HIGH-RESOLUTION PALEOENVIRONMENTAL ANALYSIS OF A PLIO-PLEISTOCENE MARINE SUCCESSION IN CALIFORNIA

The Rincon Hill section near Carpinteria, California preserves a fault-bounded block of Plio-Pleistocene sediments, which have been correlated with the Santa Barbara Formation of western Ventura-Santa Barbara Basin. This sedimentary package is well known for its ease of collecting and high diversity, with more than 210 molluscan species reported in the literature, and at least 180 species obtained through exhaustive bulk sampling. This study documents the results of high resolution (0.5 m stratigraphic intervals) faunal inventorying through the lower 90 m of the section. The objective was to do document fine-scale stratigraphic variations in faunal composition, sediment type, and taphonomic properties, such as packing density, fragmentation, and encrustation/bioerosion.

Faunal lists were collected from nearly 140 horizons, and this approach documented stratigraphic variations in the relative abundance of 105 species. It was observed that invertebrate fossils were most diverse in relatively few horizons that were characterized by densely packed, disarticulated and fragmented bioclasts in often sandy and conglomeratic sediments. Nevertheless throughout the lower 90 m of section most siltstones and mudstones contain some sparsely- or loosely-packed fossils. A basal contact with the Sisquoc Formation is marked by channelized, redeposited coarse terrigenous sediments and bioclasts. The overlying 90 m of section contain three sedimentary sequences, each one bounded by a basal coquina, overlain by mudstone, and grading upwards into siltsone. These subtle lithofacies changes appear to correlate with shifts in biofacies composition.

The rich data obtained here have been analyzed using Detrended Correspondence Analysis (DCA), Non-Metric Multidimensional Scaling (NMDS) and Cluster Analysis. The cluster analysis identified four faunal associations, which were dominated by Lirobittium sp. (undetermined), Nutricola tantilla, Cyclocardia occidentalis, and Chlamys opuntia, respectively. DCA and NMDS both identified strong faunal gradients within the data, which are associated with paleobathymetry and sediment composition. When plotted against stratigraphic position, variations in DCA values appear to depict temporal fluctuations in paleobathymetry and sediment flux at the Rincon Hill site.