corner of sec. 13, T. 

38 S., R. 2 W., and NEA 1 sec. 49, T. 38 S., R. 2 W., 

Medford 15-minute quadrangle). Although these 

patches of conglomerate initially appear to form 

interbeds within the Blue Gulch Mudstone Member, 

careful mapping has shown them to be irregular secondary subhorizontal lenses that rest unconformably 

on dipping mudstone of the Blue Gulch Mudstone 

Member. The patches form a linear trend that strikes 

about 120° and that appears to represent weathered 

blocks that have been cemented along a fault; 

travertinelike carbonate materials in the matrix and 

around some clasts suggest that heated waters from 

mineral springs may have contributed to the cementation of the blocks.

STRATIGRAPHIC RELATIONS

The Blue Gulch Mudstone Member rests conformably on the Rocky Gulch Sandstone Member in all 

areas except on the south flank of Black Mountain, 

where, in my opinion, it rests directly and possibly 

unconformably on the Ditch Creek Siltstone Member 

in areas where the Rocky Gulch Sandstone Member 

is locally absent (Nilsen and others, 1983; pi. 1). The 

lower contact is gradational to abrupt but typically 

well marked in the field.

The upper contact of the Blue Gulch Mudstone 

Member is generally a well-marked unconformity between the fine-grained Cretaceous marine rocks and 

the Tertiary nonmarine conglomeratic or volcanic

rocks. In the Medford Valley, Bear Creek valley, and 

part of the Shasta Valley areas, the Blue Gulch Mudstone Member is overlain by conglomerate, whereas 

in the Cottonwood Creek valley, Black Mountain, and 

most of the Shasta Valley areas, it is overlain by volcanic rocks. In the Bear Creek valley area northwest 

of Ashland, the erosional unconformity cuts deeply 

into the Hornbrook Formation, through the Hilt Bed 

and most of the Blue Gulch Mudstone Member; the 

Tertiary conglomerate in this area locally rests on the 

Ditch Creek Siltstone Member or Rocky Gulch Sandstone Member.

Lateral stratigraphic relations of the Blue Gulch 

Mudstone Member are poorly defined. The member 

undoubtedly extends northeastward beneath the Cascade Range but for a distance that is not known because the subsurface structure is poorly known. To 

the southeast, the Blue Gulch Mudstone Member is 

covered by Tertiary volcanic rocks in most of the 

Shasta Valley area. The Blue Gulch Mudstone Member does not appear to change lithologically in its 

southeasternmost outcrops, and thus it originally 

must have extended farther to the southeast as essentially the same lithologic unit.

To the northwest, the Blue Gulch Mudstone Member is covered by Quaternary alluvial fill in the 

Medford Valley. Farther north, the Tertiary volcanic 

rocks rest directly on basement rocks of the Klamath 

Mountains, covering the Hornbrook Formation if it 

does extend northward in the subsurface. However, 

Cretaceous strata are exposed farther to the northwest as a downfaulted block in the Grave Creek area 

of the northern Klamath Mountains. The Blue Gulch 

Mudstone Member may have originally extended at 

least as far north as the Grave Creek area, which is 

located 50 km north of the outcrops of the Hilt Bed in 

the Dark Hollow area.

THICKNESS

The thickness of the Blue Gulch Mudstone Member varies greatly over the outcrop area of the 

Hornbrook Formation, chiefly because of erosion of its 

top beneath the pre-Tertiary unconformity. At its 

type section, it is 849.87 m thick, including the 

Rancheria Gulch Sandstone Beds and the Hilt Bed. 

In the area west of Hornbrook, Peck and others 

(1956) in their type section measured an incomplete 

thickness of 552 m in strata that are equivalent to 

the Blue Gulch Mudstone Member; although this 

thickness includes the Rancheria Gulch Sandstone 

Beds, it does not include the upper part of the Blue 

Gulch Mudstone Member. Jones (1959) determined a
BLUE GULCH MUDSTONE MEMBER 65

thickness of about 1200 m for strata equivalent to the 

Blue Gulch Mudstone Member in the type area near 

Hornbrook.

Farther north, Elliott (1971) measured a thickness 

of 796 m of strata equivalent to the Blue Gulch Mudstone Member in the Bailey Hill area, near Hilt, California. Peck and others (1956) estimated a thickness 

of about 180 m for strata equivalent to the Blue 

Gulch Mudstone Member near Ashland. In the Bear 

Creek valley area, McKnight (1971) obtained a thickness of about 585 m for strata equivalent to the Blue 

Gulch Mudstone Member. I have measured a partial 

section of about 75 m of the Blue Gulch Mudstone 

Member about 5 km east of Ashland. No sections of 

the Blue Gulch Mudstone Member have been measured in the Dark Hollow area.

In summary, the Blue Gulch Mudstone Member 

appears to be about 800-900 m thick in the Cottonwood Creek valley area, where its type area is located. To the north and south its thickness has not 

been reliably measured, but it probably varies widely 

as a result of major amounts of erosion along the preTertiary unconformity. There is no evidence of major 

preerosion thinning or thickening of the member.

DEPOSITIONAL ENVIRONMENT

The Blue Gulch Mudstone Member, particularly its 

thick upper middle and lower upper parts, appears to 

have been deposited mostly in deep-marine environments. The extensive mudstone that forms the chief 

lithology contains abundant benthic foraminifers that 

suggest at least bathyal depths. Numerous ammonites and planktonic foraminifers, as well as a lack of 

massive amounts of bioturbation, support a low-energy deep-marine setting. The sandstone interbeds 

are turbidites, and their lateral continuity, organization into Bouma sequences, and lack of thinning-upward or thickening-upward cycles suggest that they 

are basin-plain turbidites. The remarkable lateral 

continuity of the Hilt Bed also supports the concept 

of deposition in a deep-marine basin-plain setting. 

The scattered conglomeratic beds that contain broken 

fossils were probably deposited by debris flows that 

moved downslope and spread out along the floor of 

the basin plain as laterally discontinuous, massive, 

locally reverse-graded beds; these beds also locally 

disrupted or deformed underlying layers of mudstone 

and interbedded turbidite sandstone.

The lower part, lower middle part, and uppermost 

part of the Blue Gulch Mudstone Member in its type 

area, however, represent deposition in shallow-marine environments. The Rancheria Gulch Sandstone

Beds in the type area are thought to have been deposited in an outer-shelf environment subject to 

storm-wave activity. The Blue Gulch Mudstone Member directly above and below the Rancheria Gulch 

Sandstone Beds is siltier and more bioturbated, contains fewer turbidite interbeds, and was probably deposited in outer-shelf to slope environments. The interval above the Rancheria Gulch Sandstone Beds 

grades upward into the true basin-plain deposits of 

the middle part of the Blue Gulch Mudstone Member 

in its type area. The uppermost part of the Blue 

Gulch Mudstone Member in its type section is again 

siltier, more massive, and more bioturbated, marking 

probably an upward change to shallower marine environments.

AGE AND CORRELATION

The age of the Blue Gulch Mudstone Member has 

been determined chiefly from shallow-marine mollusks found in the Rancheria Gulch Sandstone Beds 

and from ammonites and benthic and planktonic 

foraminifers found within the middle and upper parts 

of the member. Age determinations, however, are not 

always compatible, and the lower age limit of the 

Blue Gulch Mudstone Member remains somewhat 

problematical. No fossils have been recovered from 

the lower part of the Blue Gulch Mudstone Member 

below the Rancheria Gulch Sandstone Beds in the 

type area.

Peck and others (1956) obtained no fossil evidence 

in the type area from strata of the lower part of the 

Blue Gulch Mudstone Member that are equivalent to 

the Rancheria Gulch Sandstone Beds, nor from the 

underlying interval of siltstone and mudstone between the Rancheria Gulch Sandstone Beds and the 

top of the Rocky Gulch Sandstone Member. However, 

they did obtain abundant ammonite fossils and 

Inoceramus from the upper part of the Blue Gulch 

Mudstone Member. They concluded that this part of 

the member was middle to late Campanian in age. 

However, it must be remembered that they did not 

sample the uppermost 250 m of the Blue Gulch Mudstone Member.

Jones (1959) concluded that strata equivalent to 

the Blue Gulch Mudstone Member were of middle 

Campanian to Maestrichtian(?) age on the basis of 

ammonites and Inoceramus. Elliott (1971) also inferred a middle Campanian to Maestrichtian(?) age 

for strata equivalent to the Blue Gulch Mudstone 

Member. McKnight (1971), on the basis of fossil identifications by D.L. Jones of his collections from the 

Bear Creek valley area, inferred a Campanian or
66 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

early Maestrichtian age for strata equivalent to the 

Blue Gulch Mudstone Member.

Foraminiferal collections from the lower middle 

part of the Blue Gulch Mudstone Member above the 

Rancheria Gulch Sandstone Beds in the type area 

near Blue Gulch (USGS Mesozoic localities Mf6399 

and Mf6583) are indicative of a middle Campanian to 

early Maestrichtian age (app. II; fig. 18). Other collections from the lower part of the Blue Gulch Mudstone 

Member north of the type area and north of the outcrop limit of the Rancheria Gulch Sandstone Beds at 

the north end of the Cottonwood Creek valley area 

(USGS Mesozoic locality Mf6411) and the south end 

of the Bear Creek valley (USGS Mesozoic localities 

Mf6401 and Mf6402) are indicative of late Turonian 

to early Coniacian and of Campanian to early 

Maestrichtian ages (app. II).

Foraminiferal collections from the upper middle 

and lower upper parts of the Blue Gulch Mudstone 

Member from the type section at Blue Gulch and 

southeast of Hornbrook (USGS Mesozoic localities 

Mf6584, Mf6588, and Mf6589) are indicative of (in ascending stratigraphic order) late Santonian to late 

Campanian, Campanian, and Campanian to early 

Maestrichtian ages (app. II; fig. 18). Foraminiferal 

collections from the middle part of the Blue Gulch 

Mudstone Member north of the type area are indicative of Campanian to early Maestrichtian age at the 

north end of the Cottonwood Creek valley (USGS 

Mesozoic locality Mf6403), and Coniacian to 

Maestrichtian age near Ashland in the Bear Creek 

valley area (USGS Mesozoic localities Mf6410 and 

Mf6579).

A foraminiferal collection from near the top of the 

Blue Gulch Mudstone Member in the type section 

(USGS Mesozoic locality Mf6586) is indicative of a 

Santonian to Maestrichtian age.

Samples collected from evenly spaced intervals by 

M.A. Elliott in 1971 from his measured section of 

strata equivalent to the Blue Gulch Mudstone Member near Bailey Hill in the northern part of the Cottonwood Creek valley area were processed in 1982 by 

the U.S. Geological Survey for foraminiferal assemblages. These samples, in ascending stratigraphic order (USGS Mesozoic localities Mf6335 to Mf6352), are 

indicative of Campanian to early Maestrichtian ages 

(app. II). However, the uppermost two samples from 

this section were barren of microfossils.

