A 35-50-ft sequence of fine-grained middle Glen Rose carbonates, present in an extensive outcrop area (± 5,000 sq mi) south of the Llano uplift, contains a variety of sedimentary features resulting from relatively mild dynamic forces acting on a broad, low-relief shelf.

This distinctive rock sequence includes the following deposits in ascending order: (1) stromatolitic and rippled beds of probable intertidal origin, (2) very fossiliferous burrowed calcareous mudstone (Salenia texana beds), (3) an iron-stained Corbula martinae bed, and (4) collapse breccias resulting from vadose solution of two gypsum beds. Widespread sedimentary features confined to "key beds" include oscillation ripple marks, asymmetric current ripple marks, stromatolites, and pholad borings. Mudcracks and dinosaur tracks occur locally along diastems.

Current action was most intense on the San Marcos platform, a promontory extending from the Llano uplift, as indicated by thinner beds, the absence of one and perhaps both gypsum beds, and large asymmetric ripple marks on the Corbula bed. Southwesterly, the Corbula bed thickens and grades from shell grainstone to calcareous mudstone with indigenous Corbula. Within the basal beds, a consistent northwest-southwest alignment of oscillation ripple marks, present along 100 mi of outcrop, is a probable result of wind disturbance of very shallow water. The dominant currents flowed southwestward, as implied by the areal configuration of a sandstone-shale lens within the calcareous mudstone interval, and may have been driven by prevailing northeast winds.

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