The paleohydrologic history of coal underclays based on Pennsylvanian paleosols in eastern Tennessee

Carboniferous underclays have been interpreted as paleosols for over 150 years, but debate over their origin still persists. Underclays are common throughout Lower Pennsylvanian deposits on the Cumberland Plateau in eastern Tennessee. Through careful examination of the morphology, micromorphology and whole-rock geochemistry, it is possible to interpret the processes leading to underclay development and further our understanding of regional environmental change in the Pennsylvanian.

Underclays of the Lower Pennsylvanian Crooked Fork Group in eastern Tennessee contain morphological and geochemical features common to well-drained paleoVertisols, paleoAlfisols, and saturated paleoHistosols. Macromorphologic features such as pedogenis slickensides and angular blocky ped structure, in conjunction with micromorphologic features that include illuviated clay pore-fillings and sepic-plasmic microfabric, indicate a well-drained period early in paleosol development. Gley overprinting of vertic features, influding drab, low-chroma paleosol colors, siderite-replaced rhizocretions, sphaerosiderite and pyrite nodules, extensive leaching and translocation of alkali and alkaline earth elements, and kaolinitization of smectites and hydroxyl-interlayer cermiculite (HIV) indicate a second stage of paleosol development during which the ancient soil was saturated and experienced moderate to low Eh and low pH conditions. Finally, burial and coalification of organic material and the stratigraphic juxtaposition of marine shales and sandstones on top of terrestrial deposits indicate a third stage of underclay development. This three-stage model of underclay genesis appears to have been driven by high-order glacio-eustatic sea level fluctuations, as evidenced by stratigraphic relationships within the Crooked Fork Group.

Because Paleosols are the weathering products of rocks and unconsolidated sedimentary deposits, they can be used as proxies for determining paleoclimate. Previous studies proposed a shift in climate in North America at the Mississippian-Pennsylvanian boundary, from semi-arid to tropical or sub-tropical rainy. The chemical index of alteration without potash (CIA-K) was used as a proxy for paleo-mean annual precipitation (MAP). Using whole-rock geochemical data obtained through XRD analysis, an estimate of 1300 mm/yr of rainfall was obtained. This value is consistent with modern sub-tropical rainy environments and supported the hypothesis for climate change at the end of the Mississippian.