The available fossil age data from the Blue Gulch 

Mudstone Member provide ambiguous results. No 

fossils, either microfossils or megafossils, have been 

collected from the lower part of the Blue Gulch Mudstone Member in the type area, the siltstone-dominated interval between the top of the Rocky Gulch

Sandstone Member and the basal part of the 

Rancheria Gulch Sandstone Beds. Microfossils and 

previous collections of ammonites generally have indicated Campanian to early Maestrichtian ages; however, samples from the Dark Hollow area are indicative of ages that could be as old as late Turonian (localities FL 8 and FL 117 in figure 2 of Sliter and 

others, 1984), a sample from the Ashland area is indicative of an age that could be as old as Coniacian 

(USGS Mesozoic locality Mf6410), and a sample from 

the type area is indicative of an age that could be as 

old as Santonian (USGS Mesozoic locality Mf6586). 

In summary, the age of the Blue Gulch Mudstone 

Member appears to range from late Turonian to early 

Maestrichtian, a span of about 18 m.y. It appears to 

be oldest in the Dark Hollow area (late Turonian) 

and youngest in the Cottonwood Creek valley area 

(chiefly Campanian and early Maestrichtian), a trend 

consistent with the overall pattern of the Hornbrook 

Formation of younger ages from northwest to southeast. Because no ages have been obtained from the 

lowest part of the member below the Rancheria 

Gulch Sandstone Beds in the type area, it could possibly range downward in age in that area into the 

middle or late Coniacian.

The proposed late Turonian to early Maestrichtian 

age of the Blue Gulch Mudstone Member suggests 

that it is, at least, partly correlative with Cretaceous 

strata exposed along the southwest coast of Oregon 

the Cape Sebastian Sandstone and Hunters Cove 

Formation of Dott (1971) but younger than other 

Cretaceous strata exposed farther north in the Days 

Creek, Riddle, and Mitchell areas. It is, at least, 

partly correlative with subsurface deposits of the 

Ochoco basin (Thompson and others, 1984) adjacent 

to Mitchell but younger than the Cretaceous strata to 

the west at O*Brien.

To the south, the lower part of the Blue Gulch 

Mudstone Member is probably partly correlative with 

the upper part of the Great Valley sequence near 

Redding, which is as young as Santonian(?) (Popenoe, 

1943; Matsumoto, 1960). It should also be, at least, 

partly correlative with the upper part of the Gas 

Point Member of the Budden Canyon Formation of 

Murphy and others (1969), which crops out about 30 

km southwest of Redding and may be as young as 

early Coniacian (Murphy and others, 1969).

Farther south, the Blue Gulch Mudstone Member 

at least partly correlates with the Great Valley sequence on both the east side of the Sacramento Valley near Chico (Haggart and Ward, 1984) and on the 

west side of the Sacramento Valley (Ingersoll, 1979), 

where it is very thick. The Blue Gulch Mudstone 

Member is also correlative with parts of the Coastal
RANCHERIA GULCH SANDSTONE BEDS 67

belt terrane of the Franciscan assemblage (Evitt and 

Pierce, 1975).

RANCHERIA GULCH SANDSTONE BEDS

DEFINITION

The name "Rancheria Gulch Sandstone Beds" was 

applied by Nilsen (1984a) to outcrops of marine sandstone strata within the lower part of the Blue Gulch 

Mudstone Member of the Hornbrook Formation. The 

unit was named for exposures adjacent to Rancheria 

Gulch, located about 1.5 km southwest of Hornbrook, 

Calif. The type section was measured across a pair of 

low ridges located to the west and north of the U.S. 

Forest Service Forestry Camp, which is located west 

of Interstate Highway 5, adjacent to the HenleyHornbrook exit (figs. 8, 19; Nilsen and others, 1984, 

stop 5).

The Rancheria Gulch Sandstone Beds are restricted areally to a northwest-trending belt that is 

located about 1 km to the southwest of Hornbrook, 

between Ditch Creek on the northwest and Osburger 

Gulch on the southeast. The unit forms a lens of 

sandstone within the lower part of the Blue Gulch 

Mudstone Member. The unit is both underlain and 

overlain by mudstone of the Blue Gulch Mudstone 

Member and pinches out laterally to the northwest 

and southeast into mudstone.

The Rancheria Gulch Sandstone Beds were designated as member V of the Hornbrook Formation by 

Peck and others (1956), who did not describe the 

unit's lens-shaped geometry. D.L. Jones (unpub. mapping) mapped most of the extent of the Rancheria 

Gulch Sandstone Beds in the Hornbrook area and included them within the lower part of his unit C (of 

Hornbrook Formation) (Jones, 1959). McKnight 

(1971) did not recognize the Rancheria Gulch Sandstone Beds in the Bear Creek valley area. Elliott 

(1971; unpub. data, 1980) did not map the Rancheria 

Gulch Sandstone Beds or differentiate them within 

his Bailey Hill mudstone unit of his Hilt unit or his 

unit D of the Hornbrook Formation (fig. 6A). Nilsen 

and others (1983) mapped the Rancheria Gulch Sandstone Beds as lx and 12, two sandstone lenses within 

the lower part of their unit d of the Hornbrook 

Formation.

The Rancheria Gulch Sandstone Beds rest conformably on the lower part of the Blue Gulch Mudstone Member. The lower boundary is marked by the 

abrupt upward lithologic change from mudstone, siltstone, and some thin beds of very fine grained sandstone of the upper part of the lower Blue Gulch MudFIGURE 19. Type section of the Rancheria Gulch Sandstone Beds 

of the Blue Gulch Mudstone Member of the Hornbrook Formation. A, View northwest, showing ridge underlain by the 

Rancheria Gulch Sandstone Beds (of the Blue Gulch Mudstone 

Member) and massive quarry exposures (roadcut is about 10 

m high at bend in road). B, Hummocky cross-stratified and 

parallel-stratified fine-grained sandstone exposed in quarry, 

beds dip toward the viewer. C, Alternating beds of lightcolored hummocky cross-stratified sandstone and less resistant dark-colored bioturbated sandstone; pen in bioturbated 

interval circled for scale.
68 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

stone Member to repetitive and amalgamated beds of 

massive or hummocky cross-stratified fine-grained 

sandstone. This contact is generally clear, and there 

is little evidence of interfingering along it. The underlying mudstone and siltstone is generally massive 

and bioturbated, and it contains a few scattered beds 

of very fine grained sandstone that are generally less 

than 15 cm thick, ripple-marked, and hummocky 

cross-stratified. The overlying sandstone contains 

beds of fine-grained sandstone as thick as 4 m and 

few, if any, siltstone or mudstone interbeds in its 

lower part.

The upper contact of the Rancheria Gulch Sandstone Beds with the middle part of the Blue Gulch 

Mudstone Member is conformable and more gradational than the lower contact. It is marked by an upward change in predominant lithology from very fine 

grained to fine-grained sandstone with some interbedded mudstone and siltstone to massive siltstone 

with some thin interbeds of very fine grained sandstone. The contact is arbitrarily placed at the top of 

the highest bed of fine-grained sandstone thicker 

than 30 cm.

The Rancheria Gulch Sandstone Beds typically 

form two subparallel ridges held up by the resistant 

sandstone. Nilsen and others (1983) recognized and 

mapped two subunits of the Rancheria Gulch Sandstone Beds based on this outcrop pattern. However, 

measured sections have failed to reveal a major shale 

or siltstone interval between the sandstone ridges, 

and in this report, subunits lj and 12 of Nilsen and 

others (1983) are grouped together as one map unit 

(pi. 1).

AREAL EXTENT

The Rancheria Gulch Sandstone Beds are areally 

restricted to the southern Cottonwood Creek valleynorthern Black Mountain area, southwest of Hornbrook, California. The unit crops out between Ditch 

Creek and Osburger Gulch over a northwest-southeast distance of about 8 km. I have not recognized at 

the same stratigraphic level any detached or separated bodies of sandstone to the northwest or southeast of the main lens, so that it appears to pinch out 

laterally in a well-defined manner within mudstone 

of the Blue Gulch Mudstone Member.

The northwestern limit of outcrops of the 

Rancheria Gulch Sandstone Beds is on the narrow 

southern end of the divide between Cottonwood 

Creek and Hutton Creek (SWMSEM sec. 7, T. 47 N., 

R. 6 W., Hornbrook 15-minute quadrangle). The unit 

is very thin in this area and is truncated by a minor

northeast-trending fault that cuts through the northwest corner of sec. 18, T. 47 N., R. 6 W., Hornbrook 

15-minute quadrangle; the lens does not continue 

northwest of this fault.

The southeastern limit of outcrops of the 

Rancheria Gulch is on the northwest flank of Black 

Mountain, between Osburger Gulch and Blue Gulch. 

The last mapped patch of the unit is in the 

NW^NWM sec. 3, T. 46 N., R. 6 W., Hornbrook 

15-minute quadrangle. Because of the large area covered by landslides along the flanks of Black Mountain, it is possible that the unit may extend farther 

south along the western and southwestern flanks of 

Black Mountain. However, the Rancheria Gulch 

Sandstone Beds do not crop out along the northeastern edge of the Shasta Valley, where the Blue Gulch 

Mudstone Member crops out extensively.

TYPE SECTION

The type section of the Rancheria Gulch Sandstone 

Beds of the Blue Gulch Mudstone Member of the 

Hornbrook Formation was measured southeast of 

Rancheria Gulch in the center of the SI^SWK sec. 20, 

T. 47 N., R. 6 W., Hornbrook 15-minute quadrangle 

(fig. 8). This section begins in some roadcuts along an 

access roadway to a house, extends northeastward 

into and across a large quarry west of the U.S. Forest 

Service Forestry Camp, and continues northward 

along another road that borders the quarry. The section is about 86 m thick and is almost completely exposed (fig. 20). It consists almost wholly of very fine 

grained and fine-grained sandstone. The basal and 

upper contacts with the Blue Gulch Mudstone Member are well exposed.

Thickness 

(in meters)

Hornbrook Formation (upper part):

Blue Gulch Mudstone Member (middle part):

Siltstone, massive, bioturbated 2.00

Conformable contact.

Rancheria Gulch Sandstone Beds:

Sandstone, medium-grained, massive; abundant

molluscan fossil debris, bioturbated................. .45

Sandstone, very fine grained, and siltstone,

massive, bioturbated .......................................... .85

Sandstone, fine-grained, massive; scattered

molluscan fossils, bioturbated........................... 2.60

Siltstone, massive, bioturbated............................. 1.10

Sandstone, very fine grained, silty, massive,

bioturbated .......................................................... 1.40

Mudstone, silty, bioturbated ................................. .70

Sandstone, very fine grained, massive,

bioturbated .......................................................... 1.20
RANCHERIA GULCH SANDSTONE BEDS 69

Rancheria Gulch Sandstone Beds Continued:

Mudstone, silty, massive........................................

Sandstone, very fine grained, massive,

bioturbated ..........................................................

Sandstone, very fine grained, with thin interbeds of siltstone and mudstone.........................

Covered interval (float of siltstone and shale)....

Sandstone, fine-grained, massive, bioturbated ...

Covered interval .....................................................

Sandstone, fine-grained; hummocky crossstrata....................................................................

Sandstone, fine-grained, massive, bioturbated ...

Sandstone, fine-grained; hummocky crossstrata....................................................................

Sandstone, fine-grained, massive, bioturbated ...

Sandstone, fine-grained; hummocky crossstrata....................................................................

Sandstone, fine-grained, massive, bioturbated ...

Sandstone, fine-grained; hummocky crossstrata....................................................................

Sandstone, fine-grained, massive, bioturbated ...

Sandstone, fine-grained; hummocky crossstrata....................................................................

