Patterns of cementation along a Cenozoic normal fault: A record of paleoflow orientations

Abstract

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 characterized locally by striking patterns of elongate cementation that 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. We conclude that the 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; i.e., the orientation of outcrop-scale flow.