ORGANIC GEOCHEMICAL STUDY OF SEQUENCES OVERLYING COAL SEAMS - EXAMPLEFROM THE MANSFIELD FORMATION (LOWER PENNSYLVANIAN), INDIANA

Roof successions above two coal seams from the Mansfield Formation (Lo wer Pennsylvanian) in the Indiana portion of the Illinois Basin have b een studied with regard to sedimentary structures, organic petrology a nd organic geochemistry. The succession above the Blue Creek Member of the Mansfield Formation is typical of the lithologies covering tow-su lphur coals (< 1%) in the area studied, whereas the succession above t he unnamed Mansfield coal is typical of high-sulphur coals (> 2.%). Th e transgressive-regressive packages above both seams reflect the perio dic inundation of coastal mires by tidal flats and creeks as inferred from bioturbation and sedimentary structures such as tidal rhythmites and clay-draped ripple bedforms. Geochemistry and petrology of organic facies above the Blue Creek coal suggest that tidal flats formed inla nd in fresh-water environments. These overlying fresh water sediments prevented saline waters from invading the peat, contributing to low-su lphur content in the coal. Above the unnamed coal, trace fossils and g eochemical and petrological characteristics of organic facies suggest more unrestricted seaward depositional environment. The absence of sal ine or typically marine biomarkers above this coal is interpreted as e vidence of very shea periods of marine transgression, as there was not enough time for establishment of the precursor organisms for marine b iomarkers. However, sufficient time passed to raise SO42- concentratio n in pore waters, resulting in the formation of authigenic pyrite and sulphur incorporation into organic matter.