DESMOINESIAN COAL BEDS OF THE ILLINOIS BASIN AND MIDCONTINENT--THE BROADEST AND THICKEST TROPICAL PEATLANDS IN EARTH HISTORY

The Colchester, Springfield, and Herrin coal beds of the Illinois Basin are some of the most extensive coals in North America, if not the world. The Colchester covers an area of more than 100,000 km2, the Springfield 73,500 to 81,000 km2, and the Herrin 73,900 km2. Each has correlatives in the Midcontinent, such that their regional extent varies from 116,000 km2 to 200,000 km2. The blue-band parting of the Herrin and correlative coal beds suggests that at least the upper bench of this coal represents an expansive coeval mire across much of its area of occurrence, rather than slightly time-transgressive mires. The Colchester coal is thin, but the Springfield and Herrin coals reach nearly 3 m in thickness.

High ash yields, dominance of vitrinite macerals, and abundant lycopsids suggest that these Desmoinesian coals were deposited in mostly topogenous (ground- and surface-water fed) mires. The only modern mire complexes that are as widespread are northern-latitude mires. In these modern, cold-climate wetlands, the expanse of the mires is attributed to paludification of an expansive intracratonic area, resulting in impeded drainage, seasonal to permanent waterlogging of the impermeable substrates, and peat accumulation. Except for differences in latitude, (paludification in the Desmoinesian occurred in warm tropical climates), such a model seems applicable to the Desmoinesian coal deposits, as the most widespread coal beds are associated with the most widespread paleosols.

Tropical climate settings and eustatic transgressions during optimal cyclothemic conditions of the Late Pennsylvanian also contributed to the thickness of Desmoinesian paleomires. If a compaction ratio of 10:1 is used to interpret original peat thickness, then vast expanses of these peatlands may have been between 13 and 20 m thick. Other factors that contributed to peat thickness and expanse were (1) extremely broad floodplains along large rivers with enough flooding to hydrologically link peatlands and keep them wet, (2) accumulation in a basin surrounded by low relief, which led to minimal sediment input from the rivers, and (3) low rates of tectonic subsidence.