Abstract

Detailed mapping and stratigraphic studies in the Ridgway area, along the northwestern flank of the San Juan Mountains in Colorado, indicate that the lower Mancos Formation (Upper Cretaceous) can be subdivided into four members. From oldest to youngest, they are the Mancos₁, Juana Lopez, Mancos₂, and Niobrara Members. These members have lithologic and paleontologic content similar to their equivalents in the Denver and San Juan basins, Colorado, and Black Hills, Wyoming.

Subsurface and outcrop data to the south indicate that the study area straddles the southwestern margin of the Pennsylvanian Uncompahgre uplift Structural evolution has been controlled largely by three east-west-trending Precambrian fault blocks - from north to south, the Orvis block, the Ouray graben, and the Uncompahgre block. Recurrent movement of these fault blocks from the Paleozoic through the Cenozoic affected both sediment thickness and facies distribution. A cross section restored to the base of the Dolores Formation (Upper Triassic) indicates that the Permian Cutler Formation increases 1,050 feet in thickness across the Orvis fault, which had down-to-the-south movement, into the Ouray graben. The north-dipping monocline now present at the surface suggests that significant reversal in movement took place during the Laramide orogeny.