Carbonate - Siliciclastic Stacking Patterns in the Lower Permian Lueders Limestone, Texas: Relative Sea Level and Climate Controls on Depositional Systems

The Lower Permian Lueders Limestone is the uppermost unit of the Albany Group and directly underlies the Clear Fork Group on the Eastern Shelf of the Midland Basin. The Lueders appears to mark the maximum marine onlap of the Upper Wolfcampian – Leonardian supersequence on the Eastern Shelf and marks the end of a significant period of transition from the Pennsylvanian – Early Permian Icehouse to the latest Paleozoic “greenhouse”.

The Lueders is bound above and below by relatively thick shales with variably developed paleosols and thin sandstones in the inner platform facies tract of its type region. The Lueders itself is organized into four parasequence sets (PSS) defined by the stacking of bedset “couplets” of thinning and shallowing upward limestones with corresponding thickening upward shales. The lowest PSS represents the transgressive systems tract while the remaining PSS's compose the highstand of the Lueders sequence. The facies stacking patterns within and among the PSS's clearly indicate relative changes in sea – level and coastal onlap, from fully marine foram – mollusk grainstones (FMG) to coastal plain bays, lakes, and low-net fluvial systems.

The limestone – shale couplets do not appear to record base level changes, but instead record fluctuations in precipitation resulting in “wet-dry” cycles that are modulated by long term base level changes. Thus, the deeper portion of the PSS is dominated by bioturbated FMG (representing dry, normal marine conditions) overlain by thin, laminated carbonaceous shales with low Th/U ratios (representing wet, brackish conditions distal to detrital input). The shallow portion of the PSS contains thin, peloid – ostracode mudstones to packstones (representing dry phase bays and ponds) punctuating thick siliciclastic mudstones with high Th/U, conchostracans, and plant remains (representing wet phase coastal plain facies). Recognizing the interaction of climate and base level change is critical for the proper interpretation of these depositional systems.