CONSTRAINING THE OLIGOCENE-MIOCENE TRANSITION FROM CLOSED BASIN RIFT SEDIMENTATION DURING THE EARLIEST STAGES OF THE ANCESTRAL RIO GRANDE FLUVIAL SYSTEM, SOUTH-CENTRAL NEW MEXICO

Latest Oligocene–Miocene synorogenic strata of the Hayner Ranch and Rincon Valley formations preserve the rift history of early, endorheic (closed-basin) sedimentation in southern New Mexico and outcrop along the modern Rio Grande rift (RGR) margin in the Palomas and Jornada del Muerto basins.

The RGR preserves a near-continuous record of volcanism and sedimentation from late Eocene–Present, and although a considerable amount of previous work has focused on documenting the onset, extent, and geochemistry of Eocene–Oligocene volcanism during the early stages of the RGR, very little is known about provenance, drainage development, and sediment dispersal during the latest Oligocene–Miocene. Given the unique distribution of bedrock source areas that have been exposed as a result of rift deformation (e.g., Cenozoic volcanic fields, recycled Paleozoic–Mesozoic stratigraphy of the Colorado plateau, and Precambrian basement sources), southern New Mexico is an excellent natural laboratory to carry out this project. At the largest scale, this project will place some constraint on the endorheic conditions in rift basins just prior to river integration and the nature and timing of river-integration events in continental rifts.

The Oligo-Miocene stratigraphic record in southern New Mexico is thought to record closed-basin conditions (i.e., lacustrine- and fan-dominated sedimentation) until the system began to transition to more exorheic (open-basin) conditions (i.e., axial fluvial-dominated sedimentation) by the start of the Pleistocene. New provenance data (U-Pb detrital zircon geochronology) reveal widespread volcanic detritus in the basal parts of the Oligocene system (i.e., Hayner Ranch Formation), which likely reflects early unroofing of Oligocene rift volcanic provinces from the nearby Caballo Mountains and associated range-bounding normal faults. Upsection, the youngest parts of the Hayner Ranch and overlying Rincon Valley Formation contain detritus derived largely from Paleozoic and basement source areas with less detrital contributions from volcanic source areas. The Palomas and Jornada del Muerto basins record the earliest stage of unroofing along the Caballo rift block during the Oligo-Miocene.