RECENT RESEARCH TRENDS IN PENNSYLVANIAN CYCLOTHEMS

Classic marine transgressive-regressive Pennsylvanian cyclothems on the northern Midcontinent shelf show examples of lowstand systems tracts that appear basinward of their lowstand shorelines, hence are recognized also as complete stratigraphic sequences. Groupings of Desmoinesian through Virgilian cyclothems of three scales (major, intermediate, minor) around the major cyclothems that represent the highest sea-level stands result in 26 approximately 400-k.y.-long units that are compatible with the few correlatable radiometric dates available for this time span, and reflect the long orbital eccentricity cycle. Conodont-based correlation of the major cyclothems has been accomplished for central and eastern North America, and extended to the more recently recognized cyclothem successions in Russia and Ukraine. Although this facilitates correlation of short-term major highstand events throughout the Pennsylvanian pantropical belt, correlating sea-level fluctuations in the pantropical region with those in the cold-climate Gondwana region requires more precise radiometric dates from volcanic beds. This is because Gondwana has only endemic fossils and because the more conspicuous events in much of Gondwana are typically the glacial and periglacial deposits with tillites and dropstones that would have been widespread only during the times of greatest regression and lowstand. The six general times of greater-than-usual regression recognized from late Desmoinesian through mid-Virgilian in the Midcontinent are not yet correlated with specific glacial events on Gondwana, but two successive events of greater Midcontinent drawdown appear to correlate with greater-than-usual incision events in the Appalachian Basin. Moreover, minor geochemical fluctuations detected in the dark offshore core shales on the Midcontinent low shelf (and thought to reflect minor sea-level fluctuations related to orbital precession cycles) seem to have counterparts in the successions of minor exposure surfaces and marine inundations seen within individual major highstand units on the Appalachian high shelf. Ongoing investigations by several workers are focusing on detecting more minor sea-level fluctuations preserved within the classic major cyclothems.