The American As.sociation of Petroleum OeolosiJts Bulletin 'V. 73, No. II (November 1989), P.I307· 132S, II Fias., llllblc

Stratigraphy and Depositional Environment of Lower San Andres Formation in Subsurface and Equivalent Outcrops: Chaves, Lincoln, and Roosevelt Counties, New Mexico1

L.A . ELLIO~andJ . K . VVARllEN 3

ABSTRACT dolomite/anhydrite transition, and the accumulation and preservation of thick subaqueous evaporites north of This paper establishes correlations from subsurface to the study area. outcrop and develops a depositional model for the lower Sao Andres Formation in New Mexico. Five cyclic carbonate-evaporite sequences, numbered consecutively INTRODUCTION upward as units 1, 2, 3, 4, and 5, were identified from measurement of approximately 1, 750 ft (534 m) of sec­ The middle Permian San Andres Formation is a regres­ tion in the Hondo Canyon area of Lincoln County and sive carbonate-evaporite sequence deposited on broad examination of approximately 400 ft (122 m) of core platforms and shelves in the Permian basin (Figure 1). In from six wells in the Levelland-Slaughter trend of Chaves the Northwestern shelf province of southeast New Mex­ and Roosevelt Counties. An idealized sequence consists ico and west Texas, the cyclic lower member is a major of open-marine, restricted subtidal, intertidal, and hydrocarbon reservoir. Producing fields on the shelf are supratidal facies capped by anhydrite (in the subsurface) concentrated along an east-west line known as the or an evaporite dissolution zone (in outcrop). Lateral Levelland-Slaughter trend (Figure 2A), which is named facies changes in outcrop indicate that the Pedernal after the two largest fields in Cochran and Hockley uplift provided a locus for emergent conditi~n s at low Counties, Texas. In this trend, hydrocarbons are trapped relative sea level stands through the deposition of unit 3. where porous dolomites pinch out updip into nonporous Units 4 and 5 are incomplete, highly variable sequences. facies. With this type of stratigraphic/diagenetic trap, Lateral facies changes are not present in outcrop, indica­ detailed knowledge of environmental patterns is essential ting that the Pedernal uplift exerted little influence on the to generate successful exploration and development deposition of these units. strategies. Preserved anhydrite cores capping P 1 and P 1 sequences The Levelland-Slaughter trend projects into outcrop (equivalent to units 4 and 5) range up to 15 ft (5 m) thick, west of Roswell, New Mexico, due to a regional eastward are relatively pure, commonly lack underlying intertidal/ dip (Figure 2A). The proximity of outcrop to the produc­ supratidal carbonates, and contain some gypsum ghosts, ing trend provides a unique opportunity to study the indicating that part of the anhydrite was deposited sub­ lower San Andres Formation using both field and sub­ aqueously. We present a depositional model involving surface data. The purpose of this paper is to (1) establish both subaqueous (salina) and subaerial (sabkha) deposi­ regional correlations from the subsurface to equivalent tion of the anhydrite. The model accounts for the purity outcrops in the Hondo Canyon area of eastern Lincoln and thickness of the evaporite caps, the rarity of sub­ County, (2) describe facies from both outcrop exposures aerial exposure features, variations in the nature of the and subsurface cores, and (3) develop a regional model of deposition for the lower San Andres Formation in south­ east New Mexico. Seven detailed sections totaling @Copyright 1989. The Amencan Associahon of Petroleum Geologists. All approximately I, 750 ft (534 m) were measured (Figure rights reserved. 1Manuscript received, April4, 1988; accepted, July 6, 1989. 2B). Approximately 400ft (122m) of core from six wells 2Laura E. Tillman, Chevron U.S.A. Inc., P.O. Box 599, Denver, Colorado within the trend were also examined (Figure 2A, Thble 1) 80201 . ~ational Centre for Petroleum Geology and Geophysics, G.P.O. Box 498, to enhance understanding of the depositional and dia­ Adelaide. South Australia 5001 . genetic features observed during outcrop study and to This paper is part of a Master's thesis written by the primary author and relate these features to production characteristics. accepted by the University of Texas at Austin, Austin, Texas. Arco Exploration and Production and Tenneco Oil Company funded the study. We thank Much has been written on the San Andres Formation. Sneider Exploration, the New MexiCO Bureau of Mines and M1neral Resources, Subsurface studies include Dunlap (1967), Gratton and Dan Engles of Tenneco Oil Company, and Jack Ahlen for allowing us to study cores In their possession. Well logs were obtained with permission from the Lemay (1969), Chuber and Pusey (1972), Thdd (1976), Permian Association, Roswell New Mexico. Phelps Anderson, owner, and Jay Pitt and Scott (1981), Bein and Land (1982, 1983), Posey, foreman of the Diamond A Cattle Company, granted us access to the Ramondetta (1982a, b), Kumar and Foster {1982), Hov­ field area. Mike Tillman, Rick Otto, and Shiner were invaluable field assistants. George Martinez, Glenda Jackson. and Mark Sarene drafted and revised ~9- orka (1983, 1987), Ward et al (1986), Bebout and Harris ures In their spare time. S. D. Hovorka, A. L. Folk, D. G. Bebout, P. M. Hams. ( 1986), Cowan and Harris ( 1986), Fracasso and Hovorka J. F. Sarg, A. Evans, F. E. Kottlowskl, and A. F. Broadhead critically reviewed (1986), Fisher and Hovorka (1987), Harris and Stoudt this wortt at various stages and we are grateful for their many helpful com· ments. (1988), and Major et al (1988). Most outcrop studies of

1307 1308 Stratigraphy and Depositional Environment, Lower San Andres Formation

\ <-.\ ~- the Early Permian (Wolfcampian), the complex of --,.&.Y basins, platforms, and shelves collectively known as the c. J-:- Permian basin were fully developed (Figure I) (Galley, ' ' l I 1958; Hills, 1984). The Eastern, Northern and North­ western shelves formed a broad horseshoe around the northern Delaware and Midland basins. The Palo Duro l basin, although very shallow, was subsiding and accumu- ""'LO DURO BASIN . lating sediment throughout the Permian. The Matador arch, Roosevelt uplift, and Pedernal uplift were buried ARc~ positive features that influenced sedimentation during San Andres deposition (Milner, 1974; Cowan and Harris, 1986). During San Andres deposition, the Northwestern shelf was a very broad extensive shelf area and the site of car­ I bonate deposition. Shelf sediments were characterized by I repetitive and laterally extensive transgressive and regres- sive packages. These repeated sequences commonly are ~-- explained by glacial-eustatic sea level fluctuations and ' changing subsidence rates (Jacka et al, 1967; Silver and '" Todd, 1969; Meissner, 1972; Wanless, 1972; Fracasso and Hovorka, 1986), although tectonics and climatic changes also may have been contributing factors. Subsidence was occurring at rates to permit the shelf to remain shallow, and to permit the development of a low-angle (5° or less) shelf margin with no abrupt break in slope (Sarg and Lehmann, 1986). This shelf margin trended northeast­ southwest through the present-day Eddy and Lea Coun­ ties south and east of the study area (Silver and Todd, 1969).

