Preliminary geologic map of the Dalies quadrangle

Love, D. W., Maldonado, F., Hallett, B., Panter, K., Reynolds, C., McIntosh, W., and Dunbar, N., 1999, Preliminary geologic map of the Dalies quadrangle [abs.]: U.S. Geological Survey, Open-file Report 99-203, p. 44-46.

(download a red-blue 3-d image of Los Lunas volcano)

 The Dalies Quadrangle is in the center of the Albuquerque Basin 20 km south-southwest of Albuquerque near the rapidly expanding community of Los Lunas. It straddles the eastern edge of the Llano de Alburquerque, the valley border slopes and terraces, and the modern floodplain of the Rio Grande. Elevations range from 1505 m on the floodplain to 1815 m at the top of El Cerro de Los Lunas. Two volcanic fields are exposed at the surface: Los Lunas volcano and Cat Hills. Several north-trending and east-west-trending faults and folds offset the Llano de Alburquerque, Los Lunas volcano and younger surfaces.

Geologic units at the surface are rift-fill deposits, volcanics, and their derivative sediments. Depths to pre-rift Cretaceous rocks beneath the quadrangle in three oil-well tests are 864 m (1 Harlan Ranch) in the southeastern corner, 2200 m (Long 1 Dalies) in the southwestern corner, and 3691 m (Shell Isleta No. 1) in the northeastern corner. Russell and Snelson (1994) interpret Shell Development Company seismic lines in the vicinity, particularly a line through wells Isleta No. 1 and Isleta No. 2 farther north to show a mid-basin transfer zone with a large negative flower structure trending west-southwestward. This structure deforms some basin fill and is overlain by upper Tertiary basin fill. It is not expressed at the surface.

Two hundred m of tilted upper Tertiary and lowest Quaternary basin fill are exposed in badlands on both the northwest and southwest sides of Los Lunas volcano. Part of this basin fill can be traced northward to local facies in the Santa Fe Group of Isleta Pueblo (Maldonado and Atencio, 1998). The two mappable units are (1) QTsi, consisting of at least 25 m of sand, silt, and clay lithofacies that fill a north-south graben and/or sag structure within the Llano de Alburquerque, and (2) QTui, sand and gravel lithofacies that underlie most of the quadrangle to an unknown, but considerable depth. Reflection seismic profiles by Reynolds adjacent to Los Lunas volcano reveal flat-lying basin sediments beneath and beyond Los Lunas volcano on the east side, but tilted, faulted, and folded sediments on the northwest side of the volcano.

The quadrangle includes six of the seven basalt flows of the Cat Hills volcanic field, the oldest of which is dated by Ar/Ar at 0.098 +-0.02 and 0.11 +- 0.03Ma (Love, 1997; Maldonado and Atencio, 1998). The oldest flow lacks rugged surface topography--the lows have been filled with eolian sand and colluvium and the highs have been eroded. The more recent upper flows have rugged topographic highs and lows and locally include a lava tube.

El Cerro de Los Lunas consists of two or more volcanoes of different ages. Outlying volcanic rocks exposed south of the volcano and aeromagnetic anomalies (U. S. Geological Survey and Sander Geophysics, Ltd., 1998) show that the field is much more extensive in the subsurface. Some of the southern outliers appear to be intrusives, but do not intrude the sediments that presently surround them, perhaps indicating that they were exhumed and reburied. The older group of eruptive rocks, with a date of 3.88 +- 0.04 (Love and others, 1994) consists of a lower alkalic to calc-alkalic trachyandesite-andesite-dacite flow, vent-related agglomerate, and overlying flows. The younger volcano consists of at least three vents and four lava flows of trachyandesites. The lowest flow has an Ar/Ar date of 1.22 +- 0.01 (Love and others, 1994).

El Cerro de Los Lunas and sediments adjacent to it have undergone multiple stages of deformation, including folding, thrusting, normal faulting, and uplift. The older volcanic pile is tilted southward and has been cut by numerous faults, and partially buried by later sediments. The basal flow of the younger volcano has a fault offset of at least 30 m with the western part of the flow and underlying sediments raised 150 m above the rest of the landscape. Angular unconformities in basin-fill sediments exposed on the north side of the volcano may be related to deformation during emplacement of magma. The lowest exposed sediments are vertical and may have been tilted during the older eruptions. The overlying sediments are tilted northwestward up to 15 degees by trap-door uplift during and after the eruption of the younger volcano. Tephra from the younger eruptions is folded and thrusted (northwest vergence) and then buried by debris shed from the uplifted portion of the volcano.

Post-volcano sediments include a large alluvial apron (up to 50 m thick) on the west side of Los Lunas Volcano, eolian deposits and soils of the Llano de Alburquerque, the floodplain of the Rio Grande, valley border alluvial and eolian sediments and soils descending from the Llano de Alburquerque to the modern Rio Grande floodplain, and a Rio Grande terrace known as the Los Duranes Formation.

A combination of erosion of unconsolidated sediments over lavas, local faulting and tilting, and incision of the Rio Grande valley has caused abnormal stream courses in the southwestern and southern sides of El Cerro de Los Lunas. One meandering stream, following a tilted sediment/lava contact, has entrenched a succession of subparallel courses in lava that were abandoned as the badland-lava interface eroded lower than the stream course through lava. The stream has cut two short slot canyons 1-4 m wide and up to 7 m deep. Its former course also was affected by faults, including a small graben that caused a right-angle turn.

REFERENCES

Love, D. W., Reynolds, C. B., Hallett, B., Lozinsky, R. P., and Niemyjski, T., 1994, Sedimentation, deformation, and erosion related to Los Lunas volcanoes, central New Mexico (abs.): New Mexico Geology, v. 16, no. 3, p. 57.

Love, D. W., mapper and compiler, 1997, Preliminary geologic map of Isleta Quadrangle, Bernalillo and Valencia Counties, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Open-file Digital Geologic Map OF-DGM 13, scale 1:24,000.

Maldonado F. and Atencio A., 1998, Preliminary geologic map of the Wind Mesa Quadrangle, Bernalillo County, New Mexico: U.S. Geological Survey Open-File Report 97-740.

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.

U. S. Geological Survey and Sander Geophysics, Ltd., 1998, Digital data from the Isleta-Kirtland aeromagnetic survey, collected south of Albuquerque, New Mexico: U. S. Geological Survey Open-File Report 98-341, 1 compact disk