Abstract:
Late Cenozoic terrestrial sedimentation and volcanism
produced more than 6000 ft of complexly
interstratified rocks in the Owyhee region, southeastern
Oregon. Deposition upon a basement of
peripherally exposed Paleozoic and Mesozoic rocks
began in Miocene time and continued intermittently
through the Pleistocene. High-angle block faulting
related to the adjacent Basin and Range Province
created hundreds to thousands of feet of structural
relief. Faulting and concomitant erosion formed
north-trending basins that received Miocene, Pliocene,
and Pleistocene deposits.
The rocks described range from late Miocene to
sub-historic. They are dated through mammalian
chronology, stratigraphic relations, and potassium·
argon chronology. About 30 stratigraphic units are
discussed, of which 12 are named and defined. Extrusive
rocks are olivine-poor clinopyroxene basalts,
porphyritic andesine rhyolites, and rhyolitic welded
ash-flow tuffs. Clastic rocks are arkoses, granite-cobble
conglomerates, air-fall tuffs, and fluviatile
and lacustrine bentonitic volcaniclastic rocks variously
adulterated with plutonic detritus.