HYDROGEOLOGIC FRAMEWORK OF THE MIDDLE RIO GRANDE BASIN, NEW MEXICO, WITH EMPHASIS ON KIRK BRYAN'S CONTRIBUTION TO CONCEPTUAL-MODEL DEVELOPMENT

The Middle Rio Grande basin system (MRGBS) is a linked-series of large structural basins, with intervening basin constrictions, that occupy most of the Rio Grande rift (RGR) tectonic province in central New Mexico. Fault-block uplifts and volcanic highlands are the dominant boundary units of the basin-scale hydrogeologic framework. Major aquifers comprise coarse-grained facies of the main basin-fill unit--the Neogene Santa Fe Group (mostly fluvial), and Upper Quaternary fill of the inner Rio Grande Valley. The MRGBS extends from the Cochiti Reservoir area (SW Española Basin) to the Engle Basin (Elephant Butte area). Its’ northernmost and deepest part is the Albuquerque Basin (ABQB), with three large subbasins (Santo Domingo, Calabacillas, and Belen). The ABQB merges southward with the narrow Socorro and La Jencia basins, which are in turn linked to the broader San Marcial and northern Jornada basins. By 1950, Kirk Bryan and his graduate-student associates had developed the basic hydrogeologic-framework model that we still use in characterizing intermontane basins of the RGR region. They recognized that the fill of the linked-basin series, which formed Bryan’s Rio Grande “depression,” is a single major lithostratigraphic unit, the Santa Fe “formation”(SFF). Furthermore, they demonstrated that this unit has 3 major lithofacies assemblages: 1) piedmont-slope; 2) closed-basin-floor, associated with alluvial flats, playas, and lakes; and 3) open-basin-floor, associated with through-going fluvial systems. Investigations in the ABQB by Bryan and associates (notably F. McCann, C. Denny, H. Wright, and C. Stearns) were particularly important, because they recognized 1) that the SFF could be separated into at least 3 member-rank mapping units, 2) the crucial roles that tectonism and volcanism played during and after Santa Fe deposition, 3) the major stages of river-valley evolution following cessation of widespread basin aggradation. Finally, Bryan (1938) defined the basic elements of groundwater flow in undrained, drained, and partly drained basin-system components.