“MERGING” PENNSYLVANIAN LIMESTONES IN ILLINOIS BASIN: FORMATION AT HIGHSTAND

Two or more marine limestone beds belonging to separate depositional sequences can “merge” as intervening clastic strata thin and pinch out. For example, the Providence Limestone in the southeastern part of the basin forms by merger of Brereton, Conant, and Bankston Fork Limestones. The West Franklin Limestone, to the east, combines Piasa, Lonsdale, and Exline Limestones. The Livingston Limestone, to the north, combines limestone beds from three sequences (nomenclature is complex). Limestone layers merge onto positive areas. In some cases these are tectonic highs, such as the Sparta Shelf, where the Higginsville and St. David limestones merge. In other cases, the highs are delta platforms which only partly filled the basin, for example the Farmington delta underlying the merged West Franklin Limestone. Limestone beds commonly thicken as they merge because shallow water favored carbonate production. Clastic depocenters shifted away from the high positive areas. Coal beds generally pinch out, because positive areas were well-drained during eustatic lowstands. Paleosols exhibit features of better drainage and oxidation, such as red - green mottling, where limestones merge. Phosphatic shales thin greatly and change from black to mottled, yet retain “hot” gamma-ray log response. Merged limestone beds persist tens of kilometers inland of the termination of phosphatic shales These patterns imply that areas of merging were subaerially exposed for long periods of time and that at least in the cases studied, limestone, not phosphatic shale, records maximum flooding.