Preliminary regional compilation of geologic mapping along portions of the Albuquerque basin, Sandia mountains, and vicinity, central New Mexico
Connell, S.D., and Read, A.S., 1999, Preliminary regional compilation of geologic mapping along portions of the Albuquerque basin, Sandia mountains, and vicinity, central New Mexico, USGS Middle Rio Grande Basin Study--Proceedings of the third annual workshop, Albuquerque, New Mexico (Feb 24-25,1999).
Rapid growth of the Albuquerque-Rio Rancho metropolitan area (ARMA), the largest urban area in New Mexico, has resulted in growing concerns over the availability of groundwater resources and suitable land for development. The National Cooperative Geologic Mapping Program (NCGMP), a federally funded program administered by the U.S. Geological Survey, mandates the production of multipurpose geologic maps in areas of socioeconomic importance. Since the inception of the NCGMPs STATEMAP Program, the New Mexico Bureau of Mines and Mineral Resources (NMBMMR) has completed geologic mapping of fifteen 7.5-minute quadrangles (~ 900 sq. miles) in the Albuquerque Basin, Sandia Mountains, and adjacent eastern slope. The principal purpose of this project is to provide a current and detailed geologic framework for regional groundwater resource and geologic hazard studies in this rapidly growing region. We present a preliminary compilation of a contiguous set of nine digital geologic quadrangle maps and cross sections extending from the Rio Grande to the eastern slope of the Sandia Mountains (Fig. 1). These maps include the Alameda, Albuquerque East, Albuquerque West, Bernalillo, Placitas, Sandia Crest, Sandia Park, Sedillo, and Tijeras 7.5-minute quadrangles (NMBMMR open-file digital maps 10, 17, 3, 16, 2, 6, 1, 20, and 4, respectively). Mapping of the Proterozoic-Cenozoic rocks of the Sandia Mountains and vicinity was conducted at scales of 1:24,000 and 1:12,000; mapping of Cenozoic deposits in the Albuquerque Basin was completed at a scale of 1:24,000. These maps, excluding the Sandia Park and Sedillo quadrangles, are part of an ongoing compilation of the geology of the ARMA at a scale of 1:50,000 (Fig. 1).
Geologic mapping of the Albuquerque Basin employs a combination of lithostratigraphic, soil-morphologic, and allostratigraphic methods. Previous limitations to the development of a regionally correlative Cenozoic stratigraphic nomenclature include, the lack of good exposures and well-defined stratigraphic sections, lithologic similarity between Santa Fe Group (SFG) basin fill and younger deposits, and differences in nomenclature used by different investigators working in diverse areas over the past 60 years. Compilation of these quadrangles, integration with available borehole data, and incorporation of biostratigraphic and radioistopic data are used to develop a regionally consistent and correlative stratigraphic framework.
Deposits of the SFG have been provisionally assigned to the Sierra Ladrones and Arroyo Ojito Formations. Major revisions to previous mapping include the subdivision of piedmont and fluvial (ancestral Rio Grande) facies of the Sierra Ladrones Formation, and delineation of a western- and northern- margin facies called the Arroyo Ojito Formation (Connell et al., this volume). Younger (post-SFG) deposits are divided into several major units and subunits. Integration of borehole data with geologic mapping also delineates hydrogeologically significant subsurface stratigraphic trends (Connell et al., 1998) that constrain the distribution of buried faults and aquifers. The spatial distribution of SFG fluvial facies generally corresponds to variations in well productivity. For instance, water-supply wells completed in much of the Arroyo Ojito Formation and in piedmont facies of the Sierra Ladrones Formation are typically less productive than wells completed in fluvial deposits of the Sierra Ladrones Formation.
Mapping and compilation of the pre-Tertiary geology has refined structural geology and lithostratigraphy of the study area. Particular emphasis was placed on mapping fault systems and joint sets in an effort to provide better data for hydrogeologic studies (delineate zones of increased fracture porosity or potential impediments to flow). Many structures, such as the La Cueva fault, extend into the Albuquerque Basin where they may influence groundwater flow and basin-margin recharge. Lithostratigraphic mapping of Proterozoic and Phanerozoic rocks provides finer subdivision of potentially hydrogeologically important strata, such as on the Sedillo quadrangle, where coarse clastic units are differentiated from carbonate and shale in the Madera Formation.
Major structural elements within the study area are not vastly different than described by previous investigations (Kelley, 1977; Kelly and Northrop, 1975). However, some elements of the new maps are of notable interest. Faulting of pre-Madera Formation sediments by locally mineralized northwest-striking faults suggest these faults were related to Ancestral Rocky Mountains tectonism. Laramide structural features, such as monoclines and reverse faults with associated drag folds were, in some cases, reactivated during Tertiary extension, suggesting that these zones of crustal weakness strongly influenced the position and character of Neogene faults in the Albuquerque Basin. Another persistant feature, the Tijeras fault system may record a history of repeated reactivation since the Proterozoic. Minor faults near the Tijeras fault may be synthetic Riedel faults suggesting left-lateral motion occurred most recently along this system. Also, near Placitas at the northern end of the Sandia mountains, strata between steeply to moderately dipping faults are not significantly rotated as would be expected for listric faults like those interpreted by Russell and Snelson (1994) and Woodward and Menne (1995).
These observations and others portrayed on this new series of maps will provide a better understanding of the development of the Albuquerque Basin and Rio Grande rift. Refining our knowledge of the geology of the entire region will continue to be of vital concern as increased demand for water resources, groundwater contamination, and exposure to geologic hazards affect rapidly growing populations. This series of digitally compiled geologic maps will allow the flexibility of relatively easy revisions and rapid dissemination of continuously evolving geologic knowledge.