Abstract: Eocene Paleogeography of the Morton Anticline Region, Southwest Washington

S. Y. Johnson, W. D. Stanley

The Morton anticline is a complex, north-trending structural arch in the Cascade Range foothills of southwest Washington that provides exposure of about 1,200 m of the middle Eocene Carbonado Formation. The Carbonado consists of alternating intervals of nonmarine and shallow- to marginal-marine deposits that accumulated within a river- and tide-influenced delta. Nonmarine rocks make up about 67 percent of measured sections and include fluvial channel sandstone bodies (typically 5-20 m thick) and floodplain deposits (forming sequences as thick as 60 m) of fine-grained sandstone, mudstone, and coal. Paleocurrent data indicate western sediment transport. Shallow- to marginal-marine rocks consist mainly of bioturbated shoreface sandstone and laminated sandstone and mudstone d posited in tidal-flat or -inlet environments. Volcaniclastic rocks, including tuff, tuffaceous sandstone, and volcanic breccia-conglomerate form a minor part of the sequence and are best preserved in floodplain environments.

Although the base of the Carbonado Formation is not exposed, seismic reflection data suggest that the Carbonado may be 3-4 km thick. Magnetotelluric data further suggest that this Carbonado section is underlain by a 4-5-km thick, highly conductive unit interpreted as early Eocene marine sedimentary rock. The Carbonado Formation interfingers with and is overlain by volcanic rocks of the Northcraft Formation; the contact has about 400 m of depositional relief. The cumulative thickness of the Eocene section suggests rapid subsidence, which we attribute to regional transtension.

We propose the following paleogeographic model for the Morton anticline area. Early Eocene: Fine-grained marine rocks were deposited in pro-deltaic setting. Basinal circulation was partly restricted by offshore seamounts. Middle Eocene: Deltaic deposits of the Carbonado Formation prograded westward to the Morton area. Seamounts protected the delta from the sediment-dispersing energy of the Pacific Ocean. Late Eocene: Northcraft volcanic rocks cover the Morton area. Volcanic relief deflected fluvial-deltaic systems to the north and south, creating a favorable setting for accumulation of thick peat deposits to the west near Centralia. Post-Eocene: Transpressional deformation along the west flank of the Cascade Range res lted in complex faulting and uplift of the Morton anticline.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California