Shallow Onlap Model for Ordovician and Devonian Organic-Rich Shales, New York State
Smith, Langhorne B.
Sedimentology, 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 anoxic basins greater than 150 meters deep, may in fact have been deposited in relatively shallow water of less than 50 meters. 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 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. Black shale deposition occurred in the Appalachian Basin while areas to the west were exposed land suggesting relatively low eustatic sea level during deposition.
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 is subaerial exposure on the arches to the west of the basin. Black shale is deposited in relatively shallow water (most likely <50 meters deep) on the western, cratonward flank of the basin in what are likely to have been seasonally anoxic or dysoxic conditions. The zone of anoxia may have mainly been at or slightly above the sediment-water interface. In situ benthic fossils are abundant in some of these organic-rich shales suggesting periods when the water was tolerable for some organisms. Cross-lamination and scour surfaces in most of the shales suggest conditions above storm wave base. The black shale is most organic-rich where it was deposited in the shallowest water to the west and becomes progressively less organic rich approaching the deepest part of the basin where turbidite and gray shale deposition occurred. This 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.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013