Figure 1-Major geologic features of Permian basin, west Texas and southeast New Mexico (after Fracasso and Hovorka, SAN ANDRES STRATIGRAPHY 1986; Silver and Todd, 1969; Broadhead, 1987). Location of study area is shown as shaded hachure area. The San Andres Formation is approximately 1,200 ft (366m) thick and includes all the beds between the top of the San Andres Formation have concentrated on the the Yeso Formation (Leonardian) and the base of the exposures in the Guadalupe Mountain area south of the Grayburg Formation (middle Guadalupian). In the study study area. Boyd (1958) and Hayes (1959, 1964) were the area, the lowermost San Andres carbonate interfingers earliest workers to document the general stratigraphic with three tongues of the Glorieta Sandstone, which is framework of the San Andres shelf and shelf-edge depos­ considered part of the San Andres Formation (Kelley, its. Other more recent outcrop studies in the Guadalupe 1971; Milner, 1974). The intertonguing San Andres/ Mountain region have concentrated on the genesis of Glorieta section was interpreted by Milner (1974) as shal­ depositional facies and correlation of shelf and basin­ low marine and tidal flat deposits, and are not included in edge strata using sequence stratigraphy and biostrati­ this study. Emphasis is placed instead on the carbonates graphic markers (Sarg and Lehmann, 1986; Hinrichs et above the uppermost tongue of the Glorieta Sandstone. al, 1986; Wilde, 1986). The only two significant outcrop studies previously made in the study area are the strati­ graphic and structural work of Kelley ( 1971) and the low­ Outcrop Stratigraphy ermost San Andres environmental study of Milner (1974, 1976). Kelley (1971) first divided the outcropping San Andres Formation of southeast New Mexico into formal mem­ bers: the lower, thicker bedded Rio Bonito Member; the GEOLO(;IC HISTORY middle, thinner bedded Bonney Canyon Member; and the upper Fourmile Draw Member (Figure 3). The Four­ The Permian basin is so-named for paleogeographic mile Draw Member is composed of thinly interbedded features that existed dunng the Permian Period. The dolomites, evaporites, and sandstones, which are poorly major tectonic events that shaped the basin began during exposed and characterized by karst topography in the the Carboniferous and were associated with the conti­ Hondo Canyon area. The contact between the Fourmile nental collision between Laurasia and Gondwanaland, Draw Member and the more resistant lower members is and the resultant formation of the Ouachita-Marathon topographically defined by a break in slope between the foldbelt (Walper, 1977; Keller et al, 1980; Hills, 1984). By canyon walls and the gently rolling peneplaned surface L.A. Elliott and J. K. Warren 1309

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Figure 2-(A) Location map showing relationship of Levelland-Slaughter trend to outcrop belt. Hondo Canyon study area is out­ lined by box in San Andres outcrop. Numbered circles indicate locations of cores examined in tbis study. See Table 1 for additional core information. (B) Enlargement of Hondo Canyon study area showing location of measured sections.

above. The Fourmile Draw Member becomes increas­ exposure are abundant in these southern outcrops (e.g., ingly more evaporitic north of the Hondo Canyon, and pisolites, fenestral fabric, oversteepened algal lamina­ exposures are rare due to evaporite dissolution. South of tion). The evaporitic member appears to be laterally the Hondo Canyon, the member becomes increasingly equivalent to the oolitic upper San Andres Formation, dolomitic. Features commonly associated with subaerial described by Sarg and Lehmann (1986) in the Guadalupe 1310 Stratigraphy and Depositional Environment, lower San Andres Formation

Table 1. Core Data

Well and Field Name Location• Depth (ft) (I) Depco Oil Chaves County 8 Rose Federal Sec. 20, T5S, R25E Pecos Slope Field 1,980 ft FSL, 1,680 ft FWL 1,006-1,066

(2) Ralph Nix Oil Chaves County I Cherry Sec. 22, TIOS, R27E Diablo Field 330 ft FSL, 330ft FWL 2,097-2,128

(3) Stevens Oil Chaves County 7 Citgo State Sec. 36, T8S, R28E TWin Lakes Field 1,650 ft FNL, 1,650 ft FEL 2,596-2,655

(4) Santa Rita Exploration Chaves County 18-4 Moonshine Sec. 31, T9S, R29E 2,684-2,693 TWin Lakes Field 330ft FNL, 330ft FWL 2,723-2,741

(5) Tenneco Oil Roosevelt County I Lee Carter Sec. 22, T8S, R35E 4,701-4,733 Wildcat 660 ft FNL, 660ft FWL 4,737-4,809

(6)Dalport Oil Roosevelt County I Tenneco-Federal Sec. 18, TIS, R37E Wildcat 660 ft FNL, 660ft FWL 4,323-4,405 •FSL • from south line, FWL = from west line, FNL • from north line, FEL • from east line.

Mountains, which represents shelf margin deposition cycle (150ft or 46 m) and is capped by an evaporite disso­ following a relative drop in sea level. lution zone. Units 4 and 5 are 35 to 65-ft (11 to 20-m) In contrast to the recessive Fourmile Draw Member, thick dolomite sequences capped by dissolution zones. the Rio Bonito and Bonney Canyon Members crop out as Kelley's (1971) contact between the Rio Bonito and the resistant cliffs in the arroyo bends and along the sloping Bonney Canyon Members represents the contact between canyon walls of the Hondo Canyon. Kelley (1971, p. 10) two depositional cycles, thereby accounting for the con­ differentiated the two members, which are gradational, tact's lateral continuity in the Hondo Canyon. The con­ solely on the basis of bed thicknesses and defined the con­ tact is placed at the boundary between the evaporite tact between the two members as the "base of the first dissolution zone of unit 3 and the basal dolomite of unit prominent steepening below the gentle upland." The 4. The environmental significance of these members will Bonney Canyon Member is Jess resistant to erosion than be discussed in a following section. the Rio Bonito Member and is characterized by thinner beds of both resistant and nonresistant, sometimes cav­ ernous, carbonate strata. The Bonney Canyon Member Subsurface Stratigraphy is identified on aerial photos by a banded pattern of light and dark beds. The color contrasts are caused by differ­ In the subsurface, the San Andres Formation is ences in the permeability and amount of vegetative cover divided informally into an upper and lower member. The between strata. The Rio Bonito and Bonney Canyon contact between the two is marked by a regionally corre­ Members appear to grade southward into the lower and latable siltstone bed {2-10ft or 0.6-3 m thick) called the pi middle San Andres Formation of Sarg and Lehmann marker (Figure 5). It is recognized on logs by a sharp kick {1986) in the Guadalupe Mountain area. Here, the in the gamma curve (Dunlap, 1967; Cowan and Harris, sequence is composed of fusulinid/skeletal carbonates 1986). The upper member of the San Andres Formation and mudstones interpreted as shelf-edge and outer shelf is approximately 400-600 ft ( 122- 183 m) thick and is com­ environments, respectively. posed of thinly interbedded dolomites, evaporites, and In this study, measured sections of the Rio Bonito and siliciclastics (Dunlap, 1967; Gratton and LeMay, 1969). Bonney Canyon Members indicate that five upward­ The lower member is approximately 600-800 ft {183- shoaling depositional sequences are present. They have 244 m) thick and is composed of cyclic carbonate and been numbered consecutively as units 1-5 from the base evaporite units (Gratton and LeMay, 1969; Pitt and upward (Figures 3, 4). Unit 1 is an approximately 15ft (5 Scott, 1981; Hovorka, 1983; Cowan and Harris, 1986). In the m) thick, poorly defined dolomite sequence. Units 2 and Levelland-Slaughter trend, these cyclic units are domi­ 3 are well-developed upward-shoaling sequences consist­ nated by carbonates showing an overall regressive char­ ing of both limestone and dolomite. Unit 3 is the thickest acter with younger cycles becoming more evaporitic L.A. Elliott and J . K. Warren 1311

suggests that the transgression rate was rapid relative to the sedimentation rate. Therefore, the bases of these sequences have been assumed to be able to serve as approximate time lines for regional correlation (Milner, 1974; Fracasso and Hovorka, 1986). No other synchro­ nous units or biostratigraphic evidence has been used to

support this assumption. The four porous zones, P 1 through P 4 identified by Gratton and LeMay (1969) in Charles County, New Mexico, appear to correlate north­ ward with zones P, through P5 of Pitt and Scott (1981). Pitt and Scott (198 1) were apparently able to identify two smaller cyclic sequences within one of Gratton and

