GEOLOGIC AND GEOMORPHIC CONTROLS OF COAL DEVELOPMENT IN SOME TERTIARYROCKY-MOUNTAIN BASINS, USA

Previous investigations have not well defined the controls on the deve lopment of minable coals in fluvial environments. This study was under taken to provide a clearer understanding of these controls, particular ly in of the lower Tertiary coal-bearing deposits of the Raton and Pow der River basins in the Rocky Mountain region of the United States. In this region, large amounts of coals accumulated in swamps formed in t he flow-through fluvial systems that infilled these intermontane basin s. Extrabasinal and intrabasinal tectonism partly controlled the strat igraphic and facies distributions of minable coal deposits. The region al accumulation of coals was favored by the rapid basin subsidence cou pled with minimal uplift of the source area. During these events, coal s developed in swamps associated with anastomosed and meandering fluvi al systems and alluvial fans. The extensive and high rate of sediment input from these fluvial systems promoted the formation of ombrotrophi c, raised swamps, which produced low ash and anomalously thick coals. The petrology and palynology of these coals, and the paleobotany of th e associated sediments, suggest that ombrotrophic, raised swamps were common in the Powder River Basin, where the climate during the early T ertiary was paratropical. The paleocecology of these swamps is identic al to that of the modern ombrotrophic, raised swamps of the Baram and Mahakam Rivers of Borneo.