Abstract

Western Colorado’s major coal resources and mining activity are in rocks of Cretaceous Campanian age. The purpose of this paper is to summarize the stratigraphic framework of coal formed in the early and late Campanian regressions. During Campanian time, two major eastward regressions of the strandline occurred. The early Campanian regression deposited the type Mesaverde Group of southwestern Colorado. The late Campanian regression deposited the Pictured Cliffs and Fruitland Formations of southwestern Colorado and part of the Mesaverde Group of northwestern Colorado.

Rocks in the Durango area, southwestern Colorado, show that the early Campanian Point Lookout and Menefee Formations of the Mesaverde Group are a delta-front and delta-plain succession (the Durango delta) that prograded northeastward until submerged by the Cliff House Sandstone - Lewis Shale transgression. Menefee coal beds formed in interdistributary parts of the delta-plain.

The late Campanian regression has two components. One, in northwestern Colorado, consists primarily of the Piceance delta, which prograded southeastward and eastward to form the lower Mesaverde wedges, culminating in the Trout Creek - Rollins delta-front sandstones plus lower Williams Fork and Mount Garfield coal-bearing delta-plain deposits. Counterclockwise currents swept sand from the Piceance delta to the second regressive component, the Pictured Cliffs barrier sandstone in southwestern Colorado, behind which formed the lagoonal and coal swamp deposits of the Fruitland Formation. Lower Williams Fork and Mount Garfield coal beds formed in interdistributary areas of active lobes of the Piceance delta, while Fruitland coal beds formed in a northwest-southeast belt behind a linear barrier coastline.