An Analysis of Mixed-Layer Clay Minerals and Major Element Geochemical Trends in Middle-Upper Pennsylvanian-Aged Paleosols as a Proxy for Characterizing Basin-Wide Diagenetic Patterns and the Paleoenvironment of the Illinois Basin, U.S.A.
Pennsylvanian-aged paleosols from four Illinois Basin (IB) cores were sampled to characterize clay mineralogy, paleosol morphology, and major element geochemistry. This data combined with X-ray diffraction (XRD) and X-ray fluorescence (XRF) data from two other stratigraphically equivalent cores were used to better understand basin-wide diagenetic patterns. Paleosols unimpacted by diagenesis are considered reliable proxies for continued geochemical analyses and paleoclimate interpretations of middle to late Pennsylvanian (Desmoinesian-Missourian) low-latitude environments.
Reichweite Ordering, as determined from XRD analysis ofprofiles, indicates that diagenesis impacted the deep interior of the basin and the southern portion of the basin. Further investigation of illitization mechanisms derived from Δ° 2Θ, as determined from XRD analysis, indicate that there is a basin-wide increase in % illite in interstratified illite/smectite superstructures (I/S) in the upper Desmoinesian, followed by a decrease in % illite in I/S in the Missourian. Detailed morphology of paleosols sampled from the IB cores indicates that many exhibit gleyed, vertic, and or calcic properties. Geochemical trends of bulk matrices determined from XRF analysis indicate that alumina is prominent in IB paleosols, whereas labile element oxides, such as MgO, occur in trace to minor amounts.
Collectively, these XRD results suggest that paleosols in the southern portion of the IB were altered during burial diagenesis, perhaps due to proximity to the Reelfoot Rift-Rough Creek Graben system. Diagenetic signals were identified from paleosols lower in the stratigraphy that may have been reworked during deeper burial. Therefore, paleosols examined from the northern and eastern portions of the basin and higher in the stratigraphic succession should be considered more reliable for paleoenvironmental reconstructions. However, the results from Reichweite Ordering and % illite in I/S to determine illitization mechanisms differ; 60% of samples show conflicting results, indicating that illitization mechanism proxies should be used carefully or not at all. This study also shows through paleosol physical morphologies and subsequent classifications that some paleosols formed in wet climates whereas others formed in less wet climates. Furthermore, this study finds that chemical weathering indices from bulk paleosol matrices utilized herein show a high intensity of weathering in the Desmoinesian followed by a slight decrease in weathering intensity in the Missourian. This result aligns with previous research that states the low-latitude paleoclimate transitioned from a wet to less wet climate from the middle to late Pennsylvanian.
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McIntosh, Julia A., "An Analysis of Mixed-Layer Clay Minerals and Major Element Geochemical Trends in Middle-Upper Pennsylvanian-Aged Paleosols as a Proxy for Characterizing Basin-Wide Diagenetic Patterns and the Paleoenvironment of the Illinois Basin, U.S.A." (2018). Earth Sciences Theses and Dissertations. 6.