STRATIGRAPHY, AGE CONTROL, AND ROLE OF TECTONIC VS. PALEOCLIMATIC DRIVERS IN THE 6.5-4.5 MA DOWNSTREAM INTEGRATION OF THE ANCESTRAL RIO GRANDE IN SOUTH-CENTRAL N.M

A pivotal event in the Rio Grande rift occurred 6.5-4.5 Ma when its axial river, the Rio Grande, propagated from central to southern New Mexico and integrated surface drainage among previously closed basins. Detailed observations of outcrops and wells in the Socorro Basin (SB), an upstream locality (Española Basin, EB), and a downstream locality (Palomas Basin, PB), are synthesized with published and new age control to elucidate the manner and timing of this downstream propagation. In the EB, the ancestral Rio Grande cut extensive paleovalleys ca. 5 Ma and a 7-5 Ma unconformity extends across >90% of the basin. A pulse of sedimentation filled northern paleovalleys at 5.0-4.7 Ma, forming coalesced fluvial fans that were abandoned by renewed river incision at 4.7-4.5 Ma. The ancestral Rio Grande arrived at the northern PB by 5 Ma. Thin beds of extrabasinal sand are interbedded in uppermost playa deposits and overlain by a 10-m succession of coarsening-upward sand and pebbly sand. Then the river successively incised and backfilled two 20 m-deep paleovalleys in the northern PB during 5.0-4.87 Ma and 4.87-4.5 Ma. Near the depocenters of the PB and SB, basin-floor clays indicate internal-basin conditions that are abruptly overlain by a coarsening-upward fluvial package several tens of meters thick. We interpret downstream propagation of the Rio Grande was driven by tectonic and paleoclimatic factors that increased sediment delivery to previously closed basins through fluvial spillover. Previously interpreted lower rates of post 8 Ma basin subsidence in the San Luis Basin and EB, together with 6-5 Ma paleovalleys and unconformity development across an uplifting Jemez lineament, would have promoted sediment bypass from upstream basins to terminal playas in central and southern New Mexico. Relatively strong latest Miocene-earliest Pliocene normal faulting in southern New Mexico facilitated 6-5 Ma spillover by lowering paleo-topography immediately SSE of the Socorro Basin and between the Palomas and Engle Basins. Paleoclimatic signals of fluvial behavior are inferred in the EB and PB at 5-4 Ma based on short-lived incisional and aggradational events along the axial river and widespread coarsening in both axial and piedmont deposits.