Abstract: Tertiary Facies and Paleoenvironments Along the Coast Range and Western Cascade Margin, Western Oregon

Cynthia S. Shroba, William N. Orr

A range of early to medial Tertiary sedimentary and volcanic deposits from west-central Oregon record the early evolution of the Cascade margin. Cessation of marine deposition in the southern Willamette Basin by early Oligocene time was followed by a transgressive-regressive cycle in the central basin by early to medial Oligocene time.

Late Eocene volcaniclastic sediments within the Yachats Basalt consist of fossiliferous granule sandstones to boulder conglomerates interbedded with pillow basalts and crosscut by basaltic dikes. The source of sediment for these deposits is interpreted to be local, submarine, and not associated with the formation continental arc that was to become the Western Cascades. Fossiliferous Eocene-Oligocene volcaniclastic siltstones, sandstones and newly described, intensely fossiliferous upper conglomerates of the Eugene Formation represent shelf edge to inner shelf facies recording a volcanic pulse from the Western Cascades continental arc near the Eocene-Oligocene transition. Basaltic sandstones, siltstones and barnacle limestones of the Scotts Mills Formation represent an Oligocene transg essive sequence which followed the deposition of the uppermost Eugene Formation sediments.

These three examples exemplify the range of environments and conditions prevalent along the Cascade margin from the onset of volcanism in the Eocene, through pulses of volcanism and apparent regression near the Eocene-Oligocene boundary, to periods of relative volcanic quiescence and deposition of transgressive shoreline facies. Continental arc volcanism during the early to medial Tertiary resulted in high sediment yield and steep depositional gradients. Such conditions were conducive to rapid burial and preservation of generally underrepresented litho- and biofacies, including intensely fossiliferous proximal fan basaltic conglomerates present the Coast Range exposures, and rocky shoreline facies with hard substrate paleocommunities present in exposures along the eastern margin of th Willamette Basin.

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