Sandstone, fine-grained, massive, bioturbated; 

scattered molluscan fossils................................

Sandstone, fine-grained; hummocky crossstrata....................................................................

Sandstone, fine-grained, massive, bioturbated ...

Sandstone, fine-grained; hummocky crossstrata ....................................................................

Sandstone, fine-grained, massive, bioturbated; 

scattered molluscan fossils................................

Sandstone, fine-grained; hummocky crossstrata, bioturbated..............................................

Sandstone, fine-grained, massive, bioturbated ...

Sandstone, fine-grained; hummocky cross

strata, bioturbated..............................................

Sandstone, fine-grained, massive, bioturbated; 

scattered fossil plant fragments .......................

Sandstone, fine-grained; hummocky crossstrata, bioturbated..............................................

Sandstone, fine-grained, massive, bioturbated ...

Sandstone, fine-grained; hummocky crossstrata....................................................................

Sandstone, fine-grained, massive, bioturbated ...

Sandstone, fine-grained; hummocky crossstrata....................................................................

Sandstone, fine-grained, massive; molluscan 

fossils....................................................................

Sandstone, fine-grained; hummocky crossstrata....................................................................

Sandstone, fine-grained, massive; molluscan 

fossils....................................................................

Sandstone, fine-grained; hummocky crossstrata....................................................................

Sandstone, fine-grained, massive, bioturbated ...

Sandstone, fine-grained; hummocky crossstrata....................................................................

Sandstone, fine-grained, massive, bioturbated ...

Sandstone, fine-grained; hummocky crossstrata....................................................................

Sandstone, fine-grained, massive; scattered 

plant fossils .........................................................

Thickness 

(in meters)

.10 

.20

5.50

17.00

1.50

7.50

.30 

1.45

.20 

2.00

.20 

2.40

.15 

1.80

.25 

.40

.25 

.65

.10 

.35

.15 

.65

.70 

6.00

.30 

2.20

.70 

3.90

.25

2.90

.12

1.40

.20 

.20

.70 

.75

1.10

2.20

Thickness 

(in meters) 

Rancheria Gulch Sandstone Beds Continued:

Sandstone, fine-grained; hummocky crossstrata.................................................................... -50

Sandstone, fine-grained, massive, bioturbated,

mostly covered..................................................... 1.00

Sandstone, fine-grained; hummocky crossstrata .................................................................... .40

Covered interval (float of sandstone) ................... 3.00

Sandstone, fine-grained, massive ......................... .65

Covered interval ..................................................... .45

Sandstone, fine-grained, massive ......................... .25

Sandstone, fine-grained; hummocky crossstrata .................................................................... .10

Sandstone, fine-grained, massive, bioturbated;

scattered molluscan fossils ................................ .10

Sandstone, fine-grained; hummocky crossstrata.................................................................... .12

Sandstone, fine-grained, massive; scattered

molluscan fossils................................................. .40

Sandstone, fine-grained; hummocky crossstrata .................................................................... .60

Sandstone, fine-grained, massive ......................... .10

Sandstone, fine-grained; hummocky crossstrata.................................................................... .48

Sandstone, fine-grained, massive; scattered

molluscan fossils................................................. .70

Sandstone, fine-grained; hummocky crossstrata.................................................................... .20

Sandstone, fine-grained, massive, bioturbated... 1.35

Sandstone, very fine grained to fine-grained;

hummocky cross-strata ...................................... .20

Sandstone, fine-grained, massive ......................... .15

Total thickness of the Rancheria Gulch 

Sandstone Beds............................................ 85.77

Conformable contact.

Blue Gulch Mudstone Member (lower part): 

Siltstone and mudstone, interbedded, laminated 

and ripple-marked, bioturbated; plant fossil 

fragments............................................................. 10.00

The colors of the Rancheria Gulch Sandstone Beds 

are generally uniform. The sandstone is very light 

gray (N7) on fresh surfaces and light brown (5YR 5/6) 

or moderate yellowish brown (10YR 5/4) on weathered surfaces. The calcareous concretions are typically moderate brown (5YR 3/4) on weathered surfaces.

LITHOLOGY

The Rancheria Gulch Sandstone Beds consist 

chiefly of fine-grained sandstone. In the type section, 

strata consist predominantly of alternating beds of 

massive sandstone and hummocky cross-stratified 

sandstone, both of which commonly are bioturbated. 

The hummocky cross-stratified beds decrease in 

abundance upward; they are not present in the uppermost 15 m and may be absent from the uppermost
70 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

Base of upper part of 

Blue Gulch Mudstone Member

80 -

70 -

60 -

</J 50 -

DC

</J 

</J

LLI

* 40 - 

O

I

30 -

20 -

10 -

HCS

HCS

/?

/? HCS

y?

HCS

/?

y? 

/?

@ HCS

© 

© 

y?

GRAIN SIZE 

sh ss gravel

HCS

Middle of lower part of the 

Blue Gulch Mudstone Member

FIGURE 20. Type section of the Rancheria Gulch Sandstone Beds 

of the Blue Gulch Mudstone Member of the Hornbrook Formation. See figure 10 for explanation of symbols. Gaps in lithology represent covered intervals.

30 m. The beds of massive sandstone range from 

about 10 cm to 6 m in thickness, and the beds of 

hummocky cross-stratified sandstone range from 

about 10 cm to 1.10 m in thickness. The thickest beds 

of sandstone are generally present in the middle part 

of the type section, with thinner beds near the base 

and at the top.

The upper part of the type section contains 

interbeds of bioturbated siltstone and silty mudstone 

as thick as 1.1 m; these interbeds mark the gradational upper contact with the overlying mudstone of 

the middle part of the Blue Gulch Mudstone Member. 

In other sections, there are minor interbeds of siltstone in the lower part of the unit. Sandstone of the 

upper part of the type section is generally finer 

grained, being typically very fine grained sandstone 

rather than fine-grained sandstone. However, the uppermost bed in the type section consists of massive 

medium-grained sandstone that contains the coarsest 

grain sizes observed in the entire section. No conglomerate has been observed within the Rancheria 

Gulch Sandstone Beds.

Burrowing is common in the sandstone as well as 

in the siltstone and silty mudstone of the Rancheria 

Gulch Sandstone Beds. The burrows are typically 

solitary, but some beds (especially beds of mudstone 

and siltstone) are so thoroughly bioturbated that 

stratification is totally destroyed and individual trace 

fossils cannot be identified. The burrows are typically 

feeding burrows that penetrate subvertically into the 

beds, but some grazing traces on bedding surfaces are 

also present.

Megafossils, including pelecypods and gastropods, 

are found at several levels within the Rancheria 

Gulch Sandstone Beds. They are present both in 

growth position within the massive fine-grained 

sandstone and as transported and reworked fossil debris, commonly near the base of the hummocky crossstratified beds. The megafossils are generally more 

abundant in the lower parts of the unit, although the 

uppermost bed of medium-grained sandstone in the 

type section contains abundant transported megafossil debris. Scattered plant fossils are present 

within massive beds of sandstone as well as on bedding surfaces of hummocky cross-stratified sandstone.

STRATIGRAPHIC RELATIONS

The Rancheria Gulch Sandstone Beds form a mappable unit within the lower part of the Blue Gulch 

Mudstone Member of the Hornbrook Formation in 

the Hornbrook area. The unit conformably overlies, 

underlies, and pinches out laterally to the northwest 

and southeast into mudstone of the Blue Gulch Mudstone Member. No other lenses of sandstone or
RANCHERIA GULCH SANDSTONE BEDS 71

groups of sandstone beds have been observed in other 

outcrop areas of the lower part of the Blue Gulch 

Mudstone Member that can be correlated with the 

Rancheria Gulch Sandstone Beds. The lens probably 

also pinches out in the subsurface to the northeast, 

but there are no available drill-hole data to confirm 

this.

THICKNESS

The Rancheria Gulch Sandstone Beds range in 

thickness from 0 to at least 85.77 m, its measured 

thickness at the type section. Peck and others (1956) 

measured a section of their member V, which is 

equivalent to the Rancheria Gulch Sandstone Beds, 

on the ridge south of Rancheria Gulch in the same 

area as the type section of the Rancheria Gulch 

Sandstone Beds and obtained a thickness of 94.8 m. 

Just west of Blue Gulch, in the main part of the type 

section of the Blue Gulch Mudstone Member (center 

of sec. 33, T. 47 N., R. 6 W., Hornbrook 15-minute 

quadrangle), I measured an additional section of the 

Rancheria Gulch Sandstone Beds that is 62.55 m 

thick.

The Rancheria Gulch Sandstone Beds appear to be 

thickest in the Rancheria Gulch area. The unit thins 

rapidly northwestward, pinching out over a distance 

of about 3.5 km! It thins more ^gradually to the southeast, pinching out beneath landslides on the flanks of 

Black Mountain 4.5 to 10 km southeast of Rancheria 

Gulch. The unit thus appear to be asymmetrical in 

northwest-southeast cross section, with its thicker 

part situated toward the northwest.

DEPOSITIONAL ENVIRONMENT

The molluscan megafossils, abundant bioturbation, 

hummocky cross-stratification, and narrow range of 

clast sizes within the Rancheria Gulch Sandstone 

Beds suggest its deposition in a wave-dominated, 

high-energy, middle- to outer-shelf environment. 

Megafossils reported from the Rancheria Gulch Sandstone Beds such as the bivalve genus Meekia (Saul 

and Popenoe, 1962) were shallow-marine, infaunal 

suspension feeders that preferred a sandy substratum and a well-agitated open coastline (Sundberg, 

1980, 1982). The bivalve genus Cucullaea was an infaunal deposit feeder that preferred silty and sandy 

substrates (Rhoads and others, 1972). The bivalve genus Glycimeris was an infaunal suspension feeder indicative of paralic, high-energy environments 

(Stanley, 1970) that probably lived at water depths of 

5-100 m (Sundberg, 1982). Trace fossils include 

Ophiomorpha, Thalassinoides, and other deposit feeding burrows that are generally indicative of shallowmarine, high-energy environments (Frey and others, 

1978).

The abundant hummocky cross-stratification, 

which is commonly amalgamated in the Rancheria 

Gulch Sandstone Beds, is characteristic of storm- and 

wave-dominated sedimentation on a high-energy marine shelf (Dott and Bourgeois, 1982). The amalgamation results from frequent storm-induced currents 

below the effective wave base that scour away evidence of fair-weather deposition. Storm reworking of 

shelf sediments is also indicated by beds of hummocky cross-stratified sandstone with highly bioturbated siltstone and silty mudstone in their upper 

parts and, locally, in their lower parts; this lithologic 

association suggests that there was episodic scour 

and deposition by storm surges to yield the hummocky cross-stratified sandstone, followed by fairweather deposition of mud and silt and the return of 

burrowing organisms. The lack of conglomerate and 

of coarser grades of sand suggests that deposition 

took place on the middle to outer parts of the shelf, 

where rip currents or surf-zone processes that might 

transport coarse material were not active.

AGE AND CORRELATION

Th,e age of the Rancheria Gulch Sandstone Beds is 

constrained by megafossil collections from within the 

unit and by foraminiferal collections from the overlying mudstone of the Blue Gulch Mudstone Member. 

No foraminiferal collections have been obtained from 

within the lens itself.

