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Outer Shelf Storm Deposits of Upper Cretaceous Chico Formation, California

John S. Russell

The Kingsley Cave member of the Upper Cretaceous Chico Formation, northeastern Sacramento Valley, California, is an outer shelf sequence consisting of muddy siltstones deposited below storm wave base. Silty fine-grained sandstone interbeds, interpreted as distal tempestites, are common within this member and are typically parallel laminated and devoid of shell material. Fossil concentrations within this member are rare, occurring as internally simple lenses within fine-grained sandstone interbeds. These fossil concentrations are up to 1.0 m in thickness. The faunas within these lenses are allochthonous or, less commonly, parautochthonous. Comparisons of the faunas in these shell lenses with published faunal assemblages for the Chico Formation and other Upper Cretaceous we t coast nearshore deposits indicate that they were displaced from shallower-water environments. Lenses typically contain faunal elements of shoreface and inner shelf environments, which were located to the east during this time, as well as outer shelf faunal elements.

These fossil concentrations are sedimentologic in origin. The scoured bases and overlying hummocky cross-stratified to ripple laminated siltstones of these lenses, their relative thicknesses, and the allochthonous shallower-water faunal elements contained within them are all characteristics of proximal tempestites, normally found within inner shelf sandstones. Distal tempestites, typical of outer shelf facies, are the norm for the Kingsley Cave member, while these proximal tempestites are the exception. The former were deposited by "normal"-strength storm events, while the latter are indicative of rare, extremely high-intensity storm events. During these extremely high-intensity storms proximality trends shifted seaward (westward), causing proximal tempestites to form more offshore th n the proximal tempestites generated by normal-strength storms.

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