POTENTIAL FOR FAULT ZONE COMPARTMENTALIZATION OF GROUNDWATER AQUIFERS IN POORLY LITHIFIED, RIO GRANDE RIFT-RELATED SEDIMENTS, NEW MEXICO

Normal fault zones dissect essentially flat-lying, clastic rocks of the Neogene Santa Fe Group (SFG) in the vicinity of Albuquerque, New Mexico. There are excellent surface exposures and numerous unexposed faults identified from high-resolution aeromagnetic data. Because SFG sediments form the primary Albuquerque basin groundwater aquifer system, it is critical to understand the significant geological features that cause heterogeneity.

Field mapping of the Jemez fault in the NW part of the basin, and previous work by other researchers, show that SFG faults are characterized by distinct, variably cemented components: protolith (PL); a damage zone (DZ) marked by deformation bands; a mixed zone (MZ) dominated by tectonic mixing features; and a fault core (FC) with consistently present clay-rich "gouge" (CRG) where most of the strain has been accommodated.

To understand the permeability (k) structure of the Jemez fault, permeametry was completed on representative samples of each component. k was measured in large diameter (~8cm) cores using deionized water in a hydrostatic confining cell. Small samples (~2cm) of CRG were measured using mercury-injection capillary-pressure techniques, a petroleum-industry standard that is particularly robust for low-k rocks.

Permeametry results in sandstone PL and DZ samples give a k of ~1x10^-13 m². MZ sandstone at the FC contact has k of ~1x10^-15 m² parallel to, and ~1x10^-16 m² perpendicular to the fault. CRG k ranges from 1x10^-18 m² to 7x10^-19 m². Permeability testing by other researchers using other methods in the SFG are consistent with our results.

Field mapping, permeability testing, and geophysical modeling combine to indicate that Albuquerque basin faults are major heterogeneities due to varying combinations of juxtaposition of sedimentary units with different hydraulic properties, variable cementation of fault-related bodies, and the persistent presence of CRG with nearly six orders of magnitude lower k than PL sandstone. Fault zone geometry, in combination with k contrasts between fault and PL rocks, suggest that the faults act as barriers to groundwater flow and thus compartmentalize the SFG aquifer system. If true, this condition has significant implications for inter-compartment recharge, storage, and solute transport, particularly when the aquifer is stressed.