LEGEND LeMay's (1969) porosity zones. Cowan and Harris (1986) ~limestone recognized eight porous units in Cochran and Hockley ~..L~ Dolostone Counties of Texas. Cowan and Harris' (1986) zones 1 ··.: ::: Dissolution Zone through 3 seem to lie above the five porous units defined • Chert Nodules by Pitt and Scott (1981). To avoid confusion, the termi­ Anhydrite Nodules nology of Gratton and LeMay (1969) has been used r Fracturing throughout this paper because it is in closest proximity to ...0 our study area. North of the Levelland-Slaughter trend, z 0 the cyclic sequences become increasingly more evapo­ til 0 ritic. In the Palo Duro basin of the Texas panhandle, Fra­ ~ casso and Hovorka ( 1986) described five thick carbonate/anhydrite/halite sequences in the lower San Andres Formation. Well-log correlations indicate that these evaporite-dominated cycles show great lateral con­ tinuity in an east-west direction. Individual units can be correlated regionally over considerable distances from the Palo Duro basin westward into Debaca County, New .. Mexico (Fracasso and Hovorka, 1986). The evaporite­ ...z dominated cycles are regionally correlative southward to :l the carbonate-dominated cycles in the Levelland­ Slaughter trend. Evaporites grade into dolomite in the

UNIT lowermost cycles (P3 and P 4), and evaporites are continu­ t ous over the Matador arch and Roosevelt uplifts in youn­

WCST EAST follow are based on analyses of measured outcrop sec­ tions and core. The facies of units I, 2, and 3 are described primarily from outcrop exposures because we did not have an opportunity to examine core from this section. Unit 1 is poorly defined and consists of restricted-marine subtidal, intertidal, and supratidal facies. Units 2 and 3 are well developed upward-shoaling sequences up to 150 ft (46 m) thick consisting of (1) an open-marine subtidal facies, (2) restricted-marine subtidal facies, and (3) inter­ tidal and supratidal facies (Figure 3). Unit 3 is capped by an anhydrite dissolution zone. The facies of units 4 and 5 were observed in both core and outcrop exposures. The units consist of a highly vari­ able sequence of restricted subtidal facies and minor intertidal and supratidal facies (Figure 3). Vertical sequences range in thickness from 35 to 65 ft (11 to 20m) and are capped by anhydrite or an equivalent dissolution zone in outcrop. An idealized vertical sequence consists OISSQ(.UTION ZONE . INT£RTIDAL· SUPRATIDAL of a thin basal mudstone and restricted-marine facies 8 RESTRICTED MARINE 0 OPEN MARIN£ characterized by an upward reduction of bioturbation Figure 4-Fence diagram showing correlations between mea­ and fossils, a decrease in burrow size, an increase in the sured outcrop sections in Hondo Canyon (see Figure 2B for amount of peloids and intraclasts, and an increase in location of sections). Westward thickening of intertidal/ mudstone or grain-supported textures. Evidence for sub­ supratidal facies at expense of subtidal facies in units 2 and 3 aerial exposure is rare. can be seen between Hondo West and Border Buckle sections. A complete gradation exists from rocks of dolomite Facies thicknesses in units 4 and 5 are nearly constant through mineralogy to those of calcite mineralogy in outcrop. study area. Sunset section is not shown because it lies strati­ The mixed mineralogy is a result of both early postdepo­ graphically below zone of interest. sitional dolomitization and postburial dedolomitization. Limestone comprises approximately 200Jo of the carbon­ crop and outcrop due to slumping, karstification, and ate section in outcrop, where it is largely confined to the poor exposure. The loss of section and absence of the pi basal open-marine facies of units 2 and 3 and dissolution marker prevent correlation between outcrops and the zones at the top of units 3-5. Dolomite comprises approx­ subsurface from the top of the San Andres downward. imately 80% of the carbonate section, where it is largely The problem of evaporite dissolution can be circum­ confined to the restricted subtidal, intertidal, and supra­ vented by using the Yeso/ Glorieta contact below the San tidal facies. Andres Formation as a datum. This contact is fairly con­ The allochems, textures, and structures of the rocks sistent. Our correlations confirm that the lower San comprising the lower San Andres carbonates have been Andres section below P2 in the subsurface is correlative used to infer depositional environments. Carbonate tex­ to the Rio Bonito Member in outcrop; P 1 and P2 are cor­ tures were described using Dunham's (1962) classifica­ relative to the Bonney Canyon Member; and the interval tion. Fossiliferous wackestones and packstones are by above P1 is correlative lo the Fourmile Draw Member. far the most abundant textures composing about 70% of The anhydrite beds capping the cyclic units in the subsur­ the total carbonate section. Environmental inferences face are represented by dissolution zones in outcrop. have been drawn on the presence or absence of fossils, These correlations are supported by similarities in the their biotic diversity, and type. Other allochems include general facies characteristics and relative thickness of the pellets, peloids, intraclasts, and oolites. Allochems and cyclic sequences (Figure 8). The thickness from the top of textures alone are not the best indicators of depositional

P 1 to the top of the Yeso at the nearest subsurface control environment in a marginal marine environment due to point is approximately 100 ft (30 m) greater than that the characteristic funneling of marine sediments during found in outcrops. The thickness discrepancy can be storms and tides into intertidal and supratidal environ­ explained by (I) dissolution of interbedded anhydrite, (2) ments. Therefore, the presence or absence of distinctive westward thinning of the San Andres over the eastern structures, such as burrows, lamination, and desiccation edge of the Pedernal uplift, located beneath the study features, has aided in the environmental interpretation. area, and (3) modern erosion of the peneplaned surface Nodular anhydrite has been used to indicate supratidal above the arroyos. deposition since its discovery in sediment of the Arabian Gulf sabkha. However, evidence by Dean et al (1975) has shown that nodular anhydrite is not a reliable indicator CARBONATE LITHOFACIES of either the timing or manner of its emplacement. In the lower San Andres Formation, nodular anhydrite and The lower San Andres Formation in the study area nodular-shaped molds after anhydrite are pervasive and consists of three lithologies: (I) limestone, (2) dolostone, are found in subtidal facies as well as supratidal facies. and (3) anhydrite/gypsum. The facies descriptions that The lack of confinement to supratidal facies indicates L. A. Elliott and J. K. Warren 1313

GR DENSITY CITIES SERVICE I GOVERNMENT K been left untouched. Large unfragmented fossils up to 2 ft in. (5 em) in diameter are common, suggesting that most of the fossil material has not been transported long dis­ tances. A massive outcrop appearance, lack of bedding, and mottled textures without clearly definable bounda­ ries suggest that the sediment has been homogenized by burrowing organisms. Burrows are diagnostic of the sub­ tidal zones of modern marginal marine environments such as the Bahama tidal flats (Hardie and Ginsburg, 1977). The abundance of open-marine biota and biotur­ bated muddy texture indicates that this facies was depos­ ited in low-energy, open-marine conditions. The facies may represent deposition in a fringing bank or bank com­ plex (Read, 1985). Localized channel cuts filled with PI MARKER grain-supported sediment were found in some locations and suggest local crosscutting of this bank environment.

