New Mexico Geological Society Annual Spring Meeting — Abstracts

Small-displacement faults that cut Santa Fe Group sediments display some features common to faults in sandstone, as well as structures unique to poorly consolidated sediments. Preliminary observations of small faults in eolian sand of the lower Santa Fe Group reveal that, similar to faults in sandstone, these structures are characterized by a reduction in mean grain size and a five-fold decrease in macroporosity as compared to the host material. This deformation takes place within 1.5-2.5 mm wide zones, termed slip bands, which typically accommodate vertical displacements ofless than 30 mm. The slip bands are not cemented, but are slightly more indurated than the host sand. Petrographic examination indicates that reduction in grain size within these slip bands is accompanied by a decrease in the proportions of feldspar and lithic fragments relative to the host sand, as well as an increase in pseudomatrix and quartz relative to the host sand. Unlike faults in sandstone, the margins of slip bands are not sharp, planar structures, but are irregular. In addition, deformation resulted in fault-parallel alignment of the long axes of inequant grains, which are aligned parallel to bedding outside the fault zone. Modification of the sedimentary fabric through deformation in slip bands is expected to result in a decrease in saturated hydraulic conductivity and an increase in unsaturated hydraulic conductivity. Consequently, such deformation structures may act as barriers to fluid flow under saturated conditions, or as conduits for fluid movement in the vadose zone. These observations demonstrate a need for a better understanding of deformation structures in unconsolidated sediments.