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Inner Shelf Storm-Surge Deposits of Late Cretaceous Chico Formation, Northeastern Sacramento Valley, California

Sarah L. Baum, Rodney Watkins, John S. Russell

The early Campanian Tenmile Member of the type Chico Formation, northeastern Sacramento Valley, California, consists of 325 m of inner shelf sediments deposited by storm surge events. Storm deposits are represented by fine-grained sandstone beds 30-180 cm thick (mean = 120 cm) with a characteristic internal sequence of structures. Erosional bases are overlain by lensoidal shell lags which are in turn overlain by hummocky to swaley cross-stratified sand in the upper part of the beds, and bioturbation generally increases upward within a bed. In complete sequences, a unit of mudstone within wave-rippled sand laminae is present at the top of the bed. This sequence of structures corresponds to a change from high-energy to low-energy conditions during a single storm event. Comm n variations from the ideal sequence include a lack of basal shell lags, intensive bioturbation through most of the bed, or absence of a mudstone unit, resulting in amalgamation of beds.

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