Peck and others (1956) reported the presence of 

numerous ammonites from the upper part of the unit, 

apparently from the mudstone and siltstone beds 

that are abundant near the gradational upper contact 

with the overlying middle part of the Blue Gulch 

Mudstone Member. They collected specimens of 

Metaplacenticeras paciftcum (Smith), M. californicum 

(Anderson), Desmophyllites sp. juv., Gaudryceras cf. 

G. denmanense (Whiteaves), Pachydiscus sp., 

Eupachydiscus cf. E. perplicatus (Whiteaves), and E. 

cf. E. haradai (Jimbo) from the uppermost part of the 

beds on the south side of the Klamath River, west of 

Blue Gulch (SW^NEM sec. 33, T. 47 N., R. 6 W., 

Hornbrook 15-minute quadrangle). They also collected specimens of Inoceramus cf. /. pertenuis Meek 

and Hayden as well as several other pelecypods and 

gastropods. Anderson (1902, p. 78, 80, 90, 98) had 

previously collected some similar ammonites from the 

upper part of the Rancheria Gulch Sandstone Beds, 

or possibly from the overlying mudstone of the Blue 

Gulch Mudstone Member, near Henley. These ammonites and the accompanying Inoceramus are strongly
72 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

suggestive of a late Campanian age for the uppermost part of the Rancheria Gulch Sandstone Beds 

(Peck and others, 1956, p. 1981). Sliter and others 

(1984, fig. 2) reported the presence of late Campanian 

megafossils from the lower, middle, and upper parts 

of the Rancheria Gulch Sandstone Beds (USGS Mesozoic localities M382, M385, and M387).

Thus, the age of the Rancheria Gulch Sandstone 

Beds appears to be late Campanian, about 75 to 79 

Ma. The beds are at least partly correlative in Oregon with Cretaceous strata along the southwest 

coast the Cape Sebastian Sandstone and Hunters 

Cove Formation of Dott (1971) and with subsurface 

Cretaceous strata of the Ochoco basin near Mitchell 

(Thompson and others, 1984). In northern California, 

the beds are, at least, partly correlative with the 

Great Valley sequence near Chico (Haggart and 

Ward, 1984) and along the west side of the Sacramento Valley (Ingersoll, 1979), as well as with parts 

of the Coastal belt terrane of the Franciscan assemblage (Evitt and Pierce, 1975).

HILT BED

DEFINITION

The Hilt Bed is a prominent 2- to 5-m-thick 

marker bed of sandstone that forms the boundary between the middle and upper parts of the Blue Gulch 

Mudstone Member of the Hornbrook Formation in its 

type section (figs. 7, 18). In the type section of the 

Blue Gulch Mudstone Member, the Hilt Bed is situated about 458 m above the base of the Blue Gulch 

Mudstone Member, 294 m above the top of the 

Rancheria Gulch Sandstone Beds, and 388.5 m below 

the unconformable upper contact of the Blue Gulch 

Mudstone Member with the overlying Tertiary volcanic rocks (fig. 18).

Most of the Blue Gulch Mudstone Member, except 

for the Rancheria Gulch Sandstone Beds, consists of 

mudstone with thin to very thin interbeds of siltstone 

and sandstone. The Hilt Bed, in contrast, is a very 

thick bed of sandstone, locally almost 5 m thick (fig. 

21). It is the only very thick bed of sandstone that I 

have observed within the Blue Gulch Mudstone Member except for the beds of sandstone within the 

Rancheria Gulch Sandstone Beds and a single unique 

outcrop of a very thick individual bed of massive 

sandstone (SWKSE^ sec. 7, T. 47 N., R 6W., Hornbrook 15-minute quadrangle). This latter bed, 4.20 m 

thick, consists of massive, graded, granule conglomerate to medium-grained sandstone that contains abundant molluscan fossil debris and rip-up clasts of 

shale. The bed is wholly different in internal organization and composition from the Hilt Bed, is disrupted by cross-faults, and is not laterally persistent. 

Although this bed crops out in close proximity to the 

Hilt Bed, its distinct lithology suggests that it is a 

different areally restricted thick bed.

Because the Hilt Bed is the only very thick, laterally persistent bed of sandstone within the Blue 

Gulch Mudstone Member and is more resistant than 

the predominant mudstone lithology, it forms a 

prominent topographic bench on slopes underlain by 

the member. However, because it is offset by numerous northeast-trending faults, the Hilt Bed is rarely 

present at similar elevations on slopes for more than 

a few kilometers. The basal part of the Hilt Bed is 

abruptly scoured into underlying mudstone, whereas 

its uppermost part is gradational into overlying mudstone.

AREAL EXTENT

The Hilt Bed forms a prominent marker bed 

within the Blue Gulch Mudstone Member of the 

Hornbrook Formation from the area of Willow School 

at the north end of the Shasta Valley to the Lithia 

Spring area east of Ashland in the northern Bear 

Creek valley area (pi. 1). The Hilt Bed may also extend farther to the northwest in the northern Bear 

Creek valley area, but the Blue Gulch Mudstone 

Member there is mostly covered by Quaternary alluvium and, farther north, by Tertiary volcanic rocks. 

In addition, the Hilt Bed may extend farther south 

into the Shasta Valley area, but the lower part of the 

Blue Gulch Mudstone Member in that area is covered 

by Quaternary alluvium.

The distribution of the Hilt Bed within the main 

outcrop area of the Hornbrook Formation in the Bear 

Creek valley and Cottonwood Creek valley areas is 

shown by Nilsen and others (1983), who mapped the 

Hilt Bed as their subunit m within the lower part of 

their unit d of the Hornbrook Formation. In every 

area between the northern Shasta Valley and 

Ashland where the lower part of the Blue Gulch 

Mudstone Member is sufficiently well exposed, the 

Hilt Bed forms a mappable marker bed.

TYPE SECTION

The type section of the Hilt Bed of the Blue Gulch 

Mudstone Member of the Hornbrook Formation was 

measured in a roadcut along an access road east of 

Interstate Highway 5 in the NWKSWX sec. 24, T. 48 

N., R. 7 W., Hornbrook 15-minute quadrangle (fig. 

22; pi. 1). The roadcut is on the opposite side of Interstate Highway 5 from the store at Hilt, adjacent to 

the Hilt exit (Nilsen and others, 1984, stop 7).
HILT BED 73

Thickness 

(in meters)

Hornbrook Formation:

Blue Gulch Mudstone Member (upper part):

Mudstone, massive 

Conformable contact. 

Hilt Bed:

Sandstone, very fine grained, and siltstone, 

ripple-marked and laminated .......................

Sandstone, very fine grained, parallelstratified ..........................................................

Sandstone, fine-grained, massive .....................

Sandstone, very fine grained, parallelstratified ..........................................................

Sandstone, fine-grained, massive .....................

Sandstone, fine-grained, parallel-stratified.....

Sandstone, fine- to medium-grained, massive;

mudstone rip-up clasts; flute casts at base .

Total thickness of the Hilt Bed .................

0.10

.12 

.10

.35 

.08 

.48

2.85

4.08

Conformable but erosive contact.

Blue Gulch Mudstone Member (middle part): 

Mudstone, massive

The Hilt Bed is yellowish gray (5Y 7/2) on fresh 

surfaces and moderate yellowish brown (10YR 5/4) on 

weathered surfaces. These colors vary little over its 

entire outcrop extent.

LITHOLOGY

The Hilt Bed everywhere consists of sandstone. It 

can generally be divided into nine lithic divisions, all 

of which are present in almost every outcrop. These 

divisions correspond to the divisions that Bouma 

(1962) determined for graded turbidite beds, but they 

are vertically organized in a rather unorthodox fashion. The organization of the Hilt Bed is a compound 

^abababcde sequence, in which the basal a division is 

typically the thickest, and overlying divisions are of 

variable thickness but are generally thinner.

The lowermost a division typically consists of a 

fine- to medium-grained sandstone, commonly with

FIGURE 21. Hilt Bed of the Blue Gulch Mudstone Member of the Hornbrook Formation in its type section and adjacent areas. A and 

B, Type section of the Hilt Bed (of Blue Gulch Mudstone Member) east of Hilt overpass on Interstate Highway 5 and east of store 

at Hilt; note thin-bedded basin-plain turbidites stratigraphically above and below the Hilt Bed. Roadcut is approximately 10 m in 

height. C and D, Reference section of the Hilt Bed (of Blue Gulch Mudstone Member), west of store at Hilt. Note slump block of 

Hilt Bed (B) in right side of outcrop and fault (F) truncating right, or east, end of outcrop.
74 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

coarse-grained sandstone at its base and with rip-up 

clasts of mudstone scattered throughout. This division is typically massive and graded, with the grading most typically expressed as coarse-tail grading. 

Flute casts and groove casts are commonly present 

along the basal surface of the bed, where it clearly 

rests with erosional contact on underlying mudstone.

The overlying 6 division is typically fine-grained 

sandstone that is parallel stratified. It grades upward 

from the a division and commonly contains abundant 

mica and plant fragments.

The second a division consists of massive finegrained sandstone that is generally slightly coarser 

than the underlying 6 division. This a division generally rests with slight erosional contact on the underlying 6 division. Overlying this second a division is 

the second 6 division, which consists of parallelstratified very fine grained to fine-grained sandstone.

The upper part of the Hilt Bed consists of a complete Tabcde Bouma sequence. The a division consists 

of massive fine-grained sandstone, the 6 division of 

parallel-stratified very fine grained sandstone, the c 

division of ripple-marked very fine grained sandstone, the d division of laminated siltstone and very 

fine grained sandstone, with abundant mica flakes

B

4 -

3 -

entr

LLJ

LLJ

S

S 2-

en

en

LLJ

z

y

H 1 -

0 -

:-£>

- 

 .

=^ ' 

>

 ..'.

 *

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; ; 

. *

 '*

ir~_

rCe 

d

\. c

A*

?

i/»

'. : *

"." ;

"' '

' .

^ . 

 * .

Blue Gulch Mudstone 

Member

^^x *

>

a

3

a

b 3-

Hilt Bed of

the Blue Gulch

Mudstone Member 2 -

a

1 -

GRAIN SIZE

sh ss gravel

0 - 

Blue Gulch Mudstone

Member

- -. e

--- d

* . ' . ' X ^

'-^-^-j ^

T i i '3.

 ' '

 . . . -^

 '. J- " :.

-.'. ".

" ' '.'.'

:'- -' 

 . .*

 ';".- '. '

. . ..-

.- .'. - 

^

-z-j

^^^

b

b

a

b

a

FIGURE 22. Type section and reference section of the Hilt Bed 

of the Blue Gulch Mudstone Member of the Hornbrook Formation. A, Type section, east of store at Hilt. B, Reference 

section, west of store at Hilt. Lowercase letters a-e correspond to divisions that Bouma (1962) defined for graded 

turbidite beds. See figure 10 for explanations of symbols.

and finely comminuted plant debris, and the e division of mudstone that grades up into the overlying 

upper part of the Blue Gulch Mudstone Member. In 

most sections and in the type section shown in figure 

22, the mudstone of the e division of the Hilt Bed 

cannot be distinguished from mudstone of the overlying Blue Gulch Mudstone Member; as a result, it is 

not included within the thicknesses measured for the 

Hilt Bed.

The lowermost a division is typically the thickest 

division and forms a steep clifflike exposure. The uppermost divisions are generally thinner, are less resistant, and in some areas are partly to wholly covered by soil and colluvium derived from the overlying 

mudstone. In a few localities, however, the lowermost 

a division is missing.

