AN INFLECTION IN THE LATE PALEOZOIC PALEOGEOTHERM OF THE ILLINOIS BASIN AND ITS IMPLICATIONS FOR COALIFICATION

The Illinois Basin is a cratonic basin containing abundant high-volatile bituminous coal of Carboniferous age. Coal maturity, defined by vitrinite reflectance, is higher than expected for the calculated paleo-burial depth of coalification defined by compaction studies. In addition, the highest maturity does not occur in the depocenter, but rather occurs near a region of known hydrothermal activity at the south end of the basin. Based on these observations, previous researchers suggested that coal maturity in the Illinois Basin increased in response to the input of heat from hot fluids passing through lower Paleozoic aquifers. If this model is correct, then the paleogeotherm recorded in strata above the lower Paleozoic should be straight. Our analysis of vitrinite reflectance in deep wells suggests, however, that the paleogeotherm of the basin contains an inflection point at the Mississippian-Pennsylvanian boundary. Specifically, temperature at the time of coalification increased with depth more rapidly (60°C/km) above the Mississippian-Pennsylvanian boundary than below (14°C/km). This observation suggests that hot groundwater driving coalification migrated through coals and adjacent sandstones in the Carboniferous section—the heat was not introduced exclusively from below. Our proposal is supported by initial studies of conodonts extracted from strata adjacent to the coal, for the conodonts are pitted by hydrothermal flow. Also, initial cathodoluminescence petrography reveals that dolospar cements in upper Mississippian sandstones below the coal units are brightly colored, as would be expected if they were precipitated from hydrothermal fluids. Finally, a model of groundwater flow, constructed using the Basin2 computer program, shows that the presence of a high-permeability layer at the Mississippian-Pennsylvanian boundary could have focused hot-water flow in the upper Paleozoic section, and can explain the regional distribution of vitrinite reflectance in the basin.