Geomorphology and stratigraphy of inset fluvial deposits along the Rio Grande valley in the central Albuquerque Basin, New Mexico

Five inset levels of Pleistocene fluvial deposits indicating former and present positions of the Rio Grande are differentiated between San Felipe Pueblo and Los Lunas, New Mexico. All have coarse-grained, wellrounded cobble-gravel bases overlain by varying amounts of finer-grained and more poorly sorted sediments. This paper formalizes (with modification) three fluvial deposits informally proposed by P. W. Lambert in 1968: Los Duranes Formation, Menaul Member, and Edith Formation. This paper also introduces three new stratigraphic terms to complete the succession of fluvial deposits mapped in the Albuquerque area: the Lomatas Negras Formation, Arenal Formation, and Los Padillas Formation. A fluvially recycled volcanic ash, exposed in the oldest and highest inset deposit, the Lomatas Negras Formation, is geochemically indistinguishable from the middle Pleistocene Lava Creek B tephra (Yellowstone caldera; 640 ka). This ash places the youngest age limit on the initiation of incision of the Rio Grande valley in the Albuquerque area. The (middle Pleistocene) Edith Formation appears to be the second oldest inset deposit. The (middle Pleistocene) Los Duranes Formation aggraded during the eruption of the Albuquerque Volcanoes (156 ka) and had ceased by the time of the Cat Hills lava flow (98–110 ka). The Arenal Formation is inset into Los Duranes Formation and is late Pleistocene in age. Los Padillas Formation designates the 15–25 m of deposits beneath the inner valley floor and had reached nearly its current upper level by late Holocene time. The preserved sequence of deposits suggests that substantial shifts in stream power, sediment supply, and climate are responsible for each of the fluvial deposits associated with former and current positions of the Rio Grande.