Cataclysmic burial of Pennsylvanian Period coal swamps in the Illinois Basin

Abstract

Roof facies of widespread, Pennsylvanian Period, economically important coals in the Eastern Interior Coal Basin (Illinois Basin) commonly exhibit evidence of extensive and cataclysmic tidal sedimentation during rapid marine-flooding events. Upright Trees and smaller plants were covered and entombed by swiftly deposited, thin-bedded and laminated mud and sand. This facies has historically been interpreted as fluvial-style cross bedding. Detailed examination clearly shows that these facies consist of inclined heterolith stratifications (IHS). Entire forests of upright trunks have been documented in the roof strata of surface and underground coal mines. Detailed sedimentological analyses of mine-roof facies indicate a pervasive and significant tidal influence. In some cases, daily and semi-monthly tidal periods have been preserved within laminated facies (‘tidal rhythmites’). Based upon modern analogues, the tidal facies are indicative of hypertidal conditions. The hyper-dynamic tidal regime resulted in rapid sediment accumulation, particularly along pre-existing drainages within the ancient coastal swamp. Stratigraphic successions indicate a recurring pattern of very rapid change from widespread coal-swamp conditions to tidally-influenced deposition. The repetition of these stratigraphic phenomena throughout the Pennsylvanian Period suggests significant external controls on sea-level, probably related to Gondwanan deglaciations and resultant large-scale meltwater pulses. Despite the rapid changes of sea-level rise, the hypertidal depositional dynamics resulted in conditions whereby inundated coastal forests were buried by tidally influenced sedimentation.