STRATIGRAPHIC RELATIONS

The Hilt Bed appears to be a laterally continuous 

marker bed of gradually changing thickness that is 

conformable within the Blue Gulch Mudstone Member. It is everywhere both underlain and overlain by 

mudstone and thin-bedded sandstone of the Blue 

Gulch Mudstone Member. It probably pinches out laterally in most directions into the mudstone deposits, 

but this characteristic is not observed in outcrop.

THICKNESS

The Hilt Bed is thickest near Bailey Hill in the 

Cotton wood Creek valley area, where it is 4.71 m 

thick (SWXNEM sec. 25, T. 48 N., R. 7 W., Hornbrook 15-minute quadrangle). It generally thins progressively northwestward, and the following thicknesses for it have been successively measured north 

of the Bailey Hill area: (1) 4.08 m at the type section 

(NWMSWM sec. 24, T. 48 N., R. 7 W., Hornbrook 15- 

minute quadrangle); (2) 4.06 m at its reference section (SWXNWJ* sec. 24, T. 48 N., R. 7 W., Hornbrook 

15-minute quadrangle); (3) 3.77 m southeast of the 

town of Cole (NW^NWM sec. 23, T. 48 N., R. 7 W., 

Hornbrook 15-minute quadrangle); (4) 3.55 m northwest of Cole (NW^NW% sec. 13, T. 41 S., R. I E., 

Ashland 15-minute quadrangle); (5) 1.46 m north of 

Cole, where the basal T division is missing

sec. 12, T. 41 S., R. 1 E., Ashland 15- 

minute quadrangle; (6) 2.10 m near Steinman at the 

southeast end of the Bear Creek valley area 

(SWMNEJ4 sec. 16, T. 40 S., R. 2 E., Ashland 15- 

minute quadrangle); (7) 2.84 m near Lithia Spring 

east of Ashland (SW^SWM sec. 51, T. 39 S., R. 1 E., 

Ashland 15-minute quadrangle); and (8) 2.69 m near
SUMMAKY AND CONCLUSIONS 75

the Ashland airport, east of Ashland (NWKNWK sec. 

20, T. 39 S., R. 1 E., Ashland 15-minute quadrangle). 

Thus, with the exception of sections 5 and 6, the Hilt 

Bed gradually decreases in thickness (from 4.71 to 

2.69 m) northwest of the Bailey Hill area, a distance 

of about 29.5 km.

South of its thickest section near Bailey Hill, the 

Hilt Bed also progressively thins. The following thicknesses for it have been successively measured from 

north to south: (1) 4.05 m about 3 km south of Bailey 

Hill near bench mark 2749 in the Cottonwood Creek 

valley area (SWX sec. 31, T. 48 N., R. 6 W., 

Hornbrook 15-minute quadrangle); (2) 3.45 m near 

Hutton Creek (NWKSEX sec. 6, T. 47 N., R. 6 W., 

Hornbrook 15-minute quadrangle; (3) 3.55 m near 

Hutton Creek (SWXSEX sec. 7, T. 47 N., R. 6 W., 

Hornbrook 15-minute quadrangle); (4) 2.95 m about 1 

km northwest of Hornbrook (SEKSWX sec. 17, T. 47 

N., R. 6 W., Hornbrook 15-minute quadrangle); (5) 

2.99 m about 1 km southeast of Hornbrook (NEKSWK 

sec. 21, T. 47 N., R. 6 W., Hornbrook 15-minute quadrangle); (6) 3.00 m about 1.5 km southeast of 

Hornbrook (SWXSEK sec. 21, T. 47 N., R. 6 W., 

Hornbrook 15-minute quadrangle); (7) 3.60 m about 2 

km southeast of Hornbrook adjacent to bench mark 

2110 (SWXNEX sec. 28, T. 47 N., R. 6 W., Hornbrook 

15-minute quadrangle); (8) 3.31 m in the type section 

of the Blue Gulch Mudstone Member (NWKNWK sec. 

34, T. 47 N., R. 6 W., Hornbrook 15-minute quadrangle); and (9) 1.61 m near Willow Creek School at 

the north end of the Shasta Valley (NEKSWK sec. 7, 

T. 46 N., R. 5 W., Copco 15-minute quadrangle). 

Thus, the Hilt Bed decreases irregularly southeastward from a maximum thickness near Bailey Hill of 

4.71 m to a minimum thickness near Willow Creek 

School of 1.61 m, a distance of about 18.5 km.

DEPOSITIONAL ENVIRONMENT

The Hilt Bed represents a multiple or compound 

turbidite deposit in a deep-marine setting. The presence of the Bouma sequence within the bed, of grading within the bed, of the erosional basal contact with 

mudstone, and of sole markings at the base of the 

bed all suggest deposition by a turbidity current below wave base. The multiple or compound nature of 

the turbidite, in which three separate Bouma sequences constitute the Hilt Bed, suggests that three 

separate turbidity currents were generated (possibly 

by the same triggering event) to produce the bed. The 

absence of interbedded shale, the lateral continuity 

and extent of the three turbidites, and the incompleteness of the two lower Bouma sequences suggest 

that the three turbidity currents flowed to the basin

floor at almost the same time to produce the Hilt 

Bed.

AGE AND CORRELATION

The Hilt Bed, based on ages of benthic and planktonic foraminifers included within mudstone underlying and overlying it, is apparently of late Campanian 

age. At its type section, foraminifers from 35 m beneath the Hilt Bed are indicative of a late Santonian 

to late Campanian age (USGS Mesozoic locality 

Mf6584). Because of its deposition by turbidity currents, the Hilt Bed is probably not time transgressive 

or regressive; rather, it was probably deposited 

within a few hours to a few days, thus forming a very 

useful marker bed.

The Hilt Bed is, at least, partly correlative with 

Campanian rocks in southwestern Oregon [the Cape 

Sebastian Sandstone and the Hunters Cove Formation of Dott (1971)] and in northern California (the 

Great Valley sequence and the Coastal belt terrane of 

the Franciscan assemblage).

SUMMARY AND CONCLUSIONS

This paper summarizes the lithostratigraphic and 

biostratigraphic aspects of the Cretaceous Hornbrook 

Formation of southwestern Oregon and north-central 

California. The formation, originally named by Peck 

and others (1956) for exposures near Hornbrook, 

California, was divided into five members by Nilsen 

(1984a).

The basal member, the Klamath River Conglomerate Member, consists of discontinuous nonmarine deposits, 36.5 m thick in its type section, that rest on 

basement rocks of the Klamath Mountains. These 

strata were deposited as alluvial fans and braided 

streams. They commonly include a basal weathered 

breccia or conglomeratic grus. The deposits have been 

derived from the locally subjacent and nearby basement rocks. The member is most likely of Albian, 

Cenomanian, and Turonian age, although no age-diagnostic fossils have been obtained from it.

Shallow-marine strata of the conformably overlying Osburger Gulch Sandstone Member, 116.5 m 

thick in its type section, were deposited in shoreline 

to middle-shelf environments. Trough cross-stratified, 

medium-grained to very coarse grained, conglomeratic sandstone characterizes its lower nearshore deposits and hummocky cross-stratified, very fine 

grained to fine-grained sandstone characterizes its 

upper storm-dominated middle-shelf deposits. An 

abundant molluscan fauna is indicative of a late
76 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

Albian(?) to early Coniacian age for the unit, which 

generally is younger toward the southeast.

The conformably overlying Ditch Creek Siltstone 

Member, 61.6 m thick in its type section, was deposited in outer-shelf to upper-slope environments. It 

consists of thoroughly bioturbated, massive siltstone 

and silty very fine grained sandstone that in some 

places contains graded storm deposits of sandstone 

and thin turbidites. In the northern Shasta Valley, a 

bituminous coal bed is intercalated with molluskbearing siltstones, which implies at least local subaerial exposure. Megafossils and foraminifers from 

the member generally are indicative of an early 

Turonian to early Coniacian age, which generally is 

younger toward the southeast.

The Rocky Gulch Sandstone Member, 171.2 m 

thick in its type section, consists predominantly of 

turbidite sandstone organized into repetitive finingand thinning-upward cycles. Although various depositional environments have been proposed by others in 

the past, the Rocky Gulch Sandstone Member appears to have been deposited as an apron on a submarine slope. The upper part of the member locally 

contains interbedded siltstone that appears to be of 

contourite origin. A single collection of foraminifers 

from this member is indicative of a middle Turonian 

age, but the member apparently ranges in age from 

middle Turonian to Campanian(?).

The conformably overlying Blue Gulch Mudstone 

Member, about 849.9 m thick in its type section, consists in most areas of hemipelagic mudstone with 

thin-bedded basin-plain turbidites. In the type area of 

the Hornbrook Formation, however, it contains a 

prominent lens of shallow-marine, hummocky crossstratified, megafossiliferous sandstone within its 

lower part. This lens, the Rancheria Gulch Sandstone 

Beds, is 0-86 m thick, with a thickness of 85.8 m in 

its type section. The Rancheria Gulch Sandstone 

Beds are underlain and overlain by shallow-marine, 

bioturbated siltstone, mudstone, and very fine 

grained sandstone. Molluscan fossils from the sandstone beds are indicative of a late Campanian age. 

Most of the middle and upper parts of the Blue Gulch 

Mudstone Member in the type area consist of basinplain turbidites and hemipelagic mudstone. The 

lower, lower middle, and uppermost parts of the 

member in its type section, however, appear to represent deposition in shallower depths. A thick compound turbidite bed, the Hilt Bed, is present between 

the middle and upper parts of the Blue Gulch Mudstone Member and can be mapped for a lateral distance of about 48 km within the Hornbrook Formation. This bed is as thick as 4.71 m near Hilt and 

thins from there to the northwest and southeast.

Foraminiferal fossils from the Blue Gulch Mudstone 

Member are indicative of ages ranging from late 

Turonian to early Maestrichtian; in the type area, it 

appears to be Santonian to early Maestrichtian in 

age.

A major unconformity had been previously postulated within the type area of the Hornbrook Formation between strata equivalent to the Ditch Creek 

Siltstone Member and the Rocky Gulch Sandstone 

Member. The unconformity was thought to span the 

late Coniacian to early Campanian interval. However, the contact appears to be gradational in most 

areas and locally erosive in the type area. The varying ages reported herein from new collections of foraminifers and megafossils suggest that the unconformity, if present, is of local significance only.

The Hornbrook Formation records sedimentation 

within a basin located along the northeastern margin 

of the Klamath Mountains during the late Early and 

Late Cretaceous time. The basin may have originally 

extended farther to the northeast, connecting with 

the subsurface Ochoco basin and the Mitchell, 

Suplee, and Prairie Creek areas, as well as farther to 

the south, connecting with the Redding area and the 

Great Valley forearc basin. The petrography of framework grains of sandstone (Golia and Nilsen, 1984) 

and conglomerate clasts (Barats and others, 1984), as 

well as paleocurrents and facies associations (Nilsen, 

1984d), strongly suggest that the basement rocks of 

the Klamath Mountains to the southwest were the 

chief source terrain for the Hornbrook Formation.

