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 shales were mainly deposited on the present-day western or cratonward side of the basin, not in the deepest part or at the toe of a prograding clastic wedge as is commonly interpreted. The deepest part of the basin in both the Ordovician and the Devonian was the site of organic-poor turbidite deposition. The organic-rich black shales commonly overlie, onlap and pinch out on unconformities, some of which are demonstrably subaerial in origin. Both the Ordovician and Devonian black shales were deposited during periods of high tectonic subsidence driven by thrust-loading to the east. This black shale 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 throughout both of these shales 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 black shales in actively subsiding foreland basins. In this model, eustatic sea level is interpreted to have been relatively low and there was subaerial exposure on peripheral bulges to the west of the basin. The black shale 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. Shallow water would be warmer than deep water and the warmer water is the less oxygen it can hold in solution. 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. Because it was relatively shallow, storms and other currents influenced deposition throughout. The deepest part of the basin is the site of turbidite and organic-poor shale deposition, not black shale. A similar model may apply to other black shales worldwide.
URTeC 1581184
