Paleoclimate Controls on the Origin of Some Black Shales in Paleozoic Epeiric Seas of the Eastern United States
CECIL, C. BLAINE, and N. TERENCE EDGAR, U.S. Geological Survey, Reston, VA
Marine black shales rich in organic matter are important stratigraphic markers and are economically important as major hydrocarbon source beds and sources of metals. Some models suggest that organic matter in these shales was preserved by anoxic bottom
waters that resulted from density stratification in silled basins or deep-water conditions. Other models suggest high organic productivity in response to upwelling ocean currents as a control on the formation of shale rich in organic matter. We suggest that organic productivity in tropical epeiric seas may also be related to climate cycles.
Under dry climatic regimes, water loss from evaporation in epeiric seas is replaced by inflow of open-marine water, a condition favorable for shallow-marine carbonates. In contrast, high rainfall could slow or reverse inflow. The latter situation favors (1) influx of terrestrially derived materials including siliciclastic sediment, metals, and terrestrial organic matter, (2) increased marine biological productivity (organic fixation) because of nutrient influx, and (3) cessation of carbonate deposition and the onset of black shale deposition. Alternating wet and dry conditions are known for the Pleistocene and Holocene, and they have been suggested as a stratigraphic control for coal-bearing sequences for the Pennsylvanian System of the eastern United States. We suggest that climate c cles may also have influenced the origin of Paleozoic marine sequences including organic matter composition and metal content of black shale.
AAPG Search and Discovery Article #91005 © 1991 Eastern Section Meeting, Pittsburgh, Pennsylvania, September 8-10, 1991 (2009)