New insights on the stepwise collapse of the Carboniferous Coal Forests: Evidence from cyclothems and coniferopsid tree-stumps near the Desmoinesian-Missourian boundary in Peoria County, Illinois, USA

Abstract

The first phase in the stepwise collapse of the Carboniferous Coal Forests occurred near the Desmoinesian-Missourian boundary (early Kasimovian, ~ 307 Ma), and involved extirpation of Lycospora-producing lepidodendrids, and some other lycopsids, across most of tropical Euramerica. In this paper, we follow-up on historical reports of silicified tree-stumps in Peoria County, northwest-central Illinois, USA, which have significant implications for understanding Carboniferous Coal Forest collapse. Rooted near the paleoweathered top of the Lonsdale Limestone, and widespread across an area of ~ 250 km2, the silicified tree-stumps belong to Amyelon-type coniferopsids. A key feature of the fossil wood is the occurrence of abundant axial parenchyma arranged along irregular growth interruptions, suggestive of climatic seasonality, an inference consistent with silicic preservation. The silicified fossil forest directly underlies the Exline Limestone and Athensville Coal, the horizons that mark the US-wide loss of Lycospora, and demonstrate that lowland areas were colonized by dryland coniferopsid forests following Coal Forest collapse. Placed in a cyclothem context, the silicified fossil forest horizon lies above the Maria Creek mudstone paleosol (top of Piasa cyclothem), in which earlier d18O analyses have identified a major pulse of global warming, and coincides with the 'Hanna City' paleosol (top of Lonsdale cyclothem), which is correlative with the Seminole Sandstone, a Midcontinent incised valley-fill representative of one of the most profound glacioeustatic falls seen in the Pennsylvanian record. Our new findings therefore demonstrate that Coal Forest collapse was closely linked to intensification of glacial cycle amplitude near the Desmoinesian-Missourian boundary, involving both extreme episodes of global warming and cooling.