High–Resolution Correlation and Palynofacies of Inner Platform Carbonate–Mudrock Cycles of the Lower Permian Lueders Limestone, TX: Implications for High–Frequency Climate Change on the Eastern Shelf, Midland Basin
The Lower Permian Lueders Limestone represents the maximum marine onlap of the Upper Wolfcampian – Leonardian supersequence on the Eastern Shelf of the Midland Basin and marks the full development of greenhouse conditions during the mid – Lower Permian non – glacial period. Meter scale, carbonate – mudrock couplets are the fundamental stratal motif of the Lueders in shallow platform locations of north-central Texas. Differential stacking of these bedset cycles defines parasequence sets, systems tracts, and high – frequency (HF) sequences of the Lueders; however the correlability of individual cycles has not been tested. Delineating the spatial scale of these cycles has important implications for interpreting the processes driving this cyclicity. These inner platform cycles are dominated by thin to thick – bedded limestones with thin to very thin – bedded fissile to platy mudstone interbeds. Limestones are mollusk and foram – dominated grainstones and packstones with pervasive bioturbation. Palynofacies of the limestones are characterized by bisaccate pollen and either dominated by structured (woody) kerogen or mixed with amorphous kerogen of marine origin. Mudstones are carbonaceous and contain a generally sparse fauna of vertebrate material and ostracodes along with fenestrates and microconchids. Palynofacies of the mudstones are characterized by bisaccate pollen and often dominated by amorphous marine kerogen. High – resolution correlation of cycles within the Middle Lueders HF sequence indicate individual beds and bedsets are traceable for at least 15 km across the outcrop. Thus, the cycles do not appear to represent local (<1 – 3km), autocyclic phenomena but instead point to allocyclic controls acting over large areas of the inner platform. Palynofacies indicate enhanced production or preservation of marine algal material in the mudstones compared to the limestones. High productivity – high preservation potential of amorphous algal material points to nutrient loading and the development of a benthic oxygen minimum zone during mudstone deposition. It is here proposed that these cycles represent wet – dry climate cycles in which nutrient – rich wet phase runoff promotes estuarine circulation resulting in bottom water oxygen stress, shutting down carbonate production. Reduced dry phase runoff causes a breakdown of this estuarine circulation system, returning carbonate deposition to the platform.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014