Cenozoic incision history of the Little Colorado River and its role in carving Grand Canyon

Abstract:

Multi-stage incision/denudation history for the southwestern Colorado Plateau involved: carving of 70-30 Ma Laramide paleotopography controlled by strike valleys at the base of retreating cliffs, 25-15 Ma incision of an East Kaibab paleovalley that cut to about half the modern depth of eastern Grand Canyon, and 5-6 Ma integration of the Colorado River (CR) through Grand Canyon. A synthesis of Little Colorado River (LCR) incision history informs aspects of each episode. Refined evidence for the first two episodes is from new thermochronology modeling that shows post-Laramide NE cliff retreat and a 35-25 Ma LCR paleovalley that flowed NW across the Kaibab uplift. Next was 16-14 Ma playa/ marsh deposition of Lower Bidahochi Formation, 13-8 Ma non-deposition, and 6-8 fluvial upper Bidahochi Formation with base level defined by Hopi Buttes maars. Little denudation in the LCR valley from 16 to 6 Ma is documented by similar elevations of 16 and 6 Ma paleo baselevels. The third pulse of regional incision/denudation was associated with integration of the CR across the Vermillion Cliffs after 5-6 Ma. Earliest integration reoccupied the East Kaibab paleocanyon, inferred from thermochronology to have been at ~1500-1700 m elevation at 15 Ma. Topographically inverted dated basalt mesas in the LCR headwater region show steady incision of 43 m/Ma since 6 Ma. New 2 Ma sanidine ages for Crooked Ridge and Blue Point ash show ~100 m/Ma differential incision across the LCR knickpoint over the last ~ 2 Ma: ~400 m (~200 m/Ma) at the confluence, 400-500 m (200-250 m/Ma) at the projected confluence of Crooked Ridge and the LCR, 220 m (110 m/Ma) for topographically inverted basalt mesas in LCR tributaries in the central reaches, and 116-204 m (61-43 m/Ma) in the headwaters. Headwater uplift of the Rocky Mountains likely facilitated CR integration; post-2 Ma differential incision in the LCR may reflect a combination of regional mantle-driven epeirogenic uplift and additional mantle-driven headwater uplift of the Springerville volcanic field. Acceleration of incision in the last 100 ka is indicated by travertine U-series dating, but this may not have averaged out glacial-interglacial fluctuations. Proposed climate changes (e.g. 5-6 Ma and 2.5 Ma) likely amplified tectonic forcings.