PETROGRAPHIC CHARACTERISTICS OF TRANSGRESSIVE AND REGRESSIVE LIMESTONES OF UPPER-PENNSYLVANIAN (VIRGILIAN) OREAD CYCLOTHEM, SE KANSAS

Transgressive and regressive limestones may differ in composition and texture, because of different environmental conditions and modes of carbonate deposition associated with shelf deepening and shallowing during sea-level changes. We test this hypothesis by petrographic studies of three transgressive Leavenworth and two regressive Plattsmouth limestones in two sections 50 km apart in SE Kansas. Microscopic observation and point-counting document grain type, size, sorting, type and amount of cement and matrix of theses limestones to identify their differences to interpret depositional environments and controlling factors. Transgressive limestones contain dominantly fusulinids, but change from lime mudstone with well-preserved fossils in inner-shelf-lagoonal environment to muddy wackestone with coarse, highly abraded skeletal fragments in open shelf environment in the south, indicating increased wave energy to the south. Regressive limestones are muddy wackestones in the north and lime mudstones in the south, both indicate low-energy depositional environments. Fossils are diverse, including gastropods, phylloidal algae, crinoids, and horn corals. Similar petrographic characteristics suggest that similar environmental conditions at the two locations. Overall, transgressive limestones are thin (~50 cm), fine (φ=0.13), and strongly fine skewed, whereas regressive ones are thick (several meters), coarse (φ=-1.588), and strongly fine skewed; but both have similar degree of sorting (SD 1.49, 1.67). These characteristics reflect a giving-up mode during transgression and keep-up mode during regression of carbonate deposition.