| Abstract |
- The early to middle Eocene submarine basalts of the Crescent Formation form the
core and basement of the structurally uplifted Willapa Hills region of southwest
Washington. The formation consists of a thick sequence of predominantly subalkaline,
tholeiitic pillow basalts and breccias formed as oceanic crust and seamounts at a sea
floor spreading ridge or pull apart basin along the rifting continental margin of North
America. Radiometric (Ar⁴⁰ - Ar³⁹) dating of the pillow basalt indicates an age of
approximately 53 - 55 Ma for the formation. Foraminifera collected from mudstones
interbedded with the pillow basalts of the Crescent Formation in southwestern Pacific
County indicate a Ulatisian to lower Narizian (middle Eocene) age for the basalts and
suggest that they were erupted in lower-middle bathyal (1500 - 2000 m) water depths.
Locally associated with the pillow basalts are hyaloclastic basalt breccias and basaltic
sandstones.
Overlying, and interbedded with the basalts of the Crescent Formation is the
middle and upper Eocene McIntosh Formation, which is composed of three members
(informal). Stratigraphically from the lowest, these members are: 1) Fork Creek
member; 2) Lebam member, 3) McIntosh Volcanics member.
The Fork Creek member consists of a thick sequence of normally graded,
micaceous, coarse- to fine-grained, arkosic and lithic arkosic sandstones deposited by
sand-rich, high concentration turbidity currents in the deep marine-marginal basin. The
turbidites were fed by a fluvial system carrying granitic and metamorphic detritus
derived from erosion of Mesozoic crystalline rocks of the Okanagan Uplift region of
northeastern Washington and southeastern British Columbia, and possibly the Idaho
Batholith. Lithologically equivalent, shallow-marine and fluvial-deltaic sandstones
with quartz and chert pebbles include the middle Eocene Carbonado Formation of the
Puget Group and the upper portion of the type McIntosh Formation to the east.
Microprobe determination of composition of plagioclase microlites and phenocrysts in
volcanic fragments in the sandstones indicate a proximal volcanic source such as a pre
Western Cascade basaltic andesite (e.g. Northcraft Volcanics) or possibly Challis
Volcanics further east. Paleocurrent measurements suggest that dispersal of the
turbidity flows to the west southwest within the basin was controlled by the active
volcanic highs of the Crescent Formation. Lithofacies and foraminifers suggest that the
Fork Creek member represents a small lowstand fan deposited during a fall in relative
sea level either due to eustatic sea-level drop and/or tectonic uplift of the basin.
Numerous dikes and sill-like bodies of porphyritic basalt to gabbro intrude both
the pillow basalts of the Crescent Formation and micaceous arkosic sandstones of the
Fork Creek member. These intrusions are generally concentrated near the contact
between the two units and some appear to be fault controlled. Based upon major and
trace element compositions,the intrusions can be separated into two types. The first
type consists of subalkaline tholeiitic basalts and gabbros chemically equivalent to the
oceanic basalt of the Crescent Formation. Radiometric dating of one of these sills
which intrudes the upper portion of the Fork Creek member of the McIntosh formation is 48.7 + 0.5 Ma. These intrusions likely represent late stage events associated with
waning activity of the Crescent basalts. The second group of intrusive rocks consists
of high TiO₂ alkaline basalts similar in composition to the nearby subaerial basalt and
basaltic andesite flows of the middle (?) to upper Eocene Grays River Volcanics.
Radiometric dating of a trachybasalt dike from this group indicate an age of 41.4 + 0.7
Ma.
Magnetic polarity measurements indicate that most middle Eocene basalts of the
Crescent Formation formed at a time of normal magnetic polarity. Later intrusive
equivalents of the Crescent Formation and the Grays River Volcanics typically show
reversed magnetic polarities.
An abrupt transgressive event due to eustatic sea level rise and/ or tectonic
subsidence within the basin is recognized by the deposition of the thick sequence of
deeper marine mudstones, siltstones, and minor distal, thin bedded, micaceous arkosic
turbidites of the middle to upper Eocene Lebam member of the McIntosh Formation.
This transgression is accompanied by a landward shift in the supply of arkosic sand-rich
sediments and a hiatus of deposition of arkosic sand in this portion of the basin.
The sea level transgression may be due to thermal subsidence of the basin at the
cessation of Crescent volcanism.
Massive to weakly bedded basaltic tuff and tuff breccia of the McIntosh
Volcanics member is locally interbedded with the deep-marine strata in the upper
portion of the Lebam member. This basaltic aquagene tuff may be associated with
similar tuffs within the Grays River Volcanics elsewhere.
Fine-grained, glauconitic sandstones of the basal portion of the latest Eocene to
Oligocene Lincoln Creek Formation conformably overlie deep-marine strata of the
Lebam member. The sandstone is generally massive, bioturbated, and contains
gastropods and articulated pelecypod fossils These sandstones were deposited during
a time of low sedimentation rates and represent progradation of shallow-marine sands
during a period of highstand in relative sea level.
Thin section and quantitative analyses by permeameter of the fine- to medium-grained
arkosic sandstones of the Fork Creek member indicate relatively good porosity.
However, the formation of authigenic pore-lining nontronite (smectitic) and/or chlorite
clay rim cement, zeolites, and sparry calcite during diagenesis has significantly reduced
permeability making these well-indurated turbidite strata of marginal reservoir quality.
However, diagenetic effects such as dissolution of lithic framework grains and feldspar
is observed and may enhance the porosity and permeability of the sandstones in the
subsurface and subsequently improve the reservoir quality locally.
Hydrocarbon source rock potential of mudstones within the Fork Creek and
Lebam members indicate that these strata are generally organically lean and gas-prone
(type III kerogen) with little or no potential for formation of liquid hydrocarbons.
Mudstones are typically thermally immature (e.g. Ro < .494%), although, heating by
proximal basaltic intrusions in the area is observed to result in local thermal maturation
of some sedimentary strata into the oil window and beyond.
Northwest-trending, high-angle, right lateral, oblique-slip faults are the dominant
structural feature in the area. A subordinate set of northeast-trending faults appears to
be truncated by the northwest-trending faults, and could represent an earlier episode of
faulting or could possibly be en echelon and conjugate structures.
The structural pattern is similar to that mapped in the Mist gas field of northwest
Oregon. Northwest-trending faults commonly juxtapose arkosic sandstones of the
Fork Creek member against volcanics of the Crescent Formation and large intrusive
units or against the siltstone- and mudstone-rich overlying Lebam member. These
faults could form structural traps for hydrocarbons if present in the subsurface to the
north and east. Other possible traps include stratigraphic pinch outs and channeling of
the arkosic sand-rich turbidites of the Fork Creek member within deep marine mudstones or onlap of these reservoir sands against volcanic highs within the basin.
The thick fine-grained strata of the overlying Lebam member and the glauconitic
sandstone of the Lincoln Creek Formation could act as a seal for the "reservoir
sandstones" of the Fork Creek member.
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| Digitization Specifications |
- Master files scanned at 600 ppi (256 Grayscale, 24-bit Color) using Capture Perfect 3.082 on a Canon DR-9080C in TIF format. PDF derivative scanned at 300 ppi (256 B&W, 256 Grayscale, 24-bit Color), using Capture Perfect 3.082, on a Canon DR-9080C. CVista PdfCompressor 4.0 was used for pdf compression and textual OCR.
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