Paleocurrent Analysis of Cretaceous "Mitchell Formation," North-Central Oregon

Craig A. Sandefur, Lanny H. Fisk

Cretaceous sedimentary rocks of north-central Oregon, best exposed in the Mitchell Inlier and previously referred to the Hudspeth and Gable Creek formations, are potential petroleum source and reservoir rocks. Thus, determining their extent under the cover of Tertiary volcanics is of great importance to future petroleum exploration in the southern half of the Columbia basin. The direction of sediment transport has been previously studied by several workers but, unfortunately, with contradicting results and conclusions. Our study was undertaken to resolve the differences in interpretation. The outcropping sedimentary sequence represents part of a subsea fan complex consisting of fan-apron facies turbidites and mudstones ("Hudspeth mudstone facies") and channel-facies congl merates and sandstones ("Gable Creek conglomerate facies"). Paleocurrent data derived from both macrofabric (sole marks, flute casts, clast imbrication, and alignment of elongate plant fragments) and microfabric (sand-grain orientation and imbrication) document that sediment transport was primarily from the south-southeast toward the north-northwest. Thus, the greatest potential for petroleum production from Cretaceous sediments in north-central Oregon lies to the west and northwest of the Mitchell area in northeast-southwest elongated rift basins containing thick accumulations of presumably Cretaceous marine sediment fill, and now evidenced by large gravity and magnetic lows.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.