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SEAN D. CONNELL¹ and STEPHEN G. WELLS^2
1New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801-4796, connell@gis.nmt.edu
²Desert Research Institute, P.O. Box 60220, Reno, NV 89506-0220

Abstract-We use geologic mapping, stratigraphy, and soil morphology of Pliocene and Quaternary deposits along the northwestern flank of the Sandia Mountains, central New Mexico, to interpret the late Cenozoic tectonic evolution of the eastern margin of the Albuquerque Basin. We subdivide piedmont and fluvial deposits that are unconformable with the syn-rift basin fill of the Santa Fe Group. We also evaluate the activity of basin-margin and intrabasinal faults using this stratigraphic framework. The piedmont associated with western and northern flanks of the Sandia Mountains mark a transition between the northern Albuquerque and Santo Domingo sub-basins. The distribution of piedmont and Santa Fe Group basin-fill deposits records basinward migration of fault activity across a 6.5-km east step in the rift-margin, resulting in increased relief and subsequent dissection of older basin fill on the hanging walls of the Placitas and San Francisco faults. Along this step, the piedmont is deeply dissected and exposes some of the oldest rift-basin fill and pre-rift rocks on the hanging wall of the rift-bordering Placitas and San Francisco faults. South of this step, along the relatively steep and linear western front of the Sandia Mountains and Rincon Ridge, the piedmont is only slightly dissected and middle Pleistocene and Holocene deposits typically bury Santa Fe Group and older deposits. According the Russell and Snelson (1990 and 1994), the northern Albuquerque Basin is controlled by a major, mostly buried, west dipping, listric-normal fault named the Rio Grande fault. Their Rio Grande fault began during the late Miocene as the locus of rift-border faulting migrated basinward, resulting in displacement of 4-6 km of Cenozoic basin fill. Stratigraphic and geomorphic evidence does not support the presence of such a through-going, large-displacement structure in the study area. We interpret basinward migration of normal faulting as only a local feature, created in part, by the prominent east step between the Santo Domingo and northern Albuquerque sub-basins.

Connell, S.D., and Wells, S.G., 1999, Pliocene and Quaternary stratigraphy, soils, and geomorphology of the northern flank of the Sandia Mountains, Albuquerque Basin, Rio Grande rift, New Mexico: New Mexico Geological Society, Guidebook 50, p. 379-391.

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