STRATIGRAPHY OF PERMIAN ROCKS (continued)
Overlying the Guadalupe series is a thick mass of strata, consisting largely of evaporites, which forms the Ochoa series.8 This series is exposed in the Gypsum Plain and Rustler Hills east of the Delaware and Guadalupe Mountains (fig. 2), and only a small thickness of its basal beds is present in the area of this report. (pl. 3).
Outcrops of the series were first described by Richardson,9 who divided the rooks of the series as now known into the Castile gypsum, the Rustler limestone, and a rather indefinite unit that he termed "the red beds of the Pecos Valley." The rocks of most of the series tend to break down readily on weathering. Because of this tendency and other circumstances, the exposures are less instructive than the records of wells that have been drilled through it. Since 1925 many wells have been drilled down the dip and east of the outcrops. Study of the records has given a much more complete idea of the series and its relations than was hitherto available.10
On the basis of this later work, the Ochoa Series is now divided, in ascending order, into the Castile, Salado, Rustler, and Dewey Lake formations. The first unit is composed mainly of anhydrite, the second contains large amounts of salt, the third has many limestone beds, and the fourth consists of red beds. In the Pinal Dome Oil Co., Means No. 1 well, Loving County, 160 miles east of the Guadalupe Mountains, the series is 4,200 feet thick. Over considerable areas, the Rustler formation is unconformable on the Salado. Adams11 interprets the relations of the Castile and Salado as unconformable. The series thus comprises two or more subcycles of sedimentation. Because of the unconformity between the Salado and Rustler, most of the Salado is missing from the outcrop in the Gypsum Plain and Rustler Hills.
The Castile formation was named by Richardson12 for Castile Spring, which issues from the Gypsum Plain east of the area studied (fig. 2). On the outcrop, the formation consists largely of gypsum, an alteration product of an original deposit of anhydrite. As originally defined it was bounded above by limestones of the Rustler formation, but subsequent drilling has indicated that a great thickness of beds wedge in east of the outcrop, between the Rustler and the highest exposed underlying beds of the Castile. These upper beds, few or none of which crop out, are of evaporite facies like those beneath, but unlike them, consist dominantly of salt rather than of anhydrite.
For a time the upper beds were classed as a member of the Castile formation,13 but later the name Salado formation was proposed for them by Lang.14 As now defined, the Castile includes those beds in the lower part of the Ochoa series that are confined in their extent to the Delaware Basin and overlap the sloping surface of the Capitan limestone along its margins. The Salado includes higher beds, which occur both in the basin and beyond its margins (compare fig. 14, C and D). As thus defined, the Castile is dominantly anhydrite-bearing and the Salado is dominantly salt-bearing, but these distinctions are not absolute. In places, the Castile contains beds of salt, and toward the south the salt of the Salado tends to give place to anhydrite.15
The Castile formation consists largely of anhydrite, which is marked throughout by thin, light and dark laminae that may be varves.16 These laminae are best seen in well cores, but appear also in all reasonably fresh exposures of the formation, even where the rock has altered to gypsum. The light-colored laminae are relatively pure anhydrite; the darker are strongly bituminous and in many places calcareous. The calcareous content increases downward, so that the basal few feet, while still characteristically laminated, consist more of calcitic limestone than of anhydrite. Limestone beds a few inches thick are interbedded at wide intervals higher up. Drill records indicate that near the center of the Delaware Basin the formation has a maximum thickness of between 1,500 and 2,000 feet.
CASTILE OF AREA OF THIS REPORT
Within the area studied, the Castile formation crops out in small patches, and only the basal beds are present (pl. 3). Some small outcrops are found in the northeast part of the area, down the dip from the Bell Canyon formation, and the formation probably underlies wide areas elsewhere that are mantled by Quaternary gravels. One exposure appears a short distance southeast of United States Highway No. 62, on the north bank of a creek half a mile northeast of bench mark 4729; two others occur near Big Canyon Draw, south and northwest of the Gray Ranch. The last named locality is less than a mile from the base of the Reef Escarpment.
The formation is exposed also in several patches west of the Delaware Mountains, near the south edge of the area studied, where it has been downdropped by faulting (sec. DD', pl. 3). It is absent in the Guadalupe Mountains.
At all these localities, 25 to 50 feet of the basal beds of the formation lie on the uppermost beds of the Bell Canyon formation. At the base are a few feet of dark gray, bituminous, very thinly laminated, calcitic limestone, which emits a strong petroliferous odor when struck. Several more beds of limestone occur higher up, where they are interbedded with laminated gypsum, probably altered from an original anhydrite. Some beds are contorted into parallel, ripplelike corrugations an inch across from crest to crest.
In places, the laminated rock gives place to cavernous, bouldery masses several feet thick, of rotten, gypsiferous limestone, containing angular fragments of laminated gypsum. This rock is probably a weathering product.
Tests were made on a specimen of laminated, gypsiferous, calcitic limestone from the basal beds of the Castile formation at the locality northeast of B. M. 4729. The chemical analysis follows:
Analysis of limestone from the basal beds of the
Insoluble residue: Dark brown, with cherty particles, and some quartz, perhaps of detrital origin.
