Bruneau-Jarbidge
Eruptive Center
Owyhee Mountains
The Owyhee Mountains are underlain
by Cretaceous granitic rock and Tertiary volcanic rocks ranging from Eocene to
Miocene age. These Cretaceous granitic rocks are bordered by Cretaceous hornblende
gabbro and overlain by pre-Cretaceous metasediments and are thought to be a southward
extension of the Idaho Batholith. The Eocene-age silicic volcanics correlate with
the Challis volcanics. They are up to 33,000 feet thick and are dated at 44.7
million years. Olivine basalts and andesitic lavas of Oligocene age reach up to
3,800 feet thick and are dated at 30.6 million years. Miocene volcanic rocks of
basalt and latite are dated at 17 million years and are 3,300 feet thick. The
Miocene rocks are intruded by rhyolites. Olivine basalts and interbedded sedimentary
rocks are dated at 8 to 10.5 million years.
Bruneau-Jarbidge
Eruptive Center
Volcanic flows,
exposed by the canyon of the Bruneau River, were erupted from the Bruneau-Jarbidge
eruptive center. This eruptive center is about 5 9 miles long and lies southeast
of the scenic Bruneau Canyon. Numerous ash flows, lava flows and basalt flows
were erupted during Miocene and Pliocene time. The Banbury Basalt was erupted
from shield volcanoes and filled in the low areas. The Idavada volcanics, known
as the Cougar Point Tuff, consist of densely-welded, ash-flow tuffs and younger
rhyolite flows about 10 to 12 million years old. Altogether there are nine or
more of these welded tuff cooling units which filled a large basin. The eruptive
center was named by Bonnichsen after the Bruneau and Jarbidge Rivers which have
cut canyons in the area. The canyons expose both silicic and basalt flows.
Ash-Flow
Tuffs (Cougar Point Tuffs)
The Cougar Point
ash-flow tuffs were formed by a succession of pyroclastic eruptions that discharged
hot, vesiculated magma particles into the air. The ash was buoyed by air and transported
long distances over a smooth flat surface. The hot ash consisted of shards with
minor pumice and lithic fragments. The eruptive material flowed away from the
source on a cushion of hot air. Once the gases were dissipated, the hot mass collapsed,
compacted and cooled. The emplacement temperature is estimated to be in the vicinity
of 900 to 1,000 degrees centigrade. The ash flows coalesced to a viscous liquid
as indicated by flow marks and elongate vesicles and folds developed during the
final stages of movement.