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Description
The Osburger Gulch Sandstone Member of the Hornbrook Formation records a Cenomanian to early Coniacian southward transgression in the Jacksonville-Yreka area. It either conformably overlies the nonmarine deposits of the Klamath River Conglomerate Member of the Hornbrook Formation, or nonconformably overlies Paleozoic and Mesozoic rocks of the Klamath Mountains. The thickness of the Osburger Gulch Sandstone Member is variable and is generally 60-100 m. It has a maximum measured thickness of 131 m near Siskiyou Summit, Oregon and is a minimum of 3.5 m thick two km north of Ashland, Oregon. The member generally fines upward and is dominantly composed of sandstone, but contains conglomerate, siltstone, and shale. The Osburger Gulch Sandstone Member contains six distinctly different lithofacies that were deposited in lagoonal, nearshore, and offshore environments: (1) basal conglomeratic lithofacies, (2) bioturbated muddy sandstone lithofacies, (3) trough cross-bedded sandstone lithofacies, (4) massive conglomeratic sandstone lithofacies, (5) hummocky-stratified sandstone lithofacies, and (6) bioturbated siltstone lithofacies. The vertical succession of lithofacies shows a steady deepening trend. Factors affecting the distribution of lithofacies include both the variable lithology of the underlying rocks and changing coastal morphology. The vertical succession of sedimentary structures within the hummocky-stratified sandstone lithofacies shows small-scale fluctuations of water depth superimposed upon the overall upward-deepening trend of the sequence. Petrographic trends within the member indicate compositional gradation away from the fluvial complex represented by the braided-stream deposits of the Klamath River Conglomerate Member. Sandstone petrography suggests that the Klamath Mountains were the provenance of the Osburger Gulch Sandstone Member. The general north-northeast paleocurrent trends of this study agree with those of previous workers and help support the idea of a Klamath Mountains source terrane.
