RE-EXAMINING THE OPEN-BASIN RAISED MIRE MODEL FOR THE ORIGIN OF THE UNIQUE POWDER RIVER BASIN, WYOMING COALS AND PRESENTATION OF A CLOSED-BASIN SWAMP MODEL

“Raised swamps” first appeared in the literature 1897 with the first paleo-raised mire identified in 1962. A number of workers in the 80's invoked raised mires as the causal factor in the development of the extraordinarily thick, low ash and sulfur coals of the Powder River Basin (PRB). Since that time, the raised mire has become the mire of choice for the vast majority of workers in spite of the model satisfying but a few of the parameters.

Any model of the origin of the PRB coals must explain multiple seams with extraordinary thickness, low ash, low sulfur, and great areal extent (single seams over a thousand square miles). The model must also fit the geologic structure and history of a rapidly aggrading basin. A re-examination of the raised mire model is warranted in part by the fact that where there were only 1,000 data wells for model building in the late eighties, now there are more than 10,000.

Raised (domed), ombroptropic mires will indeed be low in ash and sulfur by virtue of being rain fed and above the floodplain. Beyond these two qualities, however, raised bogs have little to offer as a model for the origin of the PRB coals. Such wide acceptance of the raised mire model primarily on the basis of only two parameters seems unwarranted in view of the “stretch” needed to account for other characteristics of the incredible PRB coals.

The commonly accepted model for the coal producing mires of the PRB invokes an “Open Basin (flow through fluvial system)” with raised mires between channels. The author proposes a “Closed Basin (lake system)” model with planar swamps replacing the lakes during peat forming times. This requires only that the water table be lowered to, or near, the ground surface instead of being some feet above it. The closed basin swamp model is far simpler and comes much closer to meeting the characteristics of the PRB coals. Physical/ chemical processes operating at the edges of the swamp protect the interior of large swamps from flood sedimentation. Present day analogues exist including the processes within the Okefenokee Swamp and the size of the Pantanal.