The Animas River valley in southwestern Colorado exposes a geologic record that spans 1800 million years. A controversial geologic structure is exposed on the east side of the valley along U.S. Highway 550, about 12 miles north of Durango, Colorado. This structure is expressed as warped and folded Paleozoic sedimentary strata that overlie Proterozoic granites. Several competing hypotheses have been proposed to explain the origin and nature of this structure (Kilgore, 1955, Brew, 1984, Decker, personal communication, 2001). A more thorough understanding of this geologic structure is needed since it contains fractures that may influence the concentration and movement of groundwater that is currently being developed for residential use in the area.

A new hypothesis regarding the nature of this structure is proposed from detailed field mapping conducted during the summer of 2001. The structure is interpreted as a small east-west trending fault system, or half-graben, with associated synthetic and antithetic structures. Faults examined in the Paleozoic rocks are interpreted as reactivated Precambrian fractures. Movements on these faults are both compressional and extensional. Normal faults are by far the dominant structures, however, some reverse faults of minor displacement were observed due to local reversal in the dip of the fault planes. Results of this study have led to a greater understanding of the history of this geologic structure. They also show that the fault system related to this structure probably controls the groundwater resources in the adjacent development (Decker, personal communication, 2001), and the plumbing system of thermal warm springs on the west side of the valley.