Restricted-Marine Subtidal Facies

This facies is the most common rock, occupying approximately 75% of the carbonate section examined during this study. In units 2 and 3, this facies is typically interbedded with and underlain by the open-marine u :J facies and overlain by intertidal and supratidal facies. In ~ units 4 and 5, the restricted-marine subtidal facies typi­ cally occupies most of the section. Dolomitization, and micritization and dissolution of allochems have obscured GLORIETA grain, matrix, and cement relationships to varying degrees. Anhydrite and calcite are present both as cement L ·LIMESTONE 0· DOLOMITE A· ANHVOFUTE and replacement material, which further complicates Figure 5-Type log of San Andres Formation from Levelland­ environmental interpretation. Very finely to finely crys­ Slaughter trend, Chaves County, New Mexico. Gamma-ray and talline dolomite grains acting as matrix are inferred as a bulk-density curves show good definition of anhydrite and car­ replacement of the original calcium carbonate mud, and bonate lithologies. Neutron Jogs, sample cuttings, and/ or cores therefore, were classified as mud during textural are needed to further differentiate dolomite and limestone analysis. Wispy lamination ts pervasive in this facies, lithologies. Neutron logs, sample cuttings, and/or cores are probably a secondary structure resulting from burrowing needed to further differentiate dolomite and limestone li tholo­ and microstylolitization (Fracasso and Hovorka, 1986). gies. Depth is measured depth. (After Gratton and LeMay, Where dolomite-intercrystalline and moldic porosities 1969.) are abundant, this facies is the reservoir rock in the sub­ surface. The facies is represented by six subfacies: basal that the presence of anhydrite nodules cannot be used silty laminated mudstone, fossiliferous wackestone/ exclusively for a supratidal interpretation. Descriptions packstone, oolite/pellet packstone/grainstone, intra­ of individual carbonate facies follow. clastic packstone/grainstone, peloidal/intraclastic wackestone/packstone, and sparsely fossiliferous mud­ stone. Open-Marine Subtidal Facies Basal silty laminated mudstone.-This sub facies is rel­ atively uncommon in vertical sequences, but when Fossiliferous wackestone/packstone.-This facies present it is 1-3 ft (0.3-0.9 m) thick and is found at the occupies approximately 150Jo of the carbonate section base of units 4 and 5. This subfacies is typically dark examined during this study and is primarily confined to brown to black, suggestive of deposition in a poorly oxy­ the base of units 2 and 3 in outcrop. The facies is com­ genated environment, contains up to 15% quartz silt, posed of an unsorted, diverse biotic assemblage of brach­ pyrite, and organics and has a sharp, planar contact with iopods, bryozoans, crinoids, rugose coral, and the underlying unit. Structures include parallel and ripple cephalopods, all of which require normal-marine condi­ lamination and soft sediment microfaults. The high ter­ tions for life (Figure 9A). Internal structures are typically rigenous component, dark color, lack of biota, and rarity well preserved because this facies usually is not dolomit­ in vertical sequences suggest that the subfacies represents ized or leached. Brachiopods, crinoids, and rugose coral localized, low-energy, anoxic, and sediment-starved con­ commonly are preferentially replaced or partially ditions. This subfacies may represent deposition in replaced by megaquartz, but the surrounding matrix has restricted lows on the shelf during transgressions. 1314 Stratigraphy and Depositional Environment, Lower San Andres Formation

N s SUNRAY PAN AM MIDWEST HUMBLE PAN AM NMFF BROWN MORGAN FED NMBX MARSHALL -<>- • • •

- ~

LIMESTONE II m l -.:HAVEROO IOOL CHAVESr· - , ROOSEVELT---- "f~~?~~ ~~: -~~ :'_'_'_'~: '_ '_'_':~ ~~~ -- 4 LIMESTONE 2mo i LEA 0 3.2~m

Figure 6-North-south cross section through Chaveroo field, Chaves and Roosevelt Counties, New Mexico, showing northward facies change from dolomite to anhydrite in each cycle and resulting porosity pinch-outs (P1, P2, P3, and P J. Bulk density curves are displayed. (After Gratton and LeMay, 1969.)

Fossiliferous wackestone/packstone.-The fossilifer­ Oolite/pellet packstonelgrainstone.-Oolite and pellet ous wackestones and packstones are by far the most packstones and grainstones, in combination with other abundant subfacies, composing approximately 550Jo of restricted marine subfacies, occupy approximately 20% the carbonate section observed during this study. The of the carbonate section examined during this study. The subfacies is characterized by a restricted marine assem­ occurrence of this subfacies is variable within cyclic blage composed of biota, similar to those existing today, sequences, although pellet grainstones are more common which are capable of tolerating adverse environmental in the upper portions of cycles. The oolite/ pellet conditions, especially large variations in salinity. This packstone/grainstone are characteristically light to assemblage consists of thin-shelled bivalves (or possibly medium brown, suggesting deposition in a well­ brachiopods), miliolines and other complex foramini­ oxygenated environment. San Andres oolites are well fers, dasycladacean and phylloid algae, ostracods, and sorted and well rounded, with recognizable nuclei com­ possible Girvanella-like algae (Bein and Land, 1982). posed of pellets and peloids. The thickness of the concen­ Fossils are usually small and fragmented, but poorly tric rings varies from over 50% of the diameter of the sorted mixtures of whole and broken fossils up to 1.6 in. oolite to a superficial coating or rind around other (4 em) in size were also found (Figure 9B). Peloids and grains. In some oolites, the concentric structure has been intraclasts are also common. Structures include completely lost due to dolomitization or earlier recrystal­ Cruziana-type burrows, bioturbation, and Jag deposits. lization, but the oolites can be identified by their size In most rocks, bioturbation is suggested only by mottled (0.01 to 0.04 in. or 0.25 to 1.0 mm), degree of roundness, textures without clearly definable boundaries. Cruziana­ and association with better preserved oolites. A former type burrows are clearly definable in other rocks and are isopachous rim cement (now dolomite) typically sur­ similar to those described by Milner (1974, 1976) (Figure rounds the allochems. Zones of plastically deformed or 9C). The burrows range from 0.4 to 1.6 in. (1 to 4 em) in "squashed ooids," similar to those described by Bein and diameter, are oriented oblique or parallel to stratifica­ Land (1982) were observed adjacent to nodular-shaped, tion, and sometimes exhibit concentric or u-shaped inter­ well-cemented oolite zones in core (Figure 90). Bein and nal lamination. They are commonly in filled with coarser Land (1982) suggested that saline interstitial waters sediment and are similar to modern Callianassa shrimp increased the plasticity of the oolites and retarded lithifi­ burrows (Shinn et al, 1969; Garrett, 1977), which are cation. Compaction subsequently resulted in their defor­ found in quiet, shallow, subtidal environments of the mation. Pellets in the packstone/grainstone facies are Bahamas. The abundance of this subfacies, presence of a uniform in shape and range in size from 0.002 to 0.005 in. restricted-marine biota and Cruziana-type burrows, and (0.04 to 0.12 mm) in diameter and are interpreted as fecal lack of subaerial exposure features suggest that this sub­ pellets. Fossils and intraclasts are also present. Structures facies was deposited in a widespread, low-energy, include burrows, bioturbation, and, less commonly, rip­ restricted subtidal environment. ple lamination. The well-sorted, grain-supported texture L. A. Elliott and J. K. Warren 1315

w 5~ E (1: 6km ft 2]60m w > YATES PETROLEUM (1: EUREKA "uK" STATE (/) 0 Sec.20, T105, R26E u 0 0 w 0.. 2mi TOP 3km SAN ANDRES

COMPOS ITE OUTCROP WAL LACE OtL AND GAS WALLACE OIL AND GAS SECTION I PAYTON I BRITT T11S, R19E Sec.8, T11 S, R2.5E Sec.3, T115, R25E T11S, R20 E

