Origin of Phosphatic Black Shale Facies in Pennsylvanian Cyclothems of Mid-Continent North America

Continued lithologic and paleontologic considerations of Kansas Upper Pennsylvanian megacyclothems more firmly establish the basic ascending sequence of: outside (nearshore) shale--middle (transgressive) limestone--core (offshore) shale--upper (regressive) limestone--outside (nearshore) shale, as representing a single transgressive-regressive sequence; this sequence is thus considered simply a cyclothem. Lateral facies change along the 500-km Iowa-Kansas outcrop belt is greatest in upper parts of upper limestones and outside shales, as would be expected in shallow-water to shoreline deposits; it is least in middle limestones, core shales, and bases of upper limestones, as would be expected in deeper water deposits. The widespread phosphatic black shale facies that commonly accompanies maximum transgression in the core shale is explained by water becoming deep enough to develop a thermocline strong enough to prevent bottom oxygenation by wind-driven vertical circulation. Pennsylvanian position of Mid-Continent North America, in the trade-wind belt north of the paleoequator along the Appalachians, allowed establishment of large-scale quasi-estuarine circulation in the Mid-Continent epicontinental sea. Cold, deep, oxygen-poor, phosphate-rich water from the western ocean was drawn in along the bottom through the basins of West Texas eventually to upwell in the eastern Mid-Continent and replace the surface water moved westward out of the sea by the prevailing winds. Upwelling greatly increased surfac -water production of organic matter, which continually settled (while being carried westward) into the deeper incoming current, where it decayed and depleted the remaining oxygen while continually enriching the already high phosphate in a circulatory trap. In this way substantial organic matter and phosphorite were deposited on the anoxic sea bottom to produce the phosphatic black shale facies. This model for offshore phosphatic black shale deposition obviates the difficulty of explaining in shallow tropical water the combination of nonskeletal phosporite production, and widespread lateral uniformity of a quiet anoxic environment between two marine limestones. It supports large-scale Pennsylvanian transgressions and regressions in the Mid-Continent sea, but remains compatible with the local cyclic sedimentary process of delta outbuilding and abandonment along the shoreline. In fact, large-scale marine transgressions and regressions account for the widespread distribution of delta-shoreline deposits from the Appalachians to Kansas. The offshore black shale model can be expanded to a more general depositional model that not only explains the lateral variation in black-shale-be ring Pennsylvanian cyclothems from the Appalachians to West Texas, but also accounts for the scarcity of black shales in younger Pennsylvanian and Permian Mid-Continent cyclothems by suggesting that water depths at maximum transgression during that time were generally too shallow to establish an effective thermocline.