Evidence of Ancient Freshwater Tidal Deposits
Well-developed tidal rhythmites associated with tidal mudflats overlie some of the lowest-sulfur Pennsylvanian coals in the eastern part of the Illinois Basin, U.S.A. Such an observation is seemingly at odds with the traditionally held view that low-sulfur coals are not typically associated with marine-influenced roof rock in the Illinois Basin and elsewhere. This association can be explained, however, if the tidal mudflats formed in a freshwater or low-salinity marine setting. Such deposits are known from modern systems but have not been adequately documented from the rock record.
Geochemical, petrographic, and sedimentologic analyses of tidal deposits immediately above the low-sulfur (sulfur values <1%) Lower Block Coal Member of the Pennsylvanian Brazil Formation (Atokan), Daviess County, Indiana, confirm the potential for the preservation of extensive (>480 km2 area) freshwater tidal flat deposits in the rock record. A strong tidal (marine) signal, manifested as tidal rhythmites preserving small-scale neap-spring cycles, is preserved within the laminated mudstone and interbedded sandstone and mudstone immediately above the Lower Block Coal. These cycles indicate deposition within a mixed, predominantly diurnal tidal system in which sedimentation rates were as high as 1 m/yr. Carbon to sulfur ratios, macerai types, and the dominance of terrestrial organic markers within the rhythmite facies reveal that the Lower Block peat (coal) was initially onlapped by a freshwater (low salinity) tidal flat. The presence of tidal rhythmites indicates that at least mesotidal conditions prevailed during transgression.
The areal distribution, sedimentology, stratigraphy, petrography, and geochemistry of the succession from the Lower Block Coal to the next younger coal (Upper Block Coal Member of the Brazil Formation) suggest that the tidal flat facies formed within an embayed coastal setting that experienced significant rainfall and runoff typical of an ever-wet climate. The embayments likely formed by the collapse and transgression of the coastal peat mires (Lower Block Coal) and ultimately filled with mudflat and mixed sandflat and mudflat facies. Very high rainfall and associated runoff resulted in the formation of coast-hugging freshwater plumes. These plumes pushed the salt water wedge offshore and prevented it from entering the embayments until the coastal peats had already been covered by at least a meter of tidal flat mud.
The results of this study may have implications for paleogeographic reconstructions in other Carboniferous basins. Similar tide-dominated facies may have gone unrecognized in some Carboniferous successions in Europe, and elsewhere, that are currently interpreted to be ancient nonmarine mud-dominated tropical systems. Such recognition would have important sequence stratigraphic and paleogeographic implications.