Abstract

Coal bed depositional patterns in the Upper Cretaceous Emery Sandstone Member of the Mancos Shale in the Henry Mountains coal field were controlled by interacting tectonic and sedimentological processes. In many instances these processes are reflected on the surface as folds that are genetically classified as tectonic or compactional. Evaluation of several of the folds by drilling or surface measurements shows that the coal beds are thick in synclines and thin over anticlines, regardless of fold genesis. The primary constraints on coal deposition were tectonic features such as the Henry Mountains syncline. The axis of the structure is coincident with the thickest coals in the basin. Near the southeastern margin of the coal field, evidence of penecontemporaneous faulting during coal deposition exists. The fault plane of a normal, dip-slip fault is terminated upward in the coal zone, and coal thickness on the upthrown side, immediately adjacent to the fault, is significantly reduced. Several nontectonic folds mapped in the basin are superimposed on the tectonic framework; they were produced by differential compaction and are related to depositional features in tidal and fluvial sequences. The thicker, better quality coal beds occur in the swamp facies of the fluvial flood basin deposits, whereas the thinner, less pure coal beds occur in the tidal sequences.