THE NORTH AMERICAN STRATIGRAPHIC RECORD OF LATE PENNSYLVANIAN GLACIO-EUSTASY

During the Late Paleozoic Ice Age (LPIA), the Southern Hemisphere continent of Gondwana was heavily glaciated. However, the number and size of continental icesheets then in existence has been a matter of debate. The Midcontinent region of North America provides a potentially detailed record of changes in continental ice volume during the LPIA in the form of glacio-eustatic fluctuations. The Midcontinent was extensively flooded during highstand intervals (= ice-volume minima) and largely subaerially exposed during lowstand intervals (= ice-volume maxima), indicative of regular changes in continental ice mass thought to have been driven mainly by the Earth’s ~400-kyr long-eccentricity orbital cycle (Heckel, 1986, Geology, 14:330-334). We analyzed compositional variation through the Edmonds drillcore from northeastern Kansas, which spans the ~6-Myr-long interval from the mid-Desmoinesian through early Virgilian. This core contains 15 to 20 major cyclothems that record regular fluctuations in continental ice mass, representing one climate mode. Additionally, the core contains two thick siliciclastic intervals, comprised mainly of the Tacket and Weston shales, that represent extended lowstand intervals coinciding with the Desmoinesian-Missourian and Missourian-Virgilian stage boundaries, respectively. Each of these lowstand intervals contains several lithologic cycles documenting the continued influence of the 400-kyr orbital period. On this basis, we calculate that each lowstand interval lasted ~1.2 to 1.6 Myr. These lowstand intervals represent a second climate mode characterized by a lesser volume of continental ice, presumably due to melting of one or more icesheets in the Southern Hemisphere, relative to the highstand intervals that comprise the majority of the study interval. An analysis of this type, if extended over a wider stratigraphic interval, has the potential to provide a highly detailed record of changes in continental ice volume during the LPIA.