This study combines electrical resistivity tomography (ERT), geological information from boreholes and outcrops, and hydrogeologic data to investigate field-scale fault-zone cementation of the Loma Blanca Fault in the Rio Grande Rift. We collected electrical resistivity data from 16 transects and geological samples from 29 boreholes (completed as groundwater wells to 30 m depth) across and around the fault. 2D ERT profiles, whose interpretations are constrained by geological data, indicate: (1) a high resistivity zone in cemented portions of the fault below the water table, and (2) in the unsaturated zone, a low resistivity feature along the cemented portions of the fault. The high resistivity zone below the water table is consistent with a 10% reduction in porosity due to the fault zone cementation. With the same porosity in the unsaturated zone, the low resistivity feature in the cemented fault zone is consistent with saturation >0.7, in contrast to saturation 0.2-0.7 for sediment outside of the cemented fault zone. In addition, subsurface samples and ERT profiles delineate a buttress unconformity (i.e., steeply dipping erosional contact) corresponding to a paleo-valley margin. This unconformity truncates the cemented fault zone and separates Pliocene axial-fluvial sand (deposited by an ancestral Rio Grande) from late Quaternary sand and gravel (deposited by the Rio Salado, a Rio Grande tributary). The cemented fault zone in the southern portion of the study area is a hydrogeologic barrier; north of the buttress unconformity, where the cemented fault zone has been removed by erosion, the fault is not a hydrogeologic barrier. The integration of geological, geophysical, and hydrogeological observations was key to developing our understanding of this complex system, and allowed us to demonstrate the utility of ERT in detecting subsurface fault-zone cementation.

Fault-zones are an important control on fluid flow, affecting groundwater supply, hydrocarbon/contaminant migration, and waste/carbon storage. However, most current models of fault seal do not consider fault-zone cementation despite the recognition that it is common and can dramatically reduce permeability. As part of a study of field-scale fault-zone permeability and cementation, we examine the variably cemented Loma Blanca fault, a normal fault in the Rio Grande Rift. We collected electrical resistivity data from 15 parallel two-dimensional transects orthogonally crossing the fault, centered on exposures of the fault at the land surface. Inversions of the resistivity data indicate a low resistivity anomaly in the cemented portions of the fault and within the adjacent footwall; these anomalies are present in the unsaturated zone. This low resistivity signature may be an indication of a higher degree of fluid saturation resulting from greater capillary forces, both in the cemented fault (due to reduced pore sizes within the cemented material) and in the footwall (possibly due to smaller grain size). These mechanisms for generating low resistivity anomalies in both the cemented fault zone and in the footwall, suggest that the low resistivity anomalies likely correspond to regions with low permeability. The ability to characterize spatial variations in the degree of fault zone cementation with resistivity has exciting implications for improving predictive models of the hydrogeologic impacts of cementation within faults.

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