Coal-bearing intervals occurring within the Upper Cretaceous Mesaverde Group in the Piceance basin have been described by various authors. The most current and widely accepted work has the Sego, Corcoran, Cozzette, and Rollins Sandstone Members comprising the Iles Formation. The overlying Williams Fork Formation is divided into the basal Bowie Shale Member and Paonia Shale Member, with the upper remaining section undifferentiated.

Coal seams associated with the Iles Formation belong to the Black Diamond coal group. The Fairfield coal group and the South Canon coal group are part of the Bowie Shale Member. These two coal groups, continuous throughout the basin, are also called the Sommerset coals in the Sommerset coal field and the Cameo coal measures in the Grand Mesa coal field. Although priority of nomenclature dictates otherwise, established usage of the "Cameo coals" for coal seams in the Bowie Shale Member should be continued as the most appropriate nomenclature.

The basal coal seam of the proposed Cameo coal group is laterally continuous throughout the Piceance basin. A second coal seam 40-120 ft (12-37 m) above the basal coal also has large areal extent. Both coal seams, as existing and potential future pay zones, are of significant economic importance and should, in ascending order, be classified as the Cameo coal A and D seams.

The coal seams in the Paonia Shale Member, extremely variable in thickness, continuity, and quality, have been established as the Coal Ridge coal group.

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