MIOCENE SEDIMENTS AND TUFFS OF THE KEATING VALLEY, NE OREGON

Recent mapping suggests that the Tertiary sediments and tuffs unit mapped in the Keating Valley by Gilluly (1937), Brooks and others (1976), and Bailey (1990) can be divided into: 1) The Dooley Mountain Volcanic Group, an ~215 m thick unit of middle Miocene welded tuff, tuff, diamictite, siltstone and impure diatomites between the 16 m.y.-old Grande Ronde Basalt (and the Paleozoic to Mesozoic Sparta Complex and Clover Creek Group where the basalt is not present) and the 14 m.y.-old olivine basalt at the base of the Powder River Volcanics, and 2) The Keating Valley Formation, an ~75 m thick unit of late Miocene silts, sands, ash layers, and diatomite above the Powder River Volcanics and below the overlying Quaternary/Tertiary gravels, which are separated from the Keating Valley Formation by an angular unconformity. The volcanic rocks in the Dooley Mountain Volcanic Group came from the Dooley Mountain volcanic center southwest of Baker City, Oregon (Whitson, 1976). The Keating Valley Formation contains an ash layer that is the same age (8.7 ± 0.1 Ma) as the Prater Creek tuff (Ledgerwood, 2006). Fossils in the Keating Valley Formation include Aphelops (rhino) and mastodont (Brooks and others, 1976), leaves of the Keating flora (Chaney, 1959; Hoxie, 1965), gastropods, and fish that include the sucker Catostomus and an unidentified species that has some features similar to Richardsonius. The presence of the diatoms Gomphonema cholnokyites and Ophephora glangeaudi in the Keating Valley Formation diatomite indicate a drainage connection between the late Miocene lake in the Keating Valley and Lake Idaho during its Chalk Hills stage (VanLandingham, 1985; Ledgerwood, 2006), but no evidence has been found of lake deposits in the Keating Valley equivalent in age to the Pliocene Glenns Ferry stage of Lake Idaho.