EVOLUTION OF THE RIO GRANDE BY FLUVIAL SPILLOVER

The pace and direction of river integration following tectonic disruption is explored through the sedimentary record of drainage in rift basins. Rivers lengthen upstream through headward integration and stream capture, or they expand downstream through spillover. These modes of drainage integration can be tested by examining stratigraphic evidence for the interbasinal transfer of sediment. Stratigraphic data favor fluvial spillover as a dominant mechanism of drainage integration for the Rio Grande (RG), which follows much of its namesake rift in North America. The RG rift began extending by the late Oligocene and disrupted earlier drainage patterns by the middle Miocene. Persistent southward transport of fluvial deposits and a lack of widespread playa-lake sedimentation in the RG rift in northern New Mexico support surface hydrologic connections to the Albuquerque basin before 4.9 Ma, when the RG lengthened into southern New Mexico where it terminated in playa lakes in the Mesilla and Hueco basins. By 2.1 Ma, the RG flowed through extensional basins in Trans-Pecos Texas, where it terminated in the Presidio or Redford bolsons. The RG reached the Gulf of Mexico by about 0.6 Ma. Major steps in river integration reflect the interplay between subsidence and sediment supply. As basin-bounding faults grow laterally, they may eventually overlap and decrease the elevation of interbasinal topographic divides, allowing surface drainage to integrate. A late Miocene pulse of deformation may have provided structural and topographic linkage among previously isolated basins that subsequently filled as subsidence decreased. Similarities in the timing of integration in other large river systems in western North America suggest spillover was enhanced by climatically induced increases in water discharge and sediment supply that filled slowly subsiding basins and promoted sediment bypass. The pace of reintegration shows a 17-14 Ma lag between tectonic disruption and reestablishment of drainage from continental interiors.