EVOLVING LOWER PERMIAN SEQUENCE ARCHITECTURES OF THE EASTERN SHELF, MIDLAND BASIN: DEMISE OF THE LATE PALEOZOIC ICEHOUSE AND IMPACT ON PALEOEQUATORIAL DEPOSITIONAL SYSTEMS
The uppermost Pennsylvanian and Lower Permian (Asselian – lower Artinskian) upper Cisco and lower Albany groups are characterized by sequences with abrupt vertical facies transitions, thin (<4m) but well – developed open marine carbonates at maximum transgression, and well – developed lowstand paleosols and incised valley fills. These imply high – amplitude/high – frequency eustasy, resulting in sequences typical of icehouse climates. The middle and upper Albany Group and overlying lower Clear Fork Group (middle Artinskian – Kungurian) are characterized by stepped vertical facies transitions, thick (>10m) packages of marginal to open marine carbonates, well - developed meter - scale cyclicity, and poorly - developed lowstand lithofacies packages. These thick carbonate – dominated sequences imply a significant shift to low amplitude/high frequency eustasy superimposed on high amplitude/low frequency relative sea – level change characteristic of ice – free, greenhouse global climates. Although significant uncertainty remains in the dating and correlation of the glacial deposits of Gondwana and the sequences of the Midland Basin, this distinct change in stratal motif appears to coincide with the demise of the Late Paleozoic Icehouse and demonstrates the intimate linkage of depositional and climate systems during the Pennsylvanian and Permian.