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Dark, commonly black, fissile shales represent the basinal facies of the Catskill Delta complex. This dark-shale facies originated in the Appalachian Basin during the Emsian (late Early Devonian) and subsequently spread beyond the Appalachian Basin thus suggesting the importance of regional controls such as paleogeography, paleoclimate, and tectonic regime.

The Catskill delta debouched into a subsiding peripheral basin on the eastern margin of a nearly enclosed, equatorial sea. Enclosure prevented deep-water incursions from other water bodies and enhanced the production and concentration of organic matter. Existence in a warm, rainy equatorial belt increased organic productivity and probably formed a nearly permanent thermohaline water stratification that prevented vertical circulation and resulted in anaerobic bottom conditions.

Episodes of uplift and deformational loading in the rising Acadian Mountains resulted in closely following or concomitant periods of subsidence through isostatic adjustment in the adjacent peripheral basin. Hence, episodic Acadian uplift not only formed the source of the Catskill clastic wedge, but was also responsible for forming the peripheral basin that received the wedge. Basin subsidence isolated deltaic sediments from other parts of the sea, enhanced water stratification, and augmented net transgression. Rapid aggradation of sediments in nearshore areas due to transgression and the formation of a rainshadow due to uplift in the mountains caused decreased sedimentation on the delta, while concomitant basin subsidence produced abrupt deepening, strong water stratification (bottom anoxia), and migration of clastic-deficient (dark-shale) basinal environments shoreward. During intervening periods of tectonic quiescence, subsidence eventually halted, and erosion outstripped the effects of uplift in the mountains, so that the rainshadow was destroyed and abundant clastic influx resumed. As a result, the delta prograded rapidly across former basinal environments. Decreasing subsidence, shallower water, and abundant clastic influx destroyed basinal, bottom anoxia, so that marginally- to highly-oxygenated bottom conditions resumed throughout all delta environments. Five such major cycles of dark, basinal shales alternating with coarser clastics occur in the Catskill Delta complex of the Appalachian Basin. The progressive westward migration of successive cycles reflects the westward migration of Acadian deformation and accompanying basin infilling.

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