PI MARKER

TOP P 1 FOURMILE DRAW

UNIT 2

TOP GLORIETA GLORIETA SS UNIT I TONGUES

YESO YESO

GR DENS/NEUT GR DENS/NEUT GR DENS/NEUT

Figure 7- West-east cross section of San Andres Formation from outcrop to subsurface showing correlations. Note abrupt thin­ ning of upper member and loss of pi marker near Pecos River. Datum is top of Yeso Formation. and relative uncommonness of this subfacies indicate both subaerial and subtidal environments were eroded, deposition in localized, high-energy environments. The mixed together, and redeposited in a subtidal environ- typical absence of preserved current structure suggests that the environment was susceptible to periods of quies­ cence during which burrowers could destroy depositional 60m1 layering. This subfacies probably represents localized 96>m CIT!£ S SERVICE I GOVERNMENT K subtidal shoals. Subsequent bioturbation in a subtidal S

(~ 0.02 in. or ~ 0.6 mm) show a vague pelleted or gru­ 0 0 meleuse (Cayeax, 1935) structure and irregular bounda­ ries. Flat-pebble intraclasts with laminated structures are • Anhydnte present in some samples. These intraclasts are believed to oz 0•S.$01a~hon zone D Ootom•te form by the fragmentation of partially Iithified sediment L L•mes tone 3600 eroded from the sea bottom and supratidal flats and redeposited (Folk, 1959). Peloids and fossils are also present. The sub facies is typically structureless, although Figure 8-Comparison of general character and relati ve thick­ a vague fining-upward texture was observed in one loca­ ness of cyclic units between outcrop and subsurface. Composite tion. The polymictic nature of the intraclasts and the lack outcrop section was compiled by authors from Hondo Canyon of subaerial exposure features suggest that the subfacies area. Cities Service 1 Government K is type log of Gratton and represents a storm deposit, during which intraclasts from LeMay (1969). Depth is measured depth. 1316 Stratigraphy and Depositional Environment, Lower San Andres Formation

A B

c

D

F

E L. A. Elliott and J. K. Warren 1317

Figure 9-(A) Polished slab of open-marine facies containing large whole brachiopods, bryozoans, and crinoid fragments (Hondo West). (B) Polished slab of restricted marine fossiliferous wackestone/ packstone facies. In this sample, fossils range from 0.008 to 0.2 in . (0.2 to 4.0 mm) in size and are mostly unidentifiable (Horseshoe Bend). (C) Outcrop showing Cruziana-type burrows in restricted marine facies. Burrows are in filled with coarse-grained fossil debris (Spring Camp). (D) Polished slab of oolitic grain­ stone facies. Note dark nodular-shaped zones and the wispy laminated central zone. In thin section, nodular-shaped zones contain well-preserved oolites, and wispy laminated zones contain squashed ooids (Oepco 8 Rose Federal, depth 1,006 ft or 307m). (E) Polished slab of paraJiel-laminated sparsely fossiliferous mudstone. Note concentration of pinpoint replacement by anhydrite and nodular anhydrite, but absence of diagnostic subaerial exposure features (Dalport 1 Tenneco-Federal, depth 4,631 ft or 1,412 m). (F) Polished slab of intertidal/supratidal mudstone and wackestone facies. Note abundant evidence for subaerial exposure, including sheet and mud cracks, fl at-pebble intraclasts, possible algal lamination and pisolites (Tenneco 1 Lee Carter, depth 4, 754 ft or 1,449 m). All depths are measured depths.

ment. Current structures may have been lost during sub­ suggest that deposition may have occurred in a hypersa­ sequent bioturbation. line subaqueous environment. Peloidal/intraclastic wackestone/packstone.-The peloidallintraclastic wackestone and packstone sub­ Intertidal/Supratidal Facies facies, in combination with other restricted-marine sub­ facies, occupies approximately 200Jo of the carbonate This facies occurs at the top of cyclic sequences and section during this study. The sub facies is most common may be underlain, overlain, or interbedded with anhy­ within the uppermost parts of units 4 and 5, and is less drite or subtidal facies. The intertidal/supratidal facies often interbedded with other units throughout the occupies approximately 10% of the total carbonate sec­ sequence. The facies consists of peloids and intraclasts, tion examined during this study. The facies is most com­ and local concentrations of fossils of a restricted marine mon in units 1-3, and is rarely present in the upper units. assemblage. The term peloids (Bathurst, 1975) is used in The facies is represented by two dominant subfacies, this study to describe micritic allochems that lack internal mudstone/wackestone and fenestrallmudstone/pisolite structure and the consistent sizes and shapes characteris­ grainstone, both of which exhibit features characteristic tic of pellets. Their origin is uncertain. They may repre­ of an emergent environment. sent poorly preserved or micritic intraclasts and/or Mudstone/wackestone.-This sub facies is character­ micritized fossils. The presence of sparse fossils of a ized by the presence of sheet and mud cracks, scour sur­ restricted-marine assemblage, the abundance of peloids, faces, small vertical burrows, and possible root traces intraclasts, and muddy textures, and lack of subaerial (Figure 9F). Poorly sorted, irregularly shaped, and flat­ exposure features suggest a low-energy, restricted sub­ pebble intraclasts up to 0.2 in (6 mm) in diameter are also tidal to lower intertidal environment of deposition. present in some locations. Other allochems typically are Sparsely fossiliferous mudstone.-The sparsely fossil­ lacking, although small fossil fragments, peloids, and iferous mudstone, in combination with other restricted pisolites are present in some locations. Modern supra­ marine subfacies, occupies approximately 20% of the tidal and upper intertidal environments commonly are carbonate section examined during this study. This sub­ penecontemporaneously cemented and, upon desicca­ facies is commonly found in the uppermost parts of tion, form mud and sheet cracks and flat-pebble breccias sequences and interbedded with fossiliferous and pel­ (Shinn et al, 1969). Crinkly and undulatory lamination, oidal wackestones and packstones. Allochems include sometimes with oversteepened sides, is also common small bivalve fragments and other unidentifiable shell within this subfacies. The lamination has been used in debris, peloids, and intraclasts. Small horizontal bur­ combination with other diagnostic features of subaerial rows and parallel lamination are present, but subaerial exposure to infer deposition in a tidal-flat environment. exposure features are absent (Figure 9E). The origin of Fenestral mudstone!pisolite grainstone.-This sub­ the parallel lamination is uncertain. In the Arabian Gulf facies is characterized by crinkly, undulatory, and paral­ sabkha, both parallel and crinkly lamination are pro­ lel lamination with subhorizonlally aligned fenestral duced by the growth of blue-green algae and entrapment vugs. Pisolites and peloids are common, but the sub­ of sediment in the upper intertidal zone (Butler et al, facies is typically devoid of other allochems. Fenestral 1982). The lamination may also represent chemical pre­ fabric is formed in the Bahamas by desiccation shrinkage cipitation in a subaqueous environment where salinities or by the production of gas bubbles in algal and storm were often too high or oxygen content was too low for an lamination (Shinn, 1968). Laminar fenestral fabric pro­ abundance of burrowing organisms. Settling from storm vides good evidence for at least a periodic emergence. or tidal floods in very shaUow standing water or in interti­ San Andres pisolites are micritic, unsorted, and faintly dal and supratidal zones can also produce parallel lami­ laminated, with no visible nuclei. Their association with nation (Shinn et al, 1969; Hardie and Ginsburg, 1977). laminar fenestral fabric indicates that they probably Therefore, the parallel lamination by itself is not a useful formed by in-situ processes occurring in an intertidal to indicator of depositional environment. The sparse fos­ supratidal setting. The presence of crinkly and undula­ sils, peloids, and intraclasts associated with small hori­ tory lamination, fenestral fabric, and pisolites suggests zontal burrows and a lack of subaerial exposure features that this facies was deposited in a tidaJ-flat environment. 1318 Stratigraphy and Depositional Environment, Lower San Andres Formation L.A. Elliott and J . K. Warren 1319

Figure 10-(A) Polished slab of vertically oriented anhydrite nodules or gypsum ghosts indicating that anhydrite was originally deposited as bottom-nucleated, upward-growing selenite crystals (Dalportl Tenneco-Federal, depth 4,340-4,341 ft or 1,322-1 ,323 m). (B) Polished slab of burrowed pellet wackestone overlain by nodular mosaic anhydrite with irregular but sharp contact. Note purity of anhydrite and lack of an intervening intertidal/supratidal facies (Lee Carter 1, depth 4,711 ft or 1,436 m). (C) Polished slab of oolite grainstone directly overlain by nodular-mosaic anhydrite with sharp contact. Note purity of anhydrite and Jack of intervening intertidal/supratidal facies (Tenneco 1 Lee Carter, depth 4,795-4,79 6 ft or 1,461-1,462 m). (D) Polished slab of inter­ layered dolomite mudstone and anhydrite in transition zone between dolomite and anhydrite lithofacies. No diagnostic subaerial exposure features can be identified. Anhydrite could be of supratidal or subaqueous origin (Dalport l Tenneco-Federal, depth 4,353 ft or 1,327 m). All depths are measured depths.

