Development and integration of the middle Gila River in the Safford basin, southeastern Arizona

Abstract

The middle and upper Gila River crosses the tectonic and physiographic grain from western New Mexico into central Arizona. Hypotheses to explain the river's course involve filling and spilling of lakes, headward erosion, or some combination. Since 2012, we mapped the surficial geology of the eastern part of Safford Valley, southeastern Arizona, and described basin fill, river and tributary deposits. Clast counts on seven fluvial deposits in the Safford basin and detrital sanidine on two of those deposits provide insights into the timing and nature of drainages feeding into the Safford basin in the late Neogene. Our data indicate that between 7 and 3 Ma, sediment and surface water was supplied to a wide fluvio-lacustrine plain in Safford basin, fed primarily by the San Francisco River, and transported southwest and downstream into a terminal lake near San Carlos, AZ. Late in the basin-filling process between 3 and 2 Ma, the Gila River integrated with Duncan basin followed by Safford basin, contributing enough water and sediment to culminate in a maximum depositional highstand followed by integration and subsequent dissection and erosion of Safford basin fill. Five main river terrace levels ranging from 25 to 200 m above the modern river tracked incision of basin fill. This record of basin filling and dissection is consistent with downstream-directed integration via lake spillover. The spillover event probably occurred around 2 Ma, resulting in lengthening the Gila River almost 200 km, lowering base-level nearly 1 km, and permanent incision and debouchment of the Safford basin fill sediment downstream into the early Salt River valley.