The Rio Dell Formation: a Plio-Pleistocene basin slope deposit in Northern California |
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| Authors: | DAVID J W PIPER WILLIAM R NORMARK JAMES C INGLE JR |
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| Institution: | Departments of Geology and Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada, Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minn. 55455 and Department of Geology, School of Earth Science, Stanford University, Stanford, California 94305, U.S.A. |
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| Abstract: | The Rio Dell Formation (Pleistocene and Pliocene), exposed south of Eureka, California, is a prograded sequence of basinal turbidites overlain by basin slope and shelf deposits. The slope deposits studied in the Centerville Beach section accumulated in a steadily shallowing environment delineated by analysis of palaeobathymetrically significant benthonic foraminiferal biofacies in turn suggesting deposition at depths of 1000–100 m. Lower slope deposits interfinger with basinal turbidites derived from the Eel River delta to the north. Slumped blocks of silty mudstone, and associated silt and mud beds, are common. The middle slope deposits are mudstones; coarser sediments bypassed this zone. Mudstones and muddy siltstones alternate on the upper slope. Shallow depressions, probably slump scars, that have been rapidly filled by upper slope sediment are common. The transition to shelf deposits is marked by an increase in sediment grain size, in the degree of oxidation, and in the abundance of megafossils. High percentages of benthonic foraminifera displaced from shelf depths indicate that resedimentation processes are most important on the upper slope. |
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