The Hornbrook Formation is also an overlap assemblage that rests unconformably on various Paleozoic, Triassic, and Jurassic terranes that constitute 

the Klamath Mountains (Irwin, 1960, 1966, 1981, 

1984, 1985, 1989; Harper and Wright, 1984; Wright 

and Fahan, 1988). These terranes form a broad westfacing arcuate structure and are separated from each 

other by major east-dipping thrust faults. Sedimentary, metamorphic, and igneous rocks of various ages 

and degrees of structural complexity make up these 

terranes, which appear to consist mostly of fragments 

of oceanic crust and island arcs. Irwin (1981) concluded that the Paleozoic rocks of the eastern Klamath Mountains formed a volcanic arc built on ultramafic rocks that was intermittently active from the 

early Paleozoic to the Jurassic. He suggested that accretion of the various tectonic slices that form the 

western Klamath Mountains to the Paleozoic nucleus 

occurred during the Jurassic and not during previous 

time intervals. The terranes were intruded by plutons 

of Jurassic and earliest Early Cretaceous age (Irwin, 

1984; Mankinen and others, 1988), and uplift of these 

rocks took place in the Early Cretaceous to form the
REFERENCES CITED 77

Klamath source terrain for the Hornbrook Formation.

The Hornbrook Formation records a major transgression from the east or northeast onto the Klamath 

Mountains. The transgression is recorded by basal 

middle Albian marine strata in the Grave Creek 

area, upper AlbianC?) to middle Cenomanian strata in 

the Dark Hollow area, Turonian strata in the Bear 

Creek valley and Cottonwood Creek valley areas, and 

lower(?) Coniacian strata in the Black Mountain area 

(Sliter and others, 1984). This transgression may 

have continued southward to the Redding area, 

where Santonian strata rest directly on the basement 

rocks of the Klamath Mountains (Haggart, 1986a, b). 

However, these transgressive deposits probably 

record northward and eastward transgression from 

the Great Valley forearc basin. It is more likely that 

the transgressive strata of the Hornbrook Formation 

and Grave Creek area originally were continuous 

with eastward- or northeastward-transgressing 

Albian, Cenomanian, and Turonian shallow-marine 

deposits of the Mitchell, Suplee, and Grand Prairie 

Creek areas of central Oregon (fig. 1). This interpretation, however, depends upon the unproven existence of Cretaceous strata in the subsurface beneath 

the southern Cascade Range and beneath parts of the 

volcanic plateau of central Oregon to the east. The 

presence of a thick sequence of Upper Cretaceous 

strata that are partially correlative with the 

Hornbrook Formation in the subsurface Ochoco basin, however, supports this tentative conclusion (Thompson and others, 1984). During Cenozoic time, significant rotation and possible translation of the Cascade Range and parts of the volcanic plateau, as well 

as minor rotation of the Blue Mountains, might have 

caused major disruption of the postulated subsurface 

Cretaceous sequence in central Oregon (Beck, 1986; 

Beck and others, 1986; Wells and Heller, 1988).

The basin within which the Hornbrook Formation 

was deposited may have originally extended 

northeastward across much of central Oregon as a 

single large basin; alternatively, two separate Early 

to Late Cretaceous basins may have existed, the second one in the Ochoco-Mitchell area (fig. 1). In either 

case, the basin(s) would have been bounded by uplifted areas in western Idaho, eastern Oregon, northwestern Nevada, the Sierra Nevada of northern California and its continuation beneath the Modoc Plateau, and the Klamath Mountains. These areas were 

underlain by Paleozoic, Triassic, and Jurassic metamorphic rocks and Mesozoic plutons.

Because the age of emplacement of granite plutons 

of the Idaho batholith is chiefly Late Cretaceous, 

about 95-65 Ma (Hyndman, 1983; Lund and Snee, 

1988; Hyndman and Sears, 1988), the HornbrookOchoco basin most likely represents a forearc basin 

that developed between an oceanic subduction complex to the west and the batholith to the east. The 

Hornbrook-Ochoco basin was a fairly complex forearc 

basin, however, because it was bounded on the southwest by the older accreted terranes of the Klamath 

Mountains basement complex and the Jurassic and 

older plutons that intruded these terranes.

The Hornbrook Formation is overlain with angular 

unconformity by lower Tertiary nonmarine sedimentary and volcanic rocks. The surface of this unconformity cuts down at least several hundred meters 

into the upper part of the Hornbrook Formation, so 

that Tertiary strata locally rest directly on the Ditch 

Creek Siltstone Member and the Rocky Gulch Sandstone Member in the Ashland area.

The Hornbrook Formation was tilted northeastward sometime between the end of its deposition and 

the initiation of deposition of the lower Tertiary deposits, probably during post-early Maestrichtian to 

pre-Eocene time. The lower Tertiary strata and the 

Hornbrook Formation were subsequently tilted 

northeastward an additional amount, probably in the 

middle or late Tertiary. The Hornbrook strata were 

offset firstly along northwest-trending, steeply northeast-dipping normal faults and secondly along northeast-trending cross-faults characterized by varying 

types of movement (pi. 1). These faults may be related to prominent northwest-southeast-directed extensional tectonism during the Oligocene and Miocene that has been reported by Schweickert and 

Irwin (1989) in several parts of the southern Klamath Mountains.

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APPENDIXES I AND II
84 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

APPENDIX L MEGAFOSSILS COLLECTED FROM 

THE HORNBROOK FORMATION

See plate 1 for locations of samples; all collections by 

T.H. Nilsen and identifications by J.W. Miller of the U.S. 

Geological Survey. Samples are grouped in numerical order by member.

Osburger Gulch Sandstone Member

USGS Mesozoic locality M7682. Roadcut on west side of 

northbound lane of Interstate Highway 5, directly west 

of the type section of the Osburger Gulch Sandstone 

Member; collected from basal bed of lower siltstone unit 

of the Osburger Gulch Sandstone Member (SEK sec. 32, 

T. 47 N., R. 6 W., Hornbrook 15-minute quadrangle).

Gastropod: Turritella hearni Anderson

Age: Turonian

USGS Mesozoic locality M7683. Outcrop of fine- to 

medium-grained sandstone on northeast-facing slope 

about 3 km southeast of Paradise Craggy. Collected from 

basal beds of the Osburger Gulch Sandstone Member 

where they rest on the top of the Klamath River Conglomerate Member (SE^NW% sec. 26, T. 46 N., R. 6 W., 

Hornbrook 15-minute quadrangle). 

Gastropods: Calva regina Popenoe 

Glycimeris paciftcus (Anderson) 

Meekia takeona Saul and Popenoe 

Pterotrigonia evansana (Meek) 

Age: Turonian

USGS Mesozoic locality M7685. Outcrop of conglomeratic 

sandstone along west flank of crest of Soap Creek Ridge 

in outliers of the Hornbrook Formation about 8.5 km 

southwest of Yreka. Collected from basal trough crossstratified beds of the Osburger Gulch Sandstone Member 

(NE%NE% sec. 2, T. 44 N., R. 8 W., Yreka 15-minute 

quadrangle).

Peleycpods: Cucullaea sp. 

Ostrea sp.

Pachycardium remondianum (Gabb) 

Gastropod: Acteonella ovuftrnus Gabb 

Age: Turonian

Ditch Creek Siltstone Member

USGS Mesozoic locality M7686. Roadcut along the northeast side of Dark Hollow Road in upper part of the Ditch 

Creek Siltstone Member. Collected from silty very fine 

grained sandstone (SW!4 sec. 13, T. 38 S., R. 2 W., 

Medford 15-minute quadrangle). 

Pelecypods: Avicula roguensis Anderson 

Cucullaea sp.

Gastropods: Atresus liratus Gabb 

Cerithium sp.

Volutoderma mitraeformis (Gabb) 

Age: Turonian

APPENDIX II. FORAMINIFERS COLLECTED FROM 

THE HORNBROOK FORMATION

See plate 1 for locations of samples; benthic foraminiferal identifications by Kristin McDougall and William V. 

Sliter, and planktonic foraminiferal identifications by William V. Sliter. Samples are grouped in numerical order by 

member.

Ditch Creek Siltstone Member

USGS Mesozoic locality Mf650. Collected from mudstone 

exposed in floor of Cotton wood Creek (SWI4NEJ4 sec. 1, 

T. 47 N., R. 7 W., Hornbrook 15-minute quadrangle), 

Siskiyou County, Calif, (lat 41°57' N., long 122°36' W.). 

Middle part of the Ditch Creek Siltstone Member. 

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Astacolus jarvisi (Cushman) 

Bathysiphon vitta Nauss

Cribrostomoides cretaceus Cushman and Goudkoff 

Dorothia oxycona (Reuss) 

Gaudyrina tailleuri (Tappan) 

Gavelinella plummerae (Tappan) 

G. popenoi (Trujillo) 

Gyroidinoides birdi (Trujillo)

Haplophragmoides excavatus Cushman and Waters

Saccammina complanata (Franke) 

Saracenaria triangularis (d'Orbigny) 

Stilostomella impensia (Cushman) 

Inoceramus prisms 

Echinoid spines 

Age: Late Turonian to Coniacian

USGS Mesozoic locality Mf6581. Collected from natural 

outcrop of mudstone on southwest side of ridge located 

about 0.5 km northeast of Cottonwood Creek (NWI4NEI4 

sec. 1, T. 47 N., R. 7 W., Hornbrook 15-minute quadrangle), Siskiyou County, Calif, (lat 41°57' N., long 

122°36' W.). Uppermost part of the Ditch Creek Siltstone 

Member, about 2 m below contact with the Rocky Gulch 

Sandstone Member. 

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Ammodiscus cretaceus (Reuss) 

Astacolus jarvisi (Cushman) 

Bathysiphon vitta Nauss

Cribrostomoides cretaceus Cushman and Goudkoff 

Dorothia oxycona (Reuss) 

Gaudyrina tailleuri (Tappan) 

Gavelinella plummerae (Tappan) 

Gyroidinoides bandyi (Trujillo) 

Haplophragmoides excavatus Cushman and Waters

Fish debris 

Inoceramus prisms 

Radiolarians 

Age: Late Turonian to Coniacian.
APPENDIX II 85

USGS Mesozoic locality Mf6590. Collected from mudstone 

exposed in roadcuts along Siskiyou Boulevard in northwest part of Ashland, just north of the Southern Pacific 

railroad overpass (SW corner of sec. 38, T. 38 S., R. 1 E., 

Ashland 15-minute quadrangle), Jackson County, Ore. 

(lat 42°13' N., long 122°43' W.). Lower part of the Ditch 

Creek Siltstone Member. 

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Conorboides stormi Brotzen 

Cribrostomoides trifolium (Egger) 

Dentalina gracilis (d'Orbigny) 

Dorothia oxycona (Reuss) 

Gaudryina tailleuri (Tappan) 

Hoeglundina supracretacea (ten Dam) 

Inoceramus prisms 

Gastropod fragments 

Pelecypod molds

Planktonic foraminifers: Dicarinella canaliculate, 

(Reuss)

D. imbricata (Mornod) 

Marginotruncana renzi (Gandolfi) 

Praeglobotruncana Helvetica (Bolli) 

P. praehelvetica (Trujillo) 

Whiteinella baltica Douglas and Rankin 

Age: Middle Turonian

USGS Mesozoic locality M/6591. Collected from mudstone 

exposed in roadcuts along Siskiyou Boulevard in northwest part of Ashland, north of the Southern Pacific railroad overpass (SW corner of sec. 38, T. 38 S., R. 1 E., 

Ashland 15-minute quadrangle), Jackson County, Ore. 

