The Minturn Formation accumulated along the east side of the Eagle (Maroon) basin from detritus stripped from the ancestral Front Range uplift. Sedimentation began in early Des Moines time and continued through Middle and Late (?) Pennsylvanian until a wedge of clastic sediments as thick as 6,000 feet formed adjacent to the uplift in the area of Minturn, Colorado.

The Minturn consists of coarse arkose and conglomerate in outcrops along the basin margin. The formation thins toward the basin center, and the coarse sediments gradually grade laterally into finer-grained arkose, siltstone, mudstone, and shale. West of the junction of Gore Creek and Eagle River the finer-grained sediments interfinger with lenses of evaporite which project outward from the basin center. Lovering and Mallory (1962) designated the evaporite-bearing sequence the Eagle Valley Evaporite.

Some carbonate beds are intercalated with the detrital sediments in the upper three-fourths of the formation. Tweto (1949) gave member status to seven of the more persistent carbonate units in the Pando area. These members also crop out in the Minturn area along Eagle River and Gore Creek. Only the Jacque Mountain and Robinson Limestone Members extend into the area of evaporite interbedding. The other carbonate members appear to pinch out or change facies laterally in the eastern part of the area.

Limestone beds along the basin margin contain faunal assemblages that indicate normal-marine, open-shelf deposition. The fauna of the limestone beds decreases gradually toward the basin center; there the limestone consists of algal stromatolites and wave-fragmented intraclasts, indicating very shallow water or possibly an intertidal environment in that area. Petrographic studies show a general lack of grain-supported textures, widespread occurrence of micrite, and a corresponding scarcity of sparry calcite cement in essentially all of the limestone. Most fossils do not show evidence of abrasion. These factors suggest that the limestone beds in the marginal trough formed in relatively quiet water. Local zones of agitation are indicated, however, by oolite beds in the White Quail and Ja que Mountain Limestone Members.

Most sandstone bodies in the upper part of the Minturn consist of moderately to poorly sorted arkose and impure arkose. Dominant clay minerals in the sandstone and associated shale and mudstone are illite with some mixed-layer clay minerals and chlorite. Kaolinite and montmorillonite are absent or extremely sparse in most sediments.

Petrographic and field evidence suggests that the Minturn Formation accumulated in a narrow trough adjacent to the mobile, ancestral Front Range uplift. This trough apparently was connected with an epeiric sea on the northwest and possibly on the southeast. Fluvial sediments accumulated along the shoreward edge of this trough while shallow-marine arkose, shale, and other rock types formed farther seaward. Conditions favorable for limestone deposition recurred periodically.

The central part of the basin had a relatively higher elevation than the marginal trough through much of the time of Minturn deposition and served as a shallow evaporite "pan." Water of normal-marine salinity occupied the marginal trough during recurring periods but became distinctly shoal toward this central area of evaporite deposition. This is indicated by the presence of algal stromatolites and wave-fragmented intraclasts in limestone of the western part of the area. The absence of marine fossils in these limestone beds is attributed to unfavorable conditions of salinity and temperature associated with this extreme shallowing.