MIDDLE DESMOINESIAN TIDAL CHANNEL LOG JAMS AND ROOF FLORA INDICATE COAL SWAMP-TERMINAL VEGETATION DOMINATED BY SIGILLARIA, TREE AND SEED FERNS – A MISSOURIAN ASSEMBLAGE

Small channels, called 'rolls' by miners, are widespread in the middle Desmoinesian Herrin, Springfield and Danville coal seams in the Illinois basin. Eroded into the top of the coal, rolls are associated with tidally laminated gray shale roof strata, and found close to large peat-contemporaneous river systems, likely reflecting the development of mudflats along channel margins during the early phases of marine transgression. Rolls are infilled with tidally laminated gray shale, the same lithology found covering the coal. As mapped in underground mines, rolls are sinuous and occur in subparallel swarms. Individual rolls range from 10 to 100s of meters and rarely, 1,000s of meters long. They are largest and most numerous where strata overlying the coal are of coarsest grain size.

Chaotic tangles of crisscrossing, horizontal to sub-horizontal, lycopsid trees are common in many rolls. We interpret these lycopsid tree accumulations as log jams. Sigillaria is the dominant genus found in rolls and is also one of several common lycopsid trees found as upright stumps and prone logs in nearby mine roof.

We interpret rolls to be tidal channels that formed when rapid marine transgression drowned the peat swamp. Strong tidal currents scoured the peat in an estuarine setting. As tidal channels advanced inland, trees and vegetation lining the banks collapsed into the channels, lodging in bends and constricted areas, forming log jams. The vegetation and trees trapped in the channels, in and around log jams, therefore represent a spatially averaged compositional picture of the species in that section of the swamp. Multiple channels in an area may, in aggregate, be sampling many kilometers of transects through the final vegetation of the swamp.

The above observations indicate that terminal phases of all three swamp forests were characterized by extensive stands of Sigillaria growing in close association with tree and seed ferns. These assemblages bear greater similarity to the dominant peat-forming vegetation of the drier Late Pennsylvanian than to that of the Middle Pennsylvanian, and indicate these swamps experienced late-stage climate shifts from humid to wet-subhumid, with increasing rainfall seasonality, previewing larger scale climate changes that culminated in vegetational turnover at the Desmoinesian-Missourian boundary.