Geophysics of Rio Grande Basins
Geological, Geophysical, Geochemical, and Permeability Characteristics of Faults in Poorly Lithified Sediments of the Rio Grande Rift
Faults can act as barriers, conduits, or combined conduit barriers that can change the direction and quantity of groundwater flow in an aquifer. Understanding why is not straight-forward. Originally, faults in basins of the Rio Grande rift were identified by high-resolution aeromagnetic surveys that showed many more buried faults than were mapped from surface data. This led to the need to determine the cause of their geophysical expression, their physical properties, and the continuity of these properties to aid in evaluating their potential impacts on groundwater flow and storage. Several regional-scale studies have been conducted, as well as several detailed case studies aimed at better understanding of these faults.
Geophysical Expression of Intrabasin Faults
Faults in basin sediments are expressed in high-resolution aeromagnetic data that show many more shallowly buried faults than previously known throughout the basins (Figure 1). Geophysical analysis and rock magnetic-property studies indicate the primary source of the fault-related aeromagnetic anomalies is tectonic juxtaposition of strata rather than effects of secondary mineralogical and geochemical processes (Figures 2 and 3). Thus, magnetic data from the basin to outcrop to hand sample scales provide information on vertically extensive permeability heterogeneities that may be associated with aquifer compartmentalization.
- For more information on aeromagnetic expression of faults see Grauch and Hudson (2007) and Grauch and others (2009)
- For more information on the magnetic properties of faulted sediments see Hudson, Grauch, and Minor (2008) [PDF file, 3.8 MB]
- For more information on potential aquifer compartmentalization of faulted sediments see Caine and Minor (2009) [PDF file, 6.7 MB]
Figure 2. The San Ysidro fault zone case study in the northwest corner of the Albuquerque basin: High-resolution aeromagnetic data image, map-scale fault zone photograph, photo of a magnetic susceptibility meter on top of a typical outcrop of sediments, and photomicrographs of polished grain mounts of magnetite grains, one rimed with maghemite (Mgh). The meter is reading 0.227 x 10-3 SI volume units for an outcrop of rift sediments of the Tesuque Formation exposed near Santa Fe. (See Hudson, Grauch, and Minor (2008) [PDF file, 3.8 MB] and Caine and Minor (2009)). [Click on image to view full-size version.]
Figure 3. Top: Plot of grain size, rank for cementation, and magnetic susceptibility data from outcrops for each stratigraphic unit cut by the San Ysidro fault (See Hudson, Grauch, and Minor (2008) [PDF file, 3.8 MB]). These data are used to establish the relationships between the major physical properties of each sedimentary unit and its magnetic expression. Mean volume magnetic susceptibilities (SI units) generally increase upsection: from 1.7 to 2.2 x 10-4 in the pre-rift section, to 0.99 to 1.2 x 10-3 in the syn-rift Zia Formation, and 1.5-3.3 x 10-3 in the syn-rift Arroyo Ojito Formation. Bottom: Models of magnetic expression for four different geologic cross-sections across the San Ysidro fault incorporating the magnetic susceptibility data determined for the stratigraphic units above (See Grauch and others, 2006 [PDF file, 6.6 MB]). Model curves are compared with profiles extracted from the observed aeromagnetic data. To the right of each model, the geologic units juxtaposed at different levels of the fault are illustrated. The shaded bar graphs show the amount of magnetic susceptibility contrast. A bar to the right (positive contrast) or a bar to the left (negative contrast) indicates that the more magnetic unit is on the east or west side of the fault, respectively. [Click on image to view full-size version.]
