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ABSTRACT: Lower Cretaceous Nannofossil Stratigraphy of the Great Valley Sequence

T. J. Bralower

The calcareous nannofossil stratigraphy of four sections in the Great Valley Sequence of the Sacramento Valley, California, has been investigated in detail. These sections include Grindstone and Stony Creeks (Glenn County) and McCarty Creek and the Vestal Road segment of Dry Creek (both in Tehama County). The ages of the sections investigated, which have independently been studied by ammonite, pelecypod, and radiolaria biostratigraphers, range from Berriasian to Albian. Nannofossils occur rarely in these sediments and are restricted largely to mudstone deposited in the most distal portions of turbiditic sequences. Approximately 5% of the samples collected are nannofossiliferous. Assemblages contain some elements of Boreal floras but are dominated by Tethyan taxa. These as emblages are more directly comparable to those of stratotype sequences in Europe than are the other fossil groups, but the ages given are generally similar. However, two major discrepancies do exist. The results of this study indicate that the pelecypod, Buchia aff. B. okensis zone is Berriasian in age, not Tithonian as previously believed and the B. pacifica zone is not entirely Valanginian, but partly Berriasian. Nannofossil data provide useful age calibrations for ammonite stratigraphy. The stratigraphy obtained supports structural interpretations based upon Buchia, in particular that the Paskenta fault zone remained active throughout the Early Cretaceous. The study illustrates that nannofossil biostratigraphy can be effectively used to date sediments deposited in convergent margin se tings.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990