Organic geochemistry in Pennsylvanian tidally influenced sediments from SWIndiana

Tidal rhythmites are vertically stacked small-scale sedimentary structures that record daily variations in tidal current energy and are known to overl ie some low-sulfur coals in the Illinois Basin. Tidal rhythmites from the P ennsylvanian Brazil Formation in Indiana have been analyzed sedimentologica lly, petrographically, and geochemically in order to understand the charact er and distribution of organic matter (OM) preserved in an environment of d aily interactions between marine and fresh waters. The concentration of org anic matter (TOC) ranges from traces to 6.9% and sulfur rarely exceeds 0.1% in individual laminae. Angular vitrinite is the major organic matter type, accounting for 50-90% of total OM. The C/S ratio decreases as the vertical distance from the underlying coal increases. A decreasing C/S ratio couple d with decreases in Pr/Ph, Pr/n-C17, Ph/n-C18 ratios and a shift of carbon isotopic composition towards less negative values suggest an increase in sa linity from freshwater in the mudflat tidal rhythmite facies close to the c oal to brackish/marine in the sandflat tidal rhythmite facies further above from the coal. Within an interval spanning one year of deposition TOC and S values show monthly variability. On a daily scale, TOC and S oscillations are still detectable but they are of lower magnitude than on a monthly sca le. These small-scare variations are believed to reflect oscillations in wa ter salinity related to tidal cycles. (C) 1999 Elsevier Science Ltd. All ri ghts reserved.