Land Subsidence (Proceedings of the Fourth International Symposium on Land Subsidence, May 1991). IAHS Publ. no. 200,1991.

The Fort Hancock Earth Fissure System, Hudspeth County, : Uncertainties and Implications

JEFFREY R. KEATON Sergent, Hauskins & Beckwith Engineers, 4030 South 500 West, Suite 90, Salt Lake City, Utah 84123 ROY J. SHLEMON Roy J. Shlemon and Associates, P.O. Box 3066, Newport Beach, California 92663 ABSTRACT A previously unknown northwest-trending curvilinear system of discontinuous earth fissures about 150 m long was discovered in 1988 on a gently sloping alluvial apron in the Hueco Bolson about 80 km southeast of El Paso during an independent, third-party engineering geologic reconnaissance of a site being considered for disposal of low-level radioactive waste. Individual depressions comprising the fissure system are as much as 10 m long, 0.4 m wide, and 1.1m deep. The depressions display steep sidewalls and sharp edges at the ground surface in Holocene fine sandy silt floodplain deposits. Approximately 53.6 Mg of sediment have moved vertically downward creating these depressions. Excavations across die fissure system reveal voids and filled cracks which extend downward into late Pleistocene calcic soil horizons formed on older alluvial floodplain deposits. Clayey bolson-fill deposits of mid-Pleistocene to Pliocene age are present at depth, but the characteristics of the fissure system in these deposits are unknown. The cause of the fissuring is not known; it could be tectonic (aseismic fault slip, neotectonic folding, or induced by earthquake shaking) or nontectonic (subsidence, subsurface dissolution, differential compaction, desiccation, pedogenisis, or strain release from a nearby buried stream channel incision). Two additional earth fissure systems and two buried paleo-fissures were discovered in 1989 within about 1 to 2 km of the Fort Hancock fissure system. These were not within the boundaries of the proposed site in 1988, but the boundaries, expanded in 1989 in an attempt to avoid the 100-year floodplain, encompassed the additional fissures. So much uncertainty exists about the cause and location of the fissures, and the possibility of future fissuring, that serious questions could not be answered with the level of assurance necessary to satisfy conventional site selection requirements for critical facilities, such as the proposed low-level radioactive waste disposal facility.

INTRODUCTION The State of Texas established an agency in the early 1980s to provide for disposal of low-level radioactive waste at the most suitable site in Texas. The agency started by assessing the volume of waste generated within the state and screening the state to identify those areas relatively more suitable for waste disposal. Several areas of the state were identified as potentially suitable and ranked. Two preferred sites, both on private land, were identified in 1985 but were excluded from further consideration because of political pressures. The agency reacted to the exclusion by focusing on land owned by the State of Texas, most of which is located in west Texas.

281 Jeffrey R. Keaton & Ray J. Shlemon 282

By 1988 it was becoming clear to local governments in west Texas that the State's preferred site was going to be the Fort Hancock Site in southwest Hudspeth County, about 80 km southeast of downtown El Paso (Fig. 1). Several local governmental agencies (Hudspeth County, Hudspeth County Conservation and Reclamation District No.l, Hudspeth County Underground Water Conservation District No. 1, and El Paso County) retained a team of engineers and geologists to conduct an independent evaluation of the Fort Hancock Site. The team issued a three-volume report in April 1989 documenting several serious deficiencies with the site (Sergent, Hauskins & Beckwith, 1989); the Fort Hancock fissure system is one of these deficiencies.

FIG. 1 Map showing locations of key features in the Fort Hancock, Texas, area.

THE FISSURE SYSTEM The Fort Hancock fissure system is a 152-m long curvilinear series of elongated to circular topographic depressions ranging up to 1.2 m across, 9.1 m long, and 1.2 m deep (Fig. 2 and 3). The edges of the depressions are vertical to overhanging, showing very little modification by running water. Piping tunnels connecting adjacent topographic depressions are common. However, it appears that these piping tunnels represent the present positions of upward-migrating subsurface voids rather than hydraulic pipes because of the lack of fluvial erosion features and deposits in the pipes. The surface expression of the fissure system probably is no more than a few years to a few tens of years old because living creosote bushes are located on the edges of depressions and on earth bridges over piping tunnels. The edges of the depressions are sharper near the northwest end of the fissure system than near the southeast end, suggesting that the system may be propagating toward the northwest. The fissure system does not appear to be visible on aerial photographs taken by the U.S. Department of Agriculture in 1951; it may be present on photos taken in 1981, but the evidence is equivocal. The fissure system was mapped by establishing a baseline with the aid of a compass and a 30-m tape. Reference points were placed by driving 5-cm square wooden stakes 283 The Fort Hancock earth fissure system

