GEOLOGY AND GROUNDWATER IN THE ALBUQUERQUE-RIO RANCHO METROPOLITAN AREA, BERNALILLO AND SANDOVAL COUNTY, NEW MEXICO

Detailed mapping of the Albuquerque Basin, New Mexico, delineates various units of the upper Oligocene-lower Pleistocene Santa Fe Group (SFG), a major source of groundwater for communities within the Rio Grande rift. Integration of geologic mapping with hydrologic and geophysical (regional and borehole) data provides a clearer picture of basin architecture and groundwater resources. A GIS-based map compilation at a scale of 1:50,000, which encompasses 2175 km² of the Albuquerque-Rio Rancho metropolitan area and vicinity, preserves nearly all of the elements of the 1:24,000 base mapping and illustrates important regional stratigraphic and structural trends found in smaller scale compilations. Land management issues addressed by this compilation include aquifer geometry, and groundwater quality and quantity. Other issues include contamination of shallow groundwater, subsidence caused by groundwater withdrawal, delineation of hydrocompactive soils, as well as earthquake, volcanic, and landslide hazards.

The SFG overlies deformed remnants of an Oligocene sedimentary basin that formed during an earlier phase of extension. This extension was accommodated by faults that later cut the SFG. Much of the great thickness of the SFG is associated with deposition in a hydrologically closed basin that existed before Pliocene time. Local unconformities record episodic movement of some intrabasinal faults. Some of these unconformities do not extend towards basin depocenters. Many low-productivity wells in the western and northwestern parts of the map area take water from moderately cemented eolian sandstone of the lower SFG. Wells in the central and eastern parts of the basin draw water from the more productive upper SFG (Plio-Pleistocene), which was deposited by throughgoing rivers of the ancestral Rio Grande and tributaries. These deposits are a small proportion of the total basin fill, but contain the most productive aquifer zones. Ancestral Rio Grande sediments are found within 2 km of the eastern structural margin. Widespread SFG deposition ceased west of the Rio Grande Valley by 2-2.5 Ma; continued deposition of the ancestral Rio Grande and Sandia Mts piedmont occurred to the east until 1.2-0.8 Ma, when the Rio Grande began to incise.