ANHYDRITE LITHOFACIES relative lack of similar depositional textures in ancient evaporites, much of the texture seems to have been com­ Anhydrite and, to a lesser degree, gypsum are found in pletely lost as the gypsum dehydrates to form nodular the subsurface at the top of cyclic sequences. They occur and massive anhydrite upon burial (Warren and Kendall, both as massive beds up to 15 ft (5 m) thick and as thin 1985). Vertically oriented anhydrite nodules pseudo­ beds I in. (3 em) thick or less, which are intercalated with morphous after selenite (gypsum ghosts) are sometimes dolomite mudstone. Due to meteoric leaching, evaporite all that remain to indicate the original texture in some minerals are rare in outcrop. However, laterally continu­ ancient sequences (Davis and Nassichuk, 1975; Loucks ous dissolution zones cap the upper three cyclic units and and Longman, 1982). Subaqueous gypsum deposited in indicate the former presence of these anhydrite units. the coastal salinas of South Australia reach thicknesses of up to 30ft (9 m) and are evaporite dominated, contain­ ing very little matrix material (Warren and Kendall , Description 1985). The second possible mechanism for the origin of anhy­ drite calls for the diagenetic growth of calcium sulfate as The anhydrite is bluish gray to white in color and com­ an early displacement and/or replacement of carbonate posed of finely feltlike laths (0.0004 in. or 0.01 mm in matrix. This mechanism is well documented from studies width); it has nodular-mosaic and laminated fabrics of the Arabian Gulf sabkha (Kinsman, 1966, 1969, 1973, (classification of Maiklem et al, 1969). The nodular­ 1976; Shearman, 1966, 1978; Butler, 1969; Butler et al, mosaic anhydrite is typically anhydrite dominated rather 1982). In the sabkha, marine-derived ground water is than matrix dominated, with less than 150Jo impurities. concentrated by evaporation in the capillary zone and Thin stringers of dark, organic-rich, or dolomitic mud­ precipitates anhydrite as a primary and/or secondary stone typically occupy interstices between nodules. Verti­ mineral after gypsum. With continued growth, the anhy­ cally oriented anhydrite nodules are present in the drite nodules eventually coalesce to form nodular-mosaic anhydrite beds in the Dalport I Tenneco-Federal core, and contorted (enterolithic) layers. Certain diagnostic and bear striking resemblance to the gypsum ghosts criteria indicate a sabkha origin in ancient sequences described from other ancient sequences (Figure lOA) (Warren and Kendall, 1985). Algal mat lamination, (Davies and Nassichuk, 1975; Dean et al, 1975; Loucks which grows within the upper intertidal zone of a sabkha and Longman, 1982). In most sequences, the contact complex, underlies the nodular anhydrite in vertical between the anhydrite and underlying dolomite is sharp sequences. Sabkha anhydrites are inherently matrix and irregular. In some cases, the transition is marked by dominated as they grow syndepositionally within a car­ up to I ft (0.3 m) of thinly bedded anhydrite intercalated bonate matrix. The ground-water regime plays an impor­ with dolomite mudstone. Underlying carbonates facies tant role in limiting the thickness of the evaporite, which are highly variable and consist of (1) burrowed pellet can grow no thicker than the capillary zone, about 3 ft (1 wackestone (Figure lOB), (2) oolite and pellet grainstones m) thick in the Arabian Gulf sabkha (Warren and Ken­ (Figure IOC), and (3) anhydritic dolomite mudstone with dall, 1985; Patterson and Kinsman, 1982). Sediment or without algal laminations, fenestrae, sheet cracks, above the capillary zone dries and is blown away to form mud cracks, or flat pebble breccias (Figure lOD). an erosional surface (Patterson and Kinsman, 1982). Features of both salina and sabkha deposition of anhy­ drite and gypsum are presen· in the lower San Andres Interpretation Formation. Their presence indicates that the two mecha­ nisms were not mutually exclusive. Features suggestive of We have considered two mechanisms for the origin of a salina origin are most abundant in units 4 and 5. These the anhydrite examined in this study. In the first mecha­ features include (1) the presence of vertically oriented nism, bedded gypsum is deposited as a primary, bottom­ anhydrite nodules (gypsum ghosts), (2) the purity of nucleated precipitate in a shallow subaqueous much of the anhydrite and the Jack of matrix domina­ environment. This mechanism operates in the modern­ tion, (3) the lack of intertidal and supratidal facies in the day coastal salinas of South Australia where bottom­ underlying carbonates in some vertical sequences, and (4) nucleated crystals of selenite gypsum and thicknesses of up to 15ft (5 m) (Figure lOA-C). Features millimeter-laminated gypsarenite are deposited in suggestive of a sabkha origin are present in all units, but upward-shoaling sequences (Warren, 1982). Based on the are best developed in units 1-3. These features include (1) 1320 Stratigraphy and Depositional Environment, Lower San Andres Formation the well-defined shoaling-upward sequences with une­ Hovorka (1986) concluded from relationships observed quivocal intertidal and supratidal facies, (2) the matrix in core that the bulk of these evaporites were deposited domination of the supratidal facies in association with subaqueously in a very shallow, extensive brine pan or concentrations of nodular molds after anhydrite, and (3) salina. Supporting evidence for this interpretation lack of extensive evaporite solution breccias at the top of includes (1) nodular anhydrite underlain by carbonates units I and 2. This dual interpretation for the anhydrite with rro evidence for subaerial exposure, (2) nodular of the lower San Andres Formation is supported by the anhydrite overlain by bedded anhydrite containing halite variability of the underlying carbonate sequences. pseudomorphs after selenite gypsum, and (3) anhydrite overlain by bedded and chevron halite with textures iden­ tical to subaqueously deposited halite in the Baja Califor­ DEPOSITIONAL ENVIRONMENT nia salinas (Shearman, 1970). Because of the broadness and shallowness of the shelf, Regional Setting subtle topographic features appear to have had a pro­ found influence on deposition. The Pedernal uplift lay The lower San Andres Formation in New Mexico is just west of the study area and trended north-south characterized by cyclic and laterally extensive deposi­ through Lincoln County (Figure 1). Milner (1974) tional packages. These repetitive sequences have com­ showed that the uplift was a buried, but positive, feature monly been explained by glacial-eustatic sea level that influenced depositional trends during uppermost fluctuations and changing subsidence rates (Jacka et al, Yeso and Glorieta deposition. Our study further suggests 1967; Silver and Todd, 1969; Meissner, I972; Wanless, that the Pedernal uplift affected deposition through unit I972; Fracasso and Hovorka, 1986), although tectonics 3 deposition (evidence for this will be discussed in the and climatic changes may have also been contributing next section). The Matador and Roosevelt uplifts, which factors. Aside from these allocyclic controls, sedimenta­ run east-west from the Texas panhandle into Roosevelt tion rates and the geometry of the shelf and shelf-margin County, New Mexico, were also buried, but positive, also influenced sedimentation during lower San Andres structural features during lower San Andres deposition deposition. The shelf margin trended northeast­ (Cowan and Harris, 1986). Cowan and Harris (I986) southwest through the present-day Eddy and Lea Coun­ described vertical stacking of stratigraphic traps south of ties, approximately 50 mi (80 km) south and east of the the Matador arch and a change from carbonate­ Hondo Canyon area (Silver and Todd, 1969). The work dominated to evaporite-dominated deposition from of Silver and Todd (1969) and Sarg and Lehmann (1986) south to north across the arch. This indicates that it was a indicated that the shelf-to-basin relief was not great dur­ major influence on deposition in Cochran and Hockley ing lower San Andres deposition. Reefs comparable to Counties in Texas. It is less clear from this study what the younger Capitan and Goat Seep reefs were not role the Roosevelt uplift might have had during deposi­ present on the shelf margin during lower San Andres tion of the lower San Andres Formation farther to the deposition. In contrast, the geometry of the shelf margin west. The trend of the Roosevelt uplift becomes more was more like that of a carbonate ramp (Ahr, 1973), with poorly defined and the stacking of porous intervals fusulinid, skeletal, and peloidal banks vaguely defining becomes less vertical westward in Chaves County (Figure the shelf margin (Sarg and Lehmann, 1986). The earliest I). San Andres banks show periods of basinward prograda­ tion within an overall retrogradational or aggradational pattern (Sarg and Lehmann, 1986). Later banks have Depositional Model more upward-shoaling and progradational characteris­ tics. Our model of deposition for the lower San Andres For­ The shelf was an extremely broad and shallow area mation in southeast New Mexico is based on vertical and that extended from the shelf margin northward into east­ lateral facies relationships. Unit 1 in the Hondo Canyon central New Mexico and the Texas panhandle. Correla­ area is a poorly defined sequence consisting primarily of tions discussed in an earlier section strongly suggest that restricted-marine and intertidal/supratidal facies. The although carbonate deposition predominated in the poor definition of this carbonate unit is probably a result study area, evaporite deposition predominated in areas of deposition of the youngest Glorieta Sandstone tongue, north of the study area. In the Palo Duro basin, north­ which lies below unit I and was interpreted by Milner east of the study area, thick and laterally extensive (1974) to be primarily of shallow-marine origin. The carbonate/anhydrite/ halite sequences have been influx of terrigenous sand apparently prevented the described in the lower San Andres Formation (Fracasso establishment of carbonate environments for a period of and Hovorka, 1986). The halite in Palo Duro unit 4 (cor­ time. Cross-bedded sandstones and oolitic grainstones in relative to unit 3 of our study) reaches thicknesses of up the Glorieta/San Andres sequence below unit 1 (Milner, to 200 ft (61 m). Individual units can be correlated I974; Elliott, 1985) indicate that high-energy shoals may regionally from the Palo Duro basin westward into have helped provide the restriction necessary to deposit DeBaca County, New Mexico (Fracasso and Hovorka, the restricted facies in unit 1. The unit is somewhat thin­ I986). Because there is such a lack of suitable recent ana­ ner than the equivalent unit in the subsurface (Figures 7, logs for evaporite deposits of this thickness and lateral 8), suggesting that the Pedernal uplift was a positive fea­ extent, their origin is a perplexing problem. Fracasso and ture on the shelf. This uplift was influencing sedimenta- L. A. Elliott and J. K. Warren 1321 tion by providing a locus for emergent carbonate mann (1986) in the Guadalupe Mountains, although no environments during relative lowstands in sea level. biostratigraphic evidence has been used to confirm this In contrast, units 2 and 3 (correlative toP4 and Ps in the idea. subsurface) are thick, well-defined, upward-shoaling The open-marine environment gradually gave way to sequences. An idealized sequence is represented by (1) an more restricted subtidal and tidal-flat environments. open-marine facies consisting of fossiliferous wacke­ This increased restriction was caused by continued sedi­ stones and packstones, (2) a restricted marine facies con­ mentation and a relative fall in sea level, resulting in pro­ sisting of a variety of subfacies, and (3) intertidal/ gradation and a southward shift of the facies tracts. The supratidal facies. Intertidal and supratidal facies in these origin of the anhydrite (now an anhydrite dissolution units thicken westward at the expense of subtidal facies, zone) that capped unit 3 is unknown. Unequivocal out­ suggesting that an offlapping relationship still existed crop evidence for either a subaqueous or subaerial origin between the lower San Andres Formation and the is lacking due to meteoric dissolution. The interbedded Pedernal uplift (Figure 4). The topographic relief in the relationship between intertidal facies and dissolution vicinity of the Pedernal uplift was still providing a locus zones in some locations suggests that some of the anhy­ for tidal-flat and, perhaps, coastal salina deposition dur­ drite formed diagenetically within a supratidal environ­ ing relative sea level lowstands, whereas surrounding ment. However, in most locations, the dissolution zone is areas north and west were characterized by shallow­ characterized by solution breccia and is directly under­ water subtidal environments (Kottlowski, 1969; Milner, lain by restricted subtidal faci es. These features suggest 1974). Lack of core control from units 1-3 limited our that the original anhydrite was of substantial thickness ability to make an environmental interpretation for the and purity, and was not necessarily associated with subsurface east of the Hondo Canyon area. Studies by supratidal environments. Therefore, a subaqueous ori­ Cowan and Harris (1986) and Harris and Stoudt (1988) gin cannot be dismissed. Both subaerial and subaqueous indicated that emergent environments existed at this time evaporite deposition could have occurred. Hypersaline near the Matador arch in Cochran and Hockley Counties ponds within sabkha complexes and more extensive of Texas. evaporative lagoons and salinas could have been sites of A relative rise in sea level initiated the deposition of subaqueous gypsum precipitation, and anhydrite was unit 2. Water depths over the Hondo Canyon area were being precipitated subaerially within the sediments of relatively shallow and deposition occurred in a low­ arid tidal flats. energy environment as evidenced by the muddy textures In the Hondo Canyon area, units 4 and 5 are character­ of the subtidal facies at the base of the unit. A grada­ ized by thinner, highly variable sequences 35-65 ft (I 1-20 tional and interbedded relationship between the open m) thick. Equivalent units, P 1 and P 2, in the subsurface and restricted marine facies in unit 2 indicates that the are also characterized by highly variable sequences. A environment was one of changing circulation patterns highly idealized vertical sequence consists of (1) a thin, and fluctuating salinity. These changes could have been basal transgressive mudstone facies, (2) a variety of brought about by fluctuations in water depth and/ or the restricted marine subtidal facies with an upward reduc­ development of restrictive shoal environments beyond tion in the amount of bioturbation and fossils, a decrease the outcrop area. in burrow size, an increase in the amount of peloids and With continued sedimentation, and a relative fall in intraclasts, and an increase in mudstone or grain­ sea level, the subtidal environments gave way to tidal-flat supported textures, and (3) anhydrite or an equivalent environments. These emergent environments at first may dissolution zone. Unequivocal intertidal/supratidal have been locally developed over the topographically facies commonly are missing. The variability of vertical highest areas on the Pedernal uplift. Eventually, supra­ facies sequences suggests that continuous subtidal, inter­ tidal environments evolved into widespread features and tidal, and supratidal environments did not prograde uni­ prograded basinward, resulting in the development of a formly basinward, but that a complex facies mosaic of well-defined upward-shoaling sequence. Restricted sub­ environments existed. The east-west facies transitions tidal facies are interbedded with intertidal/supratidal observed in outcrops of the lower three cycles are not facies near the top of unit 2, suggesting that fluctuations present in units 4 and 5. The lack of definable transitions in water depth due either to autocyclic or allocyclic con­ indicates that as deposition continued, the Pedernal trols were stiJl occurring near the end of unit 2 deposi­ uplift exerted increasingly less influence on sedimenta­ tion. tion. By this time, the Hondo Canyon area was charac­ Unit 3 represents sedimentation following the maxi­ terized by very low relief. mum sea level rise during lower San Andres deposition, During the transgressions that initiated the deposition as evidenced by the overall thickness of the unit (150ft or of units 4 and 5, energy levels on the shelf were low. 46 m) and the presence of a thick open-marine sequence These low-energy levels resulted in the deposition of ter­ at the base of the unit. The open-marine biota indicates rigenous rich mudstones. These thin low-energy deposits that water depths were sufficient to establish and main­ most likely represent deposition in restricted lows on the tain open circulation this far north on the shelf. Muddy shelf. The lack of open-marine facies in units 4 and 5 and textures indicate that deposition took place below effec­ the relative thinness of these units compared to underly­ tive wave base. The base of unit 3 in the study area may be ing units suggest that regression resulted in a southward correlative to the major drowning surface and backstep­ shift of facies tracts and that the relative rise in sea level ping of shelf margin deposits described by Sarg and Leh- was not as great as it had been in previous cycles. Skeletal 1322 Stratigraphy and Depositional Environment, Lower San Andres Formation

