Abstract: A Late Cretaceous Molluscan-Microfossil Biochronology for the West Coast, and Its Application to Sequence Stratigraphic Analysis of the Great Valley Sequence

William P. Elder

A Late Cretaceous molluscan biochronology is developed using ammonites, inoceramid bivalves, and selected other bivalves and gastropods and is tied to the polarity-chronology and geochronology of Obradovich (1993) and integrated with published foraminiferal, coccolith, and radiolarian biostratigraphies. Higher placement of the Campanian-Maastrichtian boundary in this framework indicates that that boundary lies above the Didymoceras hornbyense Zone in the Pachydiscus catarinae Zone. In addition, the Submortoniceras chicoensis/Baculites chicoensis Zone represents more than 4 Myr, occupying essentially the entire lower Campanian and extending into the middle Campanian.

When applied to the Late Cretaceous part of the Great Valley sequence (GVS), this biochronology allows enhanced regional correlation and assessment of the stratigraphic effects of eustatic sea level. The following coeval fine-grained stratigraphic intervals of the GVS can be found in both the Sacramento and San Joaquin Valleys and appear to correspond to transgressive to highstand intervals: upper Turonian Yolo Formation/Upper Waltham/Ciervo shales, upper Santonian Dobbins/Lower Marlife shales, middle Campanian Sacramento/Upper Marlife/ Alcalde shales, upper Campanian Sawtooth/Mustang/Ragged Valley shales. This correspondence suggests eustatic control on some periods of fine-grained deposition in the GVS.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California