The evidence from sedimentary structures and the stratigraphic and biostratinomic context of shell layers are used here to describe how shell beds are formed within a sedimentary sand wave. Shell beds in the Pliocene San Nicolás Formation, within the Baja California Central Domain, were accumulated through the physical processes of gravity induced flow currents. The hydraulic regime and sedimentary differences between the up-current slope and the leeward slope part of the sand wave can be explained by flow stratification. The taphonomic differences between the fossil concentrations of the bottom and the upper layers in the leeward slope are a result of a change in the current dynamics for the basin. The leeward slope side of the sand wave represents episodic, short-term, dynamic bypassing in gravity-induced, low-flow current beds creating shell beds.
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Vol. 27 • No. 3