About This Item
Share This Item
The quantitative distribution of Miocene benthic foraminifers within the Cuyama basin, central California, demonstrates the relationship between biofacies, key species, and specific environmental factors. During the Miocene, the Cuyama basin occupied an inboard position along an active, convergent to translational, continental margin resembling the modern continental borderland off southern California. Benthic foraminiferal assemblages delineate shelf, slope, and basin plain biofacies. Migration and replacement of these biofacies with time reflect the depositional subsidence history of this Miocene basin. Initially, biofacies are broad and less structured, reflecting the influx of cosmopolitan species during early basin development. Recognizable biofacies are established quickly after the initial marine transgression and basin subsidence. As the basin fills, the number of biofacies decreases and deeper biofacies are excluded, whereas low oxygen and shelf biofacies expand. Bathymetrically displaced species are common, implying downslope transport by turbidity currents, increased sediment input, and/or tectonic activity.
Benthic foraminiferal species diagnostic of the standard California Miocene stages and zones occur commonly throughout the Cuyama basin. Among the key biostratigraphic events commonly cited for the early and middle Miocene are the "Valv. cal. flood" zone (middle Luisian) and the evolutionary succession of valvulinerids and siphogenerinids. Although these events are important stratigraphic markers, some difficulty is encountered in recognizing certain zones and chronostratigraphic sequences are boundaries as presently defined. The bathymetric distribution and biofacies associations for certain key species critically impact on the usefulness of these species for biostratigraphy. Siphogenerinids appear only in slope, lower slope, and basin plain assemblages, and individual species are re tricted to specific parts of these areas. Stratigraphic and evolutionary events based on these species are therefore limited to deeper water environments. Valvulinerids are present in shelf-edge environments as in-situ members of assemblages and as transported specimens in deeper environments. The Valv. cal. flood is obscured in sections dominated by shelf-edge environments where valvulinerids occur in large numbers throughout the middle Miocene, and is confused in lower slope and basin plain assemblages where they are concentrated as transported specimens.
Early and middle Miocene fauna distributions are complex. Sedimentary, tectonic, and oceanographic conditions strongly effect in-situ and transported occurrences of key species. These factors must be evaluated for individual basins if benthic foraminiferal zonations are to have regional applicability.
End_of_Article - Last_Page 499------------