AN OUTCROP ANALOG STUDY OF VADOSE-ZONE FLUID FLOW AND TRANSPORT IN FAULTED SEDIMENTS

As models of aquifer heterogeneity, outcrop analogs offer valuable insight into fluid flow and solute transport. Most analog studies have focused on sedimentary deposits, but paid little attention to vadose-zone behavior and almost no attention to structures like faults and clastic dikes. We investigated vadose zone flow and transport processes in an outcrop analog to faulted basin fill sedimentary deposits typical of the Basin and Range Province. The study site is in Tertiary deposits within the Bosque del Apache National Wildlife Refuge, located in the Rio Grande rift south of Socorro, New Mexico. Outcrop mapping identified the spatial distribution of eolian and fluvial sands, all of which are poorly lithified, and overlying piedmont deposits. Mapping and structural analysis also allowed us to determine the location and orientation of, and magnitude of slip associated with, numerous deformation bands (narrow zones within which pore and grain size are reduced by cataclasis) and small-displacement deformation-band fault zones. In addition, we investigated the potential influence of narrow deformation-band fault zones on flow and transport under variably saturated conditions. Saturated hydraulic conductivity for parent sand (protolith) and faults was measured with air and water permeameters and unsaturated hydraulic properties were measured with a specially equipped centrifuge. The smaller pores within deformation-band fault zones impede flow through the saturated zone, but in the vadose zone they preferentially retain water under dry climate conditions, inducing capillary wicking through the faults and enhancing dry climate recharge and vadose-zone transport. Non-vertical fault dip is shown to enhance infiltration through vadose-zone sands under wet climate conditions. These small-displacement faults act as barriers to saturated flow, as conduits through vadose-zone sands in dry climates, and as catchments in vadose-zone sands under wet climates. In this outcrop analog, structural, not sedimentary, features dominate flow and solute transport processes.