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Ancient Shelf Anoxia |
Department of Geology, University of Iowa, Iowa City, Iowa 52242, USA
Within the Middle-Upper Pennsylvanian cyclic sequence of Midcontinent North America (Iowa, Nebraska, Missouri, Kansas, Oklahoma), there are at least 20 horizons of thin (<1 m) phosphatic black shales that extend up to 600 km along outcrop. They overlie transgressive limestones that range from very thin (1–5 cm) shell beds where they lie above coals, to thin (1 m or less) skeletal calcilutites where they lie above fluviodeltaic deposits and palaeosols. The black shales underlie thicker (2–10 m) regressive limestones that are characterized by various types of shallowing-upward sequences and are typically capped by palaeosols on the northern shelf (Iowa to central Kansas). The black shales represent the deepest-water deposits within the succession of widespread eustatic inundations that were responsible for the major cyclothems. Their frequency in the cyclic sequence is estimated to be close to that of the longer of the earth’s orbital parameters that are considered to control the wax and wane of continental glaciation. They apparently formed beneath pycnoclines that were established during interglacial high-stands of sea level when the water was both warm enough (at the surface) and deep enough to maintain a thermocline for perhaps several thousand years. These thermoclines were probably augmented by haloclines in areas of greater fresh-water run-off and possibly in areas accessible to more saline bottom water.
