Field Trip Guide to the Upper Cretaceous Hornbrook Formation and Cenozoic Rocks of southern Oregon and northern California Students in SOU’s field geology course examining sandstones of the Rocky Gulch Member of the Hornbrook Formation near Hilt, California. Field Trip Leader: Bill Elliott Department of Geology, Southern Oregon University Saturday, September 8, 2007 Introduction The Klamath Mountains are an elongated north-trending geological province that occupies approximately 19,000 km2 in southwestern Oregon and northern California. The Klamath Mountains are made-up of numerous terranes that accreted during the Antler (Devonian), Sonoman (Permian to Late Triassic), and Nevadan (Jurassic to Early Cretaceous) orogenies (Mortimer, 1984). These terranes have been grouped into four metamorphic belts, from oldest (east) to youngest (west): Eastern Klamath Belt; Central Metamorphic; Western Paleozoic and Triassic; and Western Jurassic (Irwin, 1966; Irwin, 1994). In the Late Jurassic to Early Cretaceous, numerous magma bodies intruded the Klamath Mountains, including the Jurassic Mt. Ashland pluton and Early Cretaceous Grants Pass pluton (Hotz 1971; Gribble et al., 1990). During the Late Jurassic to Early Cretaceous, a subduction zone complex and forearc basin developed along the western margin of North America while folding and thrusting of Paleozoic and lower Mesozoic rocks associated with the Sevier orogeny triggered the formation of the Cordilleran foreland basin in the interior of North America (Figs. 1 and 2). The Hornbrook Formation (Upper Cretaceous) consists of a sequence of dominantly marine clastic sedimentary rocks about 1,200 meters thick exposed along the northeastern margin of the Klamath Mountains in southwestern Oregon to northern California (Fig. 3). These marine sediments are interpreted to have been deposited in a forearc basin similar to the Upper Cretaceous sediments of the Great Valley forearc in central California (Sliter et al., 1984; Nilsen, 1984, 1993). The Hornbrook Formation lies nonconformably on Paleozoic and Mesozoic igneous and metamorphic rocks of the Klamath Mountains, and is overlain disconformably by Tertiary sedimentary rocks and/or volcaniclastic sediments (Fig. 4). Generally, the Hornbrook Formation strikes N30oW to N45oW and dips 20o to 30o to the northeast on the eastern flank of the Klamath Mountains. The Hornbrook Formation is subdivided into five members, in ascending order: Klamath River Conglomerate, Osburger Gulch Sandstone, Ditch Creek Siltstone, Rocky Gulch Sandstone, and Blue Gulch Mudstone (Fig. 5; Nilsen, 1984; Nilsen 1993). The Hornbook Formation is overlain unconformably by the Payne Cliffs Formation (Eocene) in the Bear Creek Valley of southwestern Oregon or by the Colestin Formation (Late Eocene to Oligocene) in the Cottonwood Creek Valley of northern California. The Payne Cliffs Formation consists of conglomerates and sandstones with subordinate amounts of siltstone and mudrock (McKnight, 1971; 1984). In the Siskiyou Pass area of southwestern Oregon and northern California, the Colestin Formation is dominated by volcaniclastic sediments (Bestland, 1985; 1987). The lateral and vertical facies relationships between the Payne Cliffs and Colestin Formations are poorly constrained. The Colestin Formation and its unnamed equivalents in the Bear Creek Valley are overlain by the Roxy Formation (Late Oligocene to Early Miocene), which is composed of vesicular basalt flows, volcanic breccias, volcaniclastic siltstones, shales, and conglomerates, with subordinate amounts of fine-grained, planar stratified, volcaniclastic sandstones (Vance, 1984). The Colestin and Roxy Formations are approximately 1,550 m thick in southwestern Oregon and northern California. The Roxy Formation is overlain by the Wasson Formation (Early Miocene) which is composed of ash-flow tuffs, with interlayers of lavas and fluvial deposits (Vance, 1984). The Heppsie Andesite (Miocene) sits above the Wasson Formation. The Colestin, Roxy, Wasson, and Heppsie Formations collectively are referred to as the Western Cascades. 1 W 124 W 120 W 116 W 112 W 108 WAS HING Cascade Volcano TON X X Snake River Plain X Cretaceou s o Cretaceous subduction Fold & Thrust N 48 o zone complexes Belt N 44 X X Cretaceous OREGON X forearc sediments X X MONTA X Cretaceous NA X X X plutonic belts IDAHO WYOMING A X o Late Pz & Mz X N 44 o arc terranes N 40 Pre-Cretaceous NEVAD A UTAH A’ subduction zone BASI X N & R complexes ANGE PROVIN Pz arc CE o terranes X N 40 B’ B S X o a n X N 36 A X N n C Kilometers d X OLOR r ADO e a X PLATE 0 200 400 s AU X o F a N 36 100 300 u lt CALIFORNIA ARIZONA NEW Modified from Dorsey & LaMaskin (in review) MEXICO o o o o o W 124 W 120 W 116 W 112 W 108 Figure 1: Tectonic map of North America highlighting the major geologic provinces. This map is modified from Dorsey & LaMaskin (in review). TECTONIC RECONSTRUCTIONS FOR THE CRETACEOUS A (WEST) KLAMATH MOUNTAINS A’(EAST) Subduction Magmatic Complex Arc LEGEND Trench Forearc Fold & Cordilleran Cretaceous sandstone Basin Thrust Belt Foreland Basin S S L X L Cretaceous subduction X zone complexes X X Precambrian X X Mid-Cretaceous intrusive X & volcanic rocks X Basement Jurassic & Early Cret. Lithosphere Intrusive rocks Late Paleozoic & Mesozoic arc terranes B (WEST) SIERRA NEVADA MOUNTAINS B’(EAST) Pre-Creatceous subduction zone Subduction Magmatic complexes Complex Arc Paloezoic arc terranes Trench Fold & Forearc Cordilleran Paleozoic Rocks Thrust Belt Foreland Basin S Basin S L X L Proterozoic to X Paleozoic Crust with X Precambrian Mesozoic shortening X X Basement Precambrian Rocks Lithosphere Figure 2: Reconstructed tectonic cross-sections for the mid-Cretaceous of western North America. Refer to Figure 1 for the location of these cross-sections. 