STRATIGRAPHY AND GEOLOGIC HISTORY OF THE WHITNEY 7.5’ QUADRANGLE, NE OREGON

Paleozoic to Cenozoic siliciclastic, volcanic, and volcanogenic sedimentary and Quaternary alluvium, colluvium, and landslide deposits in the Whitney 7.5’ quadrangle, NE Oregon record the accretion of allochthonous terranes, widespread volcanism, and subsequent erosion and deposition. Geologic mapping and detailed stratigraphic assessment of this quadrangle adds to understanding of the geologic evolution of the area. The oldest rocks in the Whitney 7.5’ quadrangle (Paleozoic/Mesozoic Elkhorn Ridge Argillite and Melange terrane) record the suturing and subsequent plutonic (Cretaceous Bald Mountain Batholith) stitching of accreted terranes and subterrane rocks. Underlying structural weaknesses related to prior tectonic suturing of accreted terranes and subterranes are suspected to have influenced the structurally defined margins of the Whitney Basin, a half-graben that supplied accommodation for Tertiary sedimentation and volcanism. Early Eocene(?) conglomerate deposits contain gold-bearing sandstone and conglomerate that primarily accumulated in stream channels incised into the older terrane and subterrane strata. Eocene Clarno Formation volcanogenic sedimentary and andesite- and basaltic andesite-dominated volcanic strata cap these gold-bearing conglomerates with a calculated 1 km thickness. Northern Clarno Formation conglomerate deposits within the Whitney quadrangle contain rare pebble-boulder clasts derived from rocks of the Elkhorn Ridge Argillite and Bald Mountain batholith. Mapping, detailed section measuring, petrologic analyses, and paleocurrent data define the stratal geometries, provenance, and ages of the Clarno Formation in the Whitney Basin. These deposits are unconformably overlain by the dominantly andesitic rocks of the Miocene Strawberry Volcanics. Subsequent erosion of Tertiary and pre-Tertiary strata facilitated by landslide displacement of Clarno and Strawberry deposits contributed to the generation of Tertiary and Quaternary gravel and conglomerate deposits that blanket >50 km² of the Whitney and the adjacent quadrangles. Quaternary deposits consist of at least 50 m of loosely consolidated, sand- and silt-bearing, pebble-boulder gravel and conglomerate that accumulated on alluvial fans and basins.