RECONSTRUCTING PALEOTOPOGRAPHY, GEOMORPHIC PROCESSES AND LANDSCAPE EVOLUTION IN RESPONSE TO DRAINAGE BASIN INTEGRATION - SALT AND VERDE RIVERS, BASIN AND RANGE, SOUTH-CENTRAL ARIZONA: PRELIMINARY RESULTS

A fundamental gap exists in our understanding of landscape evolution in response to drainage integration within the Basin and Range. The sedimentology and geomorphology of the lower Salt and Verde River basins, Arizona, represent the interpretive backbone of the plio-pleistocene landscape evolution of this region. Early researchers note the existence of endorheic basin-playa sediments, strath terraces, pediments, and ballenas bounding these rivers. Theoretical models of the regional significance were proposed, but reinterpretation is necessary given the applicability of new methodologies (cosmogenic nuclide dating and geospatial technologies) and new conceptual models. Reinterpretation of the highest landscape deposits, based on preliminary mapping and topographic/sedimentologic relationships, suggest a similar integration process to that of the lake overflow model proposed along the lower Colorado River. Following initial integration the longitudinal profiles of the Salt and Verde grew headward until a period of geomorphic stability was reached, the Mesa terrace. Incision was rejuvenated marked by the Blue Point terrace and Lehi/modern floodplain. The current landscape reflection of drainage evolution is observed in terrace long-profiles that coalesce in the downstream direction. Strath terraces, inset beneath the integration fill terrace, are carved at the expense of bounding granitic pediments termed here the pediment-strath relationship. Pediments respond to initial integration processes with a period of aggradation and later incision, grading to the strath level. Incision abandons pediment surfaces graded to a previous base level, while new pediments attempt to regrade through lateral erosion, piracy, and knickpoint retreat. DGPS and DEMs were used to map the reinterpreted fluvial terraces and relict pediment surfaces once graded to Mesa level. Kriging methodologies were applied to reconstruct pediment surfaces in five sub-basins and, based on preliminary ages, erosion rates within these basins range from 6.7±0.9 mm/ka to 89.7±12.6 mm/ka, with an apparent positive correlation to pediment basin area and length.