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FIG. 2 Map of the Fort Hancock fissure system FIG. 3 Photograph showing showing trench and test pit locations. fissure expression at Pit A. into the ground at 15-rn intervals. While the 30-m tape was stretched on the ground, a second tape was used to measure the distance from the baseline to the surface features of the fissure system. The locations of the edges of the elements of the fissure system and the depths from the ground surface to the bottom of the ground crack were recorded. The mass of earth missing from the site along the fissure system was computed by determining the volume of the ground crack and multiplying by a total unit weight of 17.27 kN/m3 (110 lb/ft3). The volume, estimated by average-end-area and "rastrum-of- a-cone methods, was found to be 26.0 to 29.8 m3, respectively. Thus, appu»x_; .ately 53.6 Mg (58 tons) of sediment are missing from the site surface at 3ie fissure system. The fissure system was investigated by excavating two hand-dug pits and five trenches at locations shown on Fig. 2. The shallow stratigraphy of the site is shown in Fig. 4. The upper 2 m is Holocene fluvial fine sandy silt with rare gravel channel deposits; this unit has a weak calcareous A-C pédologie profile, several buried weak calcic horizons, and represents the modern floodplain alluvium. The upper fine sandy silt unit is underlain by high-energy fluvial gravel including individual boulders up to 30 cm long weighing nearly 15 kg. Well-developed (Stage V) calcrete horizons have formed in the gravelly parent material, as shown on Fig. 4.

FIG. 4 Stratigraphy exposed in the east wall of Trench 3. Wall is 6 m high. Fissure crosses photograph center from top to bottom. Jeffrey R. Keaton & RayJ. Shlemon 284

The ground crack was found to be associated with a subsurface fracture which in some places was an open void and in other places was filled with sediment (Fig. 4 and 5). In a few places, open voids were found within infilled sediment in cracks in the calcrete, suggesting several episodes of crack opening and infilling. However, no consistent evidence was found to indicate a recurring sequence of opening and infilling. Three trenches were excavated across projections of the fissure system beyond its surface expression. Cracks were found in shallow subsurface sediments in all trenches; in most cases, these cracks extended downward into the upper calcrete horizon which was exposed in the bottom of the trenches. The character of sediments filling the cracks indicate multiple occurrences of crack opening and filling.

FIG. 5 Void and filled crack in calcrete exposed in Pit A. The subsurface expression of the fissure system clearly is directly related to crack in the shallow subsurface sediments and calcrete. The presence of subsurface cracks beyond the surface expression of the fissure system, and the very youthful appearance of the ground cracks strongly suggest that propagation or expansion of the Fort Hancock fissure system should be expected in the near future. Critical questions regarding the fissure system at the site are: (a) What caused it?; (b) Why is it located where it is?; (c) Could similar fissuresystem s develop elsewhere on the site during the 500-year "design" life of a low-level radioactive waste disposal facility?; and (d) Can they be predicted with confidence?

POSSIBLE CAUSES The cause of the fissure system is unclear, but it could well be related to tectonic processes or to surface geologic processes. Tectonic processes include slip along faults unrelated to earthquakes (aseismic slip), continued development of a fold along the nearby Diablo Plateau Rim, or cracking of the ground owing to earthquake shaking. Surface geologic processes include subsidence possibly induced by water well pumping, dissolution of gypsum in a geologic formation at depth, differential compaction or collapse of the sediments under the site, shrinkage owing to desiccation of sediments under the site, volume changes caused by pedogenic processes, or strain release caused by a nearby deeply cut arroyo which is now buried. Earth fissures have been observed at various locations including arid of the , regions of limestone with active karst processes, and in areas of underground mining. The causes of earth fissures which have been investigated most extensively consist of subsidence associated with fluid (water, oil, and gas) withdrawal from porous media, oxidation of peat, solution and collapse in limestone terrain, and collapse of shallow mine workings (Griggs and Gilchrist, 1983). Many aspects of fissure formation are unclear and problematical. Some fissures have been noticed following earthquakes and have been attributed directly or indirectly to earthquake 285 The Fort Hancock earth fissure system shaking (Albritton and Smith, 1965); others have been observed in tectonically active areas, but at times when association with shaking is improbable (Shlemon and Davis, 1988; Swadley and Scott, 1988; Bell et al., 1989). Many uncertainties exist regarding fissure formation and propagation and extensive research would be required to advance scientific understanding to the point that reliable predictions could be made.

