Dike-induced thermal alteration of the Springfield Coal Member (Pennsylvanian) and adjacent clastic rocks, Illinois Basin, USA

Abstract

A basaltic dike intrusion caused a short-term and short-distance contact aureole in the vitrinite-rich Springfield Coal Member of the Carbondale Formation (Pennsylvanian) and in the underlying paleosol and overlying black shale in the Illinois Basin. Parallel series of samples from all three horizons were analyzed for vitrinite reflectance (Ro, %) and other parameters to evaluate the lateral extension of the thermal aureole. Ro values in the porous and fluid-permeable coal increase from a background value of ~0.66% at ≥4.0m distance from the dike to more than 5.0% at the intrusive contact. Ro values in the underlying paleosol increase from a background of ~0.65% at ≥3m distance to 1.74% at the dike contact. In contrast, roof rock shale close to the contact had a higher Ro value than the paleosol (Ro=2.56%). These differences cannot be explained by thermal conductivity of heat alone, or the coal having lower thermal conductivity would show lower vitrinite reflectance values. Apparently, the coal having much higher Ro values experienced enhanced lateral convective heat transfer in comparison to the denser and less permeable, but more conductive, rocks in the paleosol and overlying black shale. The advection of relatively cold fluid in the lower part of the coal seam streaming towards the dike may have provided cooling for floor rock close to the coal near the contact. In contrast, roof rock close to the contact had a higher Ro value than the paleosol (Ro=2.56%) in the absence of cooling.