A paradigm in the genesis of thick coal deposits and their unique angular relationships: A result of differential development of accomodation in the Powder River basin, Wyoming

Tertiary coal deposits in Wyoming's Powder River Basin contain the most abundant, thick, low-sulfur, low-ash, minable coal reserves in the U.S. Several of these coal deposits exceed 100 feet in thickness, and so have 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 bed 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 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 influences on differential development of accommodation within the basin, and (3) the coalification process - not compaction. The result of this study is a 2D structural reconstruction model showing the structural development of accommodation; alternating periods of clastic and organic deposition; and the development of stacked coal beds and parting geometry formation. Three supplemental files to this thesis, plates A-A', B-B', and C-C' illustrate the unique subsurface geometry of the coal deposits in the Powder River Basin. A structural reconstruction analysis was performed using cross section A-A', this analysis is the basis for the new model. There are four considerations implicit in this model: (1) the top of each coal represents a chronostratigraphic surface; (2) development of accommodation is syndepositional 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 that formed from an incompressible, organic-rich hydrogel.