Tertiary stratigraphy and sedimentation in the Young's River -Lewis and Clark River area, Clatsop County, Oregon

Abstract

Five early to middle Tertiary bedrock geologic units crop out in the study area. These include the late Eocene to early Miocene Oswald West mudstones; the Tucker Creek sandstone, Big Creek sandstone, Silver Point mudstone, and Pipeline members of the early to middle Miocene Astoria Formation; and the middle Miocene Depoe Bay Basalts. These units are locally overlain by Quaternary floodplain, alluvial terrace, and beach-ridge deposits. Oswald West mudstones consist of greater than 2, 500 feet of poorly stratified to thick-bedded, orange-yellow, tuffacec)'us, burrowed mudstones and siltstones. Inter stratified tuff beds and glauconitic strata are common. Deposition occurred primarily in an open, deep-marine environment (outer shelf to slope). The sandstone-rich middle part of the unit was deposited in a shallower marine, middle to outer shelf environment during an early Oligocene basin shoaling. Early Miocene regression and tectonism resulted in termination of Oswald West mudstone deposition. Ensuing transgression resulted in deposition of the Astoria Formation over the Oswald West mudstones with slight angular (?) unconformity. Basal deposits of the Astoria Formation include the interfingering Tucker Creek sandstone and Big Creek sandstone members. The former is a-150-foot thick unit consisting of medium- to very fine-grained, bioturbated, structureless sandstones which fine upward to siltstones and glauconitic sandstones. The 500- to 1, 000-foot thick Big Creek member is composed of basal fine-grained, structureless to laminated, locally trough cross-bedded sandstones and, in the upper part, intertonguing fine-grained sandstones, siltstones, and glauconitic sandstone. The fining-upward sequence in each unit represents transition from shallow-marine to deeper marine environments as transgression progressed. These basal sandstones may have been deposited, in part, by northward-longshore drift redistribution of Angora Peak delta front sands. The approximately 800-foot thick lower Silver Point mudstones are composed of well- stratified, rhythmically interbedded, fine-grained, micaceous, carbonaceous sandstones and dark-gray mudstones. The sandstones represent episodic turbidity current deposition in an open-marine, sublittoral (shelf), depositional environment. Turbidity flows may have originated by slumping of delta-front sands of the Angora Peak delta. With continued transgression, the deep-marine, interfingering, upper Silver Point mudstone member and Pipeline member were deposited in the study area. The thickness of each unit is greater than 2, 000 feet. Well-laminated, locally highly carbonaceous and micaceous, dark-gray mudstones are predominant in the upper Silver Point mudstones. These mudstones are hemipelagic, continental slope to outer shelf deposits. Similar deep-marine mudstone lithologies, which are complexly intertongued with thick, medium-grained, friable, arkosic sandstones, occur in the Pipeline member. The geometry, sedimentary structures, and bedding character of these sandstones are suggestive of deposition by sediment gravity flows (grain flow, fluidized flow), possibly in a submarine canyon or channelized upper submarine fan environment. Several middle Miocene dikes and sills of Depoe Bay Basalt intrude the Astoria Formation and Oswald West mudstones in the study area. In surrounding areas, extrusive Depoe Bay and younger Cape Foulweather Basalts unconformably overlie older Tertiary strata. Post-middle Miocene deformation formed east- west-trending folds and northwest- and northeast-trending high-angle faults in the study area. This deformation was accompanied by broad uplift of the Coast Range and termination of marine sedimentation in the study area. The Olney oil seep is suggestive of the possible presence of subsurface petroleum reservoirs in the study area. Further drilling is necessary to determine the authenticity of the seep. The Pipeline sandstones are potential hydrocarbon reservoirs in the nearby continental shelf.