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Abstract: Organic Petrology and Geochemistry in Pennsylvanian Tidally-Influenced Sediments from SW Indiana

MASTALERZ, MARIA, and ERIK P. KVALE

This study addresses small-scale variations of organic matter in rhythmically laminated sediments of tidal origin in the Brazil and Mansfield Formations (Pennsylvanian)in Daviess County, Indiana. The concentration of organic matter (TOC) in dark laminae ranges from 0.1 to 6.9% , and sulfur content usually does not exceed 0.1%. Small and angular fragments of vitrinite are the dominant organic matter type and they account for 50 to 90% of the total organic matter. Inertinite macerals are represented mainly by semi-fusinite that usually does not exceed 5% of total OM. Liptinite macerals occur as sporinite (pollen and spores), alginite and liptodetrinite. The weight ratio of carbon to sulfur (C/S) generally decreases as the distance from the underlying coal increases. The C/S ratio is much higher close to the Lower Block Coal (transgressive facies) with values of 138 to 30. Further away from the Lower Block Coal (regressive facies), the ratio drops to as low as 2. Pyrograms show the dominance of phenols, (alkyl)benzenes and (alkyl)naphthalenes, suggestive of lignin-like material. n-Alkanes are subordinate and the dominance of mid-chain n-alkene/n-alkane over n-C25-n-C33 suggests the input from sporinite and alginite as their source. No thiophenic or elemental sulfur have been detected on the pyrograms. Transgressive facies show usually more negative carbon isotope values (-21.582 to-23.851) than the regressive facies (-18.814 to-20.473). The Petrological and geochemical data obtained suggest that organic matter variations within the rhythmite sequence are either related to salinity changes or to short-term climatic fluctuations.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky