Hypertidal facies from the Pennsylvanian period: Eastern and Western Interior Coal Basins, USA

Abstract

Siliciclastic tidal facies have been recognized in Pennsylvanian coal measures of the Eastern Interior (Illinois) and Western Interior (Forest City) basins. In particular, rhythmic tidal laminations or tidal bundles are recorded in shale-rich, heterolithic estuarine and coastal paleofacies, as well as within tidal and fluvio-estuarine channels. The tidal facies are recurring and range from the upper Morrowan (Early Pennsylvanian) through at least the Desmoinesian (late Middle Pennsylvanian). Laminae-thickness series within tidal facies in both basins exhibit a variety of well-developed, tidal cycles that include semidiurnal, diurnal, neap-spring, apogean-perigean, and seasonal to annual periodicities. Study of modern analogs, predominantly from hypertidal settings, provides evidence to suggest the presence of elevated paleotidal ranges in the Pennsylvanian seaways in both basins. The tidal facies are best developed within transgressive systems, particularly within incised valley-fill sequences. During sediment accumulation, the extreme tidal dynamics resulted in widespread deposition of rhythmites. Cyclic rhythmites that contain high-resolution records of daily to yearly periodicities are much more locally restricted. Preservation of tidal rhythmites was likely aided by (1) rapid, high-magnitude changes in global paleosealevel, (2) strongly resonant depositional embayments, (3) formation of large tropical Pangean rivers during lowstand that were converted to estuaries during subsequent periods of glacial melting and the resultant sea-level rise, and (4) a strongly resonant, extensive global paleo-ocean. Preservation of cyclic tidal rhythmites that contain high-resolution records were likely controlled by the generation of local accommodation space via (1) peat compaction, (2) faulting, and (3) tidal and fluvial channel avulsion.