TIMING AND RATES OF CANYON INCISION AND BASIN INTEGRATION ALONG THE NORTHERN RIO GRANDE, SAN LUIS VALLEY, NEW MEXICO AND COLORADO

Previous cosmogenic ³He work demonstrated that the southern and northern San Luis basins, divided by the San Luis Hills, were not fluvially connected until at most Marine Oxygen Isotope (MOI) stage 12 (439±6 ka). The northern San Luis basin contained Lake Alamosa. The headwaters of the Rio Grande were south of the San Luis Hills. Geologic mapping has revealed three gravel deposits, overlying fine-grained sediments of the Santa Fe Group and Pliocene-Pleistocene basalt flows, prograding across the current position of the Rio Grande gorge. The beginning of coarser sedimentation indicates a system of greater fluvial energy induced by the onset of globally recognized middle Pleistocene glacial episodes. Based on the underlying Lava Creek B ash (640 ka) within the Santa Fe Group and Lake Alamosa highstand at ~ 419±13 ka, the three gravel deposits can be tentatively associated with MOI stages 16, 14, and 12. As Lake Alamosa slowly drained over several glacial periods, more than 25,000 km² of watershed was added to the Rio Grande. This ultimately eroded away any capping gravel deposits above the basalt flows, scoured the basalt, and initiated deep canyon incision. We use ³He surface exposure ages to constrain the initiation of the drainage of Lake Alamosa at ~ 419±13 ka, integration of the southern and northern San Luis basins, and headward erosion of the Rio Grande across the Sunshine Valley-Costilla Plain region. At Arroyo Hondo middle to late Pleistocene canyon incision rates are ~ 0.5 mm/yr beginning at ~ 300 ka.