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Florian Maldonado, Dave W. Love, Sean D. Connell, Janet L. Slate, Karl E. Karlstrom, and Van S. Williams
USGS Open File Report (in press), 2000

The Isleta Reservation straddles the middle of the Albuquerque basin, from the Rio Puerco eastward to the Manzano and Manzanita Mountains, just south of Albuquerque, New Mexico.  The U.S. Geological Survey, New Mexico Bureau of Mines and Mineral Resources, and the University of New Mexico have conducted geologic mapping on the Isleta Reservation and vicinity as part of the Middle Rio Grande Basin Project.  The study area comprises twelve 7.5-minute quadrangles that have been mapped at 1:24,000 scale and complied at 1:50,000 scale.  This geologic mapping has enhanced our understanding of the stratigraphy, structure, and geomorphic evolution of the middle Albuquerque basin.

Pre-Santa Fe Group rocks exposed in the map area include Proterozoic metamorphic and plutonic rocks and upper Paleozoic and Mesozoic strata.  Proterozoic crystalline rocks exposed in the Manzano and Manzanita Mountains consist of greenstone, metarhyolite, quartzite, and granite gneiss.  The mountains are capped by rocks of the Pennsylvanian Sandia and Madera formations.  Pennsylvanian through Triassic rocks (Sandia, Madera, Abo, Yeso, Glorieta, San Andres, Moenkopi, and Chinle formations) are slightly to moderately deformed and discontinuously exposed between the Coyote fault and the Hubbell Spring fault zone.  These rocks are highly faulted and folded, probably during Laramide deformation.  East-dipping Crevasse Canyon Formation (Upper Cretaceous) is exposed along the western margin of the study area.  Paleogene rocks are not exposed in the study area, but are as much as 2 km thick in the subsurface (Lozinsky, 1994).

The Santa Fe Group is divided into lower and upper units.  The lower Santa Fe Group is exposed near the southwestern corner of the study area.  These deposits consist of reddish-brown mudstone and sandstone correlated to the Popotosa Formation (Unit 1) of Lozinsky and Tedford (1991), who interpreted deposition in a basin-floor playa setting.  Just west of the southwest corner of the map area, exposures of trough-cross bedded fluvial sandstone are interbedded with the Popotosa Formation, which is intruded by the 8.3 Ma (Baldridge and others, 1987) diabase of Mohinas Mountain.  The Popotosa Formation is in fault contact to the east with deposits of the upper Santa Fe Group, which are divided into three major facies: western-margin facies derived from major tributary fluvial systems (ancestral Rio Puerco) draining the adjacent Colorado Plateau and Sierra Nacimiento; central-basin facies containing deposits of the ancestral Rio Grande axial-fluvial system; and eastern-margin facies derived from the adjacent rift-border uplifts of the southern Sandia, Manzanita and Manzano Mountains.  Six basaltic volcanic fields interfinger with upper Santa Fe Group deposits within the map area.  The Black Mesa (2.68 Ma); Isleta (2.73-2.79 Ma); Cat Mesa (3 Ma), Los Lunas (1.14 and 3.91 Ma), Wind Mesa (4 Ma) volcanics (Maldonado and others, 1999) interfinger with western basin-margin deposits.  Stratigraphic relationships of the El Cerro Tome (3.4 Ma; Bachman and Mehnert, 1978) volcanics are ambiguous, but they are probably overlain by the fluvial deposits of the central basin facies.

Geologic mapping, stratigraphic studies, and radioisotopic data indicate that the top of the Santa Fe Group is defined by at least four diachronous aggradational surfaces (Connell and others, this volume).

Younger, post-Santa Fe Group deposits are divided into fluvial, alluvial, colluvial, eolian, and spring sediments that were deposited in various geomorphic settings.  The Cat Hills basalt (98-110 ka) overlies ancestral Rio Grande deposits of the Los Duranes formation of Lambert (1968).

Dominantly north-trending faults crosscut older northwest-trending faults that segmented the Santa Fe Group into multiple sub-basins.  The western basin margin is characterized by numerous normal faults with generally down-to-the-east movement;  several shorter down-to-the west faults are antithetic to the dominant western margin faults and bound local horsts and grabens, including the Wind Mesa horst and Isleta Pueblo graben.  The eastern margin is defined by numerous down-to-the-west normal faults such as the Manzanita, Manzano Mountains, and Hubbell Spring fault zone.  The Hubbell Spring fault zone is a major intrabasinal structure that juxtaposes Permian and Triassic rocks against eastern-margin and central-basin facies.  Map relationships and aeromagnetic, gravity, and subsurface data all suggest that the surface trace of the Tijeras fault zone is either cut by, or merges with, the Hubbell Spring and southern Sandia fault zones just south of the Four Hills.  The Tijeras fault zone is not expressed southwestward across the basin.  Results of this study do not support the presence of the inferred north-trending Rio Grande fault of Russell and Snelson (1994).  We interpret an older (late Oligocene(?)-Miocene) northwest-trending fault-bounded depression between the southwestern margin of the northwest-trending Mountainview prong and the Isleta Pueblo graben that we call the Mountainview fault zone (Maldonado and others, 1999).  This northwest-trending fault zone was subsequently cut by younger generally north-trending faults during the Pliocene and Pleistocene.

References
Bachman, G.O., and Mehnert, H.H., 1978, New K-Ar dates and the late Pliocene to Holocene geomorphic history of the central Rio Grande region, New Mexico: Geological Society of America Bulletin, v. 89, p. 283-292.
Baldridge, W.S., Perry, F.K., and Shafiqullah, M., 1987, Late Cenozoic volcanism of the southeastern Colorado Plateau:  I. Volcanic geology of the Lucero area, New Mexico:  Geological Society of America Bulletin, v. 99, p. 463-470
Lambert, P.W., 1968, Quaternary stratigraphy of the Albuquerque area, New Mexico: Ph.D. theis, University of New Mexico, Albuquerque, 329 p.
Lozinsky, R.P. 1994, Cenozoic stratigraphy, sandstone petrology, and depositional history of the Albuquerque Basin, New Mexico: Geological Society of America, Special Paper 291, p. 73-81.
Lozinsky, R.P., and Tedford, R.H., 1991, Geology and paleontology of the Santa Fe Group, southwestern Albuquerque Basin, Valencia County, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Bulletin 132, 35 p.
Maldonado, F., Connell, S.D., Love, D.W., Grauch, V.J.S., Slate, J.L., McIntosh, W.C., Jackson, P.B., and Byers, F.M. Jr., 1999, Neogene geology of the Isleta Reservation and vicinity, Albuquerque Basin, central New Mexico: New Mexico Geological Society Guidebook, Albuquerque Geology 1999, p.175-188.
Russell, L.R., and Snelson, S., 1994, Structure and tectonics of the Albuquerque Basin segment of the Rio Grande rift:  Insights from reflection seismic data, in Keller, G.R., and Cather, S.M., eds., Basins of the Rio Grande rift:  Structure, stratigraphy, and tectonic setting:  Geological Society of America, Special Paper 291, p. 83-112.

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