New Mexico Geological Society Annual Spring Meeting — Abstracts

Recent work on small to moderate displacement (<4 meters) faults in poorly consolidated sediments indicates that such faults can have significantly lower permeabilities than their undeformed parent sediments. These permeability changes are created by deformational as well as diagenetic processes, such as compaction, cataclasis, and cementation. Large increases in clay-size fraction within the faulted sediments positively correlate with one to three orders ofmagnitude reduction in permeability. Although smaller displacement faults in poorly consolidated sediments have received little attention, their hydrologic importance is potentially great because they can cross-cut many of the basin-fill aquifers which supply the rapidly growing population in the western United States.

The hypothesis that permeability reduction within faulted, poorly consolidated, basin-fill sediments is significantly affected by amount and type of clay minerals was tested by investigating the composition of fault zone clay-size fraction through thin section analysis, X-ray diffraction, and by permeametry. The test focused on samples collected from uncemented, small to moderate displacement fault zones in poorly consolidated sediments of the Rio Grande rift's Santa Fe Group.

Clay mineralogy varies between faults according to lithology and deformational and diagenetic histories. Type and distribution of clay minerals can also influence the amount ofpermeability reduction within the fault zone. Swelling clays (e.g., smectites) tend to reduce fault zone permeability more than non-swelling clays. Other hydraulic properties, such as unsaturated hydraulic conductivity, may also be influenced by the clay-size fraction composition of small to moderate displacement faults in poorly consolidated sediments.