174 Sediments, Clastic (Posters)

Tuesday, 2 November 2010
8:00 AM - 6:00 PM
Hall D

OLIGOCECE-MIOCENE SEDIMENTATION AND VOLCANO-TECTONIC EVOLUTION OF THE WHITNEY BASIN, NE OREGON

Paper number: 174-27
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

Oligocene-Miocene volcanogenic deposits (unit Tvs) exposed in the Whitney Quadrangle of northeast Oregon lends insight into the volcano-tectonic and landscape evolution of the region. Apparently synchronous with deposits of the Clarno and John Day formations, Tvs consists of >1 km of laterally extensive, meters-thick beds of massive to crudely stratified, sandy-boulder conglomerate with intercalated meters-thick beds and lenses of massive to stratified and cross-stratified gravelly sandstone, lavas, and tuffs. The broad spatial extent of volcanogenic sedimentary rocks across the basin combined with the channelized nature of interbedded lavas are evidence that Tvs accumulated on broadly confined, but episodically incised, alluvial fans developed on volcanic uplands. NW trending extensional faults restrict the northern extent of Tvs while providing accommodation for deposition.

Clast counts and XRF whole-rock geochemistry indicate that gravel clasts within Tvs consist of porphyritic and vesicular andesite (~70%) and basalt (~20%) with lesser amounts of dacite and rhyolite and, more rarely, metamorphic clasts derived from the underlying Baker terrane. Gravel clasts are typically supported by a poorly sorted, fine- to coarse-grained sandstone matrix. Metamorphic clasts appear to be concentrated along the northern margins and stratigraphically lower portions of Tvs. The absence of radiometric age control has hindered attempts to correlate Tvs strata across the basin. Clast composition and internal sedimentary features indicate Tvs was derived from volcanic sources and transported by debris and hyperconcentrated flows and to a lesser extent, stream flows. Paleocurrent data from cross-stratified sand and gravel indicate streams flowed primarily to the north from a southern volcanic upland into the Whitney Basin. The mixed, complexly interstratified nature of volcanogenic sedimentary and volcanic deposits of Tvs highlights the influence of fluvial incision, volcanism, and tectonism on the evolution of the Whitney Basin.