FRONT COVER-Index map of southeastern New Mexico and western Texas showing approximate position of Capitan Limestone (reef) and area discussed in this report. Circular 184

New Mexico Bureau of Mines & Mineral Resources

A DIVISION OF NEW MEXICO INSTITUTE OF MINING & TECHNOLOGY

Regional geology of Ochoan evaporites, northern part of Delaware Basin

GO ERNYENT by ~etB@@$$~@&man Star Rte Box 1028, Corrales, New Mextco-87048

SOCORRO 1984 NEW MEXICO INSTITUTE OF MINING & TECHNOLOGY Laurence H. Lattman, Preridenr NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES Frank E. Kottlowski, Director George S. Austin, Depuq Director

BOARD OF REGENTS Ex Officio Toney Anaya, Governor of New Mexico Leonard DeLayo, Suprrinrendrnr of Public instruction Appointed Judy Floyd, President, 1977-1987, Las Cruces William G. Abbott, Secretq-Treasurer. 1961-1985. Hobbs Donald W. Morris, 1983-1989, Los Alamos Roben Lee Sanchez. 1983-1989. Albuuu~ruue Steve Tomes, 1967-1985, Socorro

Original Prinfing, 1984

Published by Authority of State of New Mexico, NMSA 1953 Sec. 63-1-4 Printed by University of New Mexico Printing Plant. May 1984 Available from New Mexico Bureau of Mines & Mineral Resources, Socorro, NM 87801 Contents

ABSTRACT 5 INTRODUCTION 5 STRATIGRAPHY 6 OCHOANSERIES 6 Castile Formation 6 Castile-Salado contact 8 Salado Formation 11 Rustler Formation 11 Dewey Lake Red Beds 13 MESOZOICROCKS OVERLYING THE OCIIOANSERIES 13 Rocks of Triassic age 14 Rocks of Cretaceous age 14 CENOZOICROCKS 14 Ogallala Formation 14 Gatuna Formation 14 SOILS 14 DISSOLUTION OF EVAPORITES 14 KARSTI'ROCESSES IN EVAPORlTES 16 THE ANCESTRALPECOS RIVER 17 DISSOLUTIONALONG THE EASTERN MARGIN OF THE DELAWAREBASIN 20 CONCLUSIONS 21 REFERENCES 22

Figures

I-Index map of southeastern New Mexico and western Texas. 6 2-Nomenclature and stratigraphic relations of Guadalupian and Ochoan rocks, Delaware Basin. 7 3Llsopach map of Castile Formation. 9 4-Isopach map of Halite 111 and Halite IV. 10 5-(in pocket)-Cross section showing local interfingering of anhydrite-halite facies. 6-(in pocket)-Cross section showing lenticular bodies of halite in upper part of Castile Formation. 7-Correlation of marker beds. 12 8-Collapse breccia in Salado Formation. 13 9-Collapse sink in Rustler gypsum. 17 10-Index map showing position of Ancestral Pecos River and isopachs of Cenozoic fill. 18 11-Collapsed channel gravel in upper part of Gatuna Formation. 19 12-Close-up of gravel shown in Figure 11. 19 13-(in pocket)-Stratigraphic cross sections showing relationship of dissolution to course of Ancestral Pecos River.

Tables

1-Marker beds in Salado Formation. 13

Abstract

The Ochoan Series (Permian) in the northern part of the Delaware Basin, southeastern New Mexico, includes in ascending order the Castile, Salado, and Rustler Formations, and the Dewey Lake Red Beds. The Castile and Salado Formations comprise a seouence of eva~oriteswhich include anhydrite, gypsum, halite, and associated potash salts. The Rustler Formation contains some halite

and minor amounts of potash minerals. These evaporites were de~ositedwithin the~ basin~ formed~~~~~~-~~ by the Capitan barrier ;eef, as well as across the rbef The evapotkes, as wull as the Capitan reef, are all subject to dissolution with resulting karst features analogous to those formed in limestone regions. An Ancestral Pecos River was the major drainage system in the western part of the Delaware Basin, New Mexico, during late Cenozoic time. That ancient river system was responsible for the formation of an extensive karst terrain along the east side of the present Pccos River in New Mexico and southward into Texas. During late Cenozoic time extensive dissolution occurred in the Salado Formation within the karst area as a result of the ground-water regime. The dissolutiun front was perched on the upper anhydrite member of the Castile Formation. On the eastern side of the Delaware Basin in New Mexico, a large collapse sink-San Simon sink-overlies the Capitan reef which is a prominent aquifer system in that area. So-called "breccia pipes" are ancient sinks which collapsed into the caverns in the reef on the northern margin of the basin. These have since been partially exhumed. The San Simon sink is presumed to be a mudern analog of these breccia pipes.

Introduction Numerous studies of the Delaware Basin have been been expressed by critics of the WIPP site that soluble undertaken with various objectives. The earliest com- rocks in the Castile might be dissolved to provide an prehensive studies were the result of exploration for unstable base for the site. For these reasons, most petroleum. Drill holes penetrated Ochoan and older effort during the present study concentrated on the Permian rocks, providing control points for correla- regional setting of the Castile. tion of stratigraphic units. Potash minerals were dis- The Salado and Rustler Formations, the other salt- covered east of Carlsbad in 1925. Development of these bearing units in the Ochoan Series, receive less at- deposits included extensive drilling programs to de- tention here. They are discussed only as background termine their extent and grade. As a result of this for the consideration of processes of subsurface dis- development, the details of the potash-bearing for- solution. mations are well understood within the potash min- Methods of study included examination of surface ing district in the north-central part of the Delaware exposures over much of the northern part of the Del- Basin. aware Basin. The relationship of Ochoan rocks to ad- More recently, intensive studies have been under- jacent deposits has been studied to evaluate the history taken in a limited area in the north-central part of the of those rocks since their deposition. Ancient drainage Delaware Basin to determine the feasibility of storing systems were studied both by field observations and radioactive waste in underground beds of salt. These examinations of well logs. More than 300 wire-line studies have provided background for the proposed logs of drill holes in the northern part of the Delaware construction of a Waste Isolation Pilot Plant (WIPP). Basin were examined. Cores were studied from drill Numerous drill holes have penetrated portions of the holes in the vicinity of the proposed WIPP site as well Ochoan Series, the salt-bearing rocks, during these as from other parts of the basin. background studies. J. W. Mercer and R. P. Snyder of the U.S. Geological The present work was undertaken to summarize Survey contributed generously their time and knowl- some of the knowledge derived from these studies edge of the Delaware Basin. J. W. Mussey of Duval and to examine the regional geology of part of the Mining Corporation provided core of the Castile For- salt-bearing Ochoan Series in a northern part of the mation for study. The manuscript was reviewed and Delaware Basin. The Castile Formation, the basal unit constructively criticized by John W. Hawley of the of the Ochoan Series, underlies the WIPP site. Minute New Mexico Bureau of Mines and Mineral Resources, details of some aspects of the Castile have been stud- and Lokesh Chaturvedi of the State of New Mexico ied in the past, but its regional stratigraphy and geo- Environmental Evaluation Group. logic setting have been neglected. Some concern has Stratigraphy Ochoan Series Ochoan rocks in the northern part of the Delaware Basin are shown in Figure 2. Rocks of Permian age in southeastern New Mexico and western Texas are divided, from oldest to young- Castile Formation est, into the Wolfcampian, Leonardian, Guadalupian, The Castile Formation, the basal unit of the Ochoan and Ochoan Series. They were deposited in marine Series, was named by Richardson (1904, p. 43) for environments in a tectonically negative area, the Del- Castile Spring in Culberson County, Texas, about 20 aware Basin (Fig. l). Rocks from Wolfcampian through mi south of the New MexiceTexas State line. For a Guadalupian age are mostly fine-grained sandstones, time the Castile was divided into a "lower" and "up- siltstones, shales, and various types of limestone. per" salt series. Lang (1935, pp. 265-267) separated During Guadalupian time, the Capitan reef was built the "upper salt series" from the Castile and named it as a massive limestone barrier which fringed the mar- the Salado Formation. gins of the Delaware Basin. As defined by the Capitan Later, an extensive bed of anhydrite, which rests rseef, the Delaware Basin was an elongate, bowl-shaped on the Capitan Limestone in places in the subsurface, dlepression. Most of early Ochoan deposition was was defined as the base of the Salado Formation (Lang, confined to this basin. Later, during Ochoan time 1939). This anhydrite was described in detail and named when the basin became filled with sediments, some the Fletcher Anhydrite Member of the Salado For- t~edslapped across the reef into adjacent areas. mation (Lang, 1942, pp. 75-78). This definition served Most of the rocks deposited during Ochoan time to restrict the Castile to the Delaware Basin. were evaporites such as anhydrite, halite, and potash However, Jones (1954, p. 109; 1973, p. 10) indicated minerals with only minor amounts of limestone, mud- that the upper part of the Castile includes a north- stone, and siltstone. The rocks of the Ochoan Series ward-thinning tongue of anhydrite which overlaps are subdivided into (ascending) Castile Formation, the Capitan and Tansill Formations outside the basin. Salado Formation, Rustler Formation, and Dewey Lake This tongue appears to include the Fletcher, which is Red Beds. Within the Delaware Basin, the Castile For- transitional with the upper part of the Castile in the mation rests on the Bell Canyon Formation of the Delaware Basin. The Fletcher is readily separated from (hdalupian Series. The stratigraphic relations of the main body of Castile anhydrite around the margin

