SPLIT COAL ORIGIN AS FLOATING PEAT MATS: SPRINGFIELD COAL, ILLINOIS

Large wedge-shaped “splits” of siltstone and shale within the Springfield Coal near the contemporaneous Galatia channel in southeastern Illinois previously were interpreted as crevasse-splays or overbank sediments laid down during peat accumulation. However, spectacular exposures in underground mines support a different origin, not previously considered, for some of the splits. Significant observations include (1) absence of roots and paleosols beneath upper coal benches, (2) absence of buried tree stumps (common elsewhere in these mines) on top of lower benches, (3) ragged coal stringers that splay off the undersides of upper benches, (4) uniform lithology above, below, and lateral to split coal, (5) abundant stringers and mats of coal within splits, and (6) prevalence of tidal rhythmites in splits. Splits that bear these features range from tens to hundreds of meters long and as thick as 15 meters. In some cases coal benches reunite, in others the coal terminates against clastic rocks.

We propose that these splits formed after Springfield peat was completely formed. The setting was an estuary as eustatic transgression drowned the peat mire. Buoyed by trapped gas and hammered by tides, upper peat layers tore free and floated upward. Silt then passively filled the space beneath the floating mats.