Transgressive vs. forced regressive peat accumulation: Response of fluvial-apron peatland floodplains to aggradational transgression vs. forced regression

Abstract

Previous work suggests the possibility that peat accumulation during relative sea-level (RSL) fall may be more common in the Quaternary and geological records than is currently commonly assumed. We investigate this further based on sequence stratigraphic analysis of successive coal-bearing cyclothems from the North China Permo-Carboniferous cratonic basin. The results show that each cyclothem changes basinward from a purely fluvial sequence to a mixed marine–nonmarine sequence. It is bounded by maximum regressive surfaces, correlative paired regional composite scour and interfluvial exposure surfaces farther landward, and correlative combined wave-ravinement and maximum regressive surfaces farther basinward. Temporally, each cyclothem transitions from a transgressive through highstand to falling-stage systems tract directly followed by the transgressive systems tract of the next cyclothem without an intervening lowstand systems tract. Depending on geomorphic conditions and characteristics of RSL rise or fall, the downstream fluvial reaches during transgression can be dominated by fluvial-fan or delta-plain channel aggradation, estuarine valley degradation, or lagoonal or fluvial-apron floodplain aggradation, or they transition from one to another. The downstream fluvial reaches during forced regression typically transition from delta plain degradation-inclusive through strandplain grade to fluvial apron aggradation-sustainable. Peat accumulation can be sustained only in fluvial-apron peatland floodplains during aggradational transgression or aggradational forced regression. Forced regressive fluvial-apron floodplain coal seams typically exhibit an overall upward decrease in mineral, ash, inertinite and sporinite contents and groundwater index, along with an increase in vitrinite content, gelification, tissue preservation and vegetation indices. Reverse trends of these properties characterize transgressive fluvial-apron floodplain coal seams.