HYDROGEOLOGY OF SAN AGUSTIN PLAINS, NM: A COMPREHENSIVE GEOLOGIC, GEOCHEMICAL, AND GEOPHYSICAL APPROACH TO UNDERSTANDING THE HYDROLOGY OF AN ENIGMATIC EXTENSIONAL BASIN

The San Agustin Plains, located between the Mogollon-Datil volcanic field and the Colorado Plateau in west-central NM, is an enigmatic extensional basin. Stretching 70 km roughly SW-NE and 20-30 km across, the San Agustin Plains include two closed topographic basins (West and East) surrounded by mountains composed of Oligocene volcanic and volcaniclastic rocks. To understand its hydrogeology, we integrated all available data to clarify the geologic framework and collected new water level, water chemistry and environmental tracer data. We conclude that the Plains are underlain by multiple, partially drained grabens whose sandy basin fill aquifer is recharged slowly from alluvial aquifers and complicated mountain block aquifers and has steady, slow flow rates.

Geologic mapping, historical electrical resistivity and reanalyzed terrain-corrected Bouguer gravity anomaly data show that the West Basin is probably a single graben and the East basin is comprised of the North graben, the C-N graben, and the White Lake graben. Mountain blocks are composed of Oligocene-aged, interbedded Mogollon-Datil Group felsic and basaltic-andesitic volcanics (300-1000 m total thickness) and Spears Group volcaniclastics (1,000s m total thickness). Well temperature logs and outcrops show some Mogollon-Datil Group members are fractured and transmit water at depth, while the Spears Group volcaniclastics are mostly aquitards. In the canyons that cut the mountains, poorly sorted-, coarse-sands form alluvial aquifers fed by ephemeral streams; few perennial springs exist in the region. Basin-fill consists of piedmont sands and gravels proximal to the mountain front that grade laterally to bolson fine sands and muds in the center of the basins.

Temperature, water chemistry, age tracers, and stable isotopes reveal two types of recharge into the basin: (1) focused recharge spilling out from alluvial aquifers, and (2) mountain block recharge from fracture networks in several different via variably connected fractures in volcanic rocks. Water levels and water chemistry spatial patterns suggest that the structural grabens of the basin are poorly interconnected. We believe that the San Agustin Plains are draining predominately southward from their southwestern corner.