Depositional environments in the Monterey Formation, Cuyama Basin, California
The Monterey Formation of the Cuyama Basin accumulated in an inboard basin within a Miocene borderland analogous to the Recent continental borderland off southern California. Analysis of benthic foraminiferal biofacies, lithology, and sedimentary structures permits recognition of several depositional environments within the Monterey Formation in this; basin. The lower member of the Monterey Formation, the Saltos Shale (early Miocene), contains basin-plain, slope, and various submarine-fan subenvironments. These predominantly terrigenous rocks are interbedded with impure carbonates composed primarily of foraminifera and calcareous nannofossils. The upper member, the Whiterock Bluff Shale (middle Miocene), contains highly biogenous deposits representing slope and basin-plain environments. Both siliceous and calcareous biogenous rocks are prominent in this unit. The complex: array of lithofacies within the Monterey Formation is the product of the interplay between terrigenous and biogenous sedimentation. Various factors control this interplay, including global sea level, climate, paleo-oceanographic changes, local tectonics, and paleogeography. The Monterey Formation in the Cuyama Basin is more terrigenous than coeval sections in more outboard basins of the Miocene borderland (as represented by the Santa Barbara Basin). These latter areas contain many lithofacies indicative of low terrigenous sedimentation rates: highly siliceous and calcareous rocks, phosphatic rocks, and highly organic rocks. The inboard Cuyama Basin acted as a sediment trap during much of the Miocene and is partly responsible for these lithofacies patterns. Both inboard and outboard basins contain variations in bulk-accumulation rates that correlate to variations in relative sea level. This study demonstrates the importance of depositional patterns, paleogeography, and multidisciplinary analysis for understanding the complex stratigraphic units found along active continental margins.