At least 55 cycles of marine inundation and withdrawal are recognized in the mid-Desmoinesian to mid-Virgilian Midcontinent outcrop sequence in North America. They range from widespread major cycles (classic cyclothems) with deep-water facies extending across the northern shelf, through intermediate cycles persisting as marine horizons across the shelf, to minor cycles developed on the lower shelf or as parts of major cycles. Biostratigraphic differentiation of the cycles should establish interbasinal correlation on a scale fine enough to allow evaluation of differential tectonics and sedimentation. Sequential groupings of cycles are more irregular than proposed megacyclothems or mesothems, but they may be obscured by the distinctness of the major cyclothems. Estimates of cycle periods range from about 40 to 120 × 103 yr for the minor cycles up to about 235 to 400 × 103 yr for the major cyclothems. The range for all cycles corresponds well to the range of periods of Earth's orbital parameters that constitute the Milankovitch insolation theory for the Pleistocene ice ages, and it further supports Gondwanan glacial control for the Pennsylvanian cycles. Even though the dominant period of the major Pennsylvanian cyclothems is up to four times longer than the dominant 100 000-yr period in the Pleistocene, the shapes of both curves display rapid marine transgression (rapid melting of ice caps) and slow interrupted regression (slow buildup of ice caps), which suggest similar linkages between the climatic effects of the orbital parameters and ice-cap formation and melting, at the two different scales, widely separated in time.
- Geological Society of America