Geophysics of the Rio Grande Basins
Middle Rio Grande Basin Geophysics - Subsurface Geology of the Santo Domingo Basin and La Bajada Constriction
Gravity and Aeromagnetic Studies
Investigations of gravity and aeromagnetic data help us understand the subsurface geologic and hydrogeologic framework of the Santo Domingo Basin. Gravity data show that the Santo Domingo Basin is bounded on the southeastern side by steeply dipping faults of large vertical displacement and on the northwestern side by a homocline or series of down-to-basin faults. The Cerrillos uplift is reflected in a north-south gravity high that includes Oligocene intrusions of the Ortiz Mountains and Cerrillos Hills, which also appear in regional magnetic data. Within the La Bajada constriction, the gravity data and associated models indicate a northeast-trending trough 10-14-km wide that projects under the Cerros del Rio volcanic field and that likely contains Santa Fe Group sediments about 1 km thick. Positive aeromagnetic anomalies in the Cerros del Rio volcanic field are associated with basalts that formed during the Gauss normal magnetic-polarity epoch (2.70-2.59 Ma), whereas similar negative anomalies are associated with basalts that formed during the Matuyama reversed magnetic-polarity epoch (2.59 Ma-776 ka).
In the western half of the Cochiti Pueblo study area, magnetic piedmont basin-fill sediments of the Cochiti Formation and axial river gravels are expressed as subtle aeromagnetic anomalies that correlate with topography. Numerous faults that offset these magnetic sediments have aeromagnetic expressions that help delineate faults where they are concealed or inferred. An extensive northeast-elongated negative anomaly in the southern part of the intrabasin La Majada graben is interpreted as a post-2.59-Ma buried flow. This interpretation implies that 525-725 m of displacement has occurred along the La Bajada fault zone during the last 2.4 m.y. This is discussed in more detail in:
Airborne and surface electromagnetic surveys were used to map changes in electrical resistivity with depth in the La Bajada constriction and Cochiti Pueblo area. These changes in resistivity are related to variations in rocks that, in turn, influence aquifers in the study area. In the eastern Cerros del Rio volcanic field, the northern boundary of the Cerrillos uplift is constrained by our electromagnetic survey results. This boundary defines the southeastern extent of the La Bajada constriction through which ground water, hydraulically connected with the Rio Grande, flows as it passes from the Española Basin into the Santo Domingo Basin. Electromagnetic data shows the largely concealed Tetilla fault zone forming the western boundary of an east dipping block of electrically conductive Mancos Shale. In the north-central part of the La Bajada constriction, a large area of lower resistivity coincides with a silt or clay lacustrine unit in upper Santa Fe Group basin-fill deposits. In the central part of the constriction, higher resistivities correspond in part with ancestral Rio Grande axial gravel deposits. On the western flank of the La Bajada constriction, our data provide constraints on the relative position of basement and on the thicknesses of Paleozoic, Mesozoic, and Tertiary sedimentary rocks on either side of the constriction-bounding Pajarito fault zone. This is discussed in more detail in: