Print this pageControls on the Spatial Distribution of Calcite Cementation in Fault Zones: Examples from the Albuquerque Basin, New Mexico
MOZLEY, PETER S., T. M. WHITWORTH, WILLIAM C. HANEBERG, LAUREL B. GOODWIN, and MICHIEL HEYNEKAMP
High-angle normal faults that cut poorly consolidated sediments in the Albuquerque Basin are commonly cemented by calcite. Elongate masses of cement in the cemented zones formed parllel to the groundwater flow direction at the time of calcite precipitation. Along some faults these features are nearly vertical, indicating either upward or downward flow along the fault. Such flow could have occurred if the fault zone were a high-permeability pathway, or if it were a moderately permeable conduit between disconnected high-permeability units (e.g., displaced coarse sandstone layers). In contrast, subhorizontal flow features in another fault cut directly across a subvertical fault zone. Typically cementation of fault zones is thought to result from preferential transport of cementing fluids along the fault. In this case, however, the fluids responsible for cementation must have flowed across the fault zone. The process responsible for such cementation is unknown: possible mechanisms include, a drop in P[CO2] due to pressure loss as fluids pass through lower permeability deformed zones, and/or membrane effects associated with deformation- induced reduction in grain size. We performed a preliminary experiment to test the validity of the membrane concept. A calcite-free sand was ground in a percussion mill and the two micron and smaller particles were separated and then sedimented in an experimental cell. A 90 percent saturated calcite solution was then pumped through the synthetic fault gouge. Small calcite crystals were observed under SEM on the surface of the gouge. These are thought to have precipitated in place due to the solute-sieving properties of the synthetic fault gouge.