| Abstract |
- Stratigraphic measurement of the 1,238-rn thick Cowlitz Formation in the
southwest Washington type section along Olequa and Stillwater creeks reveals complex
facies succession of wave- to tide-dominated deltaic sequences. The underlying, 625-rn
thick upper member of the McIntosh Formation (as mapped by Wells, 1981) is composed
of two units: a basal 130-m thick prograding offshore to marginal marine coal-bearing,
lithic-arkosic sandstone facies succession (upper McIntosh sandstone) and a thicker, 500-
m thick bathyal foraminilera-rich siltstone facies that is, in part, in fault contact with the
overlying Cowlitz Formation. The lower member of the McIntosh Formation is 375 rn
thick in Stillwater Creek, with the base not exposed in the study area.
The Cowlitz Formation is subdivided into five informal units. The basal 558-m
thick unit consists of (1) multiple prograding, wave-dominated shoreface hummocky
stratified lithic arkosic sandstone successions (unit 1A) that comprise several thickeningand
coarsening-upwards parasequences and (2) coal-bearing delta plain facies associations
(unit 1B). The 205-m thick second unit is composed of five coarsening-up stormdominated,
hummocky-bedded shelf to delta-front arkosic sandstone parasequences.
Fining-upwards subtidal, intertidal, and supratidal facies associations constitute the 170-m
thick third member. Tidal-estuarine facies in unit 3 include: (1) nested subtidal micaceous
lithic-arkosic sandstone channels, (2) cross-bedded subtidal sandstone ridges with
brackish water mollusc hash, (3) sandy and muddy accretionary-bank, and (4) coalbearing
marsh-swamp deposits. Thin basaltic volcaniclastic interbeds occur within units
one, two, and three. A 155-m thick fourth unit, consists of wave-dominated, arkosic
sandstone, shoreface to offshore bioturbated mudstone facies successions; these
successions form 10 coarsening-upward parasequences that overall define a
retrogradational parasequence set. Thick, transgressive, bioturbated outer shelfal molluscbearing
sandy siltstone and glauconitic mudstone in the Big Bend type locality along the
Cowlitz River can be correlated to this unit in the type section in Olequa Creek. The
uppermost unit consists of 150 m of deeper marine laminated siltstone, subordinate finegrained
and thin-bedded turbidites, thick amalgamated submarine-channel sandstone and
chaotic mudstone conglomerate, and slump-folded and soft-sediment deformed laminated
siltstone intervals.
Petrography of lithic-arkosic micaceous sandstones of the McIntosh and Cowlitz
formations indicates there was a distant eastern, extrabasinal acid plutonic-metamorphic
source for the arkosic component of these sandstones. The predominant quartz, feldspar,
and mica constituents of Cowlitz Formation were transported from a distant dissected arc
(such as the Idaho Batholith and metamorphic core complexes to the east) through an
ancestral Columbia River drainage system. A second, local and active intrabasinal basaltic
source (Grays River volcanics) supplied basaltic scoria and lava rock fragments to form
volcaniclastic interbeds. Some volcaniclastics were reworked and mixed with the arkosic
extrabasinal sediments in the shallow marine and nonmarine environments of units 2 and 3
of the Cowlitz Formation. Explosive calc-alkaline volcanic activity (Northcraft Formation
or Goble Volcanics) is also evident in the silicic and pumiceous tuff beds interbedded with
coal marsh/swamp strata in units 1 and 3. Paleocurrent directions indicated by crossbedded
tidal strata of unit 3 are to the north-northwest and south-southeast as a resultof
shore parallel transport and deflection around a proposed growing volcanic edifice of
Grays River volcanics to the south and southwest. A very high sedimentation rate of 1.6
m/1,000 years was calculated for the upper part of the Cowlitz Formation (units 3 to 5)
using thickness measurement and 39Ar/40Ar age dates from the Cowlitz Formation (i. e.,
from tuff in unit 3) and the easternmost locality in the unconformably overlying Grays
River volcanics at Bebe Mountain. The large influx of sediment deposited over a relatively
short time period was accommodated by this rapidly subsiding forearc basin.
In this study area, subaerial flows of the Grays River volcanics locally
unconformably overlie the Cowlitz Formation. A 38.9± 0.1 Ma 39Ar/40Ar age date from
a tuff bed in unit 3 of the underlying Cowlitz Formation (Irving et al., 1996) and three
39Ar/40Ar age dates of 38.640.40 (south Abernathy Mtn.), 37.44±0.45 (west Bebe
Mtn.), and 36.85 ± 0.46 Ma (east Bebe Mtn.) from the overlying Grays River volcanics
bracket the timing of this regional unconformity. Additionally, field mapping (this study)
and drill hole data supplied by Weyerhaeuser Company (Pauli, written communications)
show there is a valley-fill unit at the base of the Grays River volcanics exposed on the
surface and in the subsurface, respectively. These data confirm the volcanic- and
tectonically-controlled unconformable relationship of the Cowlitz Formation to the
overlying the Grays River volcanics.
The Cowlitz Formation is in disconformable contact (a tectonically forced
sequence boundary) with an overlying second, younger lowstand valley-fill unit (Toutle
Formation unit A) recognized in this study along Olequa Creek. The 265-m thick, newly
discovered, Toutle Formation in this area is subdivided into three informal units: (1) a
basal incised, non-marine valley-fill sequence (unit A), (2) a marginal marine (estuarine or
nearshore) sequence (unit B), and (3) an upper fluvial sequence (unit C). A 31.9 ± 0.4Ma
39Ar/40Ar date from a homblende-bearing pumiceous lapilli tuff in unit A indicates that
the Toutle Formation is a time equivalent of the upper fluvial member of the Oligocene
type Toutle Formation and the middle part of the Lincoln Creek Formation far from the
center of the forearc basin to the west. Unit C of the Toutle Formation grades upward into
the overlying deeper marine tuffaceous siltstone of the Lincoln Creek Formation.
Deformation in this area resulted from two plate tectonic events: (1) latest middle
Eocene highly oblique subduction that resulted in short-lived, normal faulting and
intrusion of Grays River basalt dikes along small faults and (2) rapid post mid-Miocene
oblique subduction that formed northeast-trending dextral and northwest-trending sinistral
conjugate faults and broad regional compressional folding throughout southwest
Washington. The broad open Arkansas anticline that trends northwest-southeast between
Bebe and Abernathy mountains is an eastward extension of the Willapa Hills basement
uplift to the west and is extensively cut by northeast and northwest trending faults (Plate
I). This compressional event deformed both the Cowlitz Formation and the overlying
Grays River volcanics. A similar structural pattern recognized in regional field mapping by
Wells (1981) indicates this folding event also deformed mid-Miocene volcanic and
sedimentary unit (i.e., Astoria Formation and Columbia River basalts).
Reservoir quality of the Cowlitz and upper McIntosh formations micaceous lithicarkosic
sandstones is good. These sandstones are clean, highly friable and porous except
where carbonate and smectite clay rim cements formed in the lithic arkose. Unit 5 siltstone
could act as a cap rock in the subsurface and the 1- to 10-rn thick coals could be a source
for natural gas. The McIntosh marine siltstone is another possible source for gas and the
micaceous arkosic sandstone in the upper McIntosh is a potential reservoir. Stratigraphic
pinchouts and normal and wrench fault traps are similar to the Mist gas field of northwest
Oregon.
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