BEDROCK ARCHITECTURE, NATURAL RESOURCES, AND GEOLOGIC HAZARDS OF NE PART OF THE OREGON COAST RANGE FOREARC

Recent USGS-sponsored mapping in Columbia Co., OR (Clear Ck., Vernonia, Pittsburg, Birkenfeld, Bacona 7.5-min. quads) shows that the m. Eocene Tillamook Volcanics (economic basement) is a complex of overlapping tholeiitic to subalkalic shield volcanoes that formed an oceanic island in the forearc (42-46 Ma, ⁴⁰Ar/³⁹Ar). The basalt is unconformably overlain by rocky shoreline basalt conglomerate, shelfal arkosic ss and deep marine mudst of the transgressive Hamlet Fm.

Natural gas is produced in the Mist gas field from the overlying delta front micaceous arkosic Cowlitz C&W ss capped by a deep marine mudst seal (upper Cowlitz mbr.). Locally, Grays River basalt and W. Cascade arc-derived Goble Volcanics (subsurface) and equivalent Cole Mtn. basalt interfinger with these sedimentary units. Late Eocene extension created horst and graben fault traps (e.g., Nehalem graben) on the Nehalem arch (a gravity volcanic high). These m. to u. Eocene units and faults are truncated by deep marine Eo/Oligocene Keasey Fm. tuffaceous siltst derived from the W. Cascade arc. Sager Creek Fm. arkosic turbidites filled slope channels incised into the Keasey and are overlain by deltaic arkosic ss, coals, and shelfal tuffaceous sandy siltst of the Oligocene Pittsburg Bluff Fm. The Scappoose Fm. (u. Oligo. to l. Miocene) represents continued deposition of continental micaceous arkosic detritus and influx of W. Cascade ashes. Paleovalleys incised into the Eo/Oligocene units were filled with a basalt conglomerate of Wapshilla Ridge clasts of the l. Mio. Mist mbr. (Scappoose Fm.) and with subaerial plateau-derived l. to m. Mio. Columbia River flood basalts (R₂ Wapshilla Ridge to N₂ WinterWater flow units).

Starting in late m. Mio. wrench faulting between the right-lateral Gales Creek and Clatskanie-Scappoose fault zones created conjugate sets of oblique-slip right-lateral and normal NW to W faults and subordinate left-lateral oblique-slip to normal NE faults. These small offset faults mimic older Eocene faults, continuing small block clockwise rotation. The NW faults are on strike with the Portland Hills fault zone and pull apart basins. The antecedent Nehalem River incised a narrow bedrock valley around the uplifting north-plunging Nehalem arch and in the graben in the late Neogene. Geologic hazards include landslides and floods (e.g., 1996).