Devonian-Mississippian black gas shales in east-central United States form a distinctive stratigraphic interval reflecting low clastic input, high organic productivity, and anaerobic conditions in a stratified inland equatorial sea. These conditions apparently originated because of interactions between tectonic and climatic factors unique to North America at this time.

The "Black-Shale Sea" developed over an expanse of slowly subsiding craton west of the Acadian mountains. These mountains provided the major source of clastics to the sea and periodically formed high orographic barriers crossing the paleoequator. Subsidence on the adjacent craton, particularly in the marginal foredeep (Appalachian foreland basin), accompanied uplift in the Acadian range. When uplifted, these mountains created barriers to moisture-laden, easterly trade winds, forming a rain shadow west of the mountains and reducing clastic input to the "Black-Shale Sea." Clastic input reduced, deposition of organic-rich muds dominated. Concomitant subsidence caused this sediment-starved sea to transgress over the Catskill delta and other parts of the craton.

During tectonic quiescence, subsidence slowed and the mountains were erosionally lowered sufficiently that the trade winds crossed and delivered precipitation to the mountains causing increased clastic influx and westward deltaic progradation into the "Black-Shale Sea." Seven major cycles of alternating black shale and deltaic clastics occur in the Catskill delta. Few of the progradations migrated beyond the foredeep, hence, sediment-starved conditions persisted in western parts of the sea.

The equatorial nature of the sea and progressive deepening within it created a stratified water column (pycnocline) that prevented oxygenation of deeper bottom waters. Organic-rich muds deposited there were preserved in the anaerobic conditions that resulted. Eventually, the sea deepened sufficiently that upwelling phosphate-rich, oxygen-poor, oceanic waters entered locally from the continental margin.

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