LATE CRETACEOUS THROUGH TERTIARY STRUCTURAL EVOLUTION OF THE SOUTHEASTERN FLANK OF THE SAND WASH BASIN, MOFFAT AND ROUTT COUNTIES, COLORADO

Mapping efforts by the Colorado Geological Survey along the southeastern flank of the Sand Wash Basin in northwestern Colorado shed new light on the structural evolution of the basin. Well known Laramide structural features are being documented in greater detail while previously un-documented features come to light with careful mapping of subtle structural fabrics. Northwest-trending faulted folds dominate the structural grain and are cross-cut by a series of northeast-trending flexures. The flexures likely represent surface expressions of deeper faults. Although individual offsets on the northeast flexures tend to be small, less than 30m, the features do appear to compartmentalize the main northwest structural components delineating distinct structural domains. Relationships of the features demonstrate a history of Laramide compression with a clear Neogene overprint. Neogene features include scarps developed on a Quaternary landscape and a network of 24 Ma and 8 Ma igneous centers. The igneous centers fall close to intersections of the dominating northwest grain with northeast flexures.