| Abstract |
- The upper Eocene to lower Oligocene Oswald West mudstone is
the oldest formation (informal) in the Green Mountain-Young's
River area. This 1,663 meter thick hemipelagic sequence was deposited
in a low-energy lower to upper slope environment in the Coast
Range forearc basin. The formation ranges from the late Narizian
to the early Zemorrian(?) in age and consists of thick-bedded bioturbated
foraminiferal claystone and tuffaceous siltstone. Rare
glauconitic sandstone beds also occur. In the eastern part of the
study area, the upper part of the Oswald West mudstone is interbedded
with the upper Refugian Klaskanine siltstone tongue. This
informal unit consists of thick bioturbated sandy siltstone and
silty sandstone that is a lateral deep-marine correlative of the
deltaic to shallow-marine Pittsburg Bluff Formation in the northeastern
Coast Range.
Discontinuous underthrusting of the Juan de Fuca oceanic
plate at the base of the continental slope of the North American
plate caused extensive uplift and subsidence along the Oregon
continental margin throughout the Cenozoic (Snavely et al., 1980).
Initiation of Oregon Coast Range uplift and accompanying erosion
in the early Miocene, coupled with a global low stand of sea level
(Vail and Mitchum, 1979), stripped most of the Oligocene (Zemorrian)
Oswald West strata and in places much of the uppermost
Eocene (upper Refugian) Oswald West strata in the field area, creating
an unconformity. Deformation accompanying uplift included a
system of east-west-trending, oblique-slip faults.
The Pillarian-to-Newportian Astoria Formation unconformably
overlies the Oswald West mudstone and reflects deposition offshore
from an open, storm-dominated coast during an early-to-middle
Miocene transgression. Deposition of the Big Creek sandstone
and Silver Point mudstone members of the Astoria Formation
was controlled in part by submarine paleotopography that developed
as a result of early Miocene deformation of the Oswald West strata.
The up to 200 meter thick Big Creek member varies from storm-deposited
laminated sandstone to bioturbated mollusk-bearing silty
sandstone that accumulated during fair weather conditions on the
inner to middle shelf. Overlying and perhaps in part laterally
equivalent to the Big Creek member is the up to 200 meter thick,
deeper marine Silver Point member which consists of two lithologies:
1) interbedded, micaceous, turbidite sandstones and laminated
mudstone; and 2) laminated bathyal mudstone that intertongues
with and caps the turbidite sequences. The turbidite
lithology is composed of two facies: 1) an underlying sand-rich
facies, transitional between the shallow-marine Big Creek member
and bathyal Silver Point strata, that was deposited on the outer
shelf by storm-induced turbidity currents; and 2) an overlying
sand-poor facies that was deposited at bathyal depths. The turbidite
facies channelized, and at some places removed the underlying
Big Creek member and were deposited directly over Oswald West mudstone.
The Astoria depositional sequence ranges, from inner to
outer neritic to bathyal facies and reflects continued deepening
and anoxic depositional conditions of the Astoria basin through
the middle Miocene. Big Creek and Silver Point sandstone petrology
reflects volcanic sources from an ancestral western Cascades volcanic
arc and metamorphic and granitic basement rocks farther east
via an ancestral Columbia River drainage system. Diagenetic effects
include: (a) formation of local calcite concretionary
cements; and (b) formation of pore-filling smectite from alteration
of volcanic rock fragments.
At least six middle Miocene Columbia River Basalt intrusive
episodes affected the Green Mountain-Young's River area soon after
deposition of the Astoria Formation. These basalt sills and dikes
include normally polarized and reversely polarized low Mg0 high
TiO₂, low Mg0 low TiO₂, and high Mg0 Grande Ronde basalt chemical
subtypes and two porphyritic Frenchman Springs Member basalts
(Ginkgo and Kelly Hollow(?) petrologic types). These basalt intrusions
are virtually indistinguishable, based on chemistry, from
subaerial flows of the plateau-derived Columbia River Basalt Group
subtypes at nearby Nicolai Mountain and Porter Ridge. This correlation
supports the Beeson et al. (1979) hypothesis that the intrusions
are not of local origin but formed by the invasion of the
flows into the Miocene shoreline sediments to form "invasive"
sills and dikes. Many dikes were emplaced along northeast- and
northwest-trending faults, and some (i.e., Ginkgo) cut older sills
(Grande Ronde). A laterally extensive Frenchman Springs sill occurs
under an older widespread Grande Ronde sill. From this older
over younger intrusive relationship, a mechanism of "invasion" of
sediment from overlying lava flows is difficult to envision.
A pulse of rapid subduction starting in the middle Miocene
(Snavely et al., 1980) was accompanied by renewed uplift, intensive
block faulting, and continued development of the earlier
formed Coast Range uplift. Left-oblique northeast-trending faults
and conjugate northwest-trending right-oblique faults offset
Grande Ronde and Frenchman Springs dikes and sills. This conjugate
fault pattern may reflect oblique east-west convergence
between the North American and Juan de Fuca plates.
The Silver Point mudstones and Oswald West mudstones have
high total organic carbon contents, up to 5.5%, but are thermally
immature and may act only as a source for biogenic gas(?) in the
subsurface. Suitable reservoir rocks, such as the gas-producing
upper Eocene Cowlitz Formation C & W sandstone, may pinch out before
reaching the Green Mountain-Young's River area and are yet to
be penetrated by exploration drilling. Post-middle Miocene fault
traps abound in the area, although these faults might also breach
subsurface natural gas reservoirs in the Green Mountain-Young's
River area.
|
|---|