Sedimentological and petrographical characteristics of Cretaceous strandplain coals: a model for coal accumulation from the North American Western Interior Seaway

Abstract

Coal seams formed on many Cretaceous wave-dominated strandplain sediments in North America are characterized by great lateral continuity (tens to hundreds of kilometres), substantial thicknesses (up to 12 m), relatively low ash and sulphur contents. The coals formed behind an active shoreline in areas undergoing subsidence due to shale compaction and dewatering. The zone of peat accumulation was remote from the shoreline and storm/tidal inundations and generally protected from fluvial flooding. If the rate of subsidence was too great, lakes formed and peat did not accumulate. Statistical evaluation of petrographic properties, by correspondence analysis, of the Lower Cretaceous coals show that the strandplain coals form distinctive petrographic groups that are characterized by relatively low vitrinite contents and high inertinite contents. Semifusinite and inertodetrinite dominate in the inertinite maceral group. Liptinite contents are negligible. Tissue Preservation Indices and Gelification Indices indicate for the strandplain coals a forest-type paleodepositional environment in which a relatively low water table allowed the accumulation of oxidized and partly oxidized components (fusinite and semifusinite). Significant amounts of detrital components, such as inertodetrinite and vitrinite B, are diagnostic that some transportation of the organic material took place prior to deposition. Comparison of coal facies and depositional environments from Permian coals of Australian show that the Lower Cretaceous strandplain coals have petrographic similarities to coals that were formed under regressive back-barrier conditions in the Permian. Due to differences in nomenclature, previously interpreted regressive back-barrier environments may be similar to the strandplain environments discussed here.