Laramide and Cenozoic structural and paleotopographic history of the Ouray area and the northwestern flank of the San Juan Mountains, Colorado
Regional structural mapping of the base Dakota (base Cretaceous) surface in the northwestern San Juan Mountains discloses a complex intersection of the San Juan Uplift, faulting of the Paradox Basin, and the southern end of the Uncompahgre Plateau. Uplifted areas are associated with the Ouray and Rico centers of Laramide igneous activity, but not with two other such centers. Pre-Cenozoic rocks subcrop in mappable belts, forming the northwest flank of the San Juan Uplift. A more detailed look at the Ouray area, including some deep well control, shows a structural high related to Laramide igneous activity, and at least three monoclines or kink folds each with about 300 m (1000 feet) of vertical offset, verging outward from the mountain mass. Oddly, the Paleozoic monoclinal fold near Ouray was not reactivated. The base Cenozoic is a paleotopographic surface with over 900 m (3000 ft) of relief regionally and over 360 m (1200 ft) in the Ouray area. A pronounced paleovalley passes east-west through Ouray and helps to form the Amphitheater; it is formed along the Lower Hermosa and Molas subcrop which was probably the least resistant pre-Cenozoic units in the area. This paleovalley may have drained a large ‘Imogene Basin,’ which extended west to Telluride. A second low area (Cow Creek Basin) to the northeast is separated by a high-standing Gold Hill upland underlain by Dakota Sandstone and Laramide igneous sills. South of Silverton, the pre-Cenozoic surface rises sharply; this may mark the southwestern edge of the San Juan volcanic field banked against a ‘Grenadier Upland’ formed over the Precambrian and Paleozoic rocks of the Laramide San Juan Uplift.
- Ewing, T.E., 2017, Laramide and Cenozoic structural and paleotopographic history of the Ouray area and the northwestern flank of the San Juan Mountains, Colorado, in: The Geology of the Ouray-Silverton Area, Karlstrom, Karl E.; Gonzales, David A.; Zimmerer, Matthew J.; Heizler, Matthew; Ulmer-Scholle, Dana S., New Mexico Geological Society, Guidebook, 68th Field Conference, pp. 169-178.