Physical Characteristics of Faults in Poorly Lithified Intrabasin Sediments
Faults have distinctive physical parts or components that include a core where most of the strain is accommodated, resulting in fault rocks such as gouge and breccia (Figure 4). The core is surrounded by a damage zone related to the growth of the fault manifested by open fractures, veins, and small displacement faults. However, faults in poorly lithified sediments, such as those found in basins of the Rio Grande rift, are unique compared with their counterparts formed in rock. Although they commonly have cores composed of clay-rich gouge they do not have well-developed damage zones with extensive open fractures. One of the unique components of such faults are mixed zones where sedimentary beds are entrained into the fault and deformation is accommodated by particulate flow rather than extensive brittle fracture. This process leads to the formation of persistent and continuous clay-rich fault cores, carbonate-cemented mixed zones, and the general absence of open fractures. Deformation bands, generally small faults with less than a centimeter of displacement and associated porosity and permeability reduction, do form in these types of faults. Thus, in the absence of open joints, these faults can be partial barriers to flow across the fault rather than combined conduit-barriers, causing flow to be impeded across the fault and enhanced parallel to the fault (See Caine and Minor (2009)).
Figure 4. Conceptual illustrations of the differences between faults in rock versus those in poorly lithified sediments and how they might affect groundwater flow. (See Caine, Minor, and Budahn, 2007) [Click on image to view full-size version.]
Intrabasin Fault Permeability and Interactions with Groundwater Flow
Not only are the distinctive arrangements of fault components a control on groundwater flow but several other factors can control flow. The most important factor is the permeability contrast between the sediments that are juxtaposed on either side of a fault as well as the fault rocks themselves. Although the best way to understand fault-controlled permeability is by using in-situ hydraulic test data from wells on either side of a fault, direct permeability measurements of representative samples of fault components can result in a variety of insights. It is not uncommon to measure six or more orders of magnitude permeability contrast between clay-rich fault gouge and the surrounding protolith (Figure 5). The combination of stratigraphic seals with fault seals, and various fault zone components and style of faulting (e.g., clay-rich gouge versus deformation-band-dominated faults) can lead to complex heterogeneities that cause the partial barrier behavior of faults in intrabasin sediments (Figures 6). Additionally, processes below versus above the water table (saturated versus unsaturated conditions) can result in different groundwater interactions with faults; i.e., the low-permeability materials in the unsaturated zone can effectively wick water into the saturated zone (Figure 7).
Figure 5. Plot of permeability data measured from each component of the San Ysidro fault zone shown with an example outcrop photo. Uncemented to weakly cemented samples are shown with small open symbols, and cemented and clay-rich core samples are shown with small filled symbols. Geometric means of sample permeabilities for each fault component are shown with large filled symbols. Abbreviations: fw = footwall, hw = hanging wall, pl = protolith, dz = damage zone, mz = mixed zone, geomean = geometric mean. (Modified from Caine and Minor (2009)). [Click on image to view full-size version.]
Figure 6. Schematic diagram of aquifer sealing and (or) rerouting due to faulting processes. (Modified from Caine and others, 2002) [Click on image to view full-size version.]
Figure 7. Schematic diagram of groundwater flow as impacted by faults with well-developed clay-rich gouge cores in the saturated zone versus deformation-band faults in the both the unsaturated and saturated zones. [Click on image to view full-size version.]
Regional Distribution of Clay-Rich and Other Fault Core Materials
The San Ysidro fault provides data from an exceptionally good exposure along strike and down dip that is nearly 10 kilometers in length. The core of this fault is highly continuous with the dominant fault rock type being clay-rich gouge. Nearly 200 other faults were also characterized in single to multiple outcrops throughout the study basins, and most had nearly continuous, clay-rich fault cores. See Figure 8.
Geochemistry of Fault Related Sediments and Rocks
In order to document syn- to post- tectonic changes in faults characterized in the study basins, sampling traverses were completed across faults, and the samples were analyzed for whole rock, semi-quantitative mineralogy by x-ray diffraction, and elemental chemistry by inductively coupled mass spectroscopy. Little primary geochemical change is recorded across the San Ysidro fault, except for calcite cementation. The lack of geochemical change is interpreted as a characteristic of particulate flow and a conspicuous lack of brittle fracturing (Figure 9; See Caine and Minor (2009), for details). Mineralogy of fault-related intrabasin sediments and rocks from the rift flank on the east side of the Española Basin show distinctive compositional patterns. The samples can be grouped by age and fault component (Figure 10). Proterozoic crystalline flank rocks are granitic in their protoliths with calcite poor, illite-rich fault rocks derived from them. The Paleozoic rocks are arkosic sandstone, shale, and limestone protoliths with fault rocks that indicate loss of calcite and gaining concentrations of clays. The Tertiary protoliths are quartz-rich erosional equivalents of the Protoerozoic source rocks with smectitic clay-rich fault cores and variable calcite cementation.
