This section describes, in ascending order, the rock units that enclose the Springfield Coal and Galatia paleochannel(Figure 4). To facilitate discussion of the new depositional model, two new members are proposed.
- Houchin Creek Coal
- Excello Shale Member
- Hanover Limestone Member
- Delafield Member (New)
- Galatia Member (New)
- Underclay of Springfield Coal
- Springfield Coal
- Dykersburg (Shale) Member
- Turner Mine Shale Member
- St. David Limestone Member
Houchin Creek Coal
Excello Shale Member
Hanover Limestone Member
Delafield Member (New)
Galatia Member (New)
Underclay of Springfield Coal
Dykersburg (Shale) Member
Turner Mine Shale Member
St. David Limestone Member
- The Houchin Creek Coal formed as an in situ peat deposit on a vast, level, stable coastal plain.
- The Excello Shale records rapid marine transgression to the point where bottom water became anoxic because of the absence of circulation and the abundance of plant-derived organic matter.
- The Hanover Limestone reflects restoration of normal marine circulation in deep water offshore, probably under a seasonally dry climate.
- The Delafield Member records progradation of the shoreline into brackish water under a falling sea level, essentially filling the basin with clastic sediment.
- 5. The Galatia Member fills an incised valley that was cut and filled during regression to early lowstand. The river meandered actively and carried a heavy load of sand, rapidly backfilling its meander belt.
- Springfield peat formation commenced during lowstand (maximum glaciation) under an ever-wet climate that produced a perennially high water table. Vegetation stabilized meanders of the Galatia channel, which transitioned to a black-water stream that carried only fine-grained sediment. There were no natural levees, but belts of laminated shaly coal flank the Galatia channel.
- The Dykersburg Member records the onset of transgression, which converted the Galatia channel to an estuary and drowned the peat swamp. Vigorous tidal currents dislodged floating mats of peat, creating rolls, splits, and localized major disruption of the seam. As the climate became seasonally dry, the fluvial runoff and sediment load increased. Gray Dykersburg clastics rapidly entombed the peat.
- 8 Deposition of the marine Turner Mine black shale, St. David Limestone, and younger units completed the story.
The Galatia channel provides insights into events that are not recorded in most Carboniferous cyclothems. This example indicates, in conformance with some other studies (e.g., Cecil et al. 1985, 2003b, 2014; Eros et al. 2012; Horton et al. 2012; DiMichele 2014), that glacially driven sea-level fluctuations were linked to climate changes in the tropics. It also refines our understanding of when certain events (such as the development of peat) took place within the eustatic and climatic cycle. These themes will be developed further in a later section of this report. For a more complete understanding of the Galatia channel, it is necessary to investigate other paleochannels related to the Springfield Coal.
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.