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Hydrogeology of the Questa Area

figure
Cross section through Guadalupe Mountain and part of the Rio Grande del Norte National Monument
(click for a larger version)

In 2011, the senior authors were contacted by Ron Gardiner of Questa, and Village of Questa Mayor Esther Garcia, to discuss the existing and future groundwater supply for the Village of Questa. This meeting led to the development of a plan in 2013 to perform an integrated geologic, geophysical, and hydrogeologic investigation of the Questa area by the New Mexico Bureau of Geology & Mineral Resources (NMBG), the U.S. Geological Survey (USGS), and New Mexico Tech (NMT).

The NMBG was responsible for the geologic map and geologic cross sections. The USGS was responsible for a detailed geophysical model to be incorporated into the NMBG products. NMT was responsible for providing a graduate student to develop a geochemical and groundwater flow model. This report represents the final products of the geologic and geophysical investigations conducted by the NMBG and USGS. The USGS final products have been incorporated directly into the geologic cross sections.

The objective of the study was to characterize and interpret the shallow (to a depth of approximately 5,000 ft) three-dimensional geology and preliminary hydrogeology of the Questa area. The focus of this report is to compile existing geologic and geophysical data, integrate new geophysical data, and interpret these data to construct three, detailed geologic cross sections across the Questa area. These cross sections can be used by the Village of Questa to make decisions about municipal water-well development, and can be used in the future to help in the development of a conceptual model of groundwater flow for the Questa area. Attached to this report are a location map, a preliminary geologic map and unit descriptions, tables of water wells and springs used in the study, and three detailed hydrogeologic cross sections shown at two different vertical scales. The locations of the cross sections are shown on the index map of the cross section sheet.

This technical completion report for the village of Questa NM by the NM Bureau of Geology, in close collaboration with U.S. Geological Survey, refines our understanding of the geologic architecture of the new Rio Grande del Norte National Monument. The report contains two oversize color plates; one is a preliminary geologic map of the Questa and Guadalupe Mountain quadrangles; the second contains three, parallel, geologic cross sections. These east-west sections are unique, detailed depictions of the architecture of this part of the San Luis Basin of the Rio Grande rift. They combine insights gleaned from: 1) new, detailed (1:24,000) geologic mapping through the STATEMAP and FEDMAP programs; 2) a high-resolution aeromagnetic survey; 3) new ground-based magnetic traverses; 4) a new gravity model of the San Luis Basin; 5) a new volcanologic model of the Taos Plateau volcanic field; and 6) a conceptual model of the stratigraphy and structural geology of the southern San Luis rift basin.

Important Findings

  1. The new geologic map of the Questa area has revealed many details of the complex Cenozoic history of this part of the Rio Grande rift, including the development of the Questa sub-basin, the eruptive history of the Taos Plateau volcanic field, the evolution of the Rio Grande and its gorge, the geomorphic history of the Red River valley, the spread of syntectonic alluvial fans, and the connections among tectonism, volcanism, sedimentation, and erosion.
  2. The detailed geologic mapping shows that the course of the ancestral Rio Grande has changed through time. The ancient, small, and slow-moving river was temporarily blocked by basalt ledges that were uplifted along the southwestern side of the Red River fault zone. Downstream of the fault zone, the high gravels deposited by the ancestral Red River continue southward within the area now containing the Rio Grande gorge, indicating that the headwaters of the middle-Quaternary ancestral Rio Grande watershed were not the San Luis Basin, but instead the Red River watershed. Eventually, the Red River cut down through the basalt ledge, capturing the northern Rio Grande watershed, and greatly increasing the amount of flow in the ancestral river system.
  3. Although the topographic expression of the Rio Grande rift (from the foothills of the Tusas Mountains to the foothills of the Sangre de Cristo Mountains) at the latitude of Questa is about 27 miles wide, the principal structural rift basin (from the Rio Grande gorge to the foothills of the Sangre de Cristo Mountains) is only about 6 miles wide.
  4. The central San Luis rift basin does not conform to the classic, asymmetric "trap-door" rift geometry. Rather, the deepest part of the basin (under Guadalupe Mountain) is flanked by nearly symmetrical, highly faulted, pre-rift basement rocks.
  5. The deepest basin-fill in the Questa area is about 4000 ft thick.
  6. Faults defined by geologic mapping show a good correlation with faults defined by aeromagnetic modeling. In many cases, the ends of mapped faults can be extended laterally with a high degree of confidence by using the aeromagnetic interpretations.
  7. According to the gravity model and the conceptual structural model of the area, buried, medium-displacement, normal faults are likely ubiquitous throughout the structural rift basin.
  8. The existence of single, large-displacement "master" faults along the topographic east edge of the rift is problematic. Although some large faults do separate basin fill and bedrock along the Sangre de Cristo fault zone, in general, extensional strain seems to be distributed among large numbers of complex normal faults that branch, curve, and terminate.
  9. The normal faults located within the basement rocks of the Sangre de Cristo Mountains are important rift structures, and actually represent a large component of the cumulative throw on the Sangre de Cristo fault system. These faults can be difficult to identify and characterize, due to the general lack of marker stratigraphy in the crystalline rocks.
  10. Guadalupe Mountain consists of three, distinct eruptive lobes that can be mapped at the surface and in the subsurface by high-resolution aeromagnetic methods.
  11. The subsurface shapes of the local volcanoes are complex, and represent an elaborate interaction of volcanism, sedimentation, tectonics, and erosion during Pliocene and Pleistocene time.
  12. Modern gravity modeling is a powerful, and essential, tool for drawing deep cross sections through the Rio Grande rift, especially in areas where deep boreholes are scarce.
  13. Modern, high-resolution aeromagnetic and ground-based magnetic modeling is a compelling tool for characterizing the buried structural geology and buried volcanic rocks in the Rio Grande rift.

Results

  1. Bauer, P.W., Grauch, V.J.S., Johnson, P.S., Thompson, R.A., Drenth, B.J., and Kelson, K.I., 2015, Geologic Cross Sections and Preliminary Geologic Map of the Questa Area, Taos County, New Mexico: New Mexico Bureau of Geology and Mineral Resources Open-File Report 578, 15 p. plus 2 oversize plates and appendices.

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