Cyclothems are marine transgressive-regressive sedimentary units, which were defined in the Pennsylvanian succession in the Illinois Basin, and reasonably interpreted as products of glacial eustatic fluctuation of sea level in the 1930s. Other models of deposition from diastrophic [tectonic] to sedimentary delta-shifting were proposed from the 1930s to 1960s. Early interpretations of cyclothem deposition ignored the significance of the distinctive black shale member, until the shallow-water algal-flotant model on delta lobes was developed in the 1960s. The widespread nature and phosphorite content of the black shales in the major Midcontinent cyclothems led to the deep-water circulation and upwelling model in the 1970s. Abundant and distinctive conodont faunas in the black shales [and homologous gray shales] allowed correlation of individual cyclothems along the Midcontinent outcrop belt and into the Illinois and Appalachian basins. Dominantly marine cyclothems are now recognized elsewhere around the Pennsylvanian tropical region in eastern Europe and China. The shapes of sea-level curves based on the succession of cyclothems of different extents in the Midcontinent resemble the shapes of Pleistocene glacial-eustatic sea-level curves. Groupings of smaller cycles around major cyclothems, which apparently represent the 400-kyr period of the Earth’s orbital parameters, can be calibrated with recent radiometric dates of the geologic time scale. Combining radiometric dates with recognition of the greatest sea-level drawdowns [that should correspond with the most widespread glacial deposits] will facilitate correlation with the southern polar Gondwana succession.
About speaker: Philip H. Heckel was born in 1938 in Rochester, New York. He received his B.A. in 1960 from Amherst College in western Massachusetts and his PH.D in 1966 from Rice University in Houston, Texas. From 1965-1971, he worked at the Kansas Geological Survey in Lawrence. Since then has moved to the University of Iowa in Iowa City as a professor of Sedimentology in the Geology department. Heckel retired from work in 2011, but continues to do research.
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