The Drewsey-Juntura Graben: a Middle to Late Miocene Synvolcanic Graben, Malheur and Harney county, Oregon

The middle to late Miocene Drewsey-Juntura graben located in
northwestern Malheur and northeastern Harney County, Oregon initiated
as a half graben with greatest displacement along the eastbounding
Beulah fault zone. Early subsidence was accompanied by
the accumulation of locally thick (100 to 200 m) deposits of
diatomaceous sediment upon a floor of partially eroded late
Oligocene to early Miocene volcanic rocks including the Imnaha
and Grande Ronde Basalt of the Columbia River Basalt Group.
Break up of the graben into an uplifted eastern block and a subsiding
western block coincided with the onset of basalt volcanism that
produced local shield volcanoes and palagonitic tuff cones. Andesite
lava flows and rhyolitic pyroclastic flows accompanied this phase of
volcanism. A sedimentary unit which includes the Devine Canyon
Tuff (9.68 Ma) and Rattlesnake Tuff (7.05 Ma) was deposited upon
the volcanic rocks. The graben has been offset across northweststriking
faults that appear to have left-lateral displacement. In addition
to active diatomite production, the graben is host to hot-spring
precious-metals-bearing prospects, particularly in the upper sedimentary
unit.