Cycle Symmetry and its Causes, Cisco Group (Virgilian and Wolfcampian), Texas

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ABSTRACT 181 transgressive-regressive cycles composed of nonmarine and marine carbonate and siliciclastic rocks of the Cisco Group on the Eastern Shelf, Texas, display complex characteristics at both hemicycle and full-cycle scales. They are delineated on the basis of successive changes of depositional environments, stratal boundary relations, and stratigraphic position. Transgressive and regressive stratigraphic environment gradients are defined as the magnitude of environmental shift divided by thickness for each hemicycle. They indicate the rates of lateral environmental shifts during transgression and regression. Cycle symmetry index is defined as the ratio between transgressive and regressive stratigraphic environment gradients. It provides a measure of stratigraphic response to controlli g processes. Most Cisco cycles are asymmetrical with steep transgressive and shallow regressive stratigraphic environment gradients, indicating cyclic but asymmetrical variations of major allogenic processes over a cycle interval. These systematic variations are delineated from climate-sensitive rock types, stratal thickness, and sedimentary features. The relative dominance of processes controlling accommodation (i.e., glacio-eustasy and basement subsidence) versus those controlling sedimentation (i.e., climate, upland sediment yield, and sediment supply at the depositional site) determines cycle symmetry. A formulated relation between cycle symmetry index (i.e., stratigraphic response) and rates of these processes demonstrates that transgressive sedimentation is controlled by sediment supply, whe eas regressive sedimentation is controlled by accommodation. This relation is potentially useful in quantitatively predicting stratigraphic responses to specific allogenic processes in cyclic sedimentation. Five Cisco cycle types defined by the type of component lithofacies display the stratigraphic response mainly to noncyclic allogenic and autogenic processes. The cycle types have varying magnitude, thickness , and symmetry. They also vary in lateral extent and in abundance. A process-response model of cyclic sedimentation of the Cisco Group on the Eastern Shelf is established. It emphasizes the interplay between autogenic and allogenic processes at the sub-cycle scale. Understanding interactions among glacio-eustasy, climate, shelf subsidence, sediment supply, and depositional dynamics during various stages of transgression and regression is central to a clearer comprehension of the observed variations in cycle characteristics.