Using the Spatial Distribution of Calcite Cements to Infer Paleoflow in Fault Zones: Examples from the Albuquerque Basin, New Mexico
Peter S. Mozley, Laurel B. Goodwin, Michiel Heynekamp, and
William C. Haneberg
High-angle normal faults that cut poorly consolidated sediments in the Albuquerque Basin are commonly cemented by calcite. Cemented zone thickness ranges from a few centimeters to several meters, and is primarily a function of the sediment grain size on either side of the fault. Cement is typically thickest where the host sediment is coarse grained and thinnest where it is fine grained. In addition, the fault zone is widest where it cuts coarser-grained sediments.
Elongate masses of cement characterize many of the cemented zones. The similarity of these features to elongate concretions, and a lack of reasonable alternative hypotheses, indicates that the long axes of the elongate masses formed parallel to the groundwater flow direction. Along the Sand Hill fault 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). Evidence for the latter mechanism can be seen in outcrop, where zones of cementation link the ends of offset coarse-grained layers. In contrast, subhorizontal flow features in the Sa ta Ana 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 PCO2 as fluids pass through low-permeability deformed zones, and/or chemical membrane effects associated with deformation-induced reduction in grain size.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California