SPATIAL PATTERNS IN THE FINAL FOREST OF A DROWNED PEAT MIRE, SPRINGFIELD COAL (MIDDLE PENNSYLVANIAN), ILLINOIS BASIN

The spatial distribution of plants in ecosystems is a key component to understanding landscape structure and dynamics. Underground coal mines provide unparalleled opportunities to map such spatial distribution of fossil plant assemblages in relation to their environments. We carried out such mapping in five mines of southern Illinois and southwestern Indiana. All five exploit thick, low-sulfur Springfield Coal along the margins of an ancient river channel (Galatia channel) that existed during peat formation. The mines span a distance of 125 km along the channel. Fossil plants are abundant in gray shale (Dykersburg Shale) immediately overlying the coal. The Dykersburg represents mud deposited as the Galatia channel became an estuary (fresh to slightly brackish, tidally influenced) when the coastline was drowned due to basin subsidence and/or eustatic sea-level rise. Tree stumps, particularly of the lycopsid trees Lepidodendron and Sigillaria, commonly are preserved in growth position, signifying abrupt burial of the peat. They are associated with well preserved remains of foliage and tree trunks of a variety of plant groups, indicating autochthonous and parautochthonous preservation. Sedimentation rates of several tens of cm per year are estimated by counting tidal rhythmites in the Dykersburg. Although overall species diversity is high across the study area, vegetation is clumped into compositionally distinct patches of low internal diversity. Such patches sometimes can be related to peat-swamp topography and distance from the channel. We are finding similar patterns in the slightly younger Herrin and Danville Coals of the same region.