Abstract: Transport and Deposition of the Peripheral Sedimentary Rocks of the Crescent Terrane and their Implications for Regional Paleogeography

Christopher A. Suczek

Lateral and vertical changes in the terrigenous sedimentary rocks of the Eocene to Miocene Crescent terrane document a complicated pattern of transport and deposition. The most complete section through the Crescent terrane lies along the northern Olympic Peninsula of Washington state. The basal deposits there are turbidites of the middle Eocene Blue Mountain unit, which underlie and interfinger with Crescent Formation basalts. Above the basalts, deposition was in generally bathyal water as submarine fan and ramp facies. Petrographic studies show that areas of deposition were fed by sediment from more than one source. For the Blue Mountain unit, arkosic sediment (from the Coast Plutonic Complex?) mixed on the fan with chert-rich sediment perhaps derived from the San Juan I lands terranes. For the overlying units, paleocurrents indicate flow directions from the northwest. Sediment derived from Vancouver Island reached the fan complex relatively unmixed with, for example, chert more abundant in the west for some units (e.g., Aldwell and Lyre fms.) and in the east for other units (e.g., Hoko River Fm.), although central and eastern exposures generally contain more basalt, apparently derived from local uplifts of Crescent basalt. The picture is one of a long source area generally to the north and northwest, containing rock types now found on Vancouver Island, shedding sediment of different compositions at different times and from different locations into the basin. As tectonics and erosion exposed different rocks in the source area, the sediment reaching the b sin varied. The upper units, deposited while the basin was filling and shallowing to near-shore and deltaic facies, are less lithic and may have tapped into a Coast Plutonic Complex source.

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