(lat 42°13' N., long 122°43' W.). Middle part of the Ditch 

Creek Siltstone Member. 

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Ammodiscus cretaceus (Reuss) 

Dorothia oxycona (Reuss) 

Gaudyrina tailleuri (Tappan) 

Gavelinella plummerae (Tappan) 

Hoeglundina supracretacea (ten Dam) 

Lenticulina modesta (Bandy) 

L. muensteri (Roemer) 

Echinoid fragment 

Gastropod fragments 

Ostracode

Planktonic foraminifers: Dicarinella canaliculata 

(Reuss)

D. imbricata (Mornod) 

Marginotruncana renzi (Gandolfi) 

Praeglobotruncana Helvetica (Bolli) 

P. praehelvetica (Trujillo) 

Age: Middle Turonian

USGS Mesozoic locality Mf6592. Collected from mudstone 

in roadcut along the north side of Dark Hollow Road (SW 

corner of sec. 50, T. 38 S., R. 2 W., Medford 15-minute 

quadrangle), Jackson County, Ore. (lat 42°15' N., long 

122°53' W.). Middle part of the Ditch Creek Siltstone 

Member.

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Astacolus jarvisi (Cushman)

Cribrostomoides cretaceus Cushman and Goudkoff

Dentalina gracilis (d'Orbigny)

D. legumen (Reuss)

Dorothia ellisorae (Cushman)

Gavelinella plummerae (Tappan)

G. popenoi (Trujillo)

Globorotalites multiseptus (Brotzen)

Haplophragmoides excavatus Cushman and Waters

Hoeglundina supracretacea (ten Dam)

Lenticulina californiensis Trujillo

L. modesta (Bandy)

L. pseudosecans (Cushman)

Planktonic foraminifers: Dicarinella imbricata 

(Mornod)

Marginotruncana renzi (Gandolfi)

Praeglobotruncana gibba Klaus

P. Helvetica (Bolli)

P. praehelvetica (Trujillo) 

Age: Middle Turonian

Rocky Gulch Sandstone Member

USGS Mesozoic locality Mf6587. Collected from shale 

interbedded with sandstone, exposed in roadcut along 

northeast side of Interstate Highway 5 (SWJ4 sec. 32, T. 

37 S., R. 1 W., Medford 15-minute quadrangle), Jackson 

County, Ore. (lat 42°18' N., long 122°50' W.). Middle part 

of the Rocky Gulch Sandstone Member. 

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Astacolus jarvisi (Cushman) 

Bandyella greatvalleyensis (Trujillo) 

Cribrostomoides cretaceus Cushman and Goudkoff 

Dorothia ellisorae (Cushman) 

Gaudyrina tailleuri (Tappan) 

Haplophragmoides excavatus Cushman and Waters

Lenticulina muensteri (Roemer) 

Lingulogavelinella globosa (Brotzen) 

Saccammina complanata (Franke) 

Echinoid fragments

Planktonic foraminifers: Dicarinella canaliculata 

(Reuss)

D. imbricata (Mornod) 

Marginotruncana pseudolinneiana Pessagno 

Praeglobotruncana gibba Klaus 

P. Helvetica (Bolli)

Whiteinella baltica Douglas and Rankin 

W. brittonensis (Loeblich and Tappan) 

Age: Middle Turonian

Blue Gulch Mudstone Member

USGS Mesozoic localities Mf6335 to Mf6352. Collected 

from mudstone of the Blue Gulch Mudstone Member 

where it crops out between Interstate Highway 5 and the 

top of Bailey Hill (NEH sec. 25, T. 48 N., R. 7 W., 

Hornbrook 15-minute quadrangle), Siskiyou County, Ca-
86 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

lif. (lat 41°59! N., long 122°37' W.). The samples were 

collected in stratigraphic order, with sample Mf6335 

from near the base of the section and sample MfB352 

from the top of the section. Lower to upper parts of the 

Blue Gulch Mudstone Member. 

Collector. M.A. Elliott, 1971.

Mf6335

Benthic foraminifers: Bathysiphon spp.

Cribrostomoides cretaceus Cushman and Goudkoff 

Haplophragmoides cf. H. famosus Takayanagi 

Saracenaria triangularis (d'Orbigny) 

Silicosigmoilina californica Cushman and Church

Age: Campanian to early Maestrichtian

Mf6336

Planktonic foraminifers: Globotruncana area (Cushman)

G. bulloides Vogler

G. mariei Banner and Blow

G. ventricosa White

Heterohelix rumseyensis Douglas 

Age: Middle Campanian to early Maestrichtian

Mf6337

Benthic foraminifers: Ammobaculites sp.

Bathysiphon spp.

Cribrostomoides cretaceus Cushman and Goudkoff

Dorothia oxycona (Reuss)

Gaudryina tailleuri (Tappan)

Silicosigmoilina californica Cushman and Church

Spiroplectammina chicoana Lalicker

Textularia sp.

Unidentified arenaceous species 

Age: Campanian to early Maestrichtian

Mf6338

Benthic foraminifers: Ammobaculites sp. 

Ammodiscus sp. 

Bathysiphon spp. 

Dorothia oxycona (Reuss) 

Gaudryina tailleuri (Tappan) 

Haplophragmoides cf. H. famosus Takayanagi 

Silicosigmoilina californica Cushman and Church 

Spiroplectammina chicoana Lalicker 

Textularia sp.

Trochammina texana Cushman and W~ters 

Unidentified arenaceous species 

Age: Campanian to Maestrichtian

Mf6339

Barren of microfossils

MfB340

Benthic foraminifers: Bathysiphon vitta Nauss

Cribrostomoides cretaceus Cushman and Goudkoff

ffQuinqueloculina sp.

Silicosigmoilina californica Cushman and Church 

Age: probably Campanian to early Maestrichtian

Mf6341

Benthic foraminifers: Dorothia oxycona (Reuss)

Globorotaloides michelinianus (d'Orbigny)

G. spineus (Cushman)

Gyroidinoides bandyi (Trujillo) G. nitidus (Reuss)

Nodosaria spp.

Osangularia cordieriana (d'Orbigny)

Pleurostomella subnodosa Reuss

Pyrulina apiculata (Marie)

Quadrimorphina allomorphinoides (Reuss)

Spiroplectammina chicoana Lalicker 

Planktonic foraminifers: Heterohelix glabrans

(Cushman)

Age: Campanian or early Maestrichtian, probably early 

Maestrichtian

Mf6342

Benthic foraminifers: Bathysiphon spp.

Cribrostomoides cretaceus Cushman and Goudkoff

C. spp

Dorothia bulletta (Carsey)

D. oxycona (Reuss)

Gavelinella sp.

Gaudryina tailleuri (Tappan)

Globorotaloides michelinianus (d'Orbigny)

G. spineus (Cushman)

Lenticulina sp.

Nodosaria sp.

Silicosigmoilina californica Cushman and Church

Spiroloculina cretacea Reuss

Unidentified arenaceous species 

Age: Campanian to early Maestrichtian

Mf6343

Barren of microfossils

Mf6344

Benthic foraminifers: Bathysiphon spp.

Cribrostomoides cretaceus Cushman and Goudkoff

Silicosigmoilina californica Cushman and Church

Trochammina boehmi Franke

Unidentified arenaceous species 

Age: Upper Cretaceous

Mf6345

Benthic foraminifers: Bathysiphon vitta Nauss

B. spp.

Cribrostomoides spp.

Dorothia oxycona (Reuss)

Silicosigmoilina californica Cushman and Church

Unidentified arenaceous species 

Age: Upper Cretaceous

Mf6346

Planktonic foraminifers: Globotruncana area 

(Cushman)

G. ventricosa White 

Age: Middle Campanian to early Maestrichtian
APPENDIX II 87

Mf6347

Benthic foraminifers: Cribrostornoides spp.

Bathysiphon spp.

B. vitta Nauss

Dorothia oxycona (Reuss)

Gaudryina tailleuri (Tappan)

Reophax globosus Sliter

Silicosigmoilina californica Cushman and Church

Trochammina sp.

Unidentified arenaceous species 

Age: Upper Cretaceous

Mf6348

Benthic foraminifers: Ammodiscus cretaceus (Reuss)

Bathysiphon spp.

Dorothia tailleuri (Tappan)

Silicosigmoilina californica Cushman and Church

Spiroplectammina sigmoidina Lalicker

Unidentified arenaceous species 

Age: Campanian to early Maestrichtian

Mf6349

Benthic foraminifers: Ammodiscus cretaceus (Reuss)

Bathysiphon spp.

Cribrostornoides spp.

Gaudryina tailleuri (Tappan)

Reophax globosus Sliter

Silicosigmoilina californica Cushman and Church

Spiroloculina sp.

Trochammina sp.

T. texana Cushman and Waters

Unidentified arenaceous species 

Age: Campanian to early Maestrichtian

Mf6350

No bulk material submitted

Mf6351

Barren of microfossils

Mf6352

Barren of microfossils

USGS Mesozoic locality Mf6399. Collected from mudstone 

exposed along south bank of Blue Gulch, northwest side 

of Black Mountain (SE^NE^ sec. 33, T. 47 N., R. 6 W., 

Hornbrook 15-minute quadrangle), Siskiyou County, Calif, (lat 41°53' N., long 122°32' W.). Lower middle part of 

the Blue Gulch Mudstone Member, above the Rancheria 

Gulch Sandstone Beds. 

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Astacolus jarvisi (Cushman)

Bathysiphon vitta Nauss

Cribrostomoides cretaceus Cushman and Goudkoff

Dorothia bulletta (Carsey)

Gaudryina laevigata Franke

G. tailleuri (Tappan)

Gavelinella stephensoni (Cushman)

Hoeglundina supracretacea (ten Dam)

Lenticulina davisi (Bandy)

L. muensteri (Roemer)

L. spissocostata (Cushman)

Marginulina bullata Reuss

Pseudonodorsaria manifesto (Reuss)

Reophax globosus Sliter

Saracenaria navicula (d'Orbigny)

Silicosigmoilina californica Cushman and Church

Spiroplectammina chicoana Lalicker

Stilostomella impensia (Cushman)

Planktonic foraminifers: Globotruncana area 

(Cushman)

G. bulloides Vogler

G. ventricosa White

Heterohelix rumseyensis Douglas 

Age: Middle Campanian to early Maestrichtian

USGS Mesozoic locality Mf6401. Collected from mudstone 

exposed along railroad cut west of Carter Creek 

(SWMSE^ sec. 16, T. 40 S., R. 2 E., Ashland 15-minute 

quadrangle), Jackson County, Ore. (lat 42°05' N., long 

122°35' W.). Lower part of the Blue Gulch Mudstone 

Member. 

Collector. T.H. Nilsen, 1981.

Benthic foraminifers: Astacolus cf. A. umbonatus 

Loetterle

A cf. A planiusculus (Reuss) 

Cribrostomoides cretaceus Cushman and Goudkoff 

Dorothia ellisorae (Cushman) 

D. oxycona (Reuss) 

Gavelinella popenoi (Trujillo) 

Globorotalites subconica (Morrow) 

Gyroidinoides bandyi (Trujillo) 

Trochammina texana Cushman and Waters 

T. trifolia Egger

Planktonic foraminifers: Archaeoglobigerina cretacea 

(d'Orbigny)

Dicarinella cachensis (Douglas) 

D. cf. D. imbricata (Mornod) 

Marginotruncana marginata (Reuss) 

M. pseudolinneiana Pessagno 

Whiteinella archaeocretacea Pessagno 

W. brittonensis (Loeblich and Tappan) 

Age: Late Turonian to early Coniacian

USGS Mesozoic locality Mf6402. Collected from mudstone 

exposed along railroad cut and road junction west of 

Carter Creek (SW^NE^ sec. 16, T. 40 S., R. 2 E., 

Ashland 15-minute quadrangle), Jackson County, Ore. 

