DIFFERENCES IN THE DURATION OF CARBONIFEROUS EUSTATIC CYCLES MAY EXPLAIN SOME DIFFERENCES IN COAL AND LITHOFACIES DISTRIBUTION THROUGH TIME

The concept of cyclothems and cyclic stratigraphy began in Carboniferous strata of the Illinois and Appalachian Basins, and later evolved into transgressive-regressive cycles, and more recently sequence stratigraphy. The underlying causes of the repetition of depositional facies through time, have been attributed to glacial-eustacy, tectonics, sediment flux, and climate. The parameter of time or duration per cycle, is generally not examined relative to sedimentation. Researchers who favor glacial-eustatic models generally infer fourth-order (400K), long-eccentricity-driven durations for coal-to-coal depositional cycles while tectonic modelers favor longer depositional successions related to crustal responses to crustal loading on the continental margin. Fourth-order cycles are supported by biostratigraphic correlation of Late Middle to Late Pennsylvanian and Late Mississippian strata in the Illlinois and Appalachian Basins to U.S. and international stage and series boundaries.

Although there are few absolute age dates for Pennsylvanian strata in either basin, two radiometric dates in the Early and Middle Pennsylvanian of the central Appalachian basin can be used to demonstrate that coal-to-coal depositional cycles during this time were dominated by fifth-order (100K) short-eccentricity cycles, and possibly sixth-order (41K) cycles, stacking into larger fifth-order cycles.

If the duration of primary depositional cycles changed through the Pennsylvanian, than many differences in coal thickness and continuity might be related to different durations of accumulation, sedimentation, or erosion and weathering. For example, longer lowstands in fourth-order cycles would lead to more weathering and erosion and possibly more extensive and thicker paleosols in the Late Mississippian and Late Pennsylvanian, than in the Early and Middle Pennsylvanian. Longer durations of cycles in the Late compared to the Early Pennsylvanian, might also explain why palynospore zones generally encompass one to two coals in the Late Pennsylvanian, as compared to five or more in the Early and Middle Pennsylvanian.