Carbonate sandstone bodies, attractive petroleum reservoirs, are present in significant concentrations adjacent to the shelf edge, at the shoreline, and associated with reef development. On the basis of the writer's studies of the Lower Cretaceous carbonate sequences in Texas, this paper outlines conditions whereby significant deposits of carbonate sand can be concentrated in the shallow-shelf environment, an area usually characterized by the deposition of fine-grained carbonate mud and silt.

Three conditions must be met before carbonate sands can be deposited: (1) grains must be available; (2) high energy must be present to winnow the "fines" and concentrate the coarser grains; and (3) high-energy conditions must persist long enough to concentrate the sands. The first condition is easily met because of abundant fossil or biogenic material.

The shallow shelf generally is a low-energy environment, therefore, the problem is to find a situation where high-energy conditions persist long enough to concentrate significant deposits of coarse-grained material.

The most obvious high-energy situation occurs during a marine transgression across the shelf, but Texas Lower Cretaceous sequences indicate that transgressive deposits generally are deeper water, muddy sediments.

In a regressive situation the high-energy zone shifts in front of the zone of concentration toward the shelf edge, allowing previously deposited sands to be preserved. The potential for widespread carbonate sand in the regressive phase is borne out by the presence of widespread, blanket, carbonate sandstones in Lower Cretaceous regressive sequences.

Structural elements within the shelf tend to influence bottom topography for a long time, and positive features become the site of relatively high energy and of the deposition of carbonate sand bodies surrounded by fine-grained carbonate sediments. Structural control of carbonate-sand deposition is illustrated by Lower Cretaceous Fredericksburg linear carbonate sandstone bodies coincident with the San Marcos and Concho arches in Central and West Texas.

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