North American Midcontinent Pennsylvanian cyclothems and their implications

Abstract

Abstract Cyclothems are stratigraphic sequences deposited during glacial-eustatic transgressive–regressive inundations of land. Major cyclothems are most complete on lower shelves and become separated by exposure surfaces higher on the shelf. Minor cyclothems extend only onto lower shelves or are parasequences reflecting a reversal of sea level during general phases of transgression or regression. In Midcontinent North America, highstand condensed intervals of major cyclothems are conodont-rich black shales correlated eastwards into the Illinois and Appalachian basins. The number of cyclothems decreased by loss of lesser cyclothems across these basins, signifying an eastwards increase in their depositional elevations. Generalized palaeogeographical maps show that facies successions developed across these basins during a single major interglacial transgression and regression. The control of glacial eustasy by the interaction of Earth's orbital parameters, along with radiometric dating, allows subdivision of the cyclothem succession into c. 400 kyr groupings, which reflect the longest eccentricity cycle and facilitates global correlation. Delineation of the basinward extents of regression from the disappearance of exposure surfaces elucidates the history of basin-margin development. Major cyclothems that underwent reversals of general sea-level trend display ‘splays’ of lesser cycles onto the higher shelf. Geochemical cycles ‘nested’ within deepest-water shales on low shelves appear as thin nearshore facies between exposure surfaces at their high-shelf shorelines.