Figure 9. Whole rock major rock-forming mineral and detailed clay mineral concentrations from sample traverses across the San Ysidro fault. From Caine and Minor (2009). [Click on image to view full-size version.]
Aquifer Compartmentalization by Faults
Because of the high permeability contrast between faults with clay-rich cores and the relatively high permeability siliciclastic sediments they cut, there is potential that faults may act as partial barriers to groundwater flow across the faults. Evidence for compartmentalization exists in several basins of the rift. This evidence includes numerous head drops across faults as measured in wells, and various geochemical and thermal anomalies, and is to be expected based on geologic and permeametry data (Figure 11).
Figure 11. Hydrogeologic map and cross-section near Santa Fe, New Mexico, showing groundwater elevation contours, a mapped and aeromagnetically expressed fault, and associated head drop across the fault interpreted as being due to the fault acting as a partial barrier to flow (modified from Johnson and others, 2006, and Caine, Minor, and Johnson, 2007). The schematic cartoon depicts a cross-section of a fault-compartmentalized aquifer. A clay-rich sediment layer confines the top, and faults confine the sides of the compartment; the faults are curviplanar and intersect perpendicular to the plane of the page. A schematic pumping municipal well and the potential effects of impeded recharge into the compartment are shown. [Click on image to view full-size version.]
InSAR, Ground Surface Subsidence and Uplift
Interferometric synthetic aperture radar (InSAR) is a remote sensing technology used to obtain extremely high-resolution maps of ground surface elevation (+/- several millimeters). When InSAR data is collected at multiple time intervals, subsequent images of ground surface elevation can be subtracted from one another. This results in a map of changes in ground surface elevation over time, thus showing subsidence or inflation. Pumping groundwater from municipal wells can cause ground subsidence and associated surface uplift due to recovery when pumping ceases. Rio Rancho is a suburban community in northwestern Albuquerque, New Mexico, whose water source is groundwater pumped from municipal wells. Although the recognition of fault-bounded compartments may be hampered by the sparse distribution of wells and associated time-series water level data spaced at appropriate scales, the regional hydraulic gradient being subparallel to fault strike, and variations in pumping rates and durations, InSAR data shows evidence for fault compartmentalization in the northern Albuquerque basin. Figure 12 shows 24 mm of surface uplift due to delayed aquifer recovery at municipal well 16 (red spot) after the well was pumped at ~108 gal/mo then "shut down". The image also shows that the surface-uplift anomaly is adjacent to faults. Impeded recharge of pumped, confined aquifers is ultimately dependent on hydraulic stress (pumping rate and duration) and the three-dimensional architecture (e.g., hydraulic holes), number, and geometry of faults.
Figure 12. InSAR image or Interferogram (211 day) showing ground displacement in relation to mapped faults (red lines; Hudson and others, 1999) and aeromagnetically determined faults (black lines; Grauch and Hudson, 2007) for the time period September 3, 1995, to April 1, 1996 (modified from Heywood et al., 2002). [Click on image to view full-size version.]
Caine, J.S. and Minor, S.A., 2009, Structural and geochemical characteristics of faulted sediments and inferences on the role of water in deformation, Rio Grande Rift, New Mexico: Geological Society of America Bulletin, v. 121; no. 9/10; pp. 1325-1340; doi: 10.1130/B26164.1. [PDF file, 6.7 MB]
Caine, J.S., Minor, S.A., and Budahn, J.R., 2007, Faults in Rock, Faults in Dirt, and Enigmatic Breccias in the Española Basin, New Mexico: Their Relations to Rift Architecture, Ground Water, and a Possible Impact Structure: Annual Meeting, Denver, Colorado, Geological Society of America Abstracts with Programs, v. 39, no. 6, p. 496.
