Upper Cretaceous Stratigraphy and Tectonic History of Ridgway Area, Northern San Juan Mountains, Colorado: ABSTRACT

Abstract

Detailed mapping and stratigraphic studies in the Ridgway area, along the northern flank of the San Juan Mountains in Colorado, indicate that the lower Mancos Group (Upper Cretaceous) can be subdivided into four formations. From oldest to youngest, they are the Benton Formation, Juana Lopez, Sage Breaks Shale, and Niobrara Formation. These formations have lithologic and paleontologic content similar to their equivalents in the Denver and San Juan basins and Black Hills. The underlying Benton Formation consists of 470 ft (143 m) of black shale that includes the Greenhorn Limestone Member (12 ft or 3.5 m thick) lying 300 ft (91 m) above the base. The overlying Juana Lopez is 40 ft (12 m) thick and consists of alternating layers of calcareous siltstone and black shale. The succeeding Sage Breaks is a 155-ft (47 m) calcareous shale unit. The overlying Niobrara Formation is an 85-ft (26 m) resistant, cliff-forming, calcareous shale. Subsurface and outcrop data in the south indicate that the study area straddles the margin of the Pennsylvanian Uncompahgre uplift. Structural evolution has been controlled largely by three east-west trending Precambrian fault blocks--the Ouray graben, the Orvis block, 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 End_Page 572------------------------------ Cutler Formation increases 1,050 ft (320 m) in thickness across the Orvis fault, which had down-to-the-south movement. During the Laramide orogeny, a significant reversal in movement occurred along the fault to cause the north-dipping monocline now present at the surface. End_of_Article - Last_Page 573------------