CONSTRAINING THE CAUSES OF ILLITIZATION IN PENNSYLVANIAN-AGED PALEOSOLS FROM THE ILLINOIS BASIN USING ILLITE/SMECTITE RATIOS

Evidence of illitization in the Pennsylvanian-aged underclays of the Illinois Basin (U.S.A.) can be attributed to low temperature, cyclical redox fluctuations from climatic changes during pedogenesis and high temperature, burial diagenesis. Underclays, recognized as paleosols, were sampled from six cores around the Illinois Basin in order to assess the degree of illitization throughout the Pennsylvanian-age strata. This is achieved by identifying the clay-mineral content of the samples via gycolated pretreatments followed by XRD analysis. Preliminary results indicate that cores located in the interior and deeper parts of the basin have more illite relative to smectite in I/S superstructures and also lack a high temperature influence of illitization. Cores located in the northern, much shallower, periphery of the basin have more illite relative to smectite in I/S, but less than the interior, and also lack a high temperature influence of illitization. Supplemental petrology and lithological data supports a high temperature influence of illitization in the in the deepest and southern-most portion of the basin due to evidence of fluid migration and proximity to the Reelfoot Rift-Rough Creek Graben system. We expect that illitization during pedogenesis was prominent throughout the basin, though burial diagenesis was more prominent in the southern portion of the basin. By constraining between the causes of illitization preserved in paleosols, we can determine where and when post-pedogenic alteration of clay minerals in paleosol profiles is most prominent in the Pennsylvanian strata, and thus where further geochemical studies should be limited.