Paleocene Coal
Basins
Stratigraphy and sedimentology, disciplines which investigate the organization and continuity of rocks and their depositional processes, respectively, play a key role in interpreting ancient landscapes and evolution of accompanying coal-forming peat accumulations. Close examination of the stratigraphy and sedimentology of Paleocene strata can provide clues to processes responsible for forming these layers of rocks. Moreover, careful analysis of the geometry, internal characteristics, and composition of sandstones deposited by ancient river systems provides unique insights to the lateral distribution of river channels, river flow directions, basin drainage patterns, and sources of river sediments. For instance, river sandstones that contain significant amounts of coarse granitic material suggest erosion of an exposed granitic core from mountain ranges near the river's source. From such studies of sediment provenance, interpretation of the location and extent of mountain building activity is also possible. In addition, the relationship of coal beds to river deposits provides clues to the depositional environment of peat accumulation, and to factors controlling thickness, continuity, distribution, and chemical composition of the coal beds.
At the time when Paleocene coal-forming peats accumulated, the Rocky Mountains and Northern Great Plains regions were primarily continental: marine and coastal plain environments were restricted to areas near the retreating Cannonball Sea in what is now North and South Dakota. Areas west and southwest of the Cannonball Sea, in what is now Montana and Wyoming, were in constant change as a result of continuing uplift of mountain ranges and the formation of basins between these uplifts. Through time, river systems and their associated floodplains and lakes formed a network of waterways in the basins. From early to middle Paleocene, rivers generally flowed eastward and northeastward toward the retreating seaway. However, as mountain building continued to reshape the landscape during the late Paleocene, rivers within developing basins were either ponded into lake systems or flowed through the basins toward the seaway. Coal-forming mires developed in the basins between river channels and along the lake margins. The extent and distribution of the mires were controlled partly by the size and pattern of these networks of rivers and lakes, and partly by mountain-building activity. As a result, coal beds that formed from these mires are generally discon-tinuous and "pod-shaped." Many of the thick peat accumulations, which resulted in thick coal beds, developed in multiple domed mires that sheltered the peat from floods and burial by sediments. Coal beds in the Powder River basin in excess of 60 meters thick reflect the repeated life cycle of mire growth, demise, and rejuvenation over long periods of time within tectonically-subsiding basins.