GOODWIN, Laurel B., Dept. of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801, lgoodwin@nmt.edu; HANEBERG, William C., New Mexico Bureau of Mines, 2808 Central Avenue SE, Albuquerque, NM 87106, haneberg@nmt.edu.
Deformation of poorly consolidated sediments within fault zones of the Rio Grande rift in New Mexico results in a variety of structures that have the potential to influence groundwater flow in the regional Santa Fe Group aquifer system. Faulted sands are modified by both cataclasis and rotation of grain fragments and undeformed grains, and can become heavily cemented over time. Cataclasis and rotation are accommodated by grain boundary sliding, and result in a foliation subparallel the slip plane and a lineation subparallel the slip direction. Statistical analysis of parameters measured from 3 mutually perpendicular thin sections cut from cemented, foliated fault-zone samples, with measurements of 200 grains per section, allows us to both quantify the amount of grain re-orientation due to deformation and to evaluate the influence of the deformational fabric on permeability anisotropy. The variability in orientation of clasts having a given aspect ratio is inversely proportional to the magnitude of the aspect ratio, suggesting that more elongate grains are preferentially rotated within the fault zone. Grain aspect ratios, grain cross-sectional areas, and equivalent grain radii (calculated for circular grains with the same cross-sectional area as elliptical grains) are log-normally distributed. Published grain size vs. permeability formulae suggest that pre-cementation permeability of the fault zone was on the order of 1 to 10 darcies, which is consistent with independent air permeameter studies of uncemented fault zones in the area. A simplified geometric analysis of tortuosity-controlled permeability variations associated with elliptical grains suggests that foliation / lineation-parallel permeability was about 2.5 times greater than foliation-normal permeability.
key words-- faults, fabrics, sediments, New Mexico, permeability