Tectonic Aseismic Fault Slip A probable west-northwest-trending fault was identified under the southwest corner of the Fort Hancock Site in 1986 during a geophysical survey conducted by Phillips et al. (1986). The geophysicists interpreted their data to indicate that the fault came to within at least 15 to 18 m of the ground surface. Faults in the Fort Hancock display chiefly normal separation. Some evidence exists for lateral separation on the nearby Amargosa fault in Chihuahua, Mexico (Keaton et al., 1989); however, this evidence is not consistent along the fault. The seismic reflection profiles interpreted by Phillips et al. (1986) indicate different senses of vertical separation from profile to profile and from older to younger reflecting horizons. Thus, if the interpretation by Phillips et al. (1986) is accurate, the postulated fault under the southwest part of the Fort Hancock Site must be a lateral fault. The west-northwest- trending Texas Lineament (Albritton and Smith, 1957; 1965; Henry and Price, 1985) passes virtually under the site (Fig. 6) and has been a persistent structural boundary with left-lateral transcurrent separation since Precambrian time. Consequently, if the Fort Hancock fissure system were caused by aseismic creep on the subsurface fault, it would be reasonable to expect features consistent with left-lateral shearing. The map of the fissure system (Fig. 2) shows it to be curvilinear and discontinuous. At two locations near Pit B (Fig. 2), the fissure system steps to the right; right-steppinge n echelon strands are found on shear zones with left-lateral separation. Thus, the surface expression of the fissure system does not contradict aseismic creep on the geophysically located fault under the Fort Hancock Site. The relationship between the geophysically located fault and the Texas Lineament remains unknown.

FIG. 6 Tectonic map of the Fort Hancock Site region. Modified from Sergent Sergent et al. (1989). Place names are shown on Fig. 1. Jeffrey R. Keaton & Roy J. Shlemon 286

Neotectonic Folding The site is located in the Hueco Bolson within 5 km of the Diablo Plateau Rim (Fig. 1). The transition from the Diablo Plateau to the Hueco Bolson is marked by the Diablo Rim fault zone and a monoclinal fold with relative deformation consistent with the Plateau being higher than the Bolson. The highway from El Paso, Texas, to Carlsbad, New Mexico, was surveyed in 1934, 1943, and 1958 (Reilinger et al., 1979; 1980). These surveys indicate that the Diablo Plateau is moving upward while the Hueco Bolson is moving downward. The geodetic movement of the Diablo Plateau with respect to the Hueco Bolson could cause continued development of the Diablo Rim fold which would induce horizontal extensional stresses oriented roughly perpendicular to the edge of the Diablo Plateau Rim. Such stresses would be expected to generate earth fissures roughly parallel to the Rim. The Fort Hancock fissure system is roughly parallel to the south edge of flat-lying limestone of the Diablo Plateau Rim, as well as to the Campo Grande fault, the Amargosa fault, the Texas Lineament, and the north edge of the complexly folded rocks of the Chihuahua Tectonic Belt (Fig. 6). Thus, any neotectonic stresses in the site region should be expected to cause or contribute to surface features with a northwest trend, such as the fissure system. Since the Diablo Plateau is moving upward with respect to the Hueco Bolson, neotectonic folding may be a likely candidate for the cause of the stresses leading to formation of the Fort Hancock fissure system. Earthquake-Induced Ground Failure The closest reported earth fissures to those at the Fort Hancock Site are located in alluvium of Red Light Draw (Quitman Canyon), approximately 24 km south of Sierra Blanca (Albritton and Smith, 1965). Three sets of fissures exist in the valley; two older sets and one young set. One of the older sets was attributed to the Valentine, Texas, earthquake of 1931, the epicenter of which was about 80 km east of Red Light Draw. This set consists of three main fissures aligned roughly parallel to the draw but west of it; smaller connecting fissures were oriented roughly perpendicular to the main ones. Albritton and Smith (1965) observed these fissures in 1949,17 years after the Valentine earthquake, and found cracks still partly open to depths as great as 2.1 m and widths as great as 0.9 m. These fissures were observed in 1989, and found to be up to about 0.9 m deep and as wide as 2 m (Fig. 7). An additional set of fissures in Red Light Draw not mentioned by Albritton and Smith (1965) was also observed in 1989. This set of fissures had similar character to those