FIGURE I-Index map of southeastern New Mexico and western Texas showing approximate position of Capitan Limestone (reef) and area discussed in this report. SHELF REEF DELAWARE BASIN I Dewey Lake Redbeds z Rustler Frn. a 0 I Salado Frn. o Tonsil Fm. o Castile Frn. Yotes Fm.

Seven Rivers Fm. z Bell Canyon Fm. -a a Queen Fm. 3 _I Grayburg Frn. an Cherry Canyon Fm. 4 3 (3

FIGURE 2-Diagram showing nomenclature and stratigraphic relations of Guadalupian and Ochoan rocks in northwestern part of the Delaware Basin. of the basin, where it rests on the Capitan or Tansill are not dolomitic. The average thickness of the calcite laminae Formations; but within the basin it merges with, and appears to be about 1/20 inch. Thin sections show that the thinnest laminae are made up of one layer of coarsely crys- is indistinguishable from, a sequence of thick anhy- talline calcite, and that the thicker beds are coarsely granular drite in the upper part of the Castile. limestone. Anhydrite is the dominant rock type in the Castile Formation. Halite is present as several massive beds Irregularly distributed through the normal banded zones are in the formation in the subsurface, but is much less beds of unlaminated anhydrite, ranging from '12 inch to several feet in thickness, and separated by banded zones that vary prominent than the halite in the overlying Salado For- even more widely. There is a tendency for secondary calcite mation. Limestone interlaminated in anhydrite, thin crystals to develop in fractures and to wavy ghost-like bands beds of limestone, and minor amounts of dolomite or even sprinkled about at irregular intervals, through these and magnesite are also constituents of the Castile. thicker anhydrites. The secondary calcite is almost everywhere Where the Castile Formation is exposed along the lighter-colored than that of the normal laminae. (Adams, 1944, p. 1604.) western side of the Delaware Basin, the halite beds have been either removed by dissolution or were never In addition to the primary variations in the calcite-banded deposited in that area; and the anhydrite has been anhydrite of the Castile Formation, there are many secondary altered to gypsum. At these surface exposures the irregularities. Calcite laminae disappear in nodular masses of anhydrite. Concretionary anhydrite lenses grow between the gypsum may be either massive or thinly laminated. partings and disrupt them. Bands of crinkled laminae appear Interlaminated anhydrite and limestone are dis- to writhe about behveen flat beds, and in some transverse tinctive lithologic features in the Castile Formation. zones all evidence of bedding is lost. More important still is Attempts to explain these laminae have occupied nu- the faulting, fracturing, and slipping that characterize most of merous investigators. Udden (1924) suggested that the cores and outcrops. Since most of the variations occur both at the surface and at depths of thousands of feet, it is assumed these laminae represent annual cycles of sedimenta- that mast of them were produced by early diagenetic pro- tion. Adams (1944, pp. 1604-1606) described the lam- cesses. (Adams. 1944, p. 1606.) inae in detail. He recognized that typical laminae consist of alternating bands of calcite and anhydrite. Bitu- Anderson et al. (1972) studied the petrology and minous material stains the carbonate deep brown. depositional history of the Castile Formation in great Where the calcite is absent, "brown organic bands are detail. They counted and measured approximately present along the bedding planes of the anhydrite. 260,000 couplets of calcite-laminated anhydrite in the The bituminous material, however, is nowhere mixed core of one drill hole in Culberson County, Texas. with the white anhydrite as it is with calcite" (Adams, They correlated individual laminae in this core with 1944, p. 1604). those in a core from a drill hole in Winkler County Adams observed that there are almost unlimited about 56 mi to the east. In other studies they have variations in the character of the laminae. He de- correlated individual laminae for distances as great as scribed the gross features as follows: 70 mi (Anderson et al., 1978). Although anhydrite and gypsum in the Castile For- Where evenly developed, the laminae resemble chemical "awes mation are usually thinly laminated, massive beds of with the anhydrite layers two or three times as thick as the calcite . . . Individual carbonate partings range from scattered these rock types are present in places in the upper calcite crystals on the anhydrite bedding planes to beds of part of the formation. Adams (1944, p. 1605) observed thinly laminated limestone several feet thick. The limestones that the banded anhydrite is absent in the upper 800 ft of the Castile in the northeast part of the Delaware The laminations in the anhydrite are assumed to rep- Basin. Along the western edge of the basin in New resent annual layers which resulted from evaporation Mexico, many surface exposures of the Castile include of sea water and precipitation of sulphate salts. Cal- besds of massive white gypsum (Hayes, 1964). Else- cium sulphate was deposited as anhydrite below the where in the subsurface, laminated anhydrite may wave base (King, 1947). grade upward into massive anhydrite (Adams, 1944, Dean and Anderson (1982) summarize the charac- p. 1605; Jones, 1954, p. 109; 1973, p. 9). These massive teristics of Castile lithology and discuss its deposi- beds are correlative with other beds of known Castile tional environment. They agree with earlier workers age in the subsurface and cannot be correlated with that the Delaware Basin was a deep, starved basin any unit within the overlying Salado Formation. which filled rapidly with evaporites. They note that Near all the margins of the Capitan reef in New there is no evidence of shallow water deposition and Mexico the Castile Formation consists entirely of an- that water in the Delaware Basin was deep enough hydrite or gypsum, with the exception of one or more to preserve the delicate details of individual laminae. thin beds of limestone along the western margin. Away They suggest that the initial depth of the basin "was from the margins of the basin one or more beds, or at least several hundred meters" (Dean and Anderson, lenses, of halite are interbedded with the anhydrite. 1982, pp. 330-331). Of particular interest is their ob- The number of halite beds increases towards the cen- servation that with the continuous filling of the basin ter of the basin, where as many as five discrete se- with evaporites "the factors which affect salinity . . . quences of halite are present. Locally, halite interfingers were affecting a progressively shallower basin and with thin beds of anhydrite to form complex strati- hence a smaller volume of water. The smaller the vol- gmphic units. ume of water being affected, the less buffered the Informal members of the Castile Formation have evaporative system is and therefore the greater the been described and designated by Roman numerals possibility for larger variations in salinity" [Dean and (Anderson et al., 1972) as (ascending) Basal Lime- Anderson, 1982, p. 331). Variations in salinity are in- stone, Anhydrite I, Halite I, Anhydrite 11, Halite 11, dicated in the central part of the basin by intricate Anhydrite 111, Halite 111, and Anhydrite IV. The Basal interbedding of anhydrite and halite in the upper part Limestone is too thin to be recognized in well logs of the Castile (Figs. 5 and 6 in pocket). and is not discussed in this report. An additional set There is little indication of a well-defined axis of of halite beds was recognized locally within Anhydrite deposition in the northern part of the Delaware Basin IV during the present work. These are designated during much of Castile time. The Castile averages H,dite V. from about 1,500 to 1,700 ft in thickness in the sub- Where halite beds are present in the Castile below surface, where the only complete stratigraphic sec- Anhydrite 111, the members are remarkably uniform tions are found. Locally, the Castile is more than 2,100 in thickness and stratigraphic expression. This indi- ft thick in north-central Loving County, Texas (Fig. cates that from the outset of Castile deposition through 3). Distribution of these variations in thickness ap- the deposition of Halite I1 the basin was stable over pears to be random. During the deposition of Halite much of its area. Halite I consists of almost pure hal- 111 there may have been a northwesterly trending de- ite. Halite I1 usually contains five anhydrite interbeds positional axis (Fig. 4) which shifted to the south and which are correlative over almost the entire basin changed its trend to northerly as the basin filled dur- wherever Halite I1 is represented. ing the deposition of Halite IV. During deposition of Anhydrite 111, the Delaware Basin became mildly unstable with some local differ- Castile-Salado contact ential subsidence. This is reflected in logs of drill holes The interpretation of the Castile-Salado contact is in southern Lea County, New Mexico, where within basic to an interpretation of the history of dissolution; a distance of 9 mi there is progressive thickening of consequently, this contact was studied in detail. As each stratigraphic unit above Halite 11. In a drill hole generally accepted, the contact between the Castile in sec. 18, T. 26 S., R. 34 E., the interval from the base and the overlying Salado Formations is between mas- of Anhydrite I11 to the top of the Castile is 100 ft sive beds of anhydrite in the Castile and a sequence thicker than the same interval in a drill hole to the dominated by halite, potash minerals, and thin beds west, in sec. 15, T. 26 S., R. 32 E. The thickening of anhydrite in the Salado. Adams (1944, p. 1608) and would be less remarkable if it would not be accom- other workers have described an "angular uncon- panied by an increase in halite content. This has re- formity" between the Castile and the Salado in the sulted in complex facies changes within a relatively north and east parts of the Delaware Basin. Jones short distance. These facies changes are also indicated (1954, p. 108; 1972, p. 39; 1973, p. 10) indicated that in the logs of drill holes in the same area along a the contact between the Castile and Salado is gra- north-south line (Fig. 6 in pocket). dational and interfingering. It is generally agreed that during Castile time the The present study generally confirms the conclu- Delaware Basin was a steep-sided, closed marine ba- sions outlined by Jones. There is no evidence in the sin confined by the Capitan barrier reef except for a northern part of the Delaware Basin of a disruption low passage to the southwest. The basin was con- of deposition at the end of Castile time, nor of dis- nected to the open sea by a channel through which solution between the two formations. The only places water entered the basin to replace water lost by evap- where a hiatus is found between the Castile and Sa- oration. Water from the open sea entered the basin lado Formations is where dissolution of salt in the a!$a surface layer which extended down to wave base. Salado has occurred subsequent to deposition. In the

