New Mexico Geological Society Annual Spring Meeting — Abstracts

The Sand Hill fault is a steeply dipping normal fault that cuts poorly consolidated sediments of the Albuquerque Basin, New Mexico. The fault zone, which varies in width from ~1 to 6 m, is selectively cemented by calcite. The margins of cemented areas are locally characterized by striking patterns of elongate cementation, which have a strong subvertical orientation. We have considered a variety of possible mechanisms for the formation of these elongate cements, including deformation, weathering, rotation of adjacent cemented beds, and precipitation from flowing ground water. All but the latter can be ruled out. In addition, the elongate cements closely resemble elongate concretions that form from flowing ground water in sedimentary rocks. Such concretions are common elsewhere in the Santa Fe Group, particularly in the Zia and Sierra Ladrones Formations. We conclude that the fault-zone cements precipitated from flowing ground water, and are elongate parallel to the flow direction at the time of precipitation. Thus, these elongate cements provide an important constraint on models of fluid flow along faults in poorly consolidated sediments - the orientation of outcrop-scale flow.