Geophysics of the Rio Grande Basins
Española Basin Geophysics
Aquifers within the Española basin are the primary ground-water resource of the City of Santa Fe, Española, six Pueblo nations, and surrounding urban areas. To better define and manage this important ground-water resource, we have collected and synthesized high-resolution aeromagnetic data, deep-looking electrical geophysical soundings, and gravity data compilations with geology and other geophysical information. The focus is on interpreting subsurface geology that may influence ground-water flow, quality, and storage and can be used to improve regional ground-water flow models. Our studies overlap with and extend efforts from our studies of the Middle Rio Grande basin. In addition to our studies, our scientists founded and ran the Española Basin Technical Advisory Group for 5 years to promote collaboration and information transfer. This group continues to hold annual workshops in Santa Fe, but now under different leadership.
The synthesis of geophysical data and subsurface interpretations for the southern Española basin is reported in Professional Paper 1761. Deep electrical geophysical studies are reported in Chapter F, Professional Paper 1720 and are being prepared for future publications.
- The southern Española Basin consists of a wedge of rift fill, primarily of Santa Fe Group sediments, that serves as the principal aquifer for the city of Santa Fe. The wedge thickens northward across the Rancho Viejo hinge zone south of Santa Fe. South of the hinge zone, Santa Fe Group sediments overlie the Santa Fe platform, which is composed of volcanic rocks of the Espinaso Formation and Cieneguilla volcanic complex. North of the hinge zone, Santa Fe Group sediments thicken markedly under the Pajarito Plateau.
Perspective view of the modeled elevation of the base of the Santa Fe Group in relation to topography, looking east. Topographic surface is raised above the base.
- The Santa Fe platform has local irregularities that produce variations in sediment thickness at the base of the Santa Fe Group. A large paleovalley of Santa Fe Group sediments is buried near Eldorado, south of Santa Fe. In three large areas, the low-permeability bedrock of the Santa Fe platform is missing. Here, a direct hydrologic connection between the Santa Fe Group and older, pre-Santa Fe Group sedimentary aquifers may occur. Older, pre-Santa Fe Group sedimentary rocks are thickest under the Santa Fe Platform and decrease to a thin veneer of volcanic rocks and limestone overlying crystalline basement north of the Rancho Viejo hinge zone. Thus, deep sandstone aquifers are possible under the platform but are unlikely below the Santa Fe Group in the deeper parts of the basin.
- Numerous faults offset the basin fill, more than can be recognized at the surface. The faults likely juxtapose at least 30 meters of sediments that have contrasting bulk magnetic properties. We determined that significant thicknesses of sediments with differing grain size or volcanic-clast content are juxtaposed at these faults, which may produce permeability contrasts that compartmentalize Santa Fe Group aquifers at depth. Many faults can be organized into two main fault systems (Agua Fria and Barrancos), which traverse north to north-northwest near Eldorado, Santa Fe, Pojoaque, and Española. The faults typically involve basement rocks and may serve as partial barriers, compartmentalize aquifers, or control upwelling of deep ground water.
Mapped and geophysically interpreted faults. Faults interpreted from aeromagnetic data (magenta lines) are from Grauch and Hudson (2007). Faults interpreted from gravity data (green lines) are from the horizontal-gradient magnitude of the isostatic residual gravity.
- A north-trending bedrock high, the Cerrillos uplift, is bounded on the west by the La Bajada fault, and outlined by magnetotelluric (deep-looking electrical geophysical) modeling on the north. This boundary defines the southeastern extent of the La Bajada constriction through which groundwater hydraulically connected with the Rio Grande flows as it passes from the Española Basin into the Santo Domingo Basin. The largely concealed Tetilla fault zone is also detected by the electromagnetic surveys in this area; it appears to form the western boundary of an east-dipping block of electrically conductive Mancos Shale, which may form an impermeable hydrogeologic base. Farther north, another north-northeast-trending bedrock high underlies the Cerros del Rio volcanic field at lower elevation. It could potentially form a major obstacle to westerly ground-water flow from deeper parts of the Espanola basin. The bedrock high under the Cerros del Rio volcanic field may have been a structural low in Laramide time that was later inverted to a structural high in rift time.
Resistivity model and possible nature of the bedrock high under the Cerros del Rio volcanic field from deep-looking electrical geophysical methods, from south to north.