A detailed comparison of styles of coal deposition of Tertiary rocks in the Powder River Basin shows that stratigraphic variations, areal distributions, and trends of depositional facies are the results of variations in alluvial settings. Styles of coal deposition are examined in coal-zone intervals, which are treated either as individual or multiple depositional packages. Facies stratigraphic variations display a common interrelationship of channel-levee facies bounded by floodplain and backswamp facies. Anomalously thick, minable coal deposits commonly are split into discrete coal beds separated by complexes of channel-levee facies. The areal distribution of channel deposits and laterally juxtaposed minable coal deposits display a primary north-to-south and secondary west-to-east depositional dip. Measurements of the crossbeds in channel deposits throughout the basin indicated that the direction of sediment transport was generally northeastward. The channel deposits were formed by braided, meandering, and anastomosed fluvial systems. The meandering streams are more commonly associated with backswamps of thick coal accumulation than are other fluvial systems. The south-to-north and west-to-east channel orientations of the fluvial systems are supported by subsurface net percent sandstone distributions within the basin.

The causes of the facies variations, distributions, and trends are deduced from what is generally known about modern alluvial plain environments. The Mahakam River, which drains the intermontane Kutai Basin in Borneo, is proposed as the best modern alluvial analog for the Tertiary deposits of the Powder River Basin. The alluvial plain of the Mahakam River is bordered by wet alluvial fans and consists of vast, continuous peat-forming backswamps that duplicate those of the Tertiary Powder River Basin coal swamps. In addition, the overall depositional elements of the Rio Paraiba do Sul in Brazil and the Upper Columbia and Lower Saskatchewan rivers in Canada are recognizable in the basin’s Tertiary rocks, giving rise to a hybrid model of coal deposition in the Powder River Basin. These modern analogs demonstrate that alluvial environments are areas where thick peat deposits, which are precursors of minable coal beds, can accumulate.