THE SIGNATURE OF CLIMATE IN MIDCONTINENT CYCLOTHEMS AND HOW IT REFLECTS GLOBAL TECTONICS AND ATMOSPHERIC CIRCULATION DURING THE LATE PENNSYLVANIAN-EARLY PERMIAN

The latest Pennsylvanian-earliest Permian rocks of the North American Midcontinent record a hierarchy of stratigraphic cycles deposited in a near-equatorial epeiric sea during a time of global icehouse conditions. The low relief and paleogeographic position of this platform made it particularly sensitive to high-frequency, high-amplitude eustatic and climatic changes. Two prominent orders of cyclicity can be identified in the latest Gzhelian through early Asselian succession. The larger scale is represented by composite depositional sequences, which are analogous to the cyclothems of Heckel. They show a transgressive-regressive stacking geometry of eight to thirteen meter-scale cycles, which are the finer scale in the hierarchy. The meter-scale cycles conform to the definition of depositional sequences; their transgressive systems tracts are dominated by carbonates with associated evaporites, and their highstand systems tracts are dominated by siliciclastics with associated coals. Both carbonate and siliciclastic facies suites in the meter-scale cycles show a similar range of water depths from coastal/shoaling through open-marine environments. However, the sequence stratigraphic context of the meter-scale cycles indicates that the two facies suites were not coeval, but rather, they reflect fundamentally different climatic regimes: eustatic lows were associated with arid conditions and eustatic highs with humid conditions. This alternation of climatically distinct facies suites can be explained by glacial-interglacial changes in the summertime position of the intertropical convergence zone (ITCZ). Based on analogy with the Neogene, the ITCZ did not extend as far north into Pangea during glacial conditions as it did during interglacials; this allowed the Appalachian-Variscan mountain belt to block equatorial moisture from reaching the continental interior (i.e., the Midcontinent) during lowstand and transgression. During highstand, the ITCZ swung farther north, bypassing the high mountains and providing the moisture needed to develop humid conditions in the continent's interior. The initiation of cyclothemic deposition in the Midcontinent succession may reflect the timing when the high mountains to the east drifted into a geographic position that led to interference with the ITCZ.