Structural and geochemical characteristics of faulted sediments and inferences on the role of water in deformation, Rio Grande Rift, New Mexico

Abstract

The San Ysidro fault is a spectacularly exposed normal fault located in the northwestern Albuquerque Basin of the Rio Grande Rift. This intrabasin fault is representative of many faults that formed in poorly lithi fi ed sediments throughout the rift. The fault is exposed over nearly 10 km and accommodates nearly 700 m of dip slip in sub hori zontal, silici clastic sediments. The extent of the exposure facilitates study of along-strike variations in deformation mechanisms, archi tecture, geochemistry, and permea bility. The fault is composed of structural and hydrogeologic components that include a clay-rich fault core, a calcite-cemented mixed zone, and a poorly developed damage zone primarily consisting of deformation bands. Structural textures suggest that initial defor ma tion in the fault occurred at low temperature and pressure, was within the paleosaturated zone of the evolving Rio Grande Rift, and was dominated by particulate fl ow. Little geochemical change is apparent across the fault zone other than due to secondary processes. The lack of fault-related geochemical change is interpreted to reflect the fundamental nature of water-saturated, particulate fl ow. Early mechanical entrainment of lowpermeability clays into the fault core likely caused damming of groundwater fl ow on the up-gradient , footwall side of the fault. This may have caused a pressure gradient and fl ow of calcite-saturated waters in higherpermeability , fault-entrained siliciclastic sediments, ultimately promoting their cementa tion by sparry calcite. Once developed, the cemented and clay-rich fault has likely been, and continues to be, a partial barrier to cross-fault groundwater fl ow, as suggested by petrophysical measurements. Aeromagnetic data indicate that there may be many more unmapped faults with similar lengths to the San Ysidro fault buried within Rio Grande basins. If these buried faults formed by the same processes that formed the San Ysidro fault and have persistent low-permeability cores and cemented mixed zones, they could compartmentalize the basin-fi ll aquifers more than is currently realized, particularly if pumping stresses continue to increase in response to population growth.

Fault architecture and deformation processes within poorly lithified rift sediments, Central Greece

“…This is due to the considerable architectural differences that these fault zones exhibit (e.g. Heynekamp et al, 1999;Caine and Minor, 2009) as a result of contrasting rheological properties and confining pressures (Balsamo et al, 2008) (compare Fig. 1a and b).…”

“…Despite the potential effects of faults in poorly lithified sediment at shallow depth on fluid flow (Sigda et al, 1999;Rawling et al, 2001;Minor and Hudson, 2006;Bense et al, 2008;Caine and Minor, 2009), studies specifically addressing their impacts remain sparse in relation to those in consolidated rock. In addition, the majority of previous outcrop studies of fault zones in poorly lithified sediment are from sand-dominated, continental sediments (Heynekamp et al, 1999;Sigda et al, 1999;Rawling et al, 2001;Minor and Hudson, 2006;Caine and Minor, 2009).…”

“…In addition, the majority of previous outcrop studies of fault zones in poorly lithified sediment are from sand-dominated, continental sediments (Heynekamp et al, 1999;Sigda et al, 1999;Rawling et al, 2001;Minor and Hudson, 2006;Caine and Minor, 2009). As such, there may remain a greater variation of fault hydraulic architectures in poorly lithified sediment than previously reported (e.g.…”

“…Haneberg, 1995;Mailloux et al, 1999) and by introducing permeability heterogeneities and anisotropies within rocks (Bredehoeft et al, 1992;Caine et al, 1996) and sediments (Bense and Van Balen, 2004). As a result, faults can influence the accessibility of groundwater supplies Minor and Hudson, 2006;Caine and Minor, 2009) and/or provide pathways between aquifers for contaminants such as brines (Stamatis and Voudouris, 2003;Bense and Person, 2006), and complicate environmental challenges such as the safe burial of radioactive waste (Bredehoeft, 1997). Hydrocarbon extraction Aydin, 1994, 1995;Shipton et al, 2002;Fossen et al, 2007) and the security of sequestered carbon (Chadwick et al, 2009) may also be affected by the presence of faults.…”

“…1a and b). Outcrop based studies of faults in poorly lithified sediment generally predict that faults behave as hydraulic barriers, preventing cross-fault fluid flow and potentially compartmentalising regional aquifers, but many also suggest that fault zones may simultaneously behave as conduits, allowing along-fault fluid flow (Rawling et al, 2001;Caine and Minor, 2009). Examination of flow tracers, contemporary hydrological data, and numerical modelling of groundwater flow show evidence to support this latter model (Bredehoeft et al, 1992;Person et al, 2000;Heffner and Fairley, 2006;Bense and Person, 2006;Bense et al, 2008).…”