GEOLOGY AND LANDSCAPE RECORD LATE CENOZOIC HISTORY OF RIO GRANDE RIFT NEAR ALBUQUERQUE, NEW MEXICO

The Late Cenozoic geology of the Rio Grande rift in north-central New Mexico chiefly reflects the competing contributions of fluvial/colluvial sedimentation and fluvial erosion, both of which were strongly influenced by climate change during the last 5 million years. A new compilation of the 1:100,000-scale Albuquerque map sheet shows that major features of the regional landscape are recorded by the Pliocene and Quaternary geologic units. The underlying Miocene section is variably tilted and only exposed on the rift margins; it was deposited during the most active period of rifting in closed tectonic basins by low-energy stream systems. Much intra-rift deformation, including local uplift and arching, pre-dates an erosional unconformity that marks the base of the Pliocene Series. The broad mesa west of the Rio Grande and gravel-capped pediments around the Ortiz and Jemez Mountains mark the constructional tops of coarse Pliocene beds that were deposited by higher-energy streams as a result of wetter, cooler climate than existed earlier. Closed-basin conditions persisted until Late Pliocene, marked by floods of basalt lava (2.7 to 2.5 Ma) that generally cap the Pliocene fluvial deposits. Capture of the Albuquerque valley drainage southward in Late Pliocene initiated the first through-flowing trunk stream ("ancestral" Rio Grande with its tributaries: Rio Puerco, Rio Jemez, and Galisteo Creek) and led to considerable erosion of rift-fill deposits and the surrounding mountains. Early Pleistocene initiation of the glacial-pluvial climate cycle is recorded by 1.6 to 1.2 Ma fluvial terrace-fill deposits east of the modern valley. Three additional inset terrace-fill deposits along the Rio Grande date from corresponding glacial intervals at about 640 ka, 150 ka, and 25 to 15 ka. East of the rift in the Estancia Basin, we recognize previously unreported shoreline deposits of a Pleistocene lake (probably ca. 150 ka).