Changing duration of Carboniferous cyclothems implications for coal and lithofacies distribution

Abstract

The concept of cyclothems and cyclic stratigraphy began in the U.S. Midcontinent and Appalachian Basins. Vertically repetitive alterations of lithofacies in Upper Mississippian through Upper Pennsylvanian strata in several basins have long been inferred first relative to cyclothems, then transgressive-regressive cycles, and more recently sequence stratigraphy. The underlying causes of cyclicity, lithofacies distribution, and lithofacies continuity within parts of cycles have been attributed to glacio-eustacy, tectonics, sediment flux, and climate, with different researchers stressing the importance of one over the other. A parameter which generally has been kept constant relative to sedimentation in previous research is time.

Most previous researchers have attributed Late Mississippian and Late Pennsylvanian cyclicity to fourth-order (400K), long-eccentricity glacio-eustatic cycles. Intervening Early and Middle Pennsylvanian coal-bearing cycles were thought to have the same duration. However, recent radiometric age dates for coals in the central Appalachian basin suggest that at least part of the Early and Middle Pennsylvanian sedimentary record was dominated by fifth-order (100K) short-eccentricity cycles. If the record is at all incomplete, or thick marine zones represent more than one cycle, than parts of the section may even represent sixth-order (41K) cycles, stacking into larger fifth-order cycles.

Longer lowstands in fourth-order cycles would lead to more weathering and erosion which could lead to more extensive and thicker paleosols as are seen in the Late Mississippian and Late Pennsylvanian. Longer weathering might also result in more tabular depositional surfaces and lithofacies distribution, as are typical in the Late Mississippian and Late Pennsylvanian. The average thickness and extent of coalbeds, which is thicker in the Late Pennsylvanian than Early Pennsylvanian, might also be attributed to mires of longer duration in the Late Pennsylvanian.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016