Abstract

Tertiary coal deposits in northeastern Wyoming’s Powder River Basin (PRB) contain the most abundant, thick, low-sulfur, low-ash, minable coal reserves in the United States. Several of these coal deposits exceed 100 feet in thickness, and consequently their formation has been of great interest to geologists. Several models have been proposed to explain the origin of these thick coal deposits. These models attribute the development of accommodation and the nature of coal splitting (parting geometry) to sedimentary processes (differential compaction, channel switching, and crevasse splay deposits) within specific depositional environments (raised mires, deltas, and basin-wide wetlands). Most are based on peat-to-coal compaction ratios ranging from 3:1 to 20:1 (3–20 feet of peat compacts to form 1 foot of coal).

This study proposes an alternate hypothesis that explains the genesis of thick Tertiary coal deposits on the basis of(1) chronostratigraphic correlation (sequence stratigraphy) of coal beds, (2) basement-related structural influence on differential development of accommodation within the basin, and (3) the coalification process –not compaction. A two-dimensional structural reconstruction model illustrates the structural development of accommodation, alternating periods of clastic and organic deposition, and the formation of stacked coal beds and accurate parting geometries. The model is based on four assumptions: (1) the top of each coal represents a chronostratigraphic surface; (2) development of accommodation is syndepositional with and controlled by basement faulting; (3) syndepositional and post-depositional compaction of organic and clastic sediments is minimal; and (4) thick coal deposits comprise numerous thin coal beds. Cross sections A–A’, B–B’, and C–C’ (Plates I–III) illustrate the unique subsurface geometry of the coal deposits in the PRB. A structural reconstruction analysis, performed using cross section A–A’, illustrates application of the new model.