| Abstract |
- The middle to upper Eocene Tillamook Volcanics form the volcanic core or basement rocks in the Rock Creek-Green Mountain area. They consist of a thick sequence of high TiO2 tholeiitic basalt, basaltic andesite, and andesite subaerial flows and breccias. Detailed mapping, petrography, major oxide plots, magnetic polarity, and radiometric age dating prove that the volcanic rocks that form the Green Mountain outlier are an upthrown, more fractionally differentiated basement block of Tillamook Volcanics. Geochemical plots, regional geology, and field relationships indicate that the Tillamook Volcanics erupted in a forearc tectonic setting and formed an extensive, moderate relief, rugged oceanic island of overlapping shield volcanoes. Subsequent transgression and subsidence of the Tillamook Volcanics, perhaps due to thermal cooling of the crust, resulted in deposition of upper Narizian (upper Eocene) beach to high energy inner shelf, mollusc-bearing, basaltic boulder conglomerate and basaltic sandstone of the Roy Creek member of the Hamlet formation (informal). Petrography, geochemistry, pebble count data, and stratigraphic relationships show that the coarse- to fine-grained basaltic sandstone and conglomerates were derived from the underlying Tillamook Volcanics and were probably deposited along a rocky coastline composed of sea stacks and headlands muck like Oregon's coastline today. Micaceous lithic arkosic sandstone and siltstone and minor basaltic sandstone and volcanic debris flow interbeds of the overlying Sunset Highway member of the Hamlet formation (informal) were deposited in an inner to middle shelf environment. The extrabasinal micaeous arkosic sands were derived from granitic/metamorphic source areas in eastern Oregon, eastern Washington, and Idaho. They were transported via an ancestral Columbia River drainage system to the shoreline where they interfinger with basaltic debris flows and basaltic sandstone that were locally eroded from rocky Tillamook basalt headlands. Broken and abraded molluscan fossils, parallel lamination and hummocky cross-bedding, moderately well sorted character of the sands, and sheet-like sandstone geometry indicate a high energy, storm-dominated, shallow marine depositional environment. Extensive diagenesis of the highly chemically reactive Roy Creek basaltic sandstone has resulted in significant porosity and permeability reduction after burial due to formation of chloritic and smectite clays and locally zeolite and calcite filling intergranular pore spaces. Thin section and scanning electron microscopy, however, show only minor thin smectite clay coats and scattered micaceous arkosic sandstones of the Sunset Highway member. This has resulted in the preservation of much primary intergranular porosity. Further transgression and/or subsidence of the upper Eocene shelf sands resulted in deposition of micaceous and carbonaceous silty mudstones and minor thin-bedded turbidite sandstone unit of the Sweet Home Creek member of the Hamlet formation in a middle to upper slope depositional environment. Bathyal benthic foraminiferal assemblages collected are assigned to an late Narizian age. Thin interbeds of graded micaceous arkosic sandstone with Bouma B, C, and D divisions were derived from adjacent Sunset Highway shelf deposits resuspended by storm waves and transported by turbidity flows into upper slope environments. Thin dikes and thick irregular-shaped sill-like bodies
of calc-alkaline Cole Mountain basalt intrude the Sweet Home Creek member. The coarse-crystalline (microgabbroic to gabbroic) to finely crystalline basalt is composed of phenocrysts of calcic plagioclase and augite in a clay altered intersertal to hyalopilitic groundmass. The Cole Mountain basalt is petrographically and chemically distinct from the high Ti02 and low Si02 basalt flows with pilotaxitic texture that form the older Tillamook
Volcanics. They are more similar to the arc-derived caic-alkaline Goble Volcanics of southwest Washington. Regression and/or progradation resulted in deposition of thick sheet sandstones of the Cowlitz Formation. Trough cross-laminations indicate paleocurrent transport directions were to the west and northwest prior to clockwise tectonic rotation. Parallel to trough
cross-laminations, the high degree of rounding and sorting, and the low degree of bioturbation indicate deposition occurred in a high energy, storm-dominated, inner and
middle shelf environment. Uplift of the Oregon Coast Range forearc region that was initiated in the late middle Miocene resulted in subaerial weathering and differential erosion of the softer sedimentary rocks in contrast to the resistant basaltic rock units. Minor alluvial basaltic stream gravels and sands formed along major, low gradient streams. Recent rock fall/landslide debris occurs along the south side of Quartz Creek. The regional structure consists of a northward-plunging gravity high or structural upwarp of Tillamook Volcanics that transects the middle of the thesis area. This structural high is cut by a series of major east-west
striking high-angle normal faults that are offset by a younger set of northwest-trending right-lateral and northeast-trending left-lateral oblique-slip faults. The conjugate oblique-slip pattern of faulting is characteristic of wrench faulting perhaps due to a north-south shear couple created by oblique subduction of
the Juan de Fuca oceanic plate beneath the North American continental plate. The downwarp of upper Eocene Hamlet sedimentary rocks preserved between the isolated upthrown
block of Tillamook Volcanics at Green Mountain on the north from the main Tillamook highland on the south may be a
pull-apart depression formed by oblique-slip motion on these fault-bounded upthrown volcanic basement blocks. This pattern is much like the structure of the Mist Gas
Field in the subsurface to the northeast.
Continued commercial production from the upper Eocene Cowlitz Formation (Clark and Wilson sandstone) since 1978 in the Mist Gas Field, has resulted in renewed interest in
the hydrocarbon potential of the Northwest Oregon. The micaceous sandstones of the Sunset Highway member of the Hamlet formation could also serve as an attractive deeper reservoir target based on porosity and permeability analysis and may equate to the "Clatskanie" sandstone in the subsurface at Mist. Total organic carbon values and
woody kerogen from carbonaceous mudstone from the Sweet Home Creek member suggest it as a potential source rock for wet gas in the subsurface. Local thermal heating of the
Hamlet mudstone by upper Eocene Cole Mountain basalt sills has created some high vitrinite reflectance values and represent a potential "quickflash" maturation mechanism
for gas.
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