FACIES RELATIONSHIPS OF THE MIDDLE PENNSYLVANIAN SPRINGFIELD COAL AND DYKERSBURG SHALE: CONSTRAINTS ON SEDIMENTATION, DEVELOPMENT OF COAL SPLITS AND CLIMATE CHANGE DURING TRANSGRESSION

The Galatia channel, in the southeastern part of the Illinois Basin, developed contemporaneously with the Springfield peat. Mine exposures and core data show that the coal grades from normal bright banded coal, free of partings, to an organic-rich shale within a distance of 200 to 300 m approaching the channel. The coal and organic shale transition is overlain with an erosional contact by Dykersburg Shale. These relationships suggest a reduction in channel bedload sediments during peat formation. Adjacent to the channel, upright tree stumps are rooted into the coal and encased in tidally laminated Dykersburg Shale, suggesting abrupt drowning of the peat swamp and rapid transition to a sediment-rich estuarine environment. Coal at the channel margin locally contains siltstone splits meters thick and more than a kilometer across. We interpret these splits as initiating from shaley partings. As peat was submerged, layers separated along eroded partings, floated upwards and estaurine silt was deposited between peat layers, producing split coal. Thus, siltstone splits are younger than the peat. We propose that the observed succession is best explained by a change of climate during transgression. Everwet conditions at lowstand to early transgression fostered peat development. Rapid change to seasonal wet/dry conditions and rapid transgression, reduced vegetation density, increased erosion and increased sedimentation into the Galatia estuary.