Evolution of a peat-contemporaneous channel and origin of split coal, Springfield Coal, Pennsylvanian of Illinois Basin.

 

Scott D. Elrick and John Nelson

Illinois State Geological Survey

Abstract for John's part of the talk:

EVOLUTION OF A PEAT-CONTEMPORANEOUS CHANNEL: GALATIA CHANNEL, PENNSYLVANIAN OF ILLINOIS BASIN
The Galatia channel in southern Illinois and Indiana is intimately associated with thick, low-sulfur Springfield Coal. Previous geologists envisioned a meandering river with crevasse splays that disgorged gray mud on top of the peat, like the modern “bird's foot” Mississippi delta. New information, including closely-spaced core drilling and mine entries driven across the channel, directs us to a new model of channel origin. This model conforms with the sediment climate model developed by Blaine Cecil and colleagues.

During eustatic lowstand prior to Springfield peat, the region underwent subaerial exposure, soil formation (underclay), and valley incision under a seasonal wet-dry climate. Rivers carried dominantly sand and meandered actively, as Paul Potter mapped during the early 1960s. Eventually basin subsidence and change from seasonal to ever-wet climate initiated development of Springfield peat. Dense vegetation now locked meanders in place while greatly reducing runoff. The Galatia channel became a black-water stream that carried only suspended clay and organic sediment. Peat and clay were thinly interlaminated along channel margins. Finally, eustatic sea-level rise, brought about by glacial melting, drowned the peat mire. A simultaneous return to seasonal climate suppressed upland vegetation and liberated sediment transport. The Galatia channel now became an estuary with much greater flow and sediment volume. Thick gray clay, silt and fine sand (Dykersburg Shale) rapidly buried the Springfield peat, compressing it and making space for more sediment. As previous geologists understood, Springfield Coal covered by thick Dykersburg Shale has low sulfur content because it was shielded from later sulfide-bearing marine water and sediment.

Abstract for Scott's part of the talk:
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.