Tectonic Control of Coastal Onlap Cycles, Southwest Washington

John M. Armentrout

Local coastal onlap and paleobiobathymetric curves for 14 sections define three Cenozoic depositional onlap-offlap cycles separated by regionally significant unconformities. A paleoclimatic curve for western Oregon and Washington, based on paleoecologic data sets, demonstrates that the local transgressions are coincident with cool climates and the regressions with warm climates, and are therefore not driven by glacioeustatic cycles.

Comparison of the local coastal onlap and paleobiobathymetric curves with the global Cenozoic Cycle Chart (modified Exxon Sea Level Chart--May, 1986) further demonstrates the uniqueness of the western Washington curves. The global Cenozoic Cycle Chart curve represents coastal onlap and sea level curves based on integration of both climate and tectonic variations. The nonparallel cycle pattern for southwest Washington suggests a unique tectonically forced system.

Evidence derived from stratigraphic sequences, igneous rock geochemistry, radiometric dating, remnant magnetic patterns, sandstone provenance studies, and paleogeographic reconstructions is used to identify the tectonic events controlling the local depositional cycles. The principal events are (1) middle Eocene accretion of a seamount chain; (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 subduction of remnants of the Kula Plate beneath North America.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.