(lat 42°06' N., long 122°35' W.). Lower part of the Blue 

Gulch Mudstone Member. 

Collector. T.H. Nilsen, 1981.

Benthic foraminifers: Ammobaculites alexanderi 

Cushman

Bathysiphon vitta Nauss 

B. spp. 

Cribrostomoides sp.
88 STRATIGRAPHY OF THE CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

Dorothia oxycona (Reuss) 

Gaudryina tailleuri (Tappan) 

Globorotalites michelinianus (d'Orbigny) 

Gyroidinoides bandyi Trujillo 

G. goudkojfi (TrujiUo) 

Lenticulina sp.

Silicosigmoilina californica Cushman and Church 

Trochammina boehmi Franke 

Unidentified arenaceous species

Age: Campanian to early Maestrichtian, probably 

Campanian

USGS Mesozoic locality Mf6403. Collected from mudstone 

exposed along railroad cut along east fork of Cotton wood 

Creek (SE%NE% sec. 12, T. 41 S., R. 1 E., Ashland 15- 

minute quadrangle), Jackson County, Ore. (lat 42°01' N., 

long 122°39' W.). Lower part of the Blue Gulch Mudstone 

Member. 

Collector. T.H. Nilsen, 1981.

Benthic foraminifers: Ammobaculites sp. 

Ammodiscus cretaceus (Reuss) 

Bathysiphon vitta Nauss 

B. spp.

Cyclammina sp. 

Gaudryina laevigata Franke 

G. tailleuri (Tappan) 

Reophax globosus Sliter

Silicosigmoilina californica Cushman and Church 

Spiroplectammina chicoana Lalicker 

Trochammina boehmi Franke 

Age: Campanian to early Maestrichtian

USGS Mesozoic locality Mf6410. Collected from mudstone 

exposed along Dead Indian Road east of Ashland, Ore. 

(sec. 12, T. 39 S., R. 1 E., Ashland 15-minute quadrangle), Jackson County, Ore. (lat 42°11' N., long 122°39' 

W.). Middle part of the Blue Gulch Mudstone Member. 

Collector. T.H. Nilsen, 1981.

Benthic foraminifers: Bathysiphon vitta Nauss

Cribrostomoides spp.

Cyclammina sp.

Dorothia bulletta (Carsey)

D. oxycona (Reuss)

Gaudryina laevigata (Frank)

G. tailleuri (Tappan)

Globorotalites spineus (Cushman)

Gyroidinoides bandyi (Trujillo)

Nonionella spp.

Lenticulina ovalis (Reuss)

Reophax globosus Sliter

Silicosigmoilina californica Cushman and Church

Spiroplectammina chicoana Lalicker

Trochammina boehmi Franke 

Age: Campanian to early Maestrichtian

USGS Mesozoic locality Mf6411. Collected from mudstone 

exposed along Cottonwood Creek, 0.8 km north of Soda 

Springs, Calif. (SE%NW% sec. 26, T. 48 N., R. 7 W., 

Hornbrook 15-minute quadrangle), Siskiyou County,

Calif, (lat 41°59' N., long 122°38' W.). Lower part of the 

Blue Gulch Mudstone Member. 

Collector. T.H. Nilsen, 1981.

Benthic foraminifers: Ammodiscus cretaceus (Reuss) 

Bathysiphon vitta Nauss 

B. spp.

Dorothia oxycona (Reuss) 

Gaudryina tailleuri (Tappan) 

Silicosigmoilina californica Cushman and Church 

Trochammina texana Cushman and Waters 

Age: Campanian to early Maestrichtian

USGS Mesozoic locality Mf6579. Collected from mudstone 

exposed in roadcut along southeast side of Dead Indian 

Road, about 5 km east of downtown Ashland (SW corner 

of sec. 12, T. 39 S., R. 1 E., Ashland 15-minute quadrangle), Jackson County, Ore. (lat 42°11' N., long 122°39' 

W.). Middle part of the Blue Gulch Mudstone Member. 

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Bathysiphon vitta Nauss

Cribrostomoides cretaceus Cushman and Goudkoff 

Haplophragmoides excavatus Cushman and Waters

Reophax globosus Sliter 

Age: Coniacian to Maestrichtian

USGS Mesozoic locality Mf6582. Collected from mudstone 

exposed on lower part of ridge northeast of Blue Gulch 

(SW%NW% sec. 34, T. 47 N., R. 6 W., Hornbrook 15- 

minute quadrangle), Siskiyou County, Calif, (lat 41°52' 

N., long 122°32' W.). Middle part of the Blue Gulch Mudstone Member, about 166 m stratigraphically below the 

base of the Hilt Bed. 

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Saracenaria cf. S. navicula 

(d'Orbigny)

Unidentified arenaceous species 

Inoceramus prisms 

Age: Late Cretaceous

USGS Mesozoic locality Mf6583. Collected from mudstone 

exposed on middle part of ridge northeast of Blue Gulch 

(SW%NW% sec. 34, T. 47 N., R. 6 W., Hornbrook 15- 

minute quadrangle), Siskiyou County, Calif, (lat 41°53' 

N., long 122°32' W.). Middle part of the Blue Gulch Mudstone Member, about 142 m stratigraphically below the 

base of the Hilt Bed. 

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Ammodiscus cretaceus (Reuss) 

Bathysiphon vitta Nauss

Cribrostomoides cretaceus Cushman and Goudkoff 

Dorothia bulletta (Carsey) 

D. pupa (Reuss) 

Gaudryina tailleuri (Tappan) 

Gyroidinoides goudkoffi (Trujillo) 

Rzehakina epigona (Rzehak)

Silicosigmoilina californica Cushman and Church 

Radiolarians 

Age: Campanian
APPENDIX II 89

USGS Mesozoic locality Mf6584. Collected from mudstone 

exposed on upper part of ridge northeast of Blue Gulch 

(SW^NW% sec. 34, T. 47 N., R. 6 W., Hornbrook 15- 

minute quadrangle), Siskiyou County, Calif, (lat 41°53' 

N., long 122°32' W.). Middle part of the Blue Gulch Mudstone Member, about 38 m stratigraphically below the 

base of the Hilt Bed. 

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Ammodiscus cretaceus (Reuss) 

Bathysiphon vitta Nauss

Cribrostomoides cretaceus Cushman and Goudkoff 

Gaudryina bentonensis (Carman) 

G. laevigata Franke 

G. tailleuri (Tappan) 

Gyroidinoides trujillo Sliter 

Haplophragmoides excavatus Cushman and

Waters

H. famosus Takayanagi 

Hyperammina erugata Sliter 

Lenticulina ovalis (Reuss) 

Saccammina complanata (Franke) 

Saracenaria triangularis (d'Orbigny) 

Silicosigmoilina californica Cushman and Church 

Stilostomella impensia (Cushman) 

Trochammina texana Cushman and Waters 

Planktonic foraminifer: Heterohelix rumseyensis Douglas

Inoceramus prisms 

Age: Late Santonian to late Campanian

USGS Mesozoic locality Mf6585. Collected from mudstone 

exposed on top of ridge crest northeast of Blue Gulch 

(NW%NEJ4 sec. 34, T. 47 N., R. 6 W., Hornbrook 15- 

minute quadrangle), Siskiyou County, Calif, (lat 41°54' 

N., long 122°31' W.). Upper part of the Blue Gulch Mudstone Member, about 340 m stratigraphically above the 

base of the Hilt Bed. 

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Bathysiphon vitta Nauss

Cribrostomoides cretaceus Cushman and Goudkoff 

Haplophragmoides famosus Takayanagi 

Silicosigmoilina californica Cushman and Church 

Age: Campanian to early Maestrichtian

USGS Mesozoic locality Mf6586. Collected from mudstone 

exposed in east side of roadcut near reservoir located 

about 1.5 km southwest of abandoned site of Klamathon 

(NW%SE% sec. 27, T. 47 N., R. 6 W., Hornbrook 15- 

minute quadrangle), Siskiyou County, Calif, (lat 41°53' 

N., long 122°31' W.). Uppermost part of the Blue Gulch 

Mudstone Member, about 2 m stratigraphically below 

upper contact with Tertiary volcanic rocks. 

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Bathysiphon vitta Nauss

Cribrostomoides cretaceus Cushman and Goudkoff 

Dorothia bulletta (Carsey)

Haplophragmoides excavatus Cushman and 

Waters

H. famosus Takayanagi

Silicosigmoilina californica Cushman and Church 

Age: Santonian to Maestrichtian

USGS Mesozoic locality MfS588. Collected from mudstone 

exposed in lower part of ridge east of Hornbrook 

(SW^SE^ sec. 21, T. 47 N., R. 6 W., Hornbrook 15- 

minute quadrangle), Siskiyou County, Calif, (lat 41°54' 

N., long 122°32' W.). Middle part of the Blue Gulch Mudstone Member, about 60 m stratigraphically above the 

base of the Hilt Bed. 

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Bathysiphon varans Sliter 

B. vitta Nauss

Cribrostomoides cretaceus Cushman and Goudkoff 

Dorothia pupa (Reuss) 

Gaudryina tailleuri (Tappan) 

Haplophragmoides excavatus Cushman and

Waters

H. famosus Takayanagi

Silicosigmoilina californica Cushman and Church 

Trochammina texana Cushman and Waters 

Age: Campanian

USGS Mesozoic locality Mf6589. Collected from mudstone 

exposed in middle part of ridge east of Hornbrook 

(SW^SE^ sec. 21, T. 47 N., R. 6 W., Hornbrook 15- 

minute quadrangle), Siskiyou County, Calif, (lat 41°54' 

N., long 122°32? W.). Middle part of the Blue Gulch Mudstone Member, about 126 m stratigraphically above the 

Hilt Bed. 

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Bathysiphon varans Sliter 

B. vitta Nauss

Cribrostomoides cretaceus Cushman and Goudkoff 

Dorothia bulletta (Carsey) 

Gaudryina bentonensis (Carman) 

G. laevigata Franke 

Haplophragmoides excavatus Cushman and

Waters

H. famosus Takayanagi 

Saccammina complanata (Franke) 

Silicosigmoilina californica Cushman and Church 

Trochammina texana Cushman and Waters 

Radiolarians 

Age: Campanian to early Maestrichtian

USGS Mesozoic locality Mf6593. Collected from shale in 

roadcut on east side of Dark Hollow Road, about 0.5 km 

south of South Stage Road (SE corner of sec. 50, T. 38 S., 

R. 2 W., Medford 15-minute quadrangle), Jackson 

County, Ore. (lat 42°17' N., long 122°53' W.). Upper part 

of the Blue Gulch Mudstone Member, above the Hilt 

Bed. 

Collector. T.H. Nilsen, 1982.

Benthic foraminifers: Cribrostomoides cretaceus

Cushman and Goudkoff 

Age: Indeterminate