Taphonomic gradients in shelf fossil assemblages; Pliocene Purisima Formation, California

Shell and bone beds in the Pliocene Purisima Formation of central California accumulated through dominantlyphysicalprocesses of concentration in shallow-marine facies and by ecological mechanisms in deep-shelf lithofacies. This trend reflects onshoreoffshore variations in rate of sedimentation, frequency of current reworking, and preservation of bioturbation. The shift from physical to ecological processes tracks a change from high frequency of reworking by currents in shallow water to lower rates ofphysical reworking below storm-wave base. Storm and currentgenerated shell beds decrease in abundance offshore due to decreased current strength andfrequency and to increased bioturbation. Shell deposits in deep shelf settings result from sediment starvation and such ecological processes as aggregation of shelled molluscs, encrustation of pre-existing shell lags, and repeated pulses of larval settlement. Nearshore shell beds preserve considerable information on current direction, types of bottom flows, duration of exposure of shells on the bottom, and flow regime, but little data on community trophic structure or species diversity. Offshore shell deposits, in contrast, preserve community associations and encruster-host relationships (which may be destroyed in the high-energy, nearshore region) as well as information regarding rate of sedimentation and early diagenetic conditions.

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