PENNSYLVANIAN (MOSCOVIAN-GZHELIAN; ATOKAN-VIRGILIAN) PALEOSOLS OF THE ILLINOIS BASIN: IMPLICATIONS FOR PALEOEQUATORIAL PALEOHYDROLOGY, CLIMATE CHANGE, AND GLACIOEUSTASY

The morphology, mineralogy and chemistry of underclays (i.e., paleosols) from Pennsylvanian coal-bearing strata in the Illinois Basin were investigated in order to determine the dominant pedogenic processes within a cyclic sedimentary depositional framework. The distribution and occurrence of ninety-one paleosol morphologies spanning the Pennsylvanian System in the Illinois Basin have been determined from analysis of five drill cores, ten open-pit mines and several road-cut outcrops. Paleosol morphologies range from weakly developed rooted horizons with little to no pedogenic structure, to those with well-developed pedogenic slickensides, angular blocky structure, and accumulations of pedogenic carbonate. The majority of paleosols preserve (1) low chroma (gley) colors, (2) identifiable fossil plant organic matter which increases upward, toward the interpreted paleosol surface, and (3) sphareosiderite and pyrite. These observations provide the basis for classifying these paleosols into four general pedotypes: 1) gleyed Vertisols 2) gleyed Protosols, 3) gleyed vertic Calcisols and 4) Histosols. Many paleosols are capped by laminated organic-rich siliciclastic layers deposited after pedogenesis and before peat development.

Preliminary stable isotope and elemental composition of paleopedogenic sphaerosiderites provide information about the redox state of the soil system and early diagenetic soil pore water chemistry. X-ray diffraction data from paleosol matrix in five lower Missourian underclays indicates these profiles are dominated by vermiculite, HIM, illite and kaolinite. Kaolinite increases, whereas 2:1 phyllosilicates decrease, upward through the paleosol profiles. These results are consistent with pedogenic, as well as early-burial weathering, patterns.

This study of combined field, mineralogic and geochemical data indicate significant temporal changes in paleohydrology, exclusive of marine transgression-regression cycles, within Pennsylvanian soil profiles, and provide a better understanding of the polygenetic history of these soils. Stratigraphic variations in pedogenesis across the Pennsylvanian landscape likely result from three possibly interrelated regional to global factors: 1) groundwater fluctuation, (2) climate change, and (3) glacioeustasy.