ABSTRACT: Biomarker Variations in Relation to Paleogeography in the Saltos Shale Member of the Monterey Formation, Cuyama Basin, California
Paul G. Lillis
The Miocene stratigraphy of the Cuyama basin provides an excellent opportunity to correlate geology with molecular organic geochemistry (e.g., biomarker compounds) because the paleogeography and paleobathymetry of the basin are well constrained by surface and subsurface geologic mapping and detailed micropaleontology. The Monterey Formation is composed of biogenous and terrigenous sediments that accumulated in a deep marine borderland basin adjacent to a Miocene shoreline. The lower member, the Saltos Shale, is predominantly terrigenous sediment interbedded with impure carbonates that consist of foraminifera and calcareous nannofossils. The upper member, the Whiterock Bluff Shale, is composed of highly biogenous (siliceous and carbonate) sediments. The biomarker compositi n of the Saltos Shale is dependent on the relative contributions of planktonic and benthonic organisms, bacteria, and terrigenous organic matter transported from the nearby landmass.
A general trend in biomarker distribution is observed in relation to paleogeography (i.e., proximity to shoreline). Pristane/phytane ratios, hopane/sterane ratios, oleanane/hopane ratios, and diasterane/sterane ratios are higher near the shoreline (to the east) because of increased terrigenous input. The more distal western basin sediments contain biomarkers that were predominantly derived from marine phytoplankton and bacteria. Submarine fan sediments in the Saltos Shale were deposited in the eastern basin east of the penecontemporaneous Cox fault.
Ponding of terrigenous organic matter at the base of slope is reflected by high ratios of pristane/phytane (2.7), oleanane/hopane (0.79), C29/C27 ^agr^agr^agr2OR steranes (1.56), and relatively large amounts of waxy n-alkanes (C27, C29, C31). In contrast, the prefan and postfan sediments contain lower concentrations of these terrigenous biomarkers.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990