FIG. 7 Photograph of older FIG. 8 Photograph of younger fissures in Red Light Draw. fissures in Red Light Draw. 287 The Fort Hancock earth fissure system described by Albritton and Smith (1965), but were located east of the draw, about 2.5 km east of the set described above. The younger set of fissures was discovered by local residents of Hudspeth County in September, 1985, the day following the Mexico earthquake (Sheriff Richard Love and Deputy Sheriff Rusty Wilbanks, 1989, personal communication). These fissures crossed the unsurfaced county road in Red Light Draw, making it impassable to vehicles. These fissures were observed in April, 1989, and found to form a rectilinear pattern approximately 300 m long. The surface expression of these fissures (Fig. 8) is similar to that at the Fort Hancock Site but appear to be even younger. The largest topographic depression along the young set of fissures in Red Light Draw is approximately 1.8 m wide and 1.5 m deep. The timing of the discovery of the younger fissures with respect to the occurrence of the Mexico earthquake suggests that the fissures may have been caused by earthquake shaking. This is problematical because the epicenter of the earthquake was on the west coast of Mexico, about 1600 km from Red Light Draw. Ground motions at the site should have been negligible. The authors visited these fissures again in December, 1989, and found fresh cracks in the shoulder of the county road and on the valley floor in close proximity to the fissure. These cracks clearly indicate an ongoing process that does not require earthquake shaking. Baumgardner (1990) cites a reference to a person who observed fissuresi n Quitman Canyon in 1924; therefore, he believes they are not caused by earthquake shaking.

Nontectonic Subsidence Ground cracking has been caused by subsidence induced by water well pumping. This process is well known from Phoenix, Arizona, Las Vegas, Nevada, and , Texas. Holzer (1984) notes that ground failure occurs on earth fissures and surface faults; the fissures are tensile failures while the faults are shear failures. Fissure locations can be controlled by the positions of subsurface faults even though the stresses responsible for opening the fissures are induced by groundwater withdrawal. The nearest water well to the Fort Hancock Site is approximately 5 km to the east and is a single low-volume domestic well. Two paleo-fissures were exposed in a large trench excavated at the direction of the State. One paleo-fissure is present below the well- developed calcrete (Fig. 9); the other paleo-fissure cuts the calcrete, but has no surface expression (Fig. 10). These paleo-fissures clearly are prehistoric and indicate that groundwater pumping is not necessary to produce fissures. Water has been pumped from exploration wells drilled on the site by the Authority's geological consultant. The volumes pumped from these wells has been small and it is unlikely that subsidence has been induced. If these wells did cause subsidence, it would indicate that the site is very sensitive to minor water level declines.

FIG. 9 Photograph of older paleo-fissure. FIG. 10 Photograph of younger paleo- Note multiple calcrete horizons. fissure. Note lack of surface expression. Jeffrey R. Keaton & RoyJ. Shlemon 288

Dissolution Ground cracking could be caused by dissolution of gypsum in a geologic formation under the Site. Large selenite crystals are littered on the ground surface in Alamo Arroyo, approximately 6 km southwest of the Site, in a formation known to underlie the site. If a concentrated zone of gypsum were to become dissolved, a subsurface void would be created. Such a void gradually would work its way to the ground surface, resulting in a series of cracks. If this process is responsible for the Fort Hancock fissure system, the predicting locations of future fissures would be virtually impossible.

Differential Compaction or Hydrocompaction Ground cracking could be caused by compaction of the sediments under the Site. Sediments gradually become more compact with age; ultimately becoming sedimentary rock. The process of natural compaction affects different sediments in different ways. For example, clay-rich sediment will compact a larger amount than sediment made of gravel. Some sediments are susceptible to hydrocompaction when wetted. Usually, such sediments have substantial eolian material or were deposited rapidly as debris flows or mudflows on alluvial fans, and have not been wetted since deposition. If a major change in the type of sediment occurs under the Site, cracking of the ground could occur due to differential compaction. Such an abrupt change of sediment type could be associated with a subsurface fault.

