CORRELATION AND CLIMATIC ORIGIN OF SMALL-SCALE CYCLICITY IN UPPER PENNSYLVANIAN CYCLOTHEMIC BLACK SHALES

Large glacio-eustatic fluctuations during the Late Paleozoic Ice Age resulted in deposition of laterally extensive cyclothems across the North American craton. These cyclothems contain deepwater black shales that exhibit a regular pattern of small-scale cyclicity (2-10 cm) as expressed in total organic carbon (TOC) profiles. The goals of this study are (1) to develop a high-resolution internal stratigraphic framework based on these cycles, and (2) to address the mechanism responsible for cycle genesis. In each black shale unit, the first TOC peak (>10%) correlates with a circa +10 ‰ nitrogen isotope excursion caused by lateral advection of denitrified waters from the oxygen-minimum zone of the Eastern Panthalassic Ocean (EPO). The maximum TOC concentrations (>30%) are observed in the middle of each black shale unit, representing the maximum flooding surface of the cyclothem. A total of about 10 fine-scale cycles can be traced from Oklahoma northward to Iowa and eastward to Illinois, indicating that environmental changes occurred simultaneously across broad reaches of the Midcontinent Shelf and Illinois Basin. Of the three possible controls on cycle genesis (i.e., variations of marine productivity, organic matter preservation, or siliciclastic dilution), we favor organic matter preservation as the dominant control. This inference is based on correlation of TOC content with redox conditions, as proxied by trace-metal concentrations and molar Corg/P ratios. We infer that redox variation in the deeper part of the water column depended on pycnocline strength, with the latter dependent on precipitation and runoff into the sea (i.e., climatic factors). Thus, the ultimate driver of small-scale cyclicity in cyclothemic black shales was probably quasi-periodic climate fluctuations.