CASTILE OF GYPSUM PLAIN
Outcrops of the Castile formation are much more extensive east of the area mapped. They constitute most of the Gypsum Plain, a broad belt that lies between the Delaware Mountains on the west and the Rustler Hills on the east (fig. 2). According to Adams,17 some outcrops of the overlying Salado formation are present at the eastern edge of the plain. "The only Salado outcrops are * * * discontinuous, poorly exposed patches of unbanded gypsum along the main drainage lines west of the Rustler Hills. * * * Along many of the divides, tongues of the Rustler still lap over onto the beveled edges of the Castile." Northwest of the Gypsum Plain, in the drainage of Black River, near the Reef Escarpment that forms the southeast side of the Guadalupe Mountains, the Castile formation is mostly covered with Quaternary gravels.
The Gypsum Plain consists of wide, grassy plains from which rise broad, domelike hills and ridges coated with crumbly, white, impure gypsum or gypsite (Reeves chalk of soil reports), which support a scanty growth of grass.18 On aerial photographs, these areas appear lighter colored than the adjacent outcrops of the Delaware Mountain group and Rustler formation, and show no bedrock structure.
Drainage in the gypsite hills is generally along shallow swales, thinly floored by alluvium. Some of the larger streams in the plain are entrenched as narrow, shallow canyons. For some miles on either side of these canyons, the tributaries are likewise entrenched as steep-sided arroyos. In parts of the plain, but not everywhere, the surface is dotted with sink holes, the smallest covering only a few acres, the largest a square mile or more. Some of the larger sinks contain intermittent lakes and receive the drainage of many square miles of surrounding area.
Rising above the plain are steep-sided gypsum buttes. Here and there are the features termed "castiles" by Adams.19 They are low mounds, haystack buttes, and castellate peaks which have a core, a few square feet to many acres in extent, of limestone and banded calcite. Adams interprets the core as resulting from localized secondary replacement of the original anhydrite and gypsum. The castiles are prominent feature on the aerial photographs and are widely distributed over the Gypsum Plain, either singly or in clusters.
One of the most remarkable peculiarities of the Gypsum Plain, as seen in aerial photographs, is a series of linear features, or long white streaks, extending across the plain in a nearly east-west direction. They are known only from the photographs, and have not been examined by me on the ground. Many of the linear features seem to be merely lines of coloration, without much surface relief, but many others form low, straight, distinct scarps, which offset the drainage, and across which roads are detoured. In places, pairs of scarps a quarter or half a mile apart face each other, the lower ground between being floored with alluvium. Where the linear features are most numerous and closely spaced, they impart a trellis pattern to the drainage.
The linear features begin on the west at the base of the formation or top of the Bell Canyon formation, but do not extend into the Bell Canyon formation (pl. 21). They extend eastward about halfway across the Gypsum Plain and fade out in the outcrops of the upper part of the Castile formation before the outcrops of the Rustler formation are reached. Most single features are two or three miles long, but some are 10 miles or more long.
These superficial linear features probably resulted from differential erosion of cemented east-west fractures that have developed in the anhydrites of the Castile formation. The origin of the fractures is unknown, but they seem to be confined to the lower part of the Castile formation.
During the present investigation, outcrops in the Gypsum Plain were studied in two places outside of the area mapped, one along the county road to the Nine K Ranch, about 4 miles southeast of United States Highway No. 62, and the other along the highway not far northeast of the Texas-New Mexico line, on the southwest side of the Yeso Hills (fig. 2). The beds in this region are higher in the section than those within the area studied, and probably lie several hundred feet above the base of the formation.
A specimen typical of these higher beds was collected at the first locality, from the banks of an arroyo just west of the county road (pl. 10, B). According to R. C. Wells, of the Geological Survey, it consists entirely of gypsum, not anhydrite; nevertheless, its original stratification is still well preserved. It consists of alternating light-gray gypsum bands and dark-gray, bituminous, slightly calcareous bands, each a few millimeters thick. The dark, calcareous bands stand out in low relief on weathered surfaces. Most of the laminations are straight and parallel, but in some there is a minute crenulation not shared by beds above and below. Scattered through the rock are occasional white gypsum knots as much as a quarter of an inch across, around which the laminae are bent. In describing a well core Lang20 points out similar knots, which he interprets as "initial points of alteration of anhydrite into gypsum." Because of the complete alteration of anhydrite into gypsum in the outcrop specimens, this suggestion cannot be verified.
At the second locality, the highway, in descending southwestward from the Yeso Hills, has been cut deeply into the original surface, so there are road cuts as much as 40 feet high. In these road cuts the Castile formation is wonderfully exposed. Most of the rock consists of thinly laminated gypsum, similar to that just described, but at one place there is an interbedded, dense, gray limestone 6 inches thick. An interesting feature of this locality is the contortion of the beds, which is on a much larger scale than the crenulations noted at previous localities, and involves masses 10 to 50 feet across. Most of the beds lie horizontally or dip gently, but in places they are sharply folded, and here and there they are vertical. This contortion may be related to the linear features