INVESTIGATING THE TIMING OF ILLITE FORMATION IN PENNSYLVANIAN-AGED PALEOSOLS OF THE ILLINOIS BASIN

Clay mineral mixtures in Pennsylvanian-aged paleosols from Illinois Basin cyclothems may have formed during episodes of soil formation or shallow to deep burial diagenesis or may have been inherited as detritus prior to soil formation. To assess origins of illitic clay minerals in these paleosols, three cores from the north, central, and southern portions of the Illinois Basin from the middle to upper Pennsylvanian strata were sampled (n=6) and isolated to <2.0, <0.2, and <0.1 μm clay-sized fractions (n_tot=18) by disaggregation and centrifugation. Amounts of illite relative to mixed layer illite-smectite in clay mineral mixtures were evaluated using Środoń intensity ratios from X-ray diffraction analyses indicating that swelling is common primarily in the samples from the northern core. Moreover, analysis of illite from X-ray diffraction data indicate an abundance of 1M_d over 2M₁ polytypes in all three cores. Using thermal extraction and isotopic analysis of crystalline-bound Ar and determination of K by atomic absorption spectrophotometry (of the same test portions from which the Ar was extracted), apparent K-Ar ages were ascertained for each size fraction. Illitic clay minerals exhibit apparent ages around 260 Ma in the northern core, 270 Ma in the central core, and 295 Ma in the southern core. Although strata of the Illinois basin were previously thought to be shallowly buried (1-3 km) and thus minimally impacted by burial diagenesis, this new dataset indicates that post-Pennsylvanian clay mineral alteration was likely and there is limited detrital contamination. It is possible that these post-Pennsylvanian ages in Pennsylvanian-aged paleosols may correspond with hydrothermal and diagenetic events spurred by the Alleghenian-Ouachita orogeny. This is because late Paleozoic orogenesis in North America has been temporally connected to the Illinois-Kentucky Fluorspar and Upper Mississippi Valley mineral districts with a hypothesized south to north brine migration mechanism across the Illinois Basin. These results are significant, because they call into question the general utility of phyllosilicates as reliable paleoenvironmental indicators across space and time and suggest older paleosols may in fact provide more information for diagenesis than paleoenvironment.