Virgilian strata in north-central Texas consist of repetitive sedimentary sequences called cyclothems. These cyclothems were caused by waxing and waning of glaciers in the southern hemisphere (Gondwana). During the transgressive phase, deltas were drowned, forming estuaries, and the oxygen-minimum zone responsible for the black shales in the Midland basin rose to a position well up on the shelf area, resulting in a sequence of lithofacies and corresponding biofacies which indicate a water column with stratified dissolved oxygen content. During early regression, the oxygen-minimum zone and the characteristic lithofacies and biofacies began retreating toward the Midland basin. Deltas began prograding well out onto the outer shelf, and the resulting lithofacies and biofacies depended upon the relationship between paleogeography and the prograding deltas. During maximum regression, the oxygen minimum zone was restricted to the Midland basin, and the shelf-edge carbonates were subaerially exposed, resulting in extensive freshwater diagenesis, which was responsible for enhancing secondary porosity.

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