Abstract

The Mt. Gunnison coal property is located in the Somerset coal field, Gunnison County, Colorado. Upper Cretaceous strata of the Mancos Shale and the Mesaverde Formation crop out in and around the coal property. For the purposes of this paper, the Mesaverde Formation has been divided into four members; they are, from bottom to top: the Rollins Sandstone, the lower coal member, the upper coal member, and the Barren Member.

The Mesaverde Formation, as found on the Mt. Gunnison coal property, was deposited as part of the Piceance delta system. The Piceance delta was a wave-dominated delta system that prograded to the southeast into the Late Cretaceous Western Interior seaway.

The Mancos Shale was deposited in offshore marine and prodelta settings. The Rollins Sandstone lies above the Mancos Shale and represents a delta-front/barrier-beach sandstone complex. Above the Rollins, the lower coal member represents coal-bearing sandstones, siltstones, and claystones that were deposited on the lower delta-plain. The extensive Mt. Gunnison distributary channel system was active at the end of lower coal member deposition. Above this distributary channel system, the upper coal member represents deposition of coal-bearing sandstones, siltstones, and claystones in a deltaic to fluvial-plain environment. Fluvial processes are dominant above the upper coal member, with an increase in sandstone units and a corresponding decrease in coal-bearing units.