The Galatia channel and similar paleochannels in the Illinois Basin yield valuable insights into patterns of climate and eustasy that controlled Pennsylvanian sedimentation. A dry, seasonal climate at peak interglacial highstand facilitated erosion and sediment transport from distant source areas. Deltas prograded rapidly into the Illinois Basin and filled nearly all accommodation space. At the onset of a glacial episode, sea level began to fall, exposing the delta plain. Following a tectonic trough, the Galatia river incised a deep valley. Having low gradient and traversing soft sediments, the river meandered actively. Away from the river, calcic Vertisols developed under a continuing dry, strongly seasonal climate.
Approaching the peak of a glacial episode, the climate in the basin changed from seasonally dry subhumid to ever-wet humid. Lush vegetation hemmed in the channel banks and inhibited the river from migrating laterally. The Springfield peat developed first in low places, then broadly across the lowlands. Sediment flux dropped dramatically and the Galatia became a black-water stream. Periodic floods carried fine sediment into the peat swamp as thin clastic splits.
With an abrupt climate shift, glaciers melted and the sea rose, drowning the peat swamp. The Galatia channel became an estuary, now carrying a large load of suspended sediment (whether from upstream or offshore is uncertain). Gray mud buried the peat, entombing tree stumps and other plant material. With continued transgression, offshore deposition of marine black shale and limestone ended the cycle.
The Walshville channel, contemporaneous with the younger Herrin Coal, is closely similar to the Galatia channel. The Colchester and Danville Coals have thick, low-sulfur deposits overlain by gray shale, but no channels because the preserved coal lay offshore from the river mouths. The Baker Coal lay farther inland than the other examples and no significant transgression followed peat accumulation. Thick, low-sulfur Murphysboro Coal flanks an estuary controlled by active tectonism.
Nelson, W.J., S.D. Elrick, W.A. DiMichele, and P.R. Ames, 2020, Evolution of a peat-contemporaneous channel: The Galatia channel, Middle Pennsylvanian, of the Illinois Basin: Illinois State Geological Survey, Circular 605, 85 p., 6 pls.