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.