MIDDLE PLEISTOCENE INTEGRATION OF THE SAN LUIS BASIN AND FORMATION OF THE MODERN RIO GRANDE, COLORADO AND NEW MEXICO, U.S.A

The modern Rio Grande (RG) fluvial system extends ~3,000 km from the San Juan Mountains and San Luis Basin of Colorado and New Mexico to the Gulf of Mexico (GOM). The temporospatial evolution of the fluvial system has been long debated. We present new ³He cosmogenic and U-series dating on landforms associated with the timing and processes of San Luis Basin integration and RG gorge formation, as well as a mechanism and framework for establishing a through-going RG to the GOM. Prior to ~400ka, the San Luis Basin contained Lake Alamosa, which occupied the northernmost of a series of subbasins isolated by Pliocene volcanics and the San Luis Hills intra-basin horst. Waning middle Pleistocene tectonic activity and onset of major middle Pleistocene glaciations allowed basin aggradation to exceed subsidence, causing Lake Alamosa to spillover starting at ~385 ka. Subsequent erosion of preexisting topographic barriers over three phases of protracted tributary integration (420-300 ka, 300-243 ka, and 243-130 ka) resulted in formation of deeper gorges in areas of pre-existing topography. Previous studies of GOM stratigraphy and basin geomorphology indicate an absence of mid-Miocene to middle Pleistocene sediment with a RG rift provenance. Instead, those sediments supplied a large bolson complex near El Paso, TX, which represented the southern terminus of RG rift transport through the late middle Pleistocene. Basin geomorphology to the north during this time indicates subtly inset, progradational deposits with strong calcic soils. The youngest/lowest inset, pre-RG gravel deposit is underlain by the Lava Creek B ash (~640 ka). However, a dramatic change to deep-canyon incision and regional dissection occurred after that, and continued until ~140 ka along the RG corridor and its tributaries. Canyon cutting in the Espanola Basin was initiated ~350 ka. The incorporation of water and sediment fluxes out of the SLB into downstream RG rift basins eroded subtle divides between subbasins, induced a drop in regional base levels, initiated gorge incision in southerly basins, and ultimately created the through-going Rio Grande.