PERSISTENCE OF LATE CARBONIFEROUS TROPICAL VEGETATION DURING GLACIALLY DRIVEN CLIMATIC AND SEA-LEVEL FLUCTUATIONS

The Late Carboniferous glaciation reached its peak during the latter p art of the Westphalian (late Desmoinesian). In the tropical Illinois b asin this was the time of deposition of the Carbondale and lower Modes to Formations, characterized by the cyclic repetition of major coal de posits, black shales, limestones, gray shales and sandstones. These li thological changes evidence repeated major fluctuations in climate and sealevel. Fossil floras from the tropical ever-wet to seasonally wet terrestrial lithofacies, coals and shales above coals, remain composit ionally similar throughout the 1-1.5 m.y. time interval. Floras from c oals, known from coal balls and palynology, characteristically were co mposed of three major plant communities: the wettest sites were domina ted by monocarpic lycopsids, intermediate sites were dominated by poly carpic lycopsids, and areas subject to fires and intermittent flooding were dominated by medullosans and the small lycopsid Paralycopodites. Clastic-substrate environments were dominated by ferns and pteridospe rms and conform to a single biozone, indicating their compositional un ity and distinctness from earlier and later assemblages. The compositi on of lowland floras is poorly known from times between the deposition of coals and associated terrestrial rocks, intervals during which mos t of the lowlands were flooded and marine rocks were being deposited. Consequently, the temporally intermittent recurrence of lowland floras despite repeated, widespread environmental disruption may be explaine d either by vegetational persistence and migration of floras, or repea ted disruption and reassembly. Several lines of evidence suggest persi stence as a likely explanation.