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Sequence Stratigraphy and Depositional History of the Upper Cretaceous Hornbrook Formation in Southwest Oregon and Northern California: Tectonic Implications for Western North America

Elliott, William S.1
1 Dept. of Environmental Studies, Southern Oregon University, Ashland, OR.

The Hornbrook Formation (Upper Cretaceous) is made-up of ~1,200 m of mudrocks, sandstones, and conglomerates exposed along the northeastern flank of the Klamath Mountains in southwestern Oregon and northern California. The Hornbrook Formation is subdivided into five members: Klamath River Conglomerate (oldest), Osburger Gulch Sandstone, Ditch Creek Siltstone, Rocky Gulch Sandstone, and Blue Gulch Mudstone (youngest). Detailed stratigraphic analysis of these Upper Cretaceous sediments reveals compressional, and possibly transpressional, tectonics associated with late Mesozoic mountain building in western North America.

The lower part of the Hornbrook Formation (~250 m) includes the continental Klamath River Conglomerate and the shallow marine Osburger Gulch Sandstone and Ditch Creek Siltstone Members. Detailed petrographic studies of sandstone concretions from the lower part of the Hornbrook Formation suggest a provenance consistent with denudation of a magmatic arc in the Late Cretaceous. In addition, clay mineral content of mudrocks in the lower part of the Hornbrook Formation comprises chlorite, illite, and mixed-layer illite-smectite. The composition of sandstones and mudrocks in the lower part of the Hornbrook Formation are consistent with accumulation in a subduction related forearc basin during the latter part of the Sevier orogeny.

The Ditch Creek Siltstone is truncated by a regional unconformity and is overlain by the Rocky Gulch Sandstone. Marine sandstones and conglomerates in the Rocky Gulch may be traced laterally for 10s of kilometers and contain rounded cobbles up to 35-cm in diameter. These coarse clastics are overlain by ~750 m of Blue Gulch Mudstone consisting of marine mudrock with sparse beds of sandstone. The clay mineral content of mudrocks in this interval is dominated by illite and chlorite. The provenance of sandstones and mudrocks in the upper part of the Hornbrook Formation is consistent with a continental block provenance such as the Klamath Mountains. Finally, the regional unconformity subdividing the Hornbrook Formation may correspond to significant tectonic reorganizations, specifically the shift from subduction to translation along the western edge of North America during the Late Cretaceous that may be associated with the onset of the Laramide orogeny.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009