NATURE OF DEPOSITIONAL SEQUENCES ASSOCIATED WITH THE CARBONIFEROUS-PERMIAN BOUNDARY AND ADJACENT UNITS IN THE MIDCONTINENT

Strata in the upper midcontinent that bracket the Carboniferous – Permian boundary record 3rd and 4th order mixed siliciclastic-carbonate-evaporite depositional sequences that accumulated on a broad shallow marine shelf during glacio-eustatic fluctuations. The upper midcontinent epeiric sea was relatively restricted with limited connection to the Permian Basin and open ocean. The midcontinent was located in tropical to subtropical latitudes north of the equator and gradually migrated northward throughout Permian time. The northward latitudinal shift and establishment of monsoonal circulation led to significantly drier conditions. Moreover, the narrow (~100 km) passage between the Amarillo-Wichita and the Sierra Grande uplifts shallowed from basinal to restricted shelf between Late Pennsylvanian and Early Permian during declining foreland basin tectonism and associated subsidence. In addition, an apparent shallowing upward trend in Permian strata in the upper midcontinent could have been influenced by an increase in continental elevation due to thermal uplift of the interior of Pangea. Thus, climate, tectonism, and eustacy probably acted in combination to increase the presence of evaporites during late 4th order regressions in the lower Permian and the eventual abrupt onset of extensive subaqueous evaporite accumulation during the Leonardian.