PACE AND PATTERN OF LATE CENOZOIC DRAINAGE INTEGRATION ON THE GILA RIVER, AZ AND NM

The Gila River cuts across the southeastern corner of North America’s Basin and Range province in Arizona and New Mexico. However, the modern, integrated drainage network of the Gila has only existed since approximately 5 Ma. Following the initiation of the Basin and Range Disturbance 8-12 Ma, the Gila River and many of its tributaries were internally drained, terminating in subsiding structural basins. The mechanisms for drainage integration post-5 Ma remain a point of debate - some argue steady headward erosion and integration following the opening of the Gulf of California, while others suggest a combination of headward erosion and upstream-to-downstream integration events via spillover. We provide a suite of new dates to help refine this story. Specifically, we use cosmogenic nuclide surface exposure and burial dating to constrain the pace and pattern of drainage integration in southeastern Arizona.

Previously, basin high stand remnants along the Gila River and tributary basins were thought to be roughly correlative, preserving the timing of integration-related incision. We find that widespread dissection of basin fill following integration of the Gila was neither geologically instantaneous nor neatly time transgressive from downstream to upstream as headward erosion would require. In Safford Basin, the Gila River arrived from upstream 2.8 Ma, and integrated with the Lower San Pedro at 2 Ma via either capture from headward erosion or downstream spillover across the Mescal Mountains. Aravaipa Creek Basin, a small tributary of the Lower San Pedro Basin just upstream from the junction of the Gila and the San Pedro Rivers, spilled over into the Lower San Pedro Basin ~3 Ma. Since drainage integration, the Gila system has steadily incised into regional basin fill at 30-60 m/m.y. as supported by our burial dates of Pliocene fluvial terraces and basin fill as well as surface exposure dating of mid-Pleistocene piedmont deposits within the Lower San Pedro Basin. Cosmogenic nuclide geochronometers allow us to fill an important gap in the geologic history of the Gila River, but the context for our work is the regional story compiled by a great deal of work before us. In particular, Bill Dickinson’s research on the San Pedro Trough and the broader Gila Basin guided both our early sample collection, and the interpretation of our results.