PALEOECOLOGY AND BIOTIC CONTROLS OF MIXED CARBONATE-SILICICLASTIC SEDIMENTS: A CASE STUDY FROM THE ECHINOID DOMINATED MIOCENE BUTTON BEDS OF CENTRAL CALIFORNIA

Investigating the paleoecology of mixed carbonate-siliciclastic sediments can reveal how specific organisms and their skeletal remains determine lithological characteristics of sedimentary environments by influencing grain size distributions, depositional fabrics and diagenetic processes. Such studies are aided by detailed knowledge not only of the ecology of living populations of the dominating organisms, but also of the taphonomic pathways of their skeletal remains.

The Miocene Button Bed Sandstone Member of the Temblor Formation (Central Valley, California) is well known for mass accumulations of clypeasteroid echinoids. These beds have already been studied in detailed with respect to stratigraphy, generally sedimentary environments and echinoid fabrics. In this study, there is special emphasis on: 1) the interpretation of the sand dollar populations based on actualistic comparisons, 2) detailed analyses of the specific skeletal architectures and resulting taphonomic pathways, 3) field measurements of size frequency and directional data, and 4) the influence of these sand dollars and their remains on the sedimentary environments as observed in the field and in numerous thin sections.

Various admixtures of siliciclastics and authigenic minerals are present together with skeletal remains dominated by echinoids accompanied by pectinids, oysters, barnacles and bryozoans. Preservation states of sand dollars range from well preserved complete specimens to highly fragmented test remains. Various packing fabrics and orientations are present. Of special importance are syntaxial cements in skeletal grainstones resulting from the diagenesis of highly fragmented echinoid remains. Finally, comparisons with respect to clast composition and general sedimentation patterns are made to further sand dollar mass accumulations and other echinoderm dominated sediments in the rock record.