DEPOSITIONAL FRAMEWORK OF THE BAKER COAL IN THE ILLINOIS BASIN

Like the Springfield, Herrin, and other major Illinois Basin coal beds, the Baker Coal is thickest and lowest in sulfur along a channel that flowed while the peat formed. However, geologic relationships and presumed origin for the Baker differ from the other examples.

The Baker Coal is confined to the southeastern Illinois Basin, where it locally exceeds 3 m thick and is relatively low sulfur. The Baker Coal has been called Allenby Coal in Illinois and Lower Millersburg in Indiana; the name Baker has priority. In Indiana the Baker was incorrectly identified as the Hymera Coal. Thick Baker Coal is concentrated along both margins of a contemporaneous channel (called Winslow in Indiana and Henderson in Kentucky). The coal is commonly split with layers of claystone and carbonaceous shale close to the channel and tends to thicken into pre-existing channels beneath the coal.

Unlike the Murphysboro, Springfield, and Herrin Coals, the Baker Coal lacks a thick estuarine gray shale roof close to its contemporaneous channel. Instead, the Baker is overlain everywhere by non-marine strata, largely flood plain but also fresh-water lake deposits.

We propose that a prolonged low stand of sea level set the stage for Baker Coal deposition. The entire basin area was exposed, soils formed, and fluvial channels eroded deeply into underlying strata. As climate shifted toward the ever-wet of glacial maximum and tectonic subsidence raised the water table, thick peat developed in low-lying areas, particularly along the edges of the Winslow-Henderson channel. Following glacial termination and a shift toward more seasonal climate, sea level rose, but remained low relative to the Illinois Basin such that non-marine sediments covered Baker peat. Not until the end of the next higher coal deposit was there complete de-glaciation and sea-level rise, leading to drowning of the peat-forming forest.

We speculate that the depositional setting after Baker peat formation and during glacial termination is representative of the uplands that existed upriver of the flooded estuaries of the Herrin, Springfield, Colchester and Murphysboro Coals, but now lie beyond the eroded limits of the Illinois Basin.