Subsurface evidence for the sudden integration of the Salt River across the internally drained Basin and Range Province, Arizona, USA

The dynamics of river systems are most easily studied by observing their present geomorphology exposed at Earth's surface. However, the bulk of these often long-lived transportation and depositional systems is mostly buried at depth and hidden from view. Accessing these subsurface archives is often not possible. The Phoenix ‘basin’ of Arizona, USA, with its rich collection of water well drilling information, offers a unique opportunity. Clast assemblages of sand and gravel deposits contained in subsurface drill cuttings enable estimates of sediment provenance and, importantly, reveal changes of provenance over time. This study represents more than a decade of drill cuttings analysis and presents a method for evaluating the depositional history of how the Phoenix basin developed a large external river system that crossed a long-lived internally drained basin. This configuration developed after Basin and Range faulting and extension had ceased, allowing the basin to fill to capacity with sediment. When placed in the context of a series of new isopach maps of the subsurface ancestral Salt River deposits, our clast assemblage analysis supports the conclusion that the Salt and Verde rivers appeared suddenly in the geologic record. The ancestral Salt River deposits and other nearby streams crossing the basin each possess a unique clast assemblage that can be used to ‘fingerprint’ sediments with similar sources. We use these observations to draw conclusions about the age and origin of these streams. Used in conjunction with other types of analyses such as cosmogenic burial dating and detrital zircon analyses, determining clast assemblages of drill cuttings in the context of mapping these assemblages is a powerful new method for estimating the source of sediment and for understanding how the Phoenix basin, and basins in general, fill with sediment. This information is of value in modeling both basin development and groundwater resources in similar contexts in Arizona and elsewhere.