2 o o o W123 5 W123 45’ W123 30’ Western Cascades (Paleogene) Medford Hornbrook Formation (Late Cretaceous) o XX X X Diorite & granodiorite plutons N42 15’ XX (Late Jurassic) Condrey Mountain Terrane X (Late Jurassic) X Ashland X X Rattlesnake Creek Terrane XXX (Late Triassic to Early Jurassic) X X XX Hayfork & Applegate Terranes X Mt Ashland (Permian to Early Jurassic) X X XX X Pluton Central Metamorphic Belt X X (Devonian) X X 5 o X X OREGON N42 X CALIFORNIA X Hilt XX Index Hornbrook N Map X XXX X X X X XX X X X X X X X 5 20 30 XX X X 0 10 Vesa X X Bluffs Kilometers X Yreka X Pluton Modified from Mortimer & Coleman (1984). Figure 3: Generalized geologic map of southwestern Oregon and northern California showing the extent of exposures of the Hornbrook Formation. Generally, the Hornbrook Formation strikes to the northwest along the northeast flank of the Klamath Mountains. In addition, the Hornbrook Formation is dissected by numerous northeast-trending normal faults. Figure 4: Southwest to northeast geologic cross-section near Hornbrook California showing the homoclinal package of sedimentary rocks that make-up the Hornbrook Formation. Figure from Nilsen (1993). 3 meters 1,000 STAGE Stratigraphic column of the Upper Cretaceous Hornbrook Formation 900 Hilt Bed near Yreka, California 800 LEGEND Blue Gulch Trough Cross-stratification 700 Mudstone Member Foraminiferal fossils UPPER CAMPANIAN Molluscan fossils 600 Plant fossils (leaf & wood) Shale & mudstone Rancheria Gulch Siltstone Sandstone Beds 500 Sandstone Conglomerate 400 Igneous & metamorphic rocks HORNBROOK FORMATION Rocky Gulch Figure 5: Idealized stratigraphic Sandstone Member LOWER CAMPANIAN 300 column of the Upper Cretaceous Hornbrook Formation. The basal Klamath River Conglomerate is only present in the southernmost 200 Ditch Creek portion of the study area. In the Siltstone Member Hilt and Bear Creek areas, the CONIACIAN Osburger Gulch Sandstone Osburger Gulch Member sits unconformably on 100 Sandstone Member igneous and metamorphic rocks of the Klamath Mountain province. Klamath River Stratigraphic section modified TURONIAN Conglomerate Member from Nilsen (1984). 0 4 Geologic Mapping in southern Oregon and northern California Wells (1956) published a geologic map of the Medford 15-minute quadrangle highlighting the sedimentary deposits. Smith and others (1982) produced a 1:250,000 scale geologic map of the 1o by 2o Medford quadrangle. In recent years, the focus has shifted from the compilation of regional geologic maps to conducting detailed geologic mapping of 7.5-minute quadrangles at a scale of 1:24,000. The most recent geologic mapping at this scale in southern Oregon was completed by Wiley and Smith (1993) of the Medford East, Medford West, Eagle Point, and Sams Valley quadrangles in Jackson County, Oregon. Within the next decade, geologic mapping at 1:24,000 will hopefully be initiated by the Oregon Department of Geology and Mineral Industries, followed by compilation of 1:100,000 geologic maps of southwestern Oregon. Geologic mapping in the Klamath Mountains is difficult due to the ruggedness of the terrain and limited access. Regardless, there have been numerous geologic maps and reports published summarizing the complex geology of the Klamath Mountains. In particular, Hotz published geologic maps of the Condrey Mountain 15-minute quadrangle (1967) and Yreka 15- minute quadrangle (1977) in northern California. Irwin (1994) published the first geologic map of the entire Klamath Mountains at a scale of 1:500,000. Future geologic mapping at a scale of 1:24,000 will undoubtedly provide new insights into understanding the formation of the Klamath Mountains. Structural Geology The Hornbrook Formation and the lower part of the Western Cascades Group are dissected by a series of northeast-southwest trending normal faults. One of the most significant of these structures, the Siskiyou Summit Fault, offsets stratigraphic units about 10 km. Recent structural studies along several of these faults suggest Cenozoic reactivation with oblique-slip to strike-slip motion. Between these major bounding faults, homoclinal sequence of sedimentary and volcaniclastic rocks of the Hornbrook and Colestin Formations respectively are tilted 20o to 30o to the northeast. Rocks of the upper part of the Western Cascades Group are not offset by these faults, and are only tilted 5o to 10o to the northeast.