Coal Palynology and Sedimentology in the Tongue River Member, Fort Union Formation, Powder River Basin, Wyoming

Abstract

The Paleocene Tongue River Member of the Fort Union Formation in the Powder River Basin, Wyoming, was deposited in meandering and anastomosed fluvial environments. In swamps associated with the meandering-fluvial environment, thick coals (e.g., Anderson coal) accumulated, and in swamps associated with the anastomosed-fluvial environment, thin coals (e.g., Smith coal) accumulated. Integrated palynologic and sedimentologic analyses of the Anderson and Smith coals and their associated sediments enhance our understanding of their environments of deposition. The Anderson coal accumulated as thick peat in a raised swamp that was above the localfluvial drainage level and relatively free of detrital influx. The Anderson coal swamp was vegetated by a forest of trees related to Glyptostrobus and broadleafplants whose distribution was governed by the local drainage patterns on the swamp and differential decay and accumulation of organic material. The presence nearby of a channel-levee system is recognized palynologically by the identification of pollen of riparian plants such as Pterocarya. The Smith coals accumulated as thin peats in low-lying swamps that began development marginal to flood-basin lakes and eventually encroached over the lakes. Frequent and rapid episodes of detrital influx curtailed peat accumulation. The Smith coal swamp was vegetated by a low-statured herbaceous vegetation in which Sphagnum was prominent, but later, with organic accumulation, a swamp forest consisting of plants similar to those of the Anderson coal swamp developed and replaced the herbaceous component.