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UPPER CRETACEOUS ROCKS OE PARTS of SOUTHWESTERN OREGON and NORTHERN Californiai DAFJ.AS L

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UPPER CRETACEOUS ROCKS OE PARTS of SOUTHWESTERN OREGON and NORTHERN Californiai DAFJ.AS L BULLETIN OF THE AMERICAN ASSOCIATION OF PETROLEUM GEOLOGISTS VOL. 40. NO. 8 (AUGUST, 1956). PP. 1968-1984. 4 FIGS. UPPER CRETACEOUS ROCKS OE PARTS OF SOUTHWESTERN OREGON AND NORTHERN CALIFORNIAi DAFJ.AS L. PECK,2 RALPH W. IMLAY,^ .^ND W. P. POPENOE^ Washington, D. C, and I>os Angeles, California ABSTRACT The Upper Cretaceous rocks in Siskiyou County, California, and in adjoining Jackson County, Oregon, are herein defined as the Hornbrook formation. A new name to diiferentiate them from the Chico formation in California is justified by their remote position and by their nearly complete lack of faunas in common with the Chico at its type section. The Hornbrook formation comprises at least 2,500 feet of arkosic wacke sandstone, siltstone, mudstone, and conglomerate; contains fossils of Cenomanian, Turonian, and Campanian age; and includes one unconformity- INTRODUCTION Upper Cretaceous sedimentary rocks trending northwesterly across Siskiyou County, California, and Jackson County, Oregon, have generally been referred to as the Chico group although they are more than 80 miles north of the nearest exposures of Upper Cretaceous rocks in the Sacramento Valley near Redding, Cal­ ifornia, and more than 140 miles north of the type locality of the Chico group along Chico Creek. Recently, however, F. G. Wells concluded, after field and office investigations, that the Upper Cretaceous rocks near the Oregon-California border are too far from outcrops at the Chico type locality in the Sacramento Val­ ley for the name Chico to be applicable to them (W^ells, F. G., oral communica­ tion). He felt that establishment of a new formational name for the Upper Cretaceous rocks in Jackson and Siskiyou counties would serve local mapping needs better than the term Chico group and would avoid stratigraphic misappli­ cations and misunderstandings. He decided that the proposed formation should be based on the excellent exposures near Hornbrook, California. Accordingly, to fulfill these needs, Ralph Imlay examined pertinent Creta­ ceous fossils in the collections at the National Museum and at the California Academy of Sciences. Subsequently he joined Dallas Peck for 7 days of field work in Siskiyou County, California, and 4 days of field work in adjoining parts of Oregon. Emphasis in the field was placed on determining the stratigraphic se­ quence, on obtaining critical dating fossils, and on selecting a section that would be suitable as the type of a new formation. The section selected was described and sampled, and the thicknesses of its component parts were measured by means of a clinometer. It was then remeasured with a plane table, alidade, and stadia rod. The section was selected because it is the best exposed and the most accessible in ^ Publication authorized by the director of the United States Geological Survey. ^ Geologist, United States Geological Survey, Washington, D. C, ' Geologist, University of California at Los Angeles. The writers are indebted to F. G. Wells for many suggestions during the preparation of this re­ port. Professor Howel Williams supplied a field map of the Hornbrook area and information on a plant locality in volcanic rocks of Tertiary age. Local residents, C. B. Kay of Montague, California, W. B. Pollock of Yreka, California, and Chester Fitch of Medford, Oregon, generously supplied many fossil specimens and contributed their time and knowledge in the search for more fossils. 1968 UPPER CRETACEOUS, OREGON AND CALIFORNIA 1969 the Hornbrook area. Probably not more than 10 per cent of the rocks are covered along the course measured. W. P. Popenoe did not work with Peck and Imlay in the field, but he has studied the Cretaceous rocks along the Oregon-California boundary and has contributed considerable unpublished stratigraphic and paleontologic data that are included herein. M ost of the correlations with sections in the Sacramento Valley and in southern California are based on his fossil determinations and his field investigations. In the following text, Imlay and Popenoe are responsible for all statements concerning fossils and correlation. Peck is reponsible for the remainder of the report. HISTORICAL SUMMARY Gabb (1869) established the name Chico group for the strata between what was then identified as the Martinez and the Shasta groups in the Pacific Coast area from Vancouver Island to southern California. He established the typical localities at Chico Creek, Pence's Ranch, and Tuscan Springs on the east side of the Sacramento Valley, California, and noted that the Chico group "includes all the known Cretaceous of Oregon and of the extreme northern portion of Cali­ fornia." Diller (1893) briefly described the occurrence of rocks of Chico age near Yreka, California, and on Grave Creek, Oregon (Fig. i). Dunn (1894) and Turner (1903) described in detail the lithologic character and thickness of the auriferous conglomerate at the base of the Upper Cretaceous rocks in Cottonwood Creek Valley, Siskiyou County, California. Anderson (1902) supplied fossil lists and described the gross stratigraphic features of the Upper Cretaceous rocks in Cot­ tonwood Creek Valley and in the Bear Creek Valley, Jackson County, Oregon. In 1931, Anderson described the section of Upper Cretaceous rocks near Hornbrook as follows. Their lower part consists of sandy strata having a total thickness of about 2,000 feet. At the base they contain conglomerates; higher up, but below the middle of the sandy beds there are thin bedded sandstones well filled with marine fossils, including many species of bivalves and gastropods; at the top the sandstones are overlaid by clay shales nearly 1,000 feet in thickness, forming here the upper one third of the Cretaceous sequence. At the junction of the uppermost sandstone l)etls and the overlying shales is the second notable zone of fossils, Avhich contains few of those found in the lower zone, but instead a considerable variet}' of cephalopod forms. Anderson suggested that the lower zone is about middle Turonian and that the upper zone is upper Turonian and lower Senonian. Later, Anderson (1943) published a paper in which he raised the Chico of the Pacific Coast to series rank, and divided it into three groups and nine stages. Unfortunately, his subdivisions are not tied to a type section and are very briefly described, and the name of the lowest group, Pioneer, is pre-empted. Diller and Kay (1924) briefly described the lithologic character and thickness of the Upper Cretaceous strata on Grave Creek, and Wells (1939) described the lithologic character and thickness of the Upper Cretaceous rocks in the Aledford 1970 DALLAS L. PECK, ET AL. _Jt»»00' EXPLANATION ^^ Upper CrCtOCtOUB Rocks FIG. I.—Index map of part of southwestern Oregon and northern California, showing areal dis­ tribution of Upper Cretaceous rocks. Data from Diller and Kay (1924), Wells (1939), and modified from Williams (1949). UPPER CRETACEOUS, OREGON AND CALIFORNIA 1971 Quadrangle, Oregon. Recently Williams (1949) described the location and litho- logic character of the Upper Cretaceous rocks in the Yreka Quadrangle, Cal­ ifornia. STRATIGRAPHY AND LITHOLOGY GENERAL DESCRIPTION OF HORNBROOK FORMATION Discontinuous exposures of well consolidated, well bedded, clastic sedimen­ tary rocks of Late Cretaceous age form a northwest-trending band that extends from near Yreka, California, along the valleys of Cottonwood and Bear Creeks to Grave Creek, Oregon (Fig. i). The Upper Cretaceous rocks non-conformably overlie deformed and in part metamorphosed volcanic and sedimentary rocks ranging in age from Paleozoic to Late Jurassic and granitic intrusive rocks of Late Jurassic or Early Cretaceous age. In turn, the Upper Cretaceous rocks are overlain disconformably by clastic sedimentary rocks and volcanic rocks of Eocene age. The writers propose that these Upper Cretaceous sedimentary rocks in south­ western Oregon and northernmost California be named the Hornbrook formation, and that the type section be designated as the exposures near the town of Horn- brook, California, as shown graphically in Figure 3 and as described and located in detail herein. In the vicinity of Hornbrook the beds of Upper Cretaceous age occupy the center and the western slope of the valley of Cottonwood Creek (Fig. 2). The beds strike N.NW. and dip is°-2S° NE. They unconformably overlie steeply dip­ ping argillite and mafic volcanic flows and mudflows of the Applegate group of Triassic age which have been metamorphosed to epidote-albite amphibolites. F. G. Wells (oral communication) traced the argillite and volcanic rocks into fossiliferous Triassic Applegate in the Grants Pass Quadrangle, Oregon. Overlying the Upper Cretaceous strata are non-marine silty shales and sandstones of Eocene age which are in turn overlain by lava flows, mudflows, and tuffs of Eocene age. The Cretaceous-Eocene contact was not found in the Hornbrook area, but Wells (1939) reports that in the near-by Medford Quadrangle, Oregon, the Eocene sedi­ ments "apparently rest disconformably upon Chico." The complete thickness of the Hornbrook formation near Hornbrook has not been determined. A thickness of 2,673 + 50 feet was measured, but of this the upper 615 feet of mudstone did not furnish megafossils and was not sampled for microfossils. Stratigraphically above the measured section is about 1,000 feet of poorly exposed, similar appearing mudstone that may also be late Cretaceous and which in turn is overlain by about 1,000 feet of mudstone that contains thin beds of micaceous and tuffaceous sandstone. These mudstone beds are lithologically similar to non-marine fossiliferous sediments of upper Eocene age in the Medford Quadrangle, Oregon, described by Wells (1939), and may be of the same age. The basal tuff of the overlying volcanics in the Hornbrook area has yielded fossil leaves identified by C. Condit (written communication to Howel Williams) 1972 DALLAS L. PECK, ET AL. EXPLANATION Mamber 6, Hornbrook formotion Members 5,4 ond 3, Hornbrook formotion Members 2 and I, Hornbrook formation ,25 T Strike and dip of beds * ^Line of measured sections Approximate contact Inferred contact ^5£?o > Structure Contour *H° Bas« modified from Geology by US as.
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