February 2007, Volume 29, Number 1 NEW MEXICO GEOLOGY 13 Abstract Five inset levels of Pleistocene fluvial depos- its indicating former and present posi- tions of the Rio Grande are differentiated between San Felipe Pueblo and Los Lunas, New Mexico. All have coarse-grained, well- rounded cobble-gravel bases overlain by varying amounts of finer-grained and more poorly sorted sediments. This paper formal- izes (with modification) three fluvial depos- its informally proposed by P. W. Lambert in 1968: Los Duranes Formation, Menaul Mem- ber, and Edith Formation. This paper also introduces three new stratigraphic terms to complete the succession of fluvial deposits mapped in the Albuquerque area: the Loma- tas Negras Formation, Arenal Formation, and Los Padillas Formation. A fluvially recycled volcanic ash, exposed in the oldest and high- est inset deposit, the Lomatas Negras Forma- tion, 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 Albu- querque area. The (middle Pleistocene) Edith Formation appears to be the second oldest inset deposit. The (middle Pleistocene) Los Duranes For- mation 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 Pleis- tocene in age. Los Padillas Formation des- ignates 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. Introduction The Rio Grande valley is an incised river valley (Gile et al. 1981; Hawley et al. 1995; Connell et al. 2005) that formed when the river began cutting into underlying basin fill of the Ceja and Sierra Ladrones Forma- tions (upper Santa Fe Group). Incised val- leys tend to leave a record of previous river positions in a suite of stepped terrace land- forms and deposits preserved along the borders of the valley (Gile et al. 1981). An understanding of the timing and location of valley entrenchment depends on an assess- ment of evidence of how the river aggrad- ed and first began to incise. Preservation of sedimentary facies within fluvial and alluvial deposits is critical to determining related fluvial conditions. Later episodes of valley entrenchment and partial backfill- ing obscure or obliterate evidence of earlier episodes of terrace formation. During late Pliocene time, the ancestral Rio Grande formed an axial river that flow- ed within a few kilometers of the western front of the Sandia Mountains, its course Geomorphology and stratigraphy of inset fluvial deposits along the Rio Grande valley in the central Albuquerque Basin, New Mexico Sean D. Connell, New Mexico Bureau of Geology and Mineral Resources, Albuquerque Office, New Mexico Institute of Mining and Technology, 2808 Central Ave. SE, Albuquerque, NM 87106, connell@gis.nmt.edu; David W. Love, and Nelia W. Dunbar, New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801 FIGURE 1—Schematic east-west cross section (vertical exaggeration = 5) of the Albuquerque area illustrating the difference between the stepped ter- races and fill of the Rio Grande valley and the broader interfingering of the ancestral Rio Grande facies of the Sierra Ladrones Formation (QTsa) with the piedmont member (QTsp) from the east and the Ceja Formation (Tca, Tcg) from the west. Note the faulted half-graben structure with tilted thick basin fill on hanging-wall blocks. This diagram is simplified from a cross section with well control by Connell (2006). Length of section is 46 km.