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Abstract: Environments of Deposition of Coalmont Formation Near Coalmont, Jackson, County, Colorado

Michael L. Hendricks

The middle and upper members of the Coalmont Formation near Coalmont, Colorado, are terrigenous deposits reflecting tectonic movements during the Laramide orogeny. Basin subsidence associated with uplift of the Park Range on the west supplied over 9,000 ft (2,740 m) of clastic sediments to the area from late Paleocene through early Eocene time. Palynology sampling from coals and shales indicates that deposition of the middle member in the Coalmont area began during late Paleocene time. The upper member is of early Eocene age.

The middle and upper members were deposited in channels and flood plains. The sandstones, conglomerates, and conglomeratic sandstones represent channel scour-and-fill sequences. Carbonaceous shales, siltstones, and mudstones, common to both members and increasing in number and lateral extent in the upper member, represent overbank deposits. The coals and coaly shales present in both members were deposited in swamps. The thickness of the Riach Coal Bed, 35 to 65 ft (10.6 to 19.8 m) thick in the lower part of the upper member, indicates continued subsidence of the North Park basin during early Eocene time.

Economically exploitable coals in the lower part of the upper member are generally subbituminous B in rank. Faulting in the Coalmont area can complicate coal mining.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado