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Shallow Seasonally-Anoxic Model for Organic-Rich Mudrocks and Interbedded Carbonates – An Alternative Hypothesis


Field relations and regional correlations show that organic-rich mudrocks in the Devonian Marcellus to Dunkirk Shales and the Ordovician Utica Shale of New York, which have historically been interpreted to have been deposited in deep, still, permanently anoxic basins greater than 150 meters deep, may in fact have been deposited in relatively shallow, moving water that was at least periodically not anoxic and no more than a few tens of meters deep. These black mudrocks and carbonates were mainly deposited on the present-day western or cratonward side of the basin, not in the deepest part of the basin which was the site of organic-poor turbidite deposition. The organic-rich black and interbedded shallow marine carbonates shales commonly overlie, onlap and pinch out on unconformities, some of which are demonstrably subaerial in origin. This black mudrock and limestone deposition occurred in the Appalachian Basin while areas to the west were exposed land suggesting relatively low eustatic sea level during deposition. Scour surfaces and cross-lamination are common in both Ordovician and Devonian organic-rich shales and limestones suggesting storm influence and possibly other types of currents. Benthic fossils and burrows occur in the shales indicating periods when the sea floor had sufficient oxygen levels to sustain life. These observations have led to the development of a new depositional model for organic-rich mudrock in foreland basins. In this model, eustatic sea level is interpreted to havebeenless than 50 meters deep and probably less than 30 meters. The mudrock is most organic rich where it was deposited in the shallowest water far to the west and becomes progressively less organic rich approaching what was the deepest part of the basin. This is at least in part due to progressively more dilution from clay and silt that are sourced from the highlands to the east but it may be that the longest duration of anoxic conditions occurred in the shallowest water. Seasonally anoxia or dysoxia similar that what occurs on many shallow continental shelves today may have helped to preserve organic matter but could also explain the presence of benthic fossils and bioturbation. Organic matter may be preserved in clay floccules and organo-mineralic aggregates. Because it was relatively shallow, storms and other currents influenced deposition throughout and produced low angle cross lamination. A similar model may apply to other black shales worldwide.