New Mexico Geological Society Annual Spring Meeting — Abstracts

STATEMAP geological mapping at 1:24,000 scale of the Santo Domingo Pueblo and Santo Domingo Pueblo SW 7.5' quadrangles has led to refinement and revision of stratigraphic and structural characteristics of the northern Santo Domingo basin and the margin of the Jemez Mountains volcanic field. Some of the new and revised observations have potentially important implications for understanding aquifer characteristics and for modeling recharge pathways in the northern Albuquerque basin, which includes the Santo Domingo basin. Because the northern basin margin (between the Rio Jemez and Galisteo Creek) provides an estimated 25% of the boundary inflow (mountain-front and tributary recharge, subsurface inflow from adjacent regions) to the Kernodel et al. model (1995, U.S. Geological Survey Water-Resources Investigation Report 94-4251), it is essential to better understand geological controls for groundwater pathways in this region.

A greater proportion of coarse, gravelly sediment is present in the Santo Domingo basin than was known at the time of construction of the Kernodle et al. (1995) groundwater-flow model. Upward revision of hydraulic conductivities for most model cells by an order of magnitude (to ~10^-2 cm/s) is recommended. Ancestral Rio Grande gravel and sand are widespread within the upper Santa Fe Group, extending as much as 12 km west of, and at least 5 km east of, the modern river. West of Cochiti Pueblo, these strata are intercalated with ~6.8 Ma pyroclastic deposits of the Peralta Tuff Member of the Bearhead Rhyolite and underlie a ~6.96 Ma rhyolite dome. Rather than being a thin
Plio-Pleistocene capping layer, as depicted on previous maps, this facies is at least 500 m thick, thus occupying a substantial volume of the shallow saturated zone. These data also indicate the presence of a gravel-bedload, axial river at an earlier time than maintained by most previous workers. West of the Rio Grande, the axial facies interfingers with Peralta Tuff and uppermost Miocene-lower Pleistocene Cochiti Formation. The Cochiti Formation is an at least 500-m-thick sequence of gravel and sand that is at the surface over most of western Cochiti and Santo Domingo Pueblos. Gravel dominates in northern and western outcrops and cementation is generally minor except locally near faults. These coarse strata of the Cochiti Formation are also likely more hydraulically conductive than realized in the Kernodle model, potentially leading to new views of recharge routing from the Jemez Mountains.

Intrabasinal faults, of greater number, length and displacement than represented on previous maps, may partition the aquifer at depth, accounting for water levels in the western basin that are lower than the Rio Grande. Preferential calcite or chalcedony cementation along faults is obvious at many locations and may be more prevalent at depth. A horst trends obliquely across the basin from NW to SE near Sile and exposes upper Miocene axial gravel with abundant Peralta Tuff pumice; axial facies may not be abundant below the water table in this area. We hypothesize that the trough in the water table west of the Rio Grande is caused by the presence there of a highly transmissive section, including axial-facies gravel, that conducts Jemez Mountains recharge southward while being structurally isolated from recharge by the Rio Grande.