deepest part of the basin in southeastern Lea County, nized in the Salado Formation (Jones et al., 1960; Jones, New Mexico, and northeastern Loving County, Texas, 1978; Sandia Report, 1982a, b). These distinctive bed: halite beds make up a prominent part of the upper include potash-ore horizons and thin beds of siltstone Castile. There the basal anhydrite beds in the Salado or anhydrite. They are persistent within the potash (including the Cowden Member) merge by interfin- district, and some are recognized over much of the gering with massive anhydrite of the upper Castile Delaware Basin in New Mexico and southward into (Figs. 5 and 6 in pocket). Kroenlein (1939) was aware Texas. of this stratigraphic relationship. This interpretation Marker beds are designated by informal names or is only slightly different from that of Jones (1954, p. by a numbering system. Only those prominent marker 108), who has drawn the SaladwCastile contact in the beds useful to the discussion of dissolution in the south part of the present study area on a bed of an- present report are treated here. They are listed in hydrite somewhat lower in the stratigraphic section Table 1. The intervals between marker beds on this than drawn in this report. In spite of this difference list include halite and potash minerals. Approximate in nomenclature, the interpretation of depositional thicknesses of marker beds, intervals between marker history resulting from this study is similar to that beds, and lithologic descriptions are from Sandia Re- implied by Jones. ports (1979, 1982a, b, 1983) and wire-line logs of Pa- In most places the contact between the two for- toil, Wright Federal, No. 1 (sec. 27, T. 23 s., R. 32 E.). mations is sharp. Problems of nomenclature occur The Cowden Anhydrite (see Table 1) is about 180 ft only in the area of interfingering lithologies. The de- above the base of the Salado Formation in the north- termination of formation boundaries presented here ern part of the Delaware Basin. It merges with mas- was made only after correlation of distinctive marker sive anhydrite of the Castile Formation in southwestern beds throughout the Salado and Castile Formations, Lea County, New Mexico, and northeastern Loving from the potash district in the northern part of the County, Texas. basin through numerous closely spaced drill holes to In the subsurface, the Salado Formation ranges from the southern part of the study area. The present inter- about 1,200 to 2,300 ft in thickness. Much of this var- pretation of stratigraphic relations conforms to the iation in thickness is the result of removal of rock salt model of deposition proposed by Dean and Anderson by dissolution. Surface exposures of the Salado For- (1982). They suggested that filling of the basin indi- mation consist only of brecciated gypsum embedded cates a smaller volume of water for evaporation and in clay, which is mostly a residuum from the disso- larger variation in salinity. This condition could ex- lution of salts (Fig. 8). plain the fluctuations in anhydrite-halite deposition at the close of Castile time, and the transition to dom- Rustler Formation inant halite deposition in Salado time. The Rustler Formation was named by Richardson In the southern part of Eddy County, New Mexico, (1904, p. 44) for exposures in the Rustler Hills, Cul- and southward into Texas, the boundary between the berson County, Texas, about 20 mi south of the New Castile and the overlying Salado is marked by solution Mexicc-Texas State line. In that area only the lower breccias and residue of the Salado which rest on the part of the formation is exposed. In many areas of anhydrite of the Castile. At places anhydrite beds in southeastern New Mexico much of the Rustler has the Salado collapsed onto Castile anhydrite, where been removed by dissolution and erosion, and com- they became indistinguishable from one another. In plete stratigraphic sections are found only in drill holes. these areas the base of the Salado is indicated on wire- At many places the salts in the Rustler have been line logs by a marked change in the traces of sonic removed by dissolution even in the subsurface. transit time and gamma rays. Castile traces are char- The Rustler Formation may be divided into five acteristically uniform, with only minor fluctuations. lithologic units. These include, in ascending order, an Solution breccias, collapsed beds, and residue in the unnamed sequence of reddish-brown siltstone with Salado are marked by broad fluctuations in the log interbeds of gypsum or anhydrite and halite, the Cu- traces (Fig. 7). These brecciated beds indicate a dis- lebra Dolomite Member, the Tamarisk Member, the continuity between the two formations, which de- Magenta Dolomite Member, and the Forty-niner veloped by dissolution long after they were deposited. Member. The Culebra and Magenta Members were named by W. B. Lang (Adams, 1944). The Tamarisk Salado Formation and Forty-niner Members were named by Vine (1963). The so-called "upper" salt series of early workers The Rustler ranges in thickness from a thin disso- was named the Salado Formation by Lang (1935) for lution breccia at places on the surface to more than Salado Wash in Loving County, Texas. His type lo- 550 ft in the subsurface in southwestern Lea County. cality was in a drill hole, the "Means well" (SE corner Over much of the study area it averages 300-350 ft in sec. 23, blk. C-26, P. S. L., Loving County, Texas). thickness. Much of the variation in thickness is the The Salado Formation includes halite, minor beds of result of the dissolution of salts. The basal unnamed anhydrite, and commercial deposits of potash min- member, as well as the Tamarisk and Forty-niner erals. Owing both to commercial interest and back- Members, include halite with some associated potash ground studies for construction of the WIPP site, the minerals in undisturbed sequences in the subsurface. Salado has been studied intensively in the northern At places where the salts have been dissolved, a "col- part of the Delaware Basin. lapse breccia" occupies their stratigraphic position. At

