(1) Department of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801; (2) Department of Earth Sciences, University of New Hampshire, Durham, NH 03824; (3) New Mexico Bureau of Mines and Mineral Resources, New Mexico Tech, Socorro, NM 87801
The Santa Fe Group (Oligocene-Pleistocene) is the syntectonic basin fill of the Rio Grande rift in New Mexico. Low-Mg calcite is the most volumetrically significant cement in the unit, occurring both in undeformed sediments and within some fault zones. Calcite is most abundant in ancestral Rio Grande tributary, closed-basin fluvial, and piedmont facies. It is least abundant in the ancestral Rio Grande facies.
At large scales (i.e., 2 to 10 meters vertical dimension), calcite occurs preferentially in coarse-grained undeformed sediments that had high pre-cementation saturated permeabilities relative to uncemented finer grained strata. At smaller scales (i.e., <2 meters), however, there is no clear relationship between degree of cementation and pre-cementation permeability. At these scales calcite cement principally occurs preferentially in areas that contain small amounts of early unsaturated-zone (vadose) calcite. This suggests that at large scales cementation was controlled by pre-cementation permeability, perhaps due to a greater flux of Ca++ and/or HCO3- in permeable horizons; whereas at smaller scales within high permeability units, the distribution of cementation was controlled by other factors, such as the presence of nucleating material.
Cementation in several fault zones that have been examined in detail (Sand Hill, Pilares, and Calabacillas) is largely developed in tectonically mixed zones on the basinward sides of the faults. Cementation of the adjacent damage zones is typically confined to the contact with the mixed zones, and diminishes with increasing distance from the fault cores. In areas where adjacent sediments are relatively fine-grained and clay-rich, only minor calcite cement is found within the fault zones. The preferential association of cementation with the relatively coarse-grained mixed zones suggests that, as for the undeformed sediments, fault zone cementation was in part controlled by pre-cementation saturated permeability.
Elongate calcite concretions, which record the orientation of groundwater flow at the time of calcite precipitation, are present in both undeformed strata and in fault zones. In undeformed strata the concretions are typically oriented subparallel to bedding, whereas in fault zones they are typically subvertical.