Tectonostratigraphic Sequences of a Forearc Basin: Western Oregon and Washington
John M. Armentrout
Five regionally significant u nconformities partition the Cenozoic stratigraphy of western Oregon and Washington into depositional sequences. Deposited in a forearc setting, the sequences demonstrate the dominance of tectonic events over variations in sea level and sediment flux. Middle Eocene basin margin deltaic systems prograded into a rapidly subsiding basin oversteepening the shelf edge with consequent gravity-flow event deposition of basinal fan systems. Middle to late Eocene coal measures are vertically stacked suggesting rapid accommodation along portions of the tectonic hingeline. Oligocene strata are dominantly basinal siltstone and claystone containing bathyal biofacies indicating subsidence exceeded sediment supply across most of the forearc basin. Miocene shallowing with rapid western progradation of coast plain facies was accentuated by thermal uplift with consequent erosion and associated extwsion of plateau basalts. Pliocene compression restricted deposition to northwest-southeast asins with fluvial-deltaic systems prograding northwestward into estuarine facies. The depositional sequences are biostratigraphically calibrated permitting correlation of the unconformities across the Pacific Plate to Alaska and the North American Plate to the Gulf of Mexico, demonstrating the regional scale of deformation. Within Oregon and Washington, the tectonic events forcing the unconformities are: (1) middle Eocene emplacement of a sea mount chain by sea floor spreading; (2) early late Eocene westward relocation of subduction: (3) late Eocene onset of Cascade arc volcanism; (4) late early Miocene plate readjustment due to back-arc extension in the Columbia River Plateau and Great Basin; and (5) late Pliocene to early Pleistocene northeast compression forced by continued subductio of remnants of the Kula plate.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California