~~ ~-~- these places the anhydrite is usually altered to gyp- In the ~otashdistrict in Eddv Countv.2. New Mexico, numerous so-called "marker beds" have been recog- sum. FIGURE 7-Correlation of marker beds. Thelog of the Santana well in Lea County, New Mexico, illustratesa relatively normal stratigraphic section of evaporites in the Castile and Salado Formations. The log of the Union well in Eddy County, New Mexico, illustrates dissolution of halite beds in the Salado Formation from the approximate horizon of marker bed 127 to the top of the Castile Formation, Minor dissolution has occurred in the upper part of the Salado. This well is near the eastern margin of the Ancestral Pecos drainage system. The log of the TXL well in Loving County, Texas, indicates that pervasive dissolution has occurred in both the lower and upper parts of the Salado Formation This well was drilled near the center of the Ancestral Pecos drainage system. There is no evidence of dissolution in any part of the Castile Formation in any of these drill holes. Halite 111 is absent in the TXL and Union wells duc to non-deposition. TABLE 1-The more prominent marker beds in the Salado Formation, potash district in Eddy County, New Mexico. -. Thickness Lithology -. Top of Salado Fm 180-ft interval MB 103 20 ft Anhydrite 120-ft interval MB lW 20 (+)it Anhydrite, finely crystalline, interbedded thin stringers of halite, polyhalite, and mud- stone. 200-ft interval Vaca Triste (of Adams. 1944) 10 ft Siltstone and silty mudstone, brownish, interbedded with halite. 210-ft interval Union Anhydrite 15-20 ft Anhydrite, finely crystalline, with stringers of hahte 70-90-ft interval MB 123 5-10 ft Halite and polyhalite. Distinctive sequence overlying MB 124. MB 124 5-10 ft Anhydrite, finely crystalline, laminated. May have stringers of mudstone. 390-ft interval MB 134 10-15 ft Anhydrite, gray, dense. 70-ft interval MB 136 10-15 ft Anhydrite, light gray, dense. May have interbeds of halite or polyhalite. ZW-ft interval MB 142 15 ft Anhydrite with interbeds of halite and stringers of mudstone 150-ft interval Cowden Anhydrite ca 20 ft Anhydrite, finely crystalline, laminated, may have thin interbeds or stringers of mag- nesite and mudstone. Divided into two units by intervening halite in southeastern Eddy County, New Mexico.

Both the Culebra and Magenta Members are dis- Page and Adams (1940, pp. 62-63) based on a sub- tinctive stratigraphic markers useful in estimating the surface stratigraphic section in Glasscock County, amount of dissolution which has occurred in the hal- western Texas. This formation has been called the ite-bearing members. The Culebra is a brownish-gray, Pierce Canyon Redbeds (Lang, 1935, p. 264) for ex- thinly bedded, finely crystalline dolomite. Many lay- posures in the vicinity of Pierce Canyon southeast of ers are marked by distinctive brownish vugs that range Loving, New Mexico. However, owing to the poor from 2 to 10 mm in diameter. The Culebra averages definition of the Pierce Canyon and the wider ac- 25-30 ft in thickness. ceptance of the term Dewey Lake among geologists, The Magenta Dolomite Member is a thinly lami- the US. Geological Survey has abandoned the Pierce nated dolomite with couplets of anhydrite or gypsum. Canyon and extended the term Dewey Lake to New The laminae are generally less than 10 mm thick. At Mexico for this stratigraphic interval. surface exposures the laminae are undulatory-prob- The Dewey Lake Red Beds rest conformably on the ably because of the expansion of gypsum during its Rustler Formation and consist mainly of alternating hydration from anhydrite. The dolomite laminae are thin, even beds of moderately reddish-brown to mod- light reddish-brown and the anhydrite or gypsum is erately reddish-orange siltstone and fine-grained gray. The Magenta ranges from 20 to 30 ft in thickness. sandstone. There are occasional small-scale lamina- tions and ripple marks. Many beds are mottled by Dewey Lake Red Beds greenish-gray reduction spots. Well-sorted quartz The Dewey Lake Red Beds are the uppermost for- grains constitute most of the rock. Cement is usually mation in the Ochoan Series. They were named by selenite or clay. Beds of Triassic age rest unconformably on, and overlap, the Dewey Lake Red Beds; and the thickness and regional distribution of the Dewey Lake is related directly to erosion before Triassic time. The Dewey Lake is thickest in eastern Eddy and western Lea Counties, where it is as much as 560 ft thick. It thins towards the west and is absent in some exposures where Triassic rocks lap across the westernmost line of outcrop.

Mesozoic rocks overlying the Ochoan Series To understand the post-depositional history of the Ochoan Series, it is necessary to consider the distri- FIGURE 8--Collapse breccia in Salado Formation. Breccia includes bution of overlying strata. These strata include Trias- gypsum, siltstone, and reddish clay. sic, Cretaceous, and Cenozoic rocks. R.ocks of Triassic age Ogallala is capped by the "caliche caprock," which Rocks of Triassic age are exposed along the north- was deposited by soil-forming processes. Ogallala ~rasternrim of Nash Draw and in isolated collapse- sediments were derived from the west and deposited jink deposits in the Pecos Valley. These exposures are on an irregular erosional surfaceas a series of complex twosional remnants of deposits which formerly ex- overlapping and coalescing alluvial fans. The Ogallala tended continuously northward into east-central New Formation is well represented on the northeast side Mexico. Triassic rocks are dark reddish-brown, cross- of the Delaware Basin, but it is absent west of San laminated, poorly sorted conglomeratic sandstones Simon Swale except for thin exposures at the Divide. with interbeds of dark reddish-brown sandy shale. 'They were deposited in a fluvial-deltaicdacustrine Gatuna Formation sistem whose depositional edge coincides approxi- The Gatuna Formation (early? to middle Pleisto- &lately with the western edge of the Delaware Basin cene) consists mainly of pale reddish-brown sand and (IdcGowen et al., 1977). The position of this deposi- sandy clay. Locally it contains yellowish sand and tional edge overlying the Ochoan Series indicates that lenticular bodies of river gravels. The widespread red- evaporites were exposed, or near the surface, and dish-brown sediments are presumed to have been available for dissolution during Triassic time. deposited on a flood plain. A bed of volcanic ash is present near the top of the formation on the east side Rocks of Cretaceous age of Nash Draw. This ash has been identified as Lava In regions where complete stratigraphic sections Creek Ash (Izett and Wilcox, 1982), which is about are preserved, rocks of Jurassic age overlie Triassic 600,000 years old. strata. However, rocks of Jurassic age have not been preserved, and regional stratigraphic relations indi- Soils c;~tethat they were never deposited in southeastern New ~exico: Two soils of Pleistocene age have been identified Rocks of Early Cretaceous age (Washita) are pre- in the western part of the Delaware Basin. These are served at a few localities in the Pecos Vallev as col- the Mescalero caliche and the Berino soil. lapse-sink deposits. These rocks consist of marine, The Mescalero caliche is an informal stratigraphic fossiliferous sandy limestones. At one locality, 5 mi unit named for the Mescalero Plain, a broad geo- northeast of Carlsbad, Cretaceous rocks are mingled morphic surface which lies east of the Pecos River and with collapse blocks of Culebra Dolomite and Triassic west of the High Plains in southeastern New Mexico. conglomerate. At two localities, 6 mi and 7.8 mi south- It was deposited by soil-forming processes and con- west of Whites City, Cretaceous rocks have collapsed sists of earthy, nodular to well-cemented calcareous on gypsum of the Castile Formation, and are asso- caprock. It accumulated on a relatively stable land ciated with isolated blocks of Culebra Dolomite. Trias- surface and has provided a useful datum for timing sic rock debris has not been observed at these localities. some Pleistocene and Recent events. By measuring The absence of Triassic rocks suggests that Triassic uranium series disequilibrium, it has been deter- sediments either were not deposited in this area, or mined that the Mescalero caliche began to accumulate were removed bv erosion before Cretaceous time. about 510,000 years ago and the upper crust formed about 410,000 years ago (J.N. Rosholt, written comm. 1979). Cenozoic rocks The Berino soil is an informal stratigraphic unit which consists of dark red, sandy, argillic paleosol. It over- Ogallala Formation lies the Mescalero at some places. The Berino is non- The Ogallala Formation (Miocene-Pliocene) con- calcareous. The uranium-series disequilibrium tech- s,ists largely of well sorted, yellowish-brown to light nique indicates that the Berino began to form about reddish-brown, wind-blown sand. Poorly sorted stream 350,000 (i 60,000) years ago (J. N. Rosholt, written iieposits and carbonate pans are locally present. The comm. 1979).

