EAGLE COLLAPSE CENTER: INTERPRETATION OF EVIDENCE FOR LATE CENOZOIC EVAPORITE-RELATED DEFORMATION IN THE EAGLE RIVER BASIN, COLORADO

Late Cenozoic flow and dissolution of evaporitic rocks resulted in deformation and collapse over an area of about 2,500 km2 (Eagle collapse center) in the Eagle and Colorado River basins between Vail, Dotsero, and McCoy, Colorado. The volume loss by dissolution of evaporitic rocks in this collapse region is estimated to be nearly 1,700 km3. Much of the evaporite-related deformation postdates emplacement of extensive 22-24 Ma basaltic flows that partly covered an extensive, nearly horizontal surface of low relief. Parts of this surface collapsed more than 1.3 km near the present-day Eagle and Colorado Rivers. Remnants of this surface outside the area of collapse, such as highlands of the White River uplift, the southwest flank of the Gore Range, and Basalt Mountain, stand at elevations of 2.9-3.6 km, whereas, the lowest elevations of basaltic remnants in the collapse region range from about 2.0-2.7 km. The areas of collapse are within or near known and inferred limits of the Pennsylvanian Eagle Valley Evaporite (mostly halite, gypsum, and anhydrite) that was deposited in the Central Colorado trough. Our geologic mapping and research in the Eagle collapse center delineate synclinal sags in the basaltic flows having amplitudes of 0.5-1 km, sinuous and discontinuous high-angle faults that cut basaltic flows, grabens, evaporite-cored anticlines, and an ellipsoidal fault system that drops a 30 km x 10 km mountain block of younger strata into evaporite. Fold orientations in the evaporite and collapse features as far as 20 km from the Colorado and Eagle Rivers suggest that the greater load on evaporite beneath surrounding highlands causes its lateral flow from beneath the highlands toward anticline crests in river valleys. Gravity-driven evaporite flow and removal of evaporite by dissolution in groundwater and subsequent discharge to surface waters probably combine to produce large synclinal collapse features that also characterize this region. An angular unconformity on older strata beneath the flow-capped surface records earlier deformation, possibly Laramide evaporite tectonism, and abrupt and dramatic changes in thickness of post-evaporite red-beds suggest late Paleozoic evaporite deformation. Although much of the evaporite tectonsim is late Cenozoic in age, flow and dissolution of evaporite probably is a long-term and ongoing geologic process in this region.