EVIDENCE FOR HYDROTHERMAL-FLUID-DRIVEN DIAGENESIS OF DUNKARD GROUP STRATA IN SOUTHEASTERN OHIO

Petrographic data from the Marietta Sandstone of the lower Dunkard Group (Pennsylvanian-Permian) indicate that hydrothermal fluids raised temperatures during early burial to 200˚ to 250˚C thus causing the conversion of unstable minerals to clays. These data also help explain the significant loss of porosity during compaction and the formation of bituminous coal throughout the basin with what would otherwise be insufficient burial. The Marietta Sandstone is a low-permeability quartz sandstone with high clay and mica contents, no K-feldspar, and just a few scattered grains of plagioclase. The sandstone shows significant evidence of compaction (16%-34%), including common quartz-grain suturing, but lacks any sign of quartz overgrowths. The clays occur almost exclusively as sand-grain-sized clusters rather than as layers, laminae, or cements; many clusters are clearly pseudomorphs after K-feldspar, including penetrating into adjoining quartz grains, and a few are muscovite pseudomorphs or partial replacements with remnant portions of the source muscovite or byproduct sericite preserved. The clays in all of the more equant clusters appear to be kaolinite and are interpreted as replacements of an original K-feldspar fraction that would have constituted up to 40% of the original sediment; most of the muscovite replacement is illite. The conversion of K-feldspar to kaolinite, coupled with the lack of quartz overgrowths, indicates flushing by a hydrothermal fluid that removed the excess potassium and silica. The through-flow of the fluids clearly ended as the muscovite began to break down. The partial breakdown of the muscovite fraction, coupled with no evidence for biotite degradation, requires that hydrothermal fluids raised temperatures to above 200˚ as the loss of permeability shut down the flushing.