Dissolution of evaporites

Dissolution of evaporites in the Ochoan Series in break between Castile and Salado deposition. southeastern New Mexico has been discussed by many 2. At the close of Salado and the beginning of Rustler geologists. One of the earliest attempts to place the deposition the west edge of the Delaware Basin events of evaporite deposition in geologic time was was uplifted and truncated. Subsequent erosion by Adams (1944, pp. 1622-1624), who described salt left a series of solution valleys in the potash district dissolution in the Delaware Basin and concluded that southeast of Carlsbad. These valleys were covered it occurred during geologic time as follows: and filled by Rustler sediments. 1. Dissolution occurred first during Permian time, 3. During Triassic time Salad[> salts were removed when the upper salt series of the Castile (Adams' along the east edge of the Delaware Basin from the "third group") was partially dissolved during the Pecos River northward into New Mexico. "The so- lution trough developed at this time is 800 or 900 Anderson and Kirkland (1980, pp. 66-69) described feet deep and is filled by an over-thickened section a theoretical process of "brine density flow" which of Triassic" (Adams, 1944, p. 1623). they believe had been responsible for the dissolution 4. Post-Triassic-pre-Cretaceous salt solution is not in- of salt deposits in the Delaware Basin. According to dicated in the Delaware Basin. their definition, "gravity flow of brine occurs when 5. The main period of solution in the Delaware Basin fresh water contacts the salt body and becomes more took place during the Tertiary. "The main trough dense by dissolution of salt. The differential density is roughly bordered on the east by the Pecos River, between the brine and fresh water causes movement and extends from Carlsbad, New Mexico, to Toyah of brine downward along separate and distinct path Lake in Texas" (Adams, 1944, pp. 1623-1624). At ways, but in the same fracture system or permeable one place in Reeves County, Texas, this trough is zone as the rising fresh water. The descending brine more than 1,300 ft deep. Adams suggested that the ultimately flows through the aquifer to a point of nat- sinking trough was filled with sediments as fast as ural discharge and, because the system is under arte- it formed. sian pressure, the brine flow is continuously replaced Of these episodes of dissolution only those assigned by fresher water. Sustained flow results in the en- to Triassic and Cenozoic were recognized during the largement of dissolution chambers or an advancing present work. front of dissolution. Subsequent collapse of compe- Maley and Huffington (1953) described three ex- tent beds causes brecciation." (Anderson and Kirt- tensive accumulations of Cenozoic fill in the Delaware land, 1980, p. 66.) Basin. The fill has a maximum thickness of approxi- Anderson (1981) adapted the concept of brine den- mately 1,900 ft. Localization of the fill deposits is at- sity flow to large scale depressions within the Dela- tributed to the formation of depositional basins by ware Basin as well as to the deep depressions along evaporite dissolution. They constructed an isopach the margin of the eastern reef. He assumed that there map of the total amount of salt present in the com- is communication between the lower Salado Forma- bined Castile and Salado Formations. This map in- tion, where a dissolution horizon is recognized, and dicates that the fill is generally thickest in areas where the underlying Delaware Mountain Group (Bell Can- salt deposits are thin or absent. They attributed the yon) aquifer. Fracture permeability could have de- localization of solution to tilting of the Delaware Ba- veloped within the basin through the dissolution and sin, chiefly during late Tertiary time, with the con- collapse of lower Castile salt and subsequent stoping sequent removal of salt by solution. Further, they to the stratigraphic level of the lower Salado (Ander- suggested that the local basins were formed initially son, 1981, p. 143). However, there is no evidence in by local concentration of runoff and downward per- the northern part of the Delaware Basin that the lower colation of water which gradually dissolved the un- Castile salt has been dissolved. derlying salts. Maley and Huffington (1953, p. 543) Lambert (1983, p. 83) indicates another discrepancy did not believe that the areas of deep fill represent in knderson's model. I le reasons th.~t the solute re- the former course of the Pecos River because "ac- latiunsh~psin the smdl dnivunt oiBell Cannm waters cording to this concept, the thickness of fill should could not have arisen from simple evaporite disso- represent the depth of the former stream valley."They lution. Bell Canyon waters have consistent NaiCl ra- failed to recognize that subsurface dissolution may tios which range from 0.46 to 0.56, whereas the Nal cause collapse and fill at the surface until the depth CI ratio of brines resulting from dissolution of salt is of fill is below the local base level of erosion. about 0.64 (Johnson, 1981). Anderson et al. (1972, p. 81; 1978, pp. 47-52; 1980, Understanding of dissolution in the Ochoan Series pp. 66-69) discussed dissolution of halite and de- is dependent on an understanding of regional strati- scribed "dissolution breccia" in the Castile-Salado se- graphic and geologic history, as well as on drill-hole quence in the western part of the Delaware Basin. data. The writer has previously outlined a history of Their conclusions are listed as follows (Anderson et dissolution in the Delaware Basin in New Mexico a]., 1978, p. 52): (Bachman, 1980, pp. 81-85). That outline was based 1. Every salt bed recognized on accoustical logs in on extensive field work and study of the literature. It the eastern side of the Delaware Basin has an is summarized here as follows: equivalent bed of dissolution breccia in the western Erosion and dissolution occur while land masses side of the basin, indicating that all of the are above sea level. The region of southeastern New western salt-bed margins are dissolutional rather Mexico was uplifted at the close of Permian time. It than depositional. was above sea level throughout Triassic and Jurassic 2. The dissolution of salt beds proceeded downdip times. During the Early Cretaceous the region was from the western edge of the basin, with the pre- again below sea level. Isolated occurrences along the ferred dissolution horizons occurring between the Pecos Valley in New Mexico of rocks containing Early Halite 111 salt of the Castile Formation and the 136 Cretaceous marine fossils provide evidence for that marker bed of the Salado Formation. incursion of the sea. At the close of Cretaceous time 3. Downdip dissolution within the body of evapor- the entire region was lifted above the sea level again. ite~undercut the overlying salt beds for distances In summary, the region has been above sea level for in excess of 20 mi, and more extensive dissolution a minimum of 154 million years, and below sea level at these same horizons in localized areas resulted for less than 71 million years since the end of the in large scale collapse and dissolution features such Permian. as Big Sinks depression. Erosion and dissolution were active processes along the western margin of the Delaware Basin during limestone terrains has been described extensively in Triassic time. Streams flowed easterly across the basin the literature. Irom a highland in the vicinity of the present Sacra- Limestone is composed mainly of calcium carbonate mento Mountains. The depositional wedge-edge of and is soluble in acids. Atmospheric water normally Triassic rocks was along a line trending northeasterly carries varying amounts of carbon dioxide which forms in the vicinity of the modern Pecos River. a weak carbonic acid in solution. Thus, in combina- Triassic stream deposits indicate that a water table tion with geologic time, atmospheric water dissolves was present in strata immediately overlying the Ochoan limestone to etch out the distinctive features of karst Series. Field relations of Triassic and Permian rocks landscape. about five miles northeast of Carlsbad and along the Karst features form as readily in evaporites as in east side of the Pecos River near Red Bluff Reservoir limestone, but ground water in contact with evapor- rndicate that the upper part of the Rustler Formation, ite~is usually of poor quality. Consequently, there ~ncludingthe Magenta Dolomite, was removed before has been little interest in the study of karst or sub- or during Triassic deposition. Elsewhere in the Del- terranean water systems in those regions, and rela- aware Basin, Adams (1929, p. 1050) believed that as tively few papers on karst processes in evaporites much as 400 ft of Upper Permian beds may have been have been published. Those areas where such pro- removed from the basin during Permian time. cesses are recognized include Italy, France, Germany, Jurassic deposits are not present in the Delaware Oklahoma, Texas, and New Mexico (Herak and Basin, but geologic relations in regions to the north Stringfield, eds., 1972). The rising interest in beds of and south of the basin indicate that southeastern New salt as potential isolation bodies for the storage of Mexico continued to be above sea level as an area of nuclear waste provides a new incentive for the study non-deposition throughout Jurassic time. It may be of karst in evaporites. assumed that erosion and dissolution were active pro- Acidic solutions are not a prerequisite to the solu- cesses during that time. bility of halite and associated salts. These rocks are Although Jurassic rocks are not represented in the extremely soluble in fresh water. The only require- Delaware Basin, and deposits which contain marine ment for their dissolution is unsaturated water which Cretaceous fossils did not collapse until long after the flows across the rocks through an open, circulating rocks had lithified, the association of other rock types system. The continuation of this process through geo- with the Cretaceous deposits indicates the depth of logic time leaves behind a relatively insoluble residue erosion before Cretaceous time. These rock associa- of collapsed breccia and clay. Even limited supplies of unsaturated water through geologic time are suf- ficient to create huge areas of complex solution brec- ico: cia~and karst features. The writer has examined 1. Rocks above the Culebra Dolomite were partially brecciated karst areas in evaporites over hundreds of removed in the vicinity of the modern Pecos River square miles in the desert of central Saudi Arabia, before Cretaceous time. where the annual rainfall averages less than two inches. 2. The Salado Formation was removed completely in Anhydrite alters by hydration to gypsum when ex- a belt along the western margin of the basin in posed to surface or vadose water. This process results New Mexico before Cretaceous time, allowing the in expansion of about 35% by volume, which may Culebra Formation to rest on the Castile Forma- close some fractures and prevent the passage of water tion. Much of this removal was by dissolution. (Jakucs, 1977, pp. 79-80). However, it also results in The above observations are outlined to indicate that buckling of rocks at the surface, which may open some dissolution of Ochoan evaporites occurred dur- other passages. Buckling and tension are responsible ing long periods of the geologic past. The quantitative for fracturing and jointing gypsum to open passages results of that dissolution are conjectural. The most across bedding planes. Joint sets are commonly ob- obvious evidence for dissolution consists of features served in surface exposures of gypsum. These sys- and deposits visible in the present Pecos Valley. These tems of open fractures provide paths for surface water features were formed during Cenozoic time. Some to dissolve evaporites at depth. Collapse sinks de- dissolution of evaporites in the Ochoan Series con- velop along these fractures and act as sumps for sur- face water (Fig. 9). Without fractures the permeability of either an- formation of karst terrain in limestone regions hydrite or gypsum is negligible. For example, salt interbedded in anhydrite in the subsurface Castile For- mation is well protected from dissolution by the an- Karst processes in evaporites hydrite. Contrary to the statement of Anderson et al. (1978, p. 52) that "every salt bed recognized on ac- Most studies of karst terrain, the study of landforms coustical logs in the eastern side of the Delaware Basin which develop in soluble rocks, have been conducted has an equivalent bed of dissolution breccia in the in limestone regions. These regions include areas in western side of the basin . . . ," there is no evidence Great Britain, Central Europe, Yugoslavia, Kentucky, in the present study area that subsurface beds of salt and many other parts of the world. Caves, under- present in the Castile, as that formation is here rec- ognized, have been subjected to dissolution since they were deposited. centive to the study of these features and the pro- The karst cycle begins with surface drainage. At cesses which formed them. Consequently, karst in maturity there is maximum underground drainage in River, was responsible for the dissolution of large amounts of evaporites and the formation of collapse features in southeastern New Mexico. During Pleis- tocene, and probably later Tertiary, the Ancestral Pe- cos River occupied channels to the east of its present course (Fig. 10). It flowed southeasterly across the area now cut by Pierce Canyon, where it left channel deposits. It flowed across the area of Poker Lake and built terraces along the New Mexic+Texas State line as far east as Phantom Banks. Some of the collapse features south of Pierce Canyon depicted on a struc- tural map of the Rustler Formation (Hiss, 1976) re- sulted from dissolution in this stream system. The Ancestral Pecos was a high-energy stream ca- pable of carrying pebble and cobble debris as large as Z314 inches in diameter far from its source. Igneous- rock types (porphyry) completely foreign to the Del- aware Basin are present in channel and terrace de- posits in both Pierce Canyon and in the vicinity of Phantom Banks. These rock types were traced by the writer up the Pecos and Hondo drainages to their source in the Capitan and Sierra Blanca Mountains about 100 mi northwest of Carlsbad. Thus, during some of its history theAncestra1 Pecos drainage system was far different from the modern river. Today the Pecos River is sluggish, at places it flows underground, and its erosive action is largely corrosional. The load it carries is mostly solutes de- rived from the dissolution of evaporites. Sediments, except for flocculents, settle into sinks and other dis- solution cavities in the river bed and probably are not being transported out of the evaporite karst area. The Ancestral Pecos River was an extant drainage FIGURE 9-View across collapse sink in Rustler gypsum, Nash system during middle Pleistocene, at least 600,000 Draw, eastern Eddy County, New Mexico, ahowing joint control. y.B.P. This age is indicated in Pierce Canyon and ad- jacent areas, where ancestral-river gravels are inter- bedded with sediments in the Pleistocene Gatuna caverns, sink holes, and blind valleys. At times during Formation. In Pierce Canyon the uppermost ances- the cycle dissolution fronts move laterally as perched tral-river gravels are at the top of the Gatuna, which ground-water tables in these subterranean systems. indicates only that middle Pleistocene is a minimum The cycle of dissolution is completed when all soluble age for these gravels. The maximum age of the Gatuna rocks are removed and drainage returns to the sur- Formation is not known, and it is probable that the face. The only indications of past karst processes may ancestral Pecos system was initiated in late Tertiary be remnants of insoluble residues, brecciation, and time, when erosion of the eastern slope of the Sac- disruption of intervals in the soluble portion of the rament-Sierra Blanca-Capitan uplift began. stratigraphic column. The results of this cycle can be Although the Ancestral Pecos River had a tremen- observed in some ancient deposits in southeastern dous carrying power, it also began a new episode of New Mexico. dissolution of evaporites. At first it was a through- flowing river. Then, at least as early as by middle Pleistocene, it formed a karst plain southeasterly from The Ancestral Pecos River the latitude of Malaga. In Pierce Canyon a lenticular channel gravel 780 ft across and 78 ft thick collapsed Few studies of paleokarst have been undertaken into a sink (Figs. 11, 12). The Magenta Dolomite is and most of these have been limited to discussions also collapsed into a chaotic structure in the upper of cave fillings in carbonate rocks such as those which reaches of Pierce Canyon. At the mouth of Pierce mark unconformities at some stratigraphic horizons. Canyon, near the channel of the modern Pecos River, Only recently the study of paleokarst has attracted the rim of an extensive sediment-filled collapse sink the attention of geologists involved in problems of is defined by steeply dipping beds in the Gatuna For- nuclear waste disposal. Studies in western Texas have mation. Dissolution on the karst plain initiated Poker linked regional dissolution of salt to Miocene-Pli- Lake as a collapse sink. ocene (Ogallala) stream systems (Gustavson et al., Some filled sinks described by Maley and Huffing- 1980). These ancient stream systems were responsible ton (1953) were created by the Ancestral Pecos River. for dissolving evaporites to form collapse sinks which They stated that "the thickness of fill should represent were later filled with sediments. the depth of the former stream valley" (Maley and A similar ancient river system, the Ancestral Pecos Huffington, 1953, p. 543). This observation would be

