Late Mississippian (Late Meramecian-Chesterian), Glacio-Eustatic Sequence Development on an Active Distal Foreland Ramp, Kentucky, U.S.A.
Aus Al-Tawil, J. Fred Read, 2003. "Late Mississippian (Late Meramecian-Chesterian), Glacio-Eustatic Sequence Development on an Active Distal Foreland Ramp, Kentucky, U.S.A.", Permo-Carboniferous Carbonate Platforms and Reefs, Wayne M. Ahr, Paul M. (Mitch) Harris, William A. Morgan, Ian D. Somerville
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High-resolution sequence stratigraphic cross sections (based on outcrops and shallow cores) of Mississippian (Meramecian to Chesterian) strata in the Appalachian Basin, Kentucky, U.S.A, are the marine stratigraphic record of the interaction of tectonics on a slowly subsiding distal foreland, with the Carboniferous glacio-eustatic transition into icehouse conditions. Late Mississippian paleoslopes were to the southeast into the foreland basin, probably because of thrust loading and accompanying block faulting of the distal foreland. The succession is dominated by fourth-order sequences that are only a few meters to 15 meters thick, which is an order of magnitude thinner than those of the proximal foreland. Regional disconformities marked by paleosols, caliche, micro-karsting, brecciation, tepee formation, or sharp contacts between limestone and the overlying transgressive marine shale bound fourth-order sequences. The sequences are stacked into several siliciclastic-bounded third-order or composite sequences that make up the broadly transgressive Mississippian supersequence. Facies are dominated by oolitic grainstones passing downslope into skeletal grainstone-packstone and updip into lagoonal muddy carbonates, green shale, eolianites, and rare red beds. Marine shales occur at bases of the younger sequences.
The fourth-order sequences can be correlated from the Appalachian distal foreland, into the proximal foreland, as well as into the intracratonic Illinois Basin, suggesting a eustatic origin. However, local and regional tectonics strongly influenced thickness and distribution of sequences as well as the distribution and timing of unconformities locally. The facies stacking in the lower sequences indicates that they formed under the influence of moderate sea-level changes that effectively flooded the ramp to depths of 10 m or less. This resulted in grainstones within sequences being partitioned by caliches and muddy carbonates near or at sequence boundaries. Facies stacking in the upper sequences suggests that the magnitude of fourth-order sea-level changes increased in the later Chesterian, causing flooding of the ramp to depths of tens of meters, probably synchronous with buildup of ice sheets on Gondwana. This was accompanied by a change to more humid climates, which decreased oolite deposition on the ramp and favored deposition of open marine skeletal limestones bounded by marine siliciclastic units. Such moderate-amplitude global eustasy may account for the widespread, highly partitioned grainy reservoirs typical of the Mississippian worldwide.