Sedimentologic and taphonomic implications of biogenic stratification

Biogenic stratification, a distinctive type of biogenic bedding, dominates the stratigraphy of the inner tidal flats at Cholla Bay, northern Gulf of California. Biogenic stratification is produced by infaunal, deposit-feeding callianassid shrimp and polychaetes through vertical size sorting of the sediment during feeding and burrow excavation. This bedding is distinct from biogenic graded bedding in that the transition from the lower coarse-grained interval to the upper fine-grained interval is sharp, not graded. Biogenic stratification results from high rates of biogenic sediment reworking occurring with very low rates of net background sedimentation. Fine particles (<1-2 mm) undergo continuous vertical "conveyor belt" recycling, while coarse particles (>1-2 mm) are transported downward and concentrated at depth. Over much of the inner tidal flats, biogenic sediment reworking modifies sequences initially formed by storms or tidal-channel migration, replacing them with biogenic stratification. Biogenic stratification has important paleoecologic implications. Radiocarbon dates on tidal-flat shells indicate that shells and sediment undergo extensive vertical mixing, probably due to the combined effects of storms and biogenic reworking. Tracer-shell studies, and published literature on reworking rates, suggest that biogenic reworking "injects" shells to considerable depths (at rates far exceeding the net sedimentation rate), where they accumulate as subsurface condensed shell beds. In areas unaffected by biogenic reworking, sediment bypassing causes shells to remain on or near the surface for long periods of time, resulting in surficial faunal condensation.