Evolution of a PeatContemporaneous Channel: The Galatia Channel, Middle Pennsylvanian, of the Illinois Basin

Abstract

For more than 40 years, geologists have understood that the thickness and quality of the Springfield Coal are intimately related to the Galatia channel, a paleochannel that existed contemporaneously with peat deposition. Early models envisioned a setting similar to the Mississippi delta, in which the river periodically breached its natural levees and carried crevasse splays of gray mud (Dykersburg Shale) into flanking peat swamps, shielding peat from a later influx of sulfur-bearing marine water and sediment. Using new findings and reinterpreting old maps, we present a new model for Galatia channel development. Before the formation of Springfield peat, falling sea levels exposed the Illinois Basin area to soil development and fluvial incision under a seasonal semiarid to wet subhumid climate. A "precursor" Galatia channel, carrying a bed load of sand, formed a meander belt several miles (kilometers) wide. Under a progressively more humid climate at glacial maximum, vegetation cover flourished and Springfield peat accumulation took place. As inferred under previous models, the thickest peat formed in lowlands flanking the channel. Dense, strongly rooted vegetation stabilized channel banks and restricted upstream sediment runoff. As a result, meanders became locked into place and the Galatia became a "black-water" stream that carried only fine suspended sediment. Some of this sediment was carried into peat swamps along the channel margins, creating belts of shaly coal a few hundred feet (meters) wide bordering the no-coal area. At the end of the glacial episode, the sea level rose, drowning the peat swamp and turning the Galatia channel into a broad estuary flanked by mud flats. With concurrent changes in atmospheric circulation, the Illinois Basin climate shifted from year-round rainfall to a strongly seasonal, wet–dry (monsoonal) regime. This drier climate led to reduced vegetation cover and increased soil erosion and runoff in the Galatia drainage basin. Thus, the channel carried a heavy sediment load, largely silt and fine sand. Much of this was deposited in the estuary as the Dykersburg Shale, which rapidly buried the Springfield peat to a depth of more than 98 ft (>30 m). As envisioned by earlier geologists, this thick deposit of gray, tidally influenced sediment shielded the peat from sulfur-rich marine water and sediment that invaded the area during maximum transgression. Large-scale rafting of peat during the initial stage of transgression produced "splits" and large coal-seam disruptions. Other channels developed at the same time as the Galatia. The south-flowing Sullivan channel in Indiana is probably an upstream segment of the Galatia. A large southeast-flowing system in eastcentral Illinois, named the Effingham channel, was abandoned before Springfield peat formation. The Leslie Cemetery channel in southern Indiana began as a precursor tributary to the Galatia channel that was abandoned and then reoccupied during later stages of peat formation. The poorly understood Terre Haute channel may have a similar origin. Channels and gray-shale clastic wedges similar to the Galatia channel and Dykersburg Shale affected the Murphysboro, Colchester, Herrin, Baker, and Danville Coals. Each of these presents variations on the theme, the Herrin example being most similar to the Springfield. We interpret coal (peat) as having formed during glacial maxima, when the sea level was lowest and global cooling pinned the intertropical convergence zone near the equator. The resulting ever-wet climate in the tropics maintained the consistently high water table required for the production and preservation of peat. A warming cycle brought deglaciation, rapid sealevel rise, and a change to the seasonal wet–dry tropical climate, which in turn caused rapid drowning and burial of the peat deposit. The impact of the Galatia channel and its analogues on coal thickness and quality has been understood since the 1960s. Our new findings and model of origin for these channels provide insights into the driving forces behind sedimentary cycles overlooked by most previous authors.