Erect Forests Are Evidence for Coseismic Base-level Changes in Pennsylvanian Cyclothems of the Black Warrior Basin, U.S.A.

Abstract

Examination of the plant taphonomic character and sedimentological processes responsible for preservation of an in situ, erect forest above the Pennsylvanian Blue Creek coal of the Mary Lee coal cycle, Alabama, provides evidence for rapid generation of accommodation space by coseismic subsidence. Standing vegetation is preserved at least to 4.5 m (15 ft) in height above the coal and includes lycopsids, regenerative calamites, tree ferns, and seed ferns (pteridospermous gymnosperms); the forest-floor litter is preserved as an adpression assemblage directly above the coal. Sediments entombing the standing trees, burying both the peat mire and forest-floor litter, and casting the erect vegetation consist of rhythmically bedded tidalites. Neap-spring-neap tidalite patterns indicate that entombment occurred on the order of a few decades, whereas burial of the mire and forest-floor litter happened on the order of weeks, if not days. Comparison with documented Holocene rates of eustatic and tectonic base-level changes indicates that eustatic processes alone cannot account for the generation of the accommodation required to provide a basis for the sedimentologic and taphonomic characteristics of the assemblage. Instead, coseismic subsidence of very high magnitude is determined to be the mechanism responsible for preservation. Hence, erect forests buried by estuarine tidal deposits provide evidence for rapid coseismic basinal subsidence. These criteria can be used to identify similar coseismic subsidence events beginning in the middle Paleozoic and provide constraints on the magnitude of event-driven base-level change in various basinal regimes.