Deepwater Siliciclastic Deposits and Anticlinal Growth at Grizzly Bluff, Humboldt County, California

Greg Gordon¹ and Todd Greene^2
1California State University Bakersfield, 9001 Stockdlae Hwy., Bakersfield, CA 93311
²CSU Chico, Dept. of Geological Sciences, Chico California 95929

Grizzly Bluff anticline in Humboldt County, coastal northern California, lies along the tectonically active southern margin of the onshore Eel River basin. Grizzly Bluff is home to a natural gas field, discovered in 1964, with ~25 well penetrations. Just south of the anticline, several of the stratigraphic units found at Grizzly Bluff come to outcrop (with northward dips). The older members of the Wildcat Group, including the Pullen, Eel River, and Rio Dell Formations, contain upper Miocene- to Pliocene-aged, deepwater siliciclastic deposits, ranging from turbidites to debris flows (debrites). An attempt to delineate differing deepwater deposits has been made on the basis of rock compositions (from outcrop samples and conventional core, where available) and grain sorting. By assembling a series of gross thickness isopach maps for sub-units of the aforementioned Wildcat Group members (specifically, the Rio Dell), it is possible to suggest an age for the onset of anticlinal growth at Grizzly Bluff (due to thinning/thickening depositional patterns). Currently, we have that age placed in middle-Rio Dell (upper Pliocene) time. The growth of this structure continued to influence siliciclastic deposition from that point into the Pleistocene.

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California