ALBERT

Description: 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.