SEDIMENTOLOGY OF QUATERNARY GRAVEL DEPOSITS, WHITNEY 7.5' QUADRANGLE, NE OREGON: IMPLICATIONS FOR EXHUMATION OF TERTIARY CLARNO AND JOHN DAY VOLCANIC AND PRE-TERTIARY BLUE MOUNTAINS TERRANE ROCKS

Presumed Quaternary gravel deposits in the Whitney 7.5’ Quadrangle, northeastern Oregon blanket over 30 square kilometers with at least 50 m of loosely consolidated, sand- and silt-bearing, pebble-boulder gravel. In map view these deposits are concentrated in a broad, pear shape with digitate protrusions to the west, northwest, and northeast along modern stream channels. These gravel deposits have been the focus of gold and silver exploration and extraction for much of the past century because of their extensive incorporation of Triassic-Pennsylvanian Elkhorn Ridge Argillite, a unit exposed to the west, north, and east of the Whitney Quadrangle and characterized by beds of chert, siliceous argillite, argillite with minor beds of chert-pebble conglomerate, limestone, and tuffaceous argillite. Elkhorn Ridge clasts comprise > 80% of the gravel and are subangular to subrounded. Additional gravel clasts were derived from erosion of Paleogene basalt, andesite and rhyolite and tend to be well to very well rounded. The textural maturity of these volcanic gravel clasts strongly suggests they are multi-cyclic. These clasts have no identified source areas to the west, north, or east but may have either southern sources or sources well outside the study area. The gravel beds are dominantly clast-supported and contain a minor to significant silt and clay component. No ages have been determined for the gravel but pumiceous beds may help constrain the deposit temporally. The dominance of argillite gravel clasts, stratigraphic relations with Paleogene and pre-Paleogene strata and absence of volcanic clasts derived from northern, western and eastern sources suggest that Paleogene volcanic deposits were not yet exposed, or only poorly exposed when the gravel was generated. The extensive low-angle, wedge-shapes of the beds, common clast support, and variably distributed deposits of clay and silt indicate that the gravel was transported into the area by both unconfined and confined fluvial systems. The absence of chronostratigraphic markers and paleocurrent indicators and the incorporation of multi-cyclic volcanic clasts that may have originated from possibly southerly, non-glacial sources suggests that large portions of the gravel may, in fact, pre-date the Quaternary.