Abstract

Coal occurs in some degree through nearly the entire range of Cretaceous strata in the San Juan Basin. Economically significant deposits in the southern part of the Basin are present in the Gallup Sandstone, the Dilco and Gibson Coal Members of the Crevasse Canyon Formation, and in the Cleary Coal Member of the Menefee Formation. All of these Units belong to the Mesaverde Group. In the central and northern parts of the Basin, minable and potentially minable coal occurs in the Lower and Upper Coal Members of the Menefee Formation (Mesaverde Group) and in the younger Fruitland Formation. Coal in the Dakota Sandstone may have an economic potential in the Cortez area of southwestern Colorado.

The coal is the result of accumulation of vegetal debris principally within the shore-marginal paludal environmental band. Deposition in this zone was more or less constantly shifting as the sea either invaded the land area southwestward or withdrew to the northeast. The orientation of the strand line, averaging about N 55° W, is thought to have influenced the axial alinement of individual coal lenses. Zones and individual beds deposited under transgressive conditions become younger to the southwest, and conversely regressively deposited coal beds and zones are younger to the northeast. Particularly thick accumulations of coal are likely to occur in: (1) areas of relative shoreline stability which are likely to be associated with major reversals in the direction of shoreline movement and (2) in conjunction with minor reversals in shoreline movement during major phases.