EVOLVING LOWER PERMIAN SEQUENCE ARCHITECTURES OF THE EASTERN SHELF, MIDLAND BASIN: DEMISE OF THE LATE PALEOZOIC ICEHOUSE AND IMPACT ON PALEOEQUATORIAL DEPOSITIONAL SYSTEMS

The earliest Permian (Asselian – Sakmarian) was the acme of the Late Paleozoic Icehouse, characterized by extensive glacial deposits across most of the Gondwanan sub-continents. Facies analysis and isotope data demonstrate this icehouse interval gave way to an essentially ice-free Gondwana by the middle portion of the Early Permian (Artinskian). It is reasonable to expect that the demise of Gondwanan ice centers would generate significant changes in the frequency and magnitude of eustasy. Thus, the expectation is that far – field Lower Permian depositional systems should reflect this fundamental change in global climate by evolving from icehouse – style depositional sequences to more greenhouse – style sequence architectures. This study examined the Lower Permian succession exposed on the paleoequatorial Eastern Shelf of the Midland Basin to assess the response of depositional and sequence stratigraphic systems to this global climate transition.

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.