HOPPIE, BRYCE W., and ROBERT E. GARRISON, Earth Science Board, University of California, Santa Cruz, CA

ABSTRACT: Sequence Stratigraphy of the Middle and Late Miocene Cuyama Basin

We present a sequence stratigraphic model for the Miocene Monterey (Whiterock Bluff Shale member) and Santa Maragarita Formations of the Cuyama depositional basin. These formations are products of poorly understood shelfal sedimentary processes influenced by coastal upwelling within a proximal borderland

basin. Our model represents a first step in defining these sedimentary processes by identifying the temporal and spatial characteristics of the formations in sequence stratigraphic terms. The model integrates well log correlations with all available biostratigraphic, lithostratigraphic, and seismic data. It further synthesizes regional and local tectonics with terrestrial and marine sediment supply, eustasy, and paleobathymetry.

Preliminary results suggest the lower Whiterock Bluff consists of an extensive transgressive system tract (TST) and a highstand system tract (HST) overlain by a thin lowstand system tract (LST) containing tuffaceous beds. This section also contains several zones of dolomitic concretions of probable early diagenetic origin; as noted elsewhere in the California Neogene, the concretions occur within the TST at parasequence boundaries and represent periods of elevated relative sea level. A shoaling of the basin during deposition of the upper Whiterock Bluff resulted in biosiliceous sediments deposited in a LST and TST. The overlying Santa Margarita Formation contains three LST to TST to HST sequences. While LSTs contain storm-deposited shelf sands, TSTs and HSTs contain mudstones with pri tine phosphatic sediments formed under conditions similar to those of the concretionary dolomites.

Our sequence stratigraphic model reaffirms the previously interpreted paleodepositional conditions and provides greater detail on the temporal and spatial distribution of facies within these two formations. Future work will focus on study on early diagenetic phosphates and dolomites at sequence boundaries and on Milankovitch cycles.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.