Geology of the Seaside-Young's River Falls area, Clatsop County, Oregon

Abstract

Eight Tertiary geologic units crop out in the Seaside-Young's River Falls area, including late Oligocene to early Miocene Oswald West mudstones; Angora Peak sandstone and Silver Point members of the middle Miocene Astoria Formation, and middle Miocene Depoe Bay and Cape Foulweather Basalts. Three new lithologically distinct units, the 'J' and Airplane units of the Astoria Formation and the middle Miocene Dump sandstones, are described and named informally. The Silver Point is subdivided into lower and upper tongues. These units are locally overlain by Quaternary beach ridges (Clatsop Plains) and two alluvial terraces. The 250-meter thick Oswald West mudstones are poorly stratified, yellowish orange tuffaceous burrowed mudstones. The upper part of the unit contains glauconitic sandstones, local fine -grained, well-bedded arkosic sandstones, and tuff beds. The unit was deposited in an open, deep-marine (outer shelf to upper slope) environment. Broad uplift following deposition of the Oswald West formed a minor angular unconformity with the overlying Astoria Formation. The Angora Peak sandstones are composed of over 120 meters of fossiliferous arkosic wackes and arenites deposited in a high energy, wave-dominated beach or bar environment. Gradual transgression is indicated by the gradation of pebbly coarse-grained sandstones upward into deeper marine clay-rich bioturbated fine-grained sandstones. The lower Silver Point tongue is a 'flysch-1ike" sequence of interbedded very fine-grained arkosic wackes an gray mudstones. It was deposited in a sublittoral environment by both turbidity and tidal currents on a delta slope or outer delta platform. The 'J' unit, which conformably overlies the lower Silver Point tongue, consists of approximately 150 meters of well-laminated micaceous, carbonaceous mudstones of lagoonal origin. A sequence of arkosic, coarse-grained pebbly delta distributary channel sandstones of the 'J' unit gradually prograded westward into the lagoonal environment. Rapid subsidence, perhaps by abandonment of the 'J' deltaic lobe, resulted in transgression over the study area. Over 250 meters of well-laminated yellowish brown mudstones of the Airplane tongue were deposited in an upper slope or outer shelf environment as suggested by an abundant foraminiferal assemblage. The Airplane mudstones grade into the overlying upper Silver Point tongue which is composed of approximately 120 meters of very thin interbedded dark gray mudstones and very fine-grained micaceous sandstones and siltstones. The upper Silver Point tongue was deposited at bathyal depths, possible in a low-energy pro-delta slope environment. Broad, open folding and extensive erosion occurred soon after deposition of the upper Silver Point The 50-meter thick Dump sandstones overlie several members of the Astoria Formation in apparent angular unconformity. These clay-rich very fine-grained arkosic sheet sandstones and siltstones possibly formed from erosion and redeposition of older sedimentary units. Numerous dikes, irregular sills, peperites, and submarine pillow lavas of the Depoe Bay Basalt and younger sparsely porphyritic Cape Foulweather Basalt intrude or overlie all Tertiary sedimentary units. These tholeiitic basalts are locally 150 meters thick, but pinch out within several kilometers, possibly due to lava ponding on an irregular ocean floor and/or accumulation around local volcanic centers. Sedimentary interbeds between flows suggest intermittent volcanic activity. Folding and faulting of these Oligocene and Miocene units probably coincides with uplift of the present Coast Range which resulted from convergence of the Juan de Fuca and North American plates during late Miocene time. This deformation formed northwest trending folds and two major sets of high angle faults striking northwest and northeast. The Angora Peak sandstones are potential hydrocarbon reservoirs in the nearby continental shelf. Well logs and cores from three nearby wells suggest that Eocene and Oligocene sedimentary units are too impermeable for significant hydrocarbon production.