MULTIVARIATE ANALYSIS OF MOLLUSCAN BIOFACIES OF THE PLIOCENE ETCHEGOIN GROUP, SAN JOAQUIN BASIN, CENTRAL CALIFORNIA

Binary (presence-absence) molluscan faunal data from 484 locality collections from the Pliocene Etchegoin group of the San Joaquin Basin (SJB), central California, were analyzed by multivariate statistical methods in 15 stratigraphic sample intervals representing 12-81 locality collections. Detrended Correspondence Analysis (DCA) of the 15 sample intervals was utilized to determine environmental gradients controlling the spatial and temporal distributions of mollusc species and the DCA scores analyzed by Q-mode cluster analysis of unweighted pair-group averaging of Euclidean distance and ordination by Non-Metric Dimensional Scaling (NMDS) to identify biofacies. At the temporal scale of the 15 stratigraphic sample intervals, the primary environmental gradients determined from DCA as controlling distribution of the Etchegoin group molluscan fauna were substrate, distributed along the DCA axis 1 (DC1), and paleowater depth, distributed along DCA axis 2 (DC2). Substrate categories ordered along DC1 range from mud to hard substrates, depending on the stratigraphic sample interval, although the largest substrate category consistently centered on sandy bottoms. Paleowater depths determined in this study from DC2 scores range from intertidal to ~25 m at maximum eustatic basin flooding and are generally shallower than those interpreted for the region in previous studies. Cluster analysis and NMDS resolved six bivalve-dominated and one gastropod-dominated biofacies averaging 2-6 species each primarily ranked by paleowater-depth settings associated with normal marine-adapted and brackish-tolerant to characteristically brackish-water species assemblages. Substrate-controlled spatial distributions of molluscs within these biofacies were patchy but consistent with modern coastal California nearshore-intertidal communities from comparable water depths and substrates. At eustatic highstands, habitat patchiness appeared to be at a minimum whereas at eustatic lowstands, and late in the history of the Pliocene SJB, a diverse distribution of shallow, brackish-water biofacies developed.