Desiccation Under limited circumstances, ground cracking could be caused by shrinkage owing to desiccation of sediments under the Site. Ground cracks caused in this way usually are roughly polygonal, commonly hexagonal, and rarely have sides as long as 30 m. The fissure system at the Site is curvilinear for 150 m. Shrinkage is most pronounced in clay-rich sediments, particularly smectite clay. The sediments at the Site above the calcrete are eolian and alluvial deposits derived from erosion of limestone in the cliffs on the Diablo Plateau Rim. Multiple, slightly developed buried paleosols are present in these sediments, indicating a few periods of landscape stability. These sediments are not rich is smectite and probably exhibit only small shrinkage potential even though some of the horizons contain substantial clay. The older bolson deposits at depth under the calcrete do contain significant amounts of clay, including smectite. Shrinkage cracks can be observed in those areas where the older bolson deposits crop out. However, the shrinkage normally would be distributed in such a way that systems of earth fissures, such as the Fort Hancock System, are very unlikely to form, especially since the stresses would have to be transmitted through the calcrete horizon. Nonetheless, despite the small probability of the fissure system being caused by desiccation, this process cannot be ruled out.

Pedogenesis It is well known that the process of soil formation results in sediment acquiring soil structure. The primary element of soil structure is the ped. Ped surfaces are commonly vertical, particularly in clayey parent material. The Fort Hancock fissure system is vertical but much larger than would be associated with normal pedogenic processes. Furthermore, only three active earth fissures have been found on the site; pedogenesis should be expected to produce hundreds or thousands of fissures on a smaller scale. The State's consultant has identified extensive, deeply-buried paleo- vertisols (Gustavson, 1990) but has not related these to fissure genesis.

Strain Release An arroyo cutting a deep channel through the calcrete could induce horizontal tension which could, in turn, crack the calcrete parallel to the arroyo bank. Such an arroyo, if it exists, would have been completely buried by younger sediments and would be much larger than any of the channels exposed in the trenches excavated to investigate the fissure system. Additional exploration would be required to evaluate this possible cause for the fissure system. 289 The Fort Hancock earth fissure system

DISCUSSION Critical questions regarding the Fort Hancock fissure system are: (a) What caused it?; (b) Why is it located where it is?; (c) Could similar fissuresystem s develop elsewhere on the site during the 500-year "design" life of a low-level radioactive waste disposal facility?; and (d) Can they be predicted with confidence? Based on the discussion presented in this report, we offer the following answers to these critical questions: (a) The cause of the fissure system is unknown, and likely to remain unknown, (b) The reason for the fissure system's location is unknown, but a geophysically located subsurface fault is suggestive, (c) It is not possible to rule out similar fissure systems developing elsewhere on the site during the 500-year "design" life of the proposed facility; in fact, other fissuresystem s may presently exist in the subsurface but not yet be expressed at the ground surface, (d) Predicting with confidence the locations, timing and size of future fissures, and their vertical and horizontal extent is not possible. So much uncertainty exists about the cause and location of the Fort Hancock fissure system, and the other active earth fissures on the site, that serious questions can not be answered with the level of assurance necessary to satisfy conventional site selection requirements for critical facilities, such as the proposed low-level radioactive waste disposal facility. Baumgardner (1990), on behalf of the Texas Low-Level Radioactive Waste Disposal Authority, concluded that: (a) "Three surface fissures in the study area formed as a result of surface collapse and piping along preexisting tension fractures." (b) "All three surface fissures are in local topographic lows, which indicates that concentrated overland flow is an essential component in their development." (c) "The source of tensional stress that formed the tension fractures is unknown." and (d) "Fissure development appears to be a natural geomorphic phenomenon in arid desert basins." However, Baumgardner (1990) has not offerd any opinion regarding the significance of the fissure systems to the proposed low-level radioactive waste disposal facility. The State selected the Fort Hancock Site, stating publicly that it was selected for its technical merit, despite the active fissures and other geotechnical studies indicating that it is within the 100-year floodplain and subjected to earthquake ground motions exceeding 0.6 g. The local governmental agencies in Hudspeth and El Paso counties sued the State for violating their own site selection criteria established in 1982 (e.g., the Fort Hancock Site is within a Wildlife Management Area, yet the State's criteria require such areas to be excluded from site selection consideration). The case was tried in the 34th Judicial District Court in El Paso from September 6 to 22,1990. The judge had not ruled by the time this manuscript was prepared; it is likely that the decision will be appealed and the finalrulin g made in the Texas State Supreme Court.

ACKNOWLEDGEMENTS The authors benefitted from discussions with George Beckwith, Jamie Barnes, Mary Gillam, Douglas Clark, Barbara Matz (Sergent, Hauskins & Beckwith), David B. Slemmons (consultant), and John Hawley (New Mexico Bureau of Mines & Mineral Resources). Mark Turnbough provided logistical support. Darcy Frownfelter, Special Assistant Attorney for El Paso County, authorized preparation of this report.