HYPERSALINE PONDS/TIDAL BAYS

TIDAL LOW-ENERGY OPEN M ARINE LOW-ENERGY FUSULINID/SKELETAL / BANKS

Figure 11-Biock diagram illustrating regional depositional environment during lower San Andres deposition in southeast New Mexico. Facies tracts shown here are characteristic of deposition during lowest relative sea level stands. banks, low-energy open-marine environments, and prograding as a blanket deposit from surrounding land­ oolite shoals were established farther out on the shelf ward boundaries on the shelf is not justified by the verti­ (Chuber and Pusey, 1972; Sarg and Lehmann, 1986). The cal sequences. Sabkha evaporite deposition most likely development of these shoals could have eventually pro­ occurred in a complex of environments, both diagen­ vided sufficient restrict10n for the development of an etically within supratidal sediment and subaqueously in extensive complex of lagoons and islands in the study localized hypersaline ponds and salinas. area. Alternatively, the extensiveness and shallowness of The extreme shaiJowness and extent of the shelf com­ the shelf alone may have provided enough restriction for bined with the development of more extensive supratidal the development of this complex. The broad shallow­ environments within the complex eventually led to water shelf, along with fringing banks and oolite shoals, sufficient restriction for the development of an extensive had a dampening effect on both tide and wave activity coastal salina north of the carbonate facies mosaic (Fra­ within the study area. Low-energy restricted-marine casso and Hovorka, 1986). Hypersaline carbonate facies were deposited subtidally on the shelf and within lagoons may have become increasingly more saline, the lagoons, and supratidal sedimentation and diagenetic evolving into evaporite-precipitating environments with growth of nodular anhydrite occurred on localized time. Areas within and immediately north of the study islands within the study area. area were characterized by gypsum precipitation, and During the lowest relative sea level stands, previously halite-precipitating brines existed in more distal areas. localized islands grew laterally and evolved into prograd­ The study area marks the approximate southernmost ing environments, especially in those areas where topo­ depositional limit of gypsum precipitation as evaporites graphic relief existed. Areas overlying and proximal to rapidly decrease in thickness to the south and thicken the Matador arch and Roosevelt uplift may have been into halite to the north (Figure II). particularly susceptible to this type of progradational supratidal deposition (Cowan and Harris, 1986). The lack of subaerial features in many sequences examined Implications of Model both in core and outcrop suggests that the study area was not as prone to emergence and that the supratidal islands probably remained fairly local features. The presence of In our model, deposition of carbonates and evaporites some unequivocal intertidal/supratidal facies in other was synchronous on the shelf. Due to the very low depo­ sequences is evidence that localized supratidal environ­ sitional dip on the shelf and relatively high subsidence ments did exist on the Northwestern shelf in New Mex­ rates, environments in any one locality evolved from ico. However, a continuous supratidal environment open marine to increasingly restricted. The facies tracts L. A. Elliott and J. K. Warren 1323 gradually shifted southward as relative sea level fell. This synchronous to the major drowning surface documented shift resulted in a regressive pattern, although the evapo­ by Sarg and Lehmann (1986) at the lower San Andres rite deposits themselves are primarily aggradational. shelf margin exposed in the Guadalupe Mountains. Units

This model does not provide a continuous topographic 4 and 5 in outcrop are equivalent to the P 1 and P 2 produc­ barrier on the shelf to provide restriction, to bold heavy ing zones in the Levelland-Slaughter trend. These units brines on the shelf, or to allow for the precipitation of the are thinner, highly variable sequences of restricted sub­ evaporites. Lucia (1972) calculated that the areal extent tidal and minor intertidal/supratidal dolomites and are of a restriction must be orders of magnitude (106 or capped by evaporite dissolution zones. A detailed facies greater) smaller than the areal extent of the basin before study of the anhydrite-carbonate transition is hampered evaporite deposition can occur. However, given the lack in outcrop by dissolution of the evaporites. However, of similar broad and shallow-water shelves during the sequences observed in core from the Levelland-Slaughter Holocene, it remains untested as to whether the areal trend are well preserved. A subaqueous origin for part of extent and shallowness of the Northwestern shelf alone the anhydrite is supported by (1) thicknesses up to 15 ft (5 could have provided enough restriction for deposition of m), (2) the purity of much of the anhydrite and lack of the evaporites. The formation and preservation of thick matrix domination, (3) the lack of unequivocal intertidal and complete evaporite sequences north of the study area and supratidal facies in the underlying carbonates in must have required almost continuous recharge of many vertical sequences, and (4) the presence of verti­ marine water. Recharge would be severely limited by the cally oriented anhydrite nodules. presence of an extensive emergent environment. In con­ Our model of deposition for the lower San Andres For­ trast, our model provides a mechanism for (l) the purity mation involves both subaqueous (salina) and subaerial and thickness of evaporites constituting the seal in the (sabkha) deposition of the anhydrite. In our model, sab­ Levelland-Slaughter trend, (2) the paucity of intertidal/ kha development is localized, rather than laterally exten­ supratidal facies in vertical sequences, and (3) the highly sive, within a facies mosaic of predominantly variable facies relationships in vertical sequences in units subaqueous environments. The development of supra­ 4 and 5, and (4) the accumulation and preservation of tidal environments was most likely controlled by topo­ subaqueous evaporite deposits that reach great thickness graphic relief. This model accounts for (1) the purity and (200ft or 61 m) north of the study area. This model has thickness of the evaporites, (2) the paucity of subaerial been proposed to serve as a basis for more critical and exposure features, (3) the variation in the character of the detailed examination of the lower San Andres Formation dolomite/anhydrite transition in vertical sequences, and in New Mexico and similar carbonate/evaporite depos­ (4) the accumulation and preservation of thick subaque­ its. ous evaporites north of the study area. This interpreta­ tional difference from previous San Andres studies is significant for exploration and development strategies in CONCLUSIONS the Levelland-Slaughter trend because the updip lateral change into nonporous units is the primary factor result­ ing in the trapping of hydrocarbons in the trend. The lower San Andres Formation of southeast New Mexico is a regressive carbonate-evaporite sequence deposited on the broad Northwestern shelf of the Per­ mian basin. 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