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  • The geology and natural gas potential of the Clatskanie 7.5-minute quadrangle in Columbia County, Oregon was studied utilizing a combination of geologic mapping, measuring of stratigraphic sections, diatom and molluscan fossil identifications, with subsurface geophysical data and petrographic and geochemical analyses. The stratigraphy of the study area consists of two Tertiary volcanic units (Grays River Volcanics and Columbia River Basalt Group) and four sedimentary units (Cowlitz (subsurface) Keasey, Pittsburg Bluff, and Scappoose formations) that were deposited in the Nehalem forearc basin. The major structure of the area is defined by the oblique-slip Scappoose-Clatskanie fault with both normal and dextral motion. In the east-central part of the study area, this post-Miocene fault zone, formed by a right step over in a right-lateral strike-slip system, created an extensional zone with abundant normal faulting (horsts and grabens). The faulting uplifted an isolated block of middle Eocene Grays River Volcanics subaerial flows against downdropped Miocene Columbia River Basalt in the northwestern part of the map area near the town of Clatskanie along Highway 30. This block was part of a major Grays River Volcanics paleohigh (herein named the Clatskanie High) and forms part of a regional gravity/magnetic high beneath the Columbia river. The Clatskanie High and the Windy Ridge High (subsurfance paleohigh also composed of Grays River Volcanics) in the central to southern part of the study area, restricted deposition and caused onlapping of the Cowlitz, Keasey, and Pittsburg Bluff formations. The Clatskanie High shield volcano defines the northern boundary of the Mist Gas Field. In addition, local unconformities at the base of the Keasey and Pittsburg Bluff formations progressively truncate the Cowlitz C&W sandstone reservoir and upper mudstone seal to the north. The largely unexplored central and northeastern parts of the study area near the Scappoose-Clatskanie fault zone with horst and grabens and stratigraphic pinchouts represent some of the most promising untested exploration plays in the quadrangle. Associated wtih the Clatskanie High are two new (informal) members of the Oligocene Pittsburg Bluff Formation that are defined and mapped by this study. The Conyers Creek member (informal) is a shallow-marine mollusk-bearing pebbly-basaltic litharenite (a poorly sorted nearshore to shoreface sandstone) with an interbedded fossiliferous fine pebble conglomerate of Grays River volcanic clasts. The lens-like nearshore unit was deposited on the margins of the uplifted Grays River basaltic oceanic island and, thus, restricted to an area around the paleohigh. Locally overlying the Conyers Creek member and interbedded with the East Fork member (Pittsburg Bluff Formation) is the Clatskanie Conglomerate member (the second informal member defined by this study). The 180 feet (55 meter) thick member consists of a basal, clast-supported 110 feet (33 meter) thick polymict conglomerate and upper well-consolidated 70 feet (21 meter) think micaceous arkosic sandstone. The Clatskanie Conglomerate is a lens-like fluvial deposit in the northwestern part of the study area, possibly syndepositional and restricted in deposition to the center of a growing syncline. The cobble to boulder sized clasts of quartzite, chert, metamorphics, and andesites/dacites indicate extrabasinal sources such as the Western Cascades arc and continental sources in eastern Oregon, Idaho, Montana, and eastern Washington via an ancestral Columbia River drainage system. The diatom-bearing tuffaceous clayey siltstone of the Keasey Formation and the mollusk-bearing sandy siltstone of the East Fork member (Pittsburg Bluff Formation) formed in continental slope and middle to outer shelf environments. Extensive explosive dacitic eruptions in the nearby Western Cascade arc contributed abundant ash, which washed into the forearc basin. Local Thyosira chemosynthetic pelecypods and calcareous concretionary beds indicate the local presence of either cold methane seeps or a gas hydrate system on the Pittsburg Bluff continental shelf. The Scappoose Formation consists predominantly of a basal basalt and siltstone rip-up conglomerate deposited in an incised valley during a low stand systems tract (LST). The conglomerate is overlain by a diatom-bearing shallow-marine tuffaceous siltstone of a transgressive and highstand systems tracts (TST & HST), and capped by a cross-bedded fluvial lithic arkosic sandstone of a second lowstand systems tract (LST). Chemical analysis of the basal basalt conglomerate clasts indicate that they were derived from erosion of high TiO2R2 Wapshilla Ridge flow unit of the Grande Ronde Basalt (16.5 to 15.6 Ma) of the lower to middle Miocene Columbia River Basalt Group. The Scappoose Formation also includes thin fluvial and lacustrine sandstones and tuffs interbedded with Columbia River Basalt flows. Capping these Tertiary units are three units of low-MgO Grande Ronde Basalt of the Columbia River Basalt Group. These include 1-2 basal aphyric flow(s) of the high-TiO2R2 Wapshilla Ridge flow unit, at least six flows (some invasive) of the R2 low-TiO2 Grouse Creek flow unit, and the upper sparsely glomerophyric flows of N2 Winter Water flow unit. The structural history of the region is characterized by two major tectonic events. A late Eocene extensional period prior to deposition of the Keasey Formation created normal faulting (horsts and grabens) throughout the Cowlitz Formation. These faults formed most of the structural traps in the Mist Gas Field including those drilled by producing wells in the southern part of the quadrangle near the Windy Ridge High. The post-Miocene episode was dominated by wrench fault tectonics that manifested as strike-slip oblique-slip motion on dominant northwest-trending and subordinate northeast-trending faults in the Columbia River Basalt Group flows.
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