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Abstract

Pennsylvanian (Atokan舑early Desmoinesian) parasequences in Indiana are thin (2舑13 m; 5舑40 ft) intervals that are composed of coal, siliciclastic, and carbonate-clastic units bounded by paleosols. Because the parasequences exhibit significant lateral and vertical lithologic variability and are so thin, they are difficult or impossible to discern on standard oil and gas geophysical logs. Therefore, in Indiana, regional correlations of this interval based primarily on geophysical logs and lithologic strip logs created from drill cuttings remain controversial. Detailed analyses of proprietary core from numerous locations in Daviess County in southwestern Indiana reveal that the most traceable of the parasequence facies in core are the paleosols which represent exposure surfaces that developed, in most cases, during apparent basinwide drops in relative sea level. Correlations are substantiated by detailed palynologic analyses of material collected from the bases of overlying marine-influenced flooding surfaces and by the use of thin, nearly continuous marker beds and the presence of certain biostratigraphically significant conodonts. Transgressive and regressive facies above the exposure surfaces are preserved with varying significance. The relative significance of the transgressive-regressive facies in a parasequence is, in part, related to the relative rates of changes in accommodation space and sea level. Detailed analyses of coal lithotypes and maceral compositions in two Atokan coal seams reveal that base-level rises during paleomire development were gradual in one and abrupt (catastrophic?) in another. Abrupt transgressions and the preservation of relatively thick transgressive sequences above the exposure surfaces were perhaps related to rates of mire collapse and compaction of the underlying peat (now coal) and soil (paleosol).

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