NEW USGS 7.5-MINUTE GEOLOGIC QUADRANGLES IN SOUTHERN CLATSOP COUNTY, NW OREGON, A PLANNING GUIDE TO MANAGEMENT OF NATURAL LAND AND WATER RESOURCES AND GEOLOGIC HAZARDS

New geologic mapping in NW Oregon will help planners manage hazards, resources and scenic geologic sites. Hazards include large translational landslides in mudstone units that impact highways and parks; wave erosion, slumps, debris slides, and rock falls endanger seaside dwellings and other areas. Coastal resort towns lie in tsunami inundation zones and were damaged by the 1964 Alaska earthquake tsunami. Rural floodplains were affected by the 1996 100-yr flood.

Groundwater resources are largely restricted to narrow Quaternary stream and coastal terrace aquifers some of which have water quality issues (e.g., high Fe). Eocene through Miocene mudstone units in uplands are relatively impermeable. Most urban water supplies are from springs in jointed Miocene Columbia River Basalt (CRB) submarine breccias and from consequent streams headed in basaltic watersheds.

Widespread CRB invasive units, forming dike and sill ridges, are the best quarry rock. Other natural resources include potential gas reserves offshore in the Miocene Astoria Fm and onshore in the mapped pinchout of the Eocene Cowlitz C&W sandstone, the reservoir in the Mist Gas Field. Lean gas-prone marine mudst. source rocks and thin coals are locally thermally matured by thick CRB sills. NW-, E-W- and NE-trending normal and oblique-slip faults, representing the terminus of the Mt. Angel/Gales Creek fault zone, are possible traps. The faults have a long history and may accommodate contemporary forearc strain.

Undeveloped scenic peaks with cirque-like valleys and sheer cliffs of CRB breccias are inverted topography of a submarine canyon fill. A few summits contain a remnant Pleistocene flora. CRB re-eruptive centers and invasive CRB sills and dikes now form sea stacks and cliffs, sanctuaries for intertidal life, pinnipeds, and seabirds. Natural recreational resources inland include the antecedent Nehalem River that cuts a gorge in Eocene Tillamook Volcanics with incised, fault-controlled meanders. Large-scale slump folded Astoria Fm (a delta of the ancestral Columbia R.) is associated with peperitic CRB dikes and scenic headlands of thick invasive CRB sills (e.g., Tillamook Head). These geologic features and deep-marine Zoophycos trace fossil assemblages (Oligocene Smuggler Cove Fm) form sea cliffs in state parks that should be geologic heritage sites.