HYDROGEOLOGIC FRAMEWORK OF THE WESTERN HUECO BOLSON AREA OF THE RIO GRANDE RIFT, TEXAS, NEW MEXICO, AND CHIHUAHUA: PROGRESS REPORT ON DEGITAL MODEL DEVELOPMENT

The western Hueco Bolson area of the southern Rio Grande rift (RGr) occupies parts of Trans-Pecos Texas, south-central New Mexico, USA and north-central Chihuahua, MX. It includes a long segment of the binational Rio Grande Valley and the El Paso-Ciudad Juárez metropolitan complex with a population of about 2 million. This progress report emphasizes recent work on development of digital hydrogeologic framework models and GIS datasets that integrate large amounts of geologic and geochemical information on Neogene rift-basin fill (Santa Fe Gp) and river-valley aquifer systems. The GIS format (ESRI ArcGIS®) allows integration of surface and subsurface information that can be used in groundwater-flow and hydrogeochemical modeling. Provisional hydrogeologic maps and cross-sections completed to date include a surficial map, 12 sections (to mean-sea-level), and a structure-contour map of the Santa Fe Gp base in the Paso del Norte area. The hydrogeologic framework of basin-fill aquifers is defined in terms of major lithofacies-assemblages (LFAs) that are grouped as informal hydrostratigraphic units (HSUs), and basin-boundary and intra-basin structural controls.

Late Cenozoic extensional-tectonic features that characterize the entire RGr region have had a profound influence on both basin-fill composition, and groundwater flow and chemistry in the Hueco Bolson area. Primary tectonic components are half-graben basins and flanking ranges that are linked across zones of structural accommodation. Major aquifer systems are formed by fluvial basin-floor LFAs deposited by Pliocene ancestral-river distributaries, and inset-channel deposits in the inner valley of the Late Quaternary Rio Grande. These poorly consolidated sediments are grouped into upper to middle Santa Fe and river-valley HSUs. Horizontal hydraulic conductivities commonly range from 3-30 m/day, and basin-fill fluvial sequences are as much as 300 m thick and 30 km wide. In marked contrast, the inner river-valley fill is less that 30m thick and 9 km wide. Except for deeply buried eolian-sand facies, subjacent middle to lower Santa Fe basin-floor deposits (Miocene) and intertonguing piedmont-slope LFAs have much lower aquifer potential because of finer matrix, and greater consolidation and cementation.