Washington-Oregon Tertiary: Evidence for Influence of Eustatic Controls on Coastal Sedimentation

Abstract

Tertiary sediments deposited along coastal Washington-Oregon extend from onshore to the base of the continental slope and beyond (Nitinat and Astoria deep sea fans). At least five Tertiary depocenters have been identified on the outer continental shelf, where maximum sedimentary section thicknesses of over 8,000 m (26,000 ft) are indicated. Stratigraphic sequences are strongly cyclical and evidence indicates that they are grounded by long-term regional hiatuses. Although previous interpretations attribute these stratigraphic sequences to four depositional/orogenic cycles, there is evidence that eustatic sea level changes may have had a major influence on Tertiary sedimentation along the Pacific Northwest Coast. Stratigraphic sequences are mostly marine, and contain larger fossil invertebrates, foraminifers, calcareous nannofossils, and siliceous microplankton, which can be dated and correlated both to California provincial stages and to worldwide chronostratigraphic standards. Stratigraphic correlation charts plotted on an absolute time scale display remarkable concordance with the supercycles and eustatic curve of the Haq-Vail Cenozoic global cycle chart. Along coastal Washington-Oregon Eocene to early Oligocene, early through middle Miocene, and Pliocene-Pleistocene sea level maximae are represented by extensive marine deposits; although both the late Oligocene (Chattian-late Zemorrian) and the late Miocene (Tortonian-late Mohnian) regressions are marked by major regional unconformities. Recognition thatmore » depositional sequences in this region were at least in part eustatically controlled is important because it could greatly simplify future subsurface geologic interpretations.« less