because it is part of the hydrologic investigation of the WIPP site (Mercer and Orr, 1977; Mercer, 1983). The "brine aquifer" has been monitored in three drill holes (designated H-1, H-2c, H-3; Mercer and Orr, 1977, pp. 56-70) in the immediate vicinity of the WIPP site. Water gains access to the system through fractures in recharge areas as far north as Bear Grass Draw (T. 18 S., R. 30 E.) about 30 mi north of Malaga Bend, and probably in Clayton Basin and Nash Draw (Mercer, 1983, pp. 49-53) where it flows across salt beds before reaching the observation wells. By the time it reaches these observation wells, it is near sat- uration and dissolution is minimal. In the vicinity of the WIPP site, brine in this aquifer flows westerly from the site and towards Nash Draw (Mercer and FIGURE 11-Collapsed channel gravel in upper part of Gatuna Gonzales, 1981, p. 128; Mercer, 1983, p. 51). Fom~ationin Pierce Canyon, Eddy County, New Mexico. Channel is 780 ft across and 78 ft thick. The Mesraleru caliche engulfs the Although salt beds in the Rustler Formation have upper bed of the channel. been dissolved in Nash Draw, resulting in chaotic stratigraphy to the west of the WIPP site (Mercer, 1983, p. 45, fig. 5), "east of Nash Draw where the relevant if the valley had been cut entirely by erosion. Rustler Formation is covered and protected by the However, karst valleys may dissolve their course far Dewey Lake Red Beds and younger rocks, structures below the level of surface drainage. Repeated epi- are less chaotic and the Rustler attains a more regular sodes of dissolution and collapse followed by filling character" (Mercer, 1983, p. 45). In the three obser- of the collapsed feature are capable of creating deep, vation wells at the WlPP site, salt is intact in the Rus- sediment-filled sumps beneath, and adjacent to, the tler Formation overlying the brine aquifer. However, river bed. it is apparent that dissolution of adjacent salt beds Salt has been dissolved from the basal part of the would occur if a steady supply of unsaturated water Salado Formation in a belt to the east of the Pecos was introduced into the system. The brine aquifer is River and southward from the latitude of Malaga and an example of a hydrologic system which has under- Pierce Canyon. This belt coincides with the course of cut a formation. the Ancestral Pecos River (Fig. 10, Fig. 13 in pocket). The relative impermeability of an anhydrite ex- Along this belt dissolution has removed beds in the plains the persistent zone of dissolution in the lower stratigraphic section to varying horizons within the part of the Salado Formation in the belt east of the formation. Residue, breccia, and collapsed beds of Pecos River. During Pleistocene time, when water anhydrite are the only remnants of the dissolution was more plentiful, it penetrated fractures along the process. At most places dissolution has occurred from western side of the Delaware Basin and upstream the top of the Castile to the 137 marker bed in the along the Pecos River. The water percolated down- Salado and at the base of the Rustler Formation. Salt ward to the impermeable anhydrite at the top of the beds with their intervening anhydrite marker beds are recognized" and well vreserved in the Salado For- mation above the 137 horizon (Fig. 7). However, at places along this belt salt beds throughout the Salado have been dissolved (Fig. 13 in pocket). The remain- ing beds are a lumble of residue and breccia with marker anhydrite beds collapsed into an indistin- guishable mass. The selective dissolution in the Salado between the

