TEPHROSTRATIGRAPHY OF ASH FALL TUFFS FROM THE YELLOWSTONE HOTSPOT IN SEDIMENTARY INTERBEDS OF THE COLUMBIA RIVER BASALT GROUP

Three large igneous systems were simultaneously active in the Pacific Northwest during the Miocene: Columbia River flood basalts, the Cascade subduction zone, and the Yellowstone hotspot. The latter two are characterized by voluminous explosive silicic eruptions whose tephra were widely distributed and preserved in depositional basins throughout the western U.S., including sedimentary interbeds in the Columbia River Basalt Group. It is possible to distinguish between Cascade and hotspot sources of ash fall tuffs on the basis of the composition of glass, and for tephra younger than 15 Ma, on the basis of isotopic ratios of Nd. Although conventional wisdom has held that volcaniclastic material in interbeds of the Columbia Plateau Basalt Group is derived from nearby Cascade volcanoes, chemical analyses reveal the presence of numerous ash fall tuffs derived from the Yellowstone hotspot system.

Individual unaltered silicic ash fall tuffs can be distinguished on the basis of the unique composition of their glass, and thus can be traced across depositional basins and used to correlate sedimentary interbeds. The earliest recognized ash fall tuff is an unnamed, 15.4 Ma unit found in sediments at Whitebird, Idaho, and in a sedimentary interbed near Asotin, Washington. Two additional ash fall tuffs have been identified at Whitebird with ages of 15.2 and 13.9 Ma. The 11.8 Ma Ibex Peak ash (=? Unit VII of the Cougar Point Tuff) has been identified in 7 localities in south-central Washington and in one locality across the Columbia River in Oregon. Additional ash fall tuffs identified in sedimentary interbeds include the Logan Ranch (11.8 Ma), Unit XI of the Cougar Point Tuff (11.3 Ma), Unit XIII of the Cougar Point Tuff (=tuff of Big Bluff, 10.9 Ma), Tuff of Wooden Shoe Butte (10.1 Ma) and the Hazen ash (9.8 Ma).