CALIBRATION OF MIDDLE TO LATE PENNSYLVANIAN TIME SCALE USING SUCCESSION OF GLACIAL-EUSTATIC MIDCONTINENT CYCLOTHEMS

The Middle to Late Pennsylvanian succession of glacial-eustatic cyclothems in midcontinent North America provides a record of transgressions and regressions of various scales that reflect periodic control by Earth's orbital parameters, as filtered through the complex system of climate changes severe enough to cause waxing and waning of Gondwanan ice sheets. Although the exact linkages between the parameters and ice volume are not yet known (even for the Pleistocene), the cyclic succession can be used, along with carefully considered assumptions, to calibrate the time scale with the increasingly precise radiometric dates that are now becoming available in biostratigraphically constrained successions. Pennsylvanian major cyclothems (recognized by deep-water facies across the Midcontinent and correlation into the Illinois and Appalachian basins) occur throughout most of the succession, separated by various numbers of intermediate and minor cyclothems. Although there is no set sequence of cyclothem scale, various assumed 400-kyr [long-eccentricity] groupings of lesser cyclothems around the well-defined major cyclothems were utilized to estimate ages of stage boundaries as more dates became available. The first attempt placed the Carboniferous-Permian boundary at 290 Ma, but as more accurate, earlier C-P boundary radiometric dates (now 298.4 Ma) became available, cyclothems were regrouped into fewer, more inclusive 400-kyr groupings to the point that three quite precise radiometric dates (two correlated from the southern Urals and one correlated from the Appalachians) are consistent with the regional and global stage boundaries that are positioned in the time scale by the cyclothem calibration. In addition, minor cycles detected geochemically by Algeo et al. (2004) within a slowly deposited offshore shale on the lower shelf in Kansas, can be recognized also in the shoreline equivalent of an analogous marine unit on the high shelf in Ohio, where Nadon and Kelly (2004) illustrated several minor cycles separated by exposure surfaces within a major Appalachian marine unit that is primarily equivalent to only an offshore shale in Kansas. So far it appears that these minor, presumably precession-controlled [~20-kyr] cycles may number about 20 in each presumably 400-kyr-long eccentricity grouping.