Late Pennsylvanian and Early Permian Black Shale-Limestone Bed-Set Couplets of the Eastern Shelf, Midland Basin (Texas): Climate-Driven Redox Cycles of the Inner Platform Realm

This study describes exposures of Upper Pennsylvanian (Kasimovian) and Lower Permian (Artinskian) inner platform limestone - dominated sequences of the Eastern Shelf of the Midland Basin. These units display black shale - limestone couplets that are interpreted as fluctuations in benthic redox conditions driven by climate - controlled changes in platform circulation.

The Kasimovian lower Winchell Limestone at the US180 outcrop is bounded above and below by gray silty shales and is dominated by phylloid algal wackestone to packstone in the lower 5m, overlain by 2.5m of skeletal packstone to grainstone. Thin, discontinuous (2-5cm) gray to black shale bands subdivide the limestones into 20 - 50cm thick beds. Fenestrate bryozoans and brachiopods are not uncommon in the shale bands. Spectral gamma ray signatures display K and Th minima in the middle of the limestone succession. However, U displays strong peaks associated with the black shale bands within the limestone succession.

The Artinskian lower and middle Lueders Limestone sequences in the type region are dominated by foram - mollusk packstones and grainstones beds 20-80cm thick separated by continuous, 2-10cm thick gray and black shale bands with sparse to common fauna including vertebrates, ostracodes, microconchids, bivalves, and rare bryozoans. Spectral gamma ray profiles are similar to the Winchell in that K and Th minima in the best developed limestones are associated with broad U highs with distinct peaks from the black shale bands.

The U - enriched black shale bands observed within the clean limestones of the Winchell and Lueders imply periodic changes to benthic redox conditions during the carbonate - dominated highstands on the Eastern Shelf. The proposed mechanism involves wet - dry climate cycles in which increased runoff during wet phases produce a freshwater surface plume, promoting a halocline across the inner platform. Associated increased terrestrial nutrient load promotes higher planktic productivity, decreasing the euphotic zone and increasing rain of organic debris to the benthic zone where decomposition produces oxygen stress. This effectively shuts down carbonate production. During dry phases, the surface plume decreases, the halocline breaks down, and nutrient loads decrease, allowing the return of benthic carbonate production. This type of lithologic cyclicity can impart significant vertical heterogeneity to carbonate reservoirs as seen in the Clear Fork of the Permian Basin.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California