Sedimentary Structures and Depositional Environments of Diatomaceous Strata of Sisquoc Formation, Santa Maria Area, California

Pedro C. Ramirez

The upper Miocene to lower Pliocene Sisquoc Formation in the Santa Maria area includes laminated to massive diatomites and diatomaceous mudstones. Sedimentary structures indicate that deposition probably occurred under both anaerobic and aerobic conditions in a slope environment periodically subjected to gentle bottom current action. Abundant small-scale slump structures, angular discontinuities, and fault-graded beds suggest deposition in a slope environment. Microscours, preferred alignment of microfossils, thin silt laminae, and concentrations of micro-fossil fragments indicate gentle bottom currents. Thin carbonate beds, in places overlain by phosphatic pebbles and bone fragments, probably formed during episodes of strong current action or nondeposition. Alternating m ssive and laminated intervals indicate fluctuating oxygen levels in the bottom water impinging on the slope. Larger bodied trace fossils such as Thallassinoides probably formed during intervals of high-oxygen levels, but the smaller Chondrites formed during low-oxygen levels. Calculated sedimentation rates for the Sisquoc, which are relatively high (200-500 m/m.y.), probably reflect the introduction of a large amount of terrigenous debris onto the slope environment following a lowstand of sea level and/or increased tectonism.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.