REFERENCES Albritton, C. C. & Smith, J. F., Jr. (1965) Geology of the Sierra Blanca area, Hudspeth County, Texas. U. S. Geological Survey Professional Paper 479, 131 p. Albritton, C. C. & Smith, J. R., Jr. (1957) The Texas Lineament. Mexico City, 20th International Geological Congress [1956], Section 5, Relaciones entre de tectonica y la sedimentecia. Baumgardner, R. W., Jr. (1990) Geomorphology of the Hueco Bolson in the vicinity of the proposed low-level radioactive waste disposal site, Hudspeth County, Texas. Jeffrey R. Keaton & Roy J. Shlemon 290

Austin, Texas, The University of Texas Bureau of Economie Geology Final Contract Report for Texas Low-Level Radioactive Waste Disposal Authority, 98 p. Griggs, G. B. & Gilchrist, J. A. (1983) Geologic hazards, resources and environmental planning. Belmont, CA, Wadsworth Publishing Co. 502 p. Gustavson, T. C. (1990) Sedimentary fades, depositional environments, andpaleosols of the Upper Tertiary Fort Hancock Formation and the Tertiary-Quaternary Camp Rice Formation, Hueco Bolson, West Texas. Austin, Texas, The University of Texas Bureau of Economic Geology Final Contract Report for Texas Low-Level Radioactive Waste Disposal Authority, 94 p. Henry, C. D. & Price, J. G. (1985) Summary of the tectonic development of Trans-Pecos Texas. Austin, Texas, The University of Texas Bureau of Economic Geology Miscellaneous Map no. 36. Holzer, T. L. (1984) Ground failure induced by groundwater withdrawal from unconsolidated sediment, in Holzer, T. L., éd., Man-induced ground failure. Boulder, CO, Geological Society of America Reviews in Engineering Geology, v. VI, p. 67 - 105. Keaton, J. R., Shlemon, R. J., Slemmons, D. B., Barnes, J. R. & Clark, D. G. (1989) The Amargosa fault - a major late Quaternary intraplate structure in northern Chihuahua, Mexico: Geological Society of America Abstracts with Programs, v. 21, no. 6, p. A148. McClure, C. R., Jr. & Hatheway, A. W. (1979) An overview of nuclear power plant siting and licensing, in Hatheway, A. W., and McClure, C. R., Jr., eds., Geology in the siting of nuclear power plants. Boulder, CO, Geological Society of America Reviews in Engineering Geology, v. IV, p. 3 - 12. Phillips, J. D., Dean, D. R. & Riherd, P. S. (1986) Preliminary seismic reflection study of the Fort Hancock area in Hudspeth County, Texas. Austin, Texas, The University of Texas Institute for Geophysics Technical Report TR-45, Final Report for the Texas Low-Level Radioactive Waste Disposal Authority, Contract No. IAC (86-87)-0994, 35 p. Reilinger, R. E., Brown, L. D. & Powers, D. (1980) New evidence for tectonic uplift in the Diablo Plateau region, west Texas: Geophysical Research Letters, v. 7, no. 3, p. 181 - 184. Reilinger, R. E., Brown, L. D., Oliver, J. E. & York, J. E. (1979) Recent vertical crustal movements from leveling observations in the vicinity of the Rift. in Riecker, R. E., éd., Rio Grande Rift, tectonics and magmatism. Washington, D.C., American Geophysical Union, p. 223 - 236. Sergent, HausMns & Beckwith (1989) Preliminary geologic and hydrologie evaluation of the Fort Hancock Site (NTP-S34), Hudspeth County Texas, for the disposal of Low-Level Radioactive Waste. Unpublished consultant's report prepared for Hudspeth County, Texas, Hudspeth County Conservation adn Reclamation District No. 1, Hudspeth County Underground Water Conservation District No. 1 and El Paso County, Texas, El Paso SHE Job No. E88-4008B, 3 volumes. Shlemon, R. J. & Davis, P. (1988) Ground fissures in the Rancho California area, Riverside County, CA. Geological Society of America Abstracts with Program, v. 20, no. 7. p. A145. Swadley, W. C. & Scott, R. B. (1988) Modern fissures in the Pahranagat shear system northeastern Nevada. EOS, v. 69, no. 44, p. 1459.