CENOZOIC EVOLUTION OF THE WHITNEY BASIN, NE OREGON, INFERRED FROM METAMORPHIC BAKER TERRANE- AND CLARNO AND JOHN DAY FORMATION-DERIVED GRAVEL DEPOSITS

Gravel-rich, Cenozoic sedimentary deposits in the Whitney Basin, Blue Mountains Province (BMP), Oregon, provide information about the depositional history of local basins, are a potential analogue for non-traditional siliciclastic hydrocarbon prospects, and have implications for locations of economic concentrations of placer gold. Detailed geologic mapping and clast counts, augmented by major/trace element XRF analyses, are used to identify five geologic and structural elements in the Whitney Basin. The first element is the Triassic-Pennsylvanian Elkhorn Ridge Argillite (ERA), a siliceous argillite, with minor beds of fossiliferous chert-pebble conglomerate, limestone, and tuffaceous argillite, which are part of the accreted Baker Terrane. ERA bedrock lies in the northern Bourne sub-terrane. The second element consists of local, northwest-trending, dextral-normal faults that mimic the Brother’s Fault Zone, a larger regional fault system bordering the BMP ~200km to the southwest. Active since at least the Oligocene, these local faults produced escarpments on the northern margin of the basin as evidenced by stratigraphic and spatial relationships between rocks of the ERA and younger depositional units. Tg, the third element, is a gravel-rich Oligocene-Miocene deposit, characterized by rounded pebble to boulder, quartz-rich metamorphic and igneous clasts, and a weakly consolidated, porous, sand-rich, often gold-bearing matrix. The extent and generally planar stratification of Tg deposits suggest it accumulated in broad braided channels that unconformably overlie ERA bedrock. The fourth element, Tvs, is an Oligocene-Miocene volcaniclastic, non-gold bearing gravel-rich deposit generated by a north-flowing alluvial fan and fluvial system, that is interstratified with basalt flows and reworked airfall deposits. Tvs filled the Whitney Basin, capped the fault escarpment to the north, and overlies the ERA and Tg. The fifth and youngest element, Quaternary-Tertiary gravel, QTg, consists of intermixed subangular to subrounded ERA gravel, lesser subrounded to rounded silicified andesitic Tg gravel, and minor rounded basalt and tuff Tvs gravel clasts. QTg is a low-relief, lobe-shaped deposit generated by Quaternary alluvial fan and fluvial activity.