available to a through-flowing system of underground water, which was perched on the Castile anhydrite. This dissolution front undercut beds in the Salado immediately overlying the Castile. An analogous hy- drologic situation is present in the northern part of the Delaware Basin, in the vicinity of the WlPP site. There a hydrologic system-the so-called "brine aqui- fer"-is present at the contact between Salado salt and the overlying Rustler Formation (Mercer, 1983, p. 47). Brine is perched on a thin, clayey residue at the top of the Salado Formation. It flows laterally at various rates. This aquifer has been studied inten- sively for many years,-first because it transports con- FIGURE ~~-cIos~-u~"iew in channel deposit shown in taminating brine to the Pecos River, and secondly Figure 11. Gravel is cemented by lime carbonate. Castile Formation, where it was perched and could Dissolution along the eastern margin only move laterally. The halite beds in the Salado became vulnerable to dissolution. Perched water ta- of the Delaware Basin bits are common in karst regions, and it is not nec- The Capitan reef is a major aquifer around the mar- essary to explain this dissolution by the rather complex gin of the Delaware Basin, and has been influential "brine density flow" mechanism proposed by Ander- in the process of dissolution along the eastern margin scan and Kirkland (1980). of the basin. An area of thick Cenozoic fill has been The Castile anhydrite has been an effective barrier outlined near the New MexiceTexas State line on the to dissolution. At places where Halite I11 is present east side of the basin (Maley and Huffington, 1953, beneath the dissolution zone in the Salado, it has pl. 1). However, the major portion of this fill is to the remained untouched by dissolution. The more deeply south, in Winkler County, Texas, where salt has been buried Halites 1 and I1 are likewise intact. Contrary dissolved from the Salado Formation directly above to the conclusion of Anderson et al. (1978, p. 52), the reef. Salt was probably never deposited there in based on the situation farther south in Texas, that "all the Castile Formation. Drill holes adjacent to the reef western salt-bed margins are dissolutional rather than in Lea County, New Mexico, do not indicate that dis- depositional," Castile salt-bed margins in the western solution has altered the stratigraphic section either part of the basin in New Mexico are depositional. To above, or adjacent to, the reef. th~enorth, in the vicinity of the WIPP site, close ex- Farther north in Lea County, New Mexico, the San amination of cores indicates that there is no solution Simon sink is a prominent collapse feature that over- breccia in the Castile anhydrite in the projected in- lies the reef. This sink has been drilled as part of the terval of Halite 111. It can only be concluded that Halite background studies for the WIPP site. Unfortunately, 111 was never deposited in that area. it was drilled only to a depth of about 800 ft. Cenozoic Since Pleistocene time, the Pecos River has mi- sand and clay comprise the upper 545 ft and rest on grated to the west of its ancient course. It is now collapsed Triassic red beds. As the roots of this col- entrenched in a series of karst features which include lapse sink were not penetrated, it is impossible to collapse sinks and caves. At places in Eddy County interpret the mechanism by which the sink formed, the river flows primarily underground. or the horizon in the subsurface which has been dis- Although the Mescalero caliche did not form on an solved to allow the sink to collapse. absolute plane, a comparison was made of elevations The San Simon sink is extremely restricted geo- on the Mescalero at places where it has engulfed an- graphically. Exploratory drill holes for oil and gas in cestral stream deposits. For example, the elevation of surrounding areas (including the large San Simon Swale ancient river deposits near Carlsbad is 3,170 ft, whereas depression) show no dissolution of underlying eva- a Pierce Canyon their elevation is 3,050 ft. This in- porite~and indicate only limited areas of Cenozoic dicates a gradient of about 5.2 ft per mile between fill. For these reasons the San Simon sink cannot be C:arlsbad and Pierce Canyon, which corresponds to compared with the broad area of dissolution along the gradient of the modern river. However, the ele- the east side of the Pecos River south of Pierce Can- vation of terrace and overbank deposits at Phantom yon. It is more readily compared with the breccia Banks is 3,180 ft, 10 ft higher than at Carlsbad and pipes which overlie the Capitan reef on the northern 130 ft higher than at Pierce Canyon. This suggests margin of the Delaware Basin (Bachman, 1980, pp. that the area in the vicinity of Pierce Canyon and as 61-73; Snyder and Gard, 1982). These features are also far north as Carlsbad has subsided since modern en- restricted to relatively small chimneys, which may trenchment of the river. Some areas in the vicinity of have collapsed into a cavern in the Capitan reef itself Fierce Canyon may remain undermined and continue (Snyder and Gard, 1982, p.62). They represent the to subside. In fact, some features along the Pecos core of a feature reminiscent of the present-day San River which superficially resemble oxbows (cutoff Simon sink. The San Simon sink itself is presumed to meanders) are actually controlled by relatively recent be an actively collapsing breccia pipe. The thick Ce- collapse of the surface. However, the modern en- nozoic fill along the eastern margin of the Delaware trenchment of the Pecos River insures that dissolution Basin in Texas may represent localities where the karst on the scale of that in the vicinity of Pierce Canyon process of breccia chimney formation has been carried during the Pleistocene has ceased. to maturity. Conclusions Since the end of Permian (Ochoan) time, evaporites Formation during Cenozoic time. Salt beds in the Cas- in the Delaware Basin in New Mexico have had many tile Formation in the northern part of the Delaware exposures to atmospheric waters which have dis- Basin have not been subject to the dissolution pro- solved some of the more soluble portions of these cess. Dissolution on the east side of the Delaware rocks. However, major dissolution has been restricted Basin in New Mexico has been limited to pipe-like to areas where river systems have initiated karst sys- collapse sinks which overlie the Capitan aquifer. tems, or to limited areas which overlie the Capitan Anderson (1982) regards the Waste Isolation Pilot reef aquifer. There is no evidence for deep dissolution Plant site as an area predisposed to deep-seated dis- in the Castile Formation in the central part of the solution. He concludes that salt is missing "from the basin. horizon of the repository, at localities not too distant An Ancestral Pecos River system developed during from the site, and by a largely unknown dissolution Cenozoic time along the western side of the Delaware process." The present work indicates that dissolution Basin. This ancient river system was responsible for along the western margin of the Delaware Basin was the development of an extensive karst terrain along by karst processes which were active during late Ter- the east side of the Pecos River in southeastern Eddy tiary or Pleistocene time under hydrologic conditions County, New Mexico, and southward into Loving radically different from those existing today. The so- County, Texas. At that time salt beds of the Salado called "deep dissolution" was nothing more than an Formation were dissolved and removed completely advanced state of solution and fill (Lee, 1925; Bach- at some places. At other places the dissolution was man, 1980). The Tertiary and Pleistocene hydrologic selective, restricted mostly to the basal part of the conditions no longer exist except along the present Salado Formation. All the dissolution in the ancient Pecos channel, and the probability of further disso- Pecos River system was the result of a dissolution lution in the proximity of the repository horizon is front perched on the upper anhydrite in the Castile remote.

Selected conversion factors*

Length Pressure, stress inches, in cmt,mctcrs, cm Ib ln~'(= lbhn'), pi, kg Cm ( = kfi'"'') ieet, it meters, m Ib in ' atmorphrrcr. atm yards. yds ... Ib in newtons (Njlm:, N m ? statute mdrs, mi k,lomctcrs. krn atm kg cm ' fathoms m atm mm of Hg (at CF C) angstroms. A cm inches of Hg (at 0" C) kg cm A micmmctcrs, ,Lm ban. h kg Cm b dynes cm cm? b atm m2 b ""g"pasc"ls. Mi% ml Density km' Ib ln ' (= lblm') m2 Viscosity dCWS hectares ha p"1sC" Ill gr cm ' wc ' or dynci crn : Volume (wet and dry) Discharge in' US. gal mi" ', gpm 6.308 x 10 ' I sec ft' RPm 6 30R x I0 ' rn'src-I yde' ft' iec ' 2 832 n 10 ? m' icr fluid ounces Hydraulic conductivity auam US. gal day ' it ' US. gallons, gal Permeability US. gal darclrs acreit Transmissivity barrels (oil), bbl US gal day' it ' 1.438 x 10 ' m' hec ' Weight, mass US gal mu'it ' 2 072 x 10 ' 1 src ' m ' ounces avoirdupm, avdp 2 839 x 11)' grams, gr Magnetic field intensity my ounces, or 3 1103 x 10' gr gdusses 1.1) x 10' gammas ~ounds,Ib 4536 x It! ' kilograms. kg Enerav, heal long tons 1016 metrlc tons. ",I ~nt;~hthermal units. BTU 2.52 n 10 I C~IO~~CS,C~I short tons 9.(!78 x It!-' ml BTU 1.0758 x 10' khgrrm-mctcrs, kgm "I mtr 3 43 x 111' parts per million, ppm BTU Ib~' 5.56 x 10 ' cal kg ' Velocity Temperature ft sec-' (= ftlsec) 3 1148 x 10 ' m src ' (= m rec) 'C + 273 1.11 OK (Krlwn) mi hr-' 16093 km hr~ "C + 17.78 1 H "F (Fahrenheit) mi hi ' 4 470 x li! ' m src ' ~F - 32 59 'C (Crlslua)

'Divide by the factor number to revrrpe conversions. Exponents: for example 4.047 x 10 (see acres) = 4,047; 9.29 x lo-? (see ft') = (1.0929.

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FIGURE 5-N-S cross section showing local interfingering of anhydrite-halite facies in Castile Formation and interfingering of Salado Halite facies with Castile Anhydrite.

FIGURE 6-E-W cross section showing lenticular bodies of halite in upper part of Castile Formation.

FIGURE 13Stratigraphic cross sections showing relationship of dissolution to course of Ancestral Pecos River.