Controls on the origin of the Devonian-Mississippian oil and gas shales, east-central United States

An abundance of organic matter, development of bottom anoxia in a stratified water column, and absence of major clastic dilution are the principal factors controlling the accumulation of organic-rich black shales. During the Devonian and Early Mississippian, such shales became abundant across much of east-central United States, apparently controlled by a unique coincidence of paleoclimatic, paleogeographic and tectonic factors. Like most major black-shale depositional episodes, Devonian-Mississippian deposits were formed during a time of global transgressive and greenhouse states. During this time, the east-central United States was located in the subtropical trade-wind belt where marine and terrestrial organic productivity would have been high and where surface waters would rarely have become cool enough to overturn and disrupt the water stratification necessary to generate bottom anoxia. Location in the trade-wind belt may also have enhanced upwelling which would have increased organic productivity and the likelihood of anoxia. Perhaps most importantly, however, black-shale deposition coincided with early phases of tectonism, when, because of rapid deformational loading in the orogen, subsidence in the adjacent Appalachian foreland basin far outstripped sedimentation. In the resulting deep-water conditions, water stratification and accompanying anoxia were soon established. Because most of the deformational load at this stage was subaqueous, there was no major source of clastic dilution, so that clastic starvation prevailed and largely organic-rich sediments accumulated. Thus hydrocarbon source beds of broad extent like the Devonian-Mississippian black shales, are rarely attributable to a single cause. They typically result from a complex interplay of factors unique to a given time and place.