EOCENE-OLIGOCENE FLORAS AND IONE STRATIGRAPHY AT UPPER DUTCH DIGGINGS MINE, LA PORTE, CALIFORNIA

The Oligocene ash flow deposits in eastern California and western Nevada are a valuable repository of information on the evolution of the western United States during the Paleogene. The deposits fill paleo-channels eroded into the Ione Formation by a yet to be fully understood river system in the Eocene-Oligocene, and are overlain by the Miocene Lovejoy Basalt. The tuffs are thought to have originated from the ancestral Cascade volcano arc in western Nevada. Upper Dutch Diggings is a site where extensive hydraulic mining took place about 1 km northwest of the town of La Porte, along Rabbit Creek. The approximately 35 m cliff face exposed by the mining reveals a single 5 m by 5 m outcrop of a green-grey tuff that contains an Oligocene paleoflora. The fossils are abundant and immaculately preserved, including detailed cuticle. The remaining face of the cliff displays typical stratigraphy of the Eocene upper Ione Formation, dominated with nearly flat-laying layers of clay and siltstone. Cross-bedding, lenses, pinch-outs, and non-continuous layers of coal are present along with Eocene plant fossils of the Chalk Bluff Flora.

Detailed stratigraphy of the Upper Dutch Diggings Mine will aid in broader studies of the Eocene-Oligocene erosion surface and drainage system, which will in turn provide important clues to questions regarding the amount of offset of faults in the Basin and Range, and the amount of Paleogene tilting and uplift of the Sierra Nevada. Clearly distinguishing the locations of the two separate fossil leaf floras of this locale will also aid in determining the paleoelevations of both the Sierra Nevada and the Basin and Range.