HYDROGEOLOGIC FRAMEWORK OF BASIN-FILL AQUIFERS IN THE SOUTHWESTERN NEW MEXICO REGION—INTEGRATING FRAMEWORK COMPONENTS USING GIS

The NM Water Resources Research Institute in cooperation with the US Environmental Protection Agency has recently developed GIS coverages that integrate large amounts of hydrogeologic information on basin-fill aquifers in the International Boundary region of southwestern New Mexico. This region includes adjacent areas of southeastern Arizona, and Chihuahua and Sonora, Mexico. The GIS format allows 3-D integration of surface and subsurface information that can be used in numerical groundwater-flow modelling and geochemical-system interpretation. Emphasis of this presentation is on the hydrogeologic framework of five intermontane basin systems in the region west of the Mesilla Basin. From east to west, these basin systems are the Mimbres-lower Casas Grandes, Hachita-Moscos, Playas-San Basilio, Animas-Cloverdale, and San Bernardino.

The basin-fill hydrogeologic framework is defined on the basis of (1) composition of lithofacies assemblages (LFAs), which are organized into hydrostratigraphic units (HSUs), and (2) the nature of basin-boundary and intra-basin structural controls. Major aquifers are formed by coarser-grained LFAs associated with the ancestral Casas Grandes, Mimbres, and Animas fluvial systems. These poorly consolidated units are informal subdivisions of the Plio-Pleistocene upper Gila Group and overlying basin-floor deposits. Typical horizontal hydraulic conductivities range from 1-10 m/day, and units are usually less than 150 m thick. Contiguous piedmont-slope LFAs and underlying middle and lower Gila Group HSUs (mostly Miocene) have much lower aquifer potential because of finer matrix texture, and more consolidation and cementation. Complex half-graben basins that are linked across zones of structural accommodation have a profound effect on both local and regional groundwater flow. All basins, except San Bernardino, have surface-flow regimes discharging to subbasins with topographic closure and playa lakes. The latter areas were sites of large perennial lakes during Late Quaternary pluvial intervals. Groundwater flow in most subbasins, however, partly drains to lower contiguous basins with terminal sinks or river valleys.