Caine, J.S., Minor, S.A. and Johnson, P., 2007, Preliminary characterization of fault zones and potential impacts on ground-water resources: Española Basin, New Mexico (abstr): Geologic and Hydrogeologic Framework of the Española Basin - Proceedings of the 6th Annual Española Basin Workshop, Santa Fe, New Mexico, March 6, 2007, New Mexico Bureau of Geology and Mineral Resources Open-File Report 508, p. 4.
Caine, J.S., Minor, S.A., Grauch, V.J.S., and Hudson, M.R., 2002, Potential for fault zone compartmentalization of groundwater aquifers in poorly lithified, Rio Grande rift-related sediments, New Mexico: Geological Society of America Abstracts with Programs, v. 34, no. 4, p. 59.
Grauch, V.J.S., Hudson, M.R., Minor, S.A., and Caine, J.S., 2006, Sources of along-strike variation in magnetic anomalies related to intrasedimentary faults: A case study from the Rio Grande Rift, USA: Exploration Geophysics, v. 37, pp. 372-378. [PDF file, 6.5 MB]
Grauch, V.J.S. and Hudson, M.R., 2007, Guides to understanding the aeromagnetic expression of faults in sedimentary basins: Lessons learned from the central Rio Grande rift, New Mexico: Geosphere, v. 3; no. 6; pp. 596-623, doi: 10.1130/GES00128.1.
Grauch, V.J.S., Phillips, J.D., Koning, D.J., Johnson, P.S., and Bankey, Viki, 2009, Geophysical interpretations of the southern Española Basin, New Mexico, that contribute to understanding its hydrogeologic framework: U.S. Geological Survey Professional Paper 1761, 88 p.
Heywood, C.E., Galloway, D.L., and Stork, S.V., 2002, Ground Displacements Caused by Aquifer-System Water-Level Variations Observed Using Interferometric Synthetic Aperture Radar near Albuquerque, New Mexico: U.S. Geological Survey Water-Resources Investigations Report 02-4235, 24 p.
Hudson, M.R., Grauch, V.J.S., Minor, S.A., and Personius, S.F., 1999, Preliminary characterization of faults in the Middle Rio Grande basin: U.S. Geological Survey Middle Rio Grande Basin Study-Proceedings of the Third Annual Workshop, Albuquerque, New Mexico, February 24-25, 1999: U. S. Geological Survey Open-File Report 99-0203, pp. 40-41.
Hudson, M.R., Grauch, V.J.S., and Minor, S.A., 2008, Rock magnetic characterization of faulted sediments with associated magnetic anomalies in the Albuquerque Basin, Rio Grande rift, New Mexico: Geological Society of America Bulletin, v. 120; no. 5/6; pp. 641-658, doi: 10.1130/B26213.1. [PDF file, 3.8 MB]
Johnson, P. S., Timmons, S., Whiteis, J., Gillard, L., and Hoffman, G., 2006, Preliminary Results Of Geochemical Characterization Of Groundwater In The Southern Española Basin, New Mexico: Geologic and Hydrogeologic Framework of the Española Basin -Proceedings of the 5th Annual Española Basin Workshop, Santa Fe, New Mexico, March 7-8, 2006, U.S. Geological Survey Open-File Report 2006-1134, p. 4.
Minor, S.A. and Hudson, M.R., 2006, Regional Survey of Structural Properties and Cementation Patterns of Fault Zones in the Northern Part of the Albuquerque Basin, New Mexico: Implications for Ground-Water Flow: U.S. Geological Survey Professional Paper 1719, 32 p.
Sigda, J. M. and Wilson, J. L., 2003, Are faults preferential flow paths through semiarid and arid vadose zones?: Water Resources Research, v. 39, no. 8, p. 1225, doi: 10.1029/2002WR001406.
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