DECIPHERING GENETIC CONTROLS ON THE OCCURRENCE, DISTRIBUTION, AND CHARACTER OF FINE-GRAINED ROCKS (MUDSTONE/SHALE)-- AN INTEGRATED SEQUENCE-STRATIGRAPHIC ANALYSIS OF THE CENOMANIAN-TURONIAN OF NORTH AMERICA
Detailed examination of thin-section, core, and outcrop samples reveals these strata accumulated in several depositional environments with wide variations in primary biogenic productivity, bottom-energy and oxygen levels, and sediment accumulation modes and rates. Scour surfaces, as well as wave, current, and combined-flow ripples, graded beds, fossil and pyrite lags, and wave-enhanced sediment gravity flow beds record benthic energy levels commonly capable of transporting sand-sized grains. In-situ body and trace fossils indicate occasional and recurring periods of benthic oxygenation during a frequently dysoxic interval— benthic foraminifera as well as articulated epi-benthic and shallow-burrowing pelecypods (Pteriidae, Pectinidae, Inoceramidae); ichnogenera: ‘mantle-and-swirl’ traces, Planolites, Chondrites, Helminthopsis, Paleophycus, Phycosiphon, and sparse Zoophycus.
Lithofacies stack systematically to form parasequences whose attributes record the distal effects of shoreline progradation. Detailed correlation shows variations in parasequence thickness in some intervals of up to 200% over 5 km spans. Significant variations in rock properties at the parasequence and bedset scale appear to be due to local changes in rates of primary biogenic production and of clastic dilution, as well as interactions with local paleobathymetry.
Integrating all these physical, biogenic, and chemical data indicates the Eagle Ford Group accumulated as a transgressive and highstand sequence set in a series of mid- to outer-shelf basins. These depocenters were regionally isolated from major siliciclastic influx by a major transgression over intricate paleobathymetry influenced by benthic carbonate bioherm complexes formed on the edge of the opening Gulf of Mexico oceanic basin. Stratal thickness and character were further influenced at the local scale by evolving paleobathymetry due to salt movement and syn-depositional faulting.