| Abstract |
- Six early to middle Tertiary geologic units crop out in the
Saddle and Humbug Mountain area. They include the late Eocene to
early Miocene Oswald West muds tones, the lower Silver Point and
the upper Silver Point tongues of the middle Miocene Astoria Formation,
and the middle Miocene Depoe Bay and Cape Foulweather Basalts.
One new lithologically distinct unit, the Falls Creek member of the
Oswald West muds tones, is mapped, described, and named informally
herein. These Tertiary units are locally overlain by Quaternary
stream alluvium and landslide deposits.
The Oswald West mudstones consist of more than 500 meters of
thickly bedded grayish to yellowish orange bioturbated tuffaceous
siltstones and mudstones interstratified with minor glauconitic sandstones
and tuff beds. An open-marine, deep-water, slightly reducing
depositional environment (outer shelf-continental slope) is indicated
by Foraminifera, trace fossils, glauconite, and the predominantly
fine-grained character of the unit. Deposition of the Falls Creek
silts tone member which contains a very shallow water molluscan
fossil assemblage probably occurred during gradual shallowing near
the end of the late Oligocene, possibly due to adjacent deltaic progradation.
An angular unconformity separates the Oswald West muds tones
from the overlying lower Silver Point tongue and suggests that a
broad uplift occurred during an early Miocene hiatus. Thin feldspathic
sandstone lenses occur locally above the unconformity and
mark the beginning of a middle Miocene marine transgression during
lower Silver Point time.
The approximately 300-meter thick Silver Point member is composed
of two intertonguing lithosomes. The lower tongue consists of
rhythmically bedded, light gray, laminated, micaceous and carbonaceous
sandstones and dark gray siltstones. Graded bedding and partial
Bouma sequences suggest that these sandstones were deposited by
turbidity currents, possibly on a delta slope or outer delta platform.
Paleocurrent measurements and facies patterns indicate that these
turbidity flows originated at a south to southeasterly source, probably
by slumping off the nearby Angora Peak delta front. Thick-bedded,
structureless, arkosic sandstones which locally interfinger with the
basal part of the lower Silver Point tongue probably also reflect redeposition
of "clean" delta front sheet sands into deeper water delta
slope environments.
The lower Silver Point tongue grades upward into the 200-meter
thick upper Silver Point tongue which consists of dark gray structureless
to finely laminated mudstones and rare thin sandstone beds.
Foraminifera and the overall fine-grained lithology indicate that this
unit was deposited under deep marine (upper bathyal), low energy
conditions and reflects continuing marine transgression over the
region.
Postulated underthrusting caused by convergence of the Juan de
Fuca oceanic plate and the North American continental plate along the
middle Miocene Oregon continental margin (Ku lm and Fowler, 1974a,
1974b), may have caused the uplift and high-angle faulting and development
of an unconformity which followed upper Silver Point marine
deposition. Rapid subsidence and marine transgression ended this
short-lived erosion period as evidenced by the local eruption of over
600 meters of palagonitized Depoe Bay submarine basaltic breccias
and pillow lavas and more than 200 meters of sparsely porphyritic
Cape Foulweather submarine pillow lavas. The Depoe Bay Basalt lies
with angular unconformity over the faulted Silver Point member and
Oswald West mudstones. A local basaltic conglomerate interbed
within the Depoe Bay breccias (near the base) suggests that some
early Depoe Bay volcanic buildups developed above wave base. Chemical
analyses show that these basalts are tholeiites and are comparable
to the type Cape Foulweather and Depoe Bay petrologic-types along
the central Oregon Coast.
All Tertiary sedimentary units are intruded by numerous dikes,
sills, and irregular-shaped plutons. Regional dike swarms and local
feeder dikes to Depoe Bay Basalt eruptive centers such as Saddle
Mountain, commonly, either parallel or coincide with northeasttrending
or northwest-trending lineaments and high-angle faults, suggesting
a structural control for the emplacement of the intrusives
and location of the volcanic centers. The largest fault in the study
area (Humbug Mountain fault) is co-linear with the northwest-trending
Gales Creek Fault and another unnamed fault southeast of the study
area. In total, these three faults form an almost continuous line of
faults across the northern Oregon Coast Range for a distance of 100
km.
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