Graduate Thesis Or Dissertation


Geology of the Green Mountain-Young's River area, Clatsop County, northwest Oregon Public Deposited


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  • 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.
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