The Mineralogy and Geochemistry of Illinois Basin Paleosols
The geochemical compositions of minerals in paleosols are assumed to be homogenous and attained through chemical weathering processes during episodes of pedogenesis in the geologic past. As such, the geochemical composition of minerals from paleosols may be used to reconstruct ancient climates and environments. One problem with this assumption is that paleosols were initially clastic sedimentary rocks or substrate prior to pedogenesis, thus it is likely that some portion of the paleosol is still composed of detrital minerals. Secondly, conditions may change in a sedimentary basin as the basin is buried and later uplifted, potentially triggering new mineralization or transformation of preexisting minerals in a paleosol.
This dissertation investigates the genesis of minerals in underclays from Pennsylvanian-strata preserved in the Illinois Basin, which are interpreted to be paleosols. X-ray diffraction analyses of phyllosilicates from paleosol matrices find that mixed-layer illite-smectite is abundant, contains R1 stacking orders, and >60% illite in illite-smectite. These data suggest that phyllosilicates in Illinois Basin have been subject to illitization, which may have occurred during pedogenesis, acid leaching from overlying coal seams, burial diagenesis, or any combination of these. Additional analysis of X-ray patterns finds evidence for 1M, 1Md, and 2M1 illite polytypes. Paleosol clay-sized fractions possess K-Ar age values around 290‒260 Ma, which act as weighted averages of combined authigenic and detrital illite in the sample. Illite polytype and K-Ar age values demonstrate the presence of detrital and diagenetic components in the paleosols. Mineral precipitation temperatures derived from the elemental chemistry and stable oxygen and hydrogen isotopic composition of phyllosilicates and Δ47 values from calcites exceed 35°C. These temperatures are warmer than hypothesized paleoclimate conditions for this low latitude, low altitude locality in the Pennsylvanian. The impacts of igneous intrusions, hydrothermal fluid flow, protracted burial, and groundwater on the mineralogy and geochemistry of paleosol minerals in the Illinois Basin are considered herein but remain unresolved.
Collectively, the results presented in this dissertation help to determine the possible contribution of detrital input and pedogenic and diagenetic processes to the geochemical signatures of mineral assemblages in paleosols, providing insights into the general utility of minerals from paleosols for paleoenvironmental and basin evolution reconstructions.
W. Crawford Elliott
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McIntosh, Julia A., "The Mineralogy and Geochemistry of Illinois Basin Paleosols: Recognizing Detrital, Pedogenic, and Diagenetic Minerals" (2022). Earth Sciences Theses and Dissertations. 24.