| Abstract |
- The oldest rocks of the Sturgill Peak area are herein named the
grassy slope phyllites, and are of Permian (?) age. They are
divided into a phyllitic member, consisting of complexly folded and
highly sheared phyllites and phyllitic cherts, and a limestone member,
containing sheared limestone, silicified limestone, and chert. The
grassy slope phyllites are in fault contact with the underlying
sedimentary rocks of Jurassic age. The grassy slope phyllites
may correlate with the Burnt River Schist of northeast Oregon.
Andesitic to dacitic welded tuffs and volcanic flows of the Upper
Triassic Seven Devils Volcanics have a minimum thickness of 3,200
feet in the Sturgill Peak area. Rare interbeds of metaconglomerate,
volcaniclastic wackes, and metachert are present.
The Jurassic Sturgill Peak rhyolite welded tuff was deposited
unconformably on the Seven Devils Volcanics, The welded ash flow
origin of the unit is documented by relict eutaxitic textures. The unit
is up to 700 feet thick, but is characterized by highly variable thick,
nesses that are attributed to an interval of erosion that followed
deposition of the welded tuff.
The Mann Creek slates, an estimated 3, 000 feet in thickness,
consist predominantly of slates, with lithic and feldspathic wackes,
and limestone. They tectonically overlie the rhyolite welded tuff
and Seven Devils Volcanics. The contact is marked, in part, by the
conglomeratic schist lithology that was formed by shearing. The
conglomeratic schist is correlated with the Red and Green Conglomerate
(Brooks, 1967) and shearing is attributed to faulting along the
Bayhorse Thrust (Livingston, 1932). The Mann Creek slates are
correlated with the Lucile Series of the Cuddy Mountains (Slater,
1969) on the basis of similar lithology.
Between Early to Middle Jurassic and Early Cretaceous time, a
small serpentinite body was emplaced along the thrust fault that
separates the Mann Creek slates and the grassy slope phyllites. The
occurrence of ultramafic rocks suggests that this fault may be of
regional importance.
The Sturgill Peak intrusive complex is believed to have been
emplaced in Early Cretaceous time. Members of the complex, in
order of emplacement, are hybrid gabbro (containing secondary
quartz), quartz diorite, granodiorite, and mafic and felsic dikes and
quartz veins. The Early Cretaceous age is based on two 120 million
year K-Ar age dates from the granodioritic phase, as reported in
Henricksen, Skurla, and Field (1972), Because of problems involving
the dated rock, the complex could be older. Intrusion of the complex
resulted in contact metamorphism of the country rocks to the
hornblende-hornfels facies, and the formation of minor hydrothermal
mineral deposits containing small amounts of copper, silver, and gold.
Uplift and erosion during Cretaceous and Tertiary time exposed
the intrusive complex. In Early to Middle Miocene time (Waters,
1961) flows of Picture Gorge Basalt completely buried the basement
rocks of the Sturgill Peak area. In Middle to Late Miocene to Early
Pliocene time (Waters, 1961) flows of Yakima Basalt were deposited,
with slight angular unconformity, on the underlying Picture Gorge
Formation.
Pre-Tertiary rocks were uplifted above the Columbia River
Basalts by major northwest and northeast-trending normal faults in
late Tertiary and Quaternary time.
In terms of plate tectonic concepts, the Seven Devils Volcanics,
Mann Creek slates, and grassy slope phyllites are considered to be an
island arc, arc-trench gap sediments, and deep sea sediments,
respectively. Late Permian to Middle (?) Jurassic deformation is
related to the separation of North American and African plates.
Assuming the 120 million year age is correct, the Sturgill Peak
intrusive complex may be related to subduction along the western
margin of the North American plate in Early Cretaceous time, perhaps
predating the separation of the European and North American
plates.
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