Provenance in drainage integration research: Case studies from the Phoenix metropolitan area, south-central Arizona

Studies of the evolution of drainage systems in extensional settings like the Basin and Range Province of western North America benefit from well drilling as a means of acquiring valuable insight. Cuttings from two wells drilled into sediments of the Phoenix metropolitan region, Arizona, USA, offer new insights into the drainage history of the Salt and Verde river drainage basins. Analyses of detrital zircons with U[sbnd]Pb dating reveal a different signature for Ancestral Salt River Deposits (ASRD) as compared to the underlying basin fill. Trace element, ⁸⁷Sr/⁸⁶Sr ratios, and electron microprobe analyses of basalt fragments in the basal deposits of the ASRD in two different wells from Mesa, Arizona, show matches for outcrop sources near Bartlett Dam in the Verde drainage and near Stewart Mountain Dam in the Salt drainage. This indicates that the Salt and Verde rivers were transporting these basalts when the Salt River first flowed into the metropolitan Phoenix area. We employed tephrochronology to determine that the 3.3-Ma Nomlaki tuff accumulated in closed-basin playa deposits located near the present-day junction of the Salt and Verde rivers, providing a maximum-limiting age for the integration of both rivers. Because the age of mountains crossed by the Salt and Verde rivers pre-date <3.3 Ma by tens of millions of years, we rule out antecedence and superimposition as possible mechanisms to explain these transverse drainages. Multiple lines of evidence presented here are inconsistent with drainage piracy from headward erosion for both drainages: (i) headward erosion would have eroded analyzed basalt clasts from outcrop positions prior to drainage integration, and yet these clasts only occur in the lowest deposits of the ASRD; (ii) headward erosion would not be expected to create transverse streams in two distinct river drainages at the same time, and yet basalt clasts eroded from outcrops in the Salt and Verde rivers arrived together in the basal layer of the ASRD deposits, within the temporal resolution of 3 m sampling interval for well cuttings; (iii) headward erosion of the Verde River from the Nomlaki Tuff (providing maximum age of river integration at 3.3 Ma) to the breached Verde Formation-depositing lake at 2.5 Ma would require an extraordinarily fast rate of >12 cm/yr across multiple bedrock uplands; and (iv) Mescal Limestone clasts on the highest Salt River strath terrace cannot be explained by headward erosion. However, all analyzed evidence are consistent with the process of lake overflow. Based on our findings and the use of geomorphic criteria (Douglass et al., 2009), we conclude that lake overflow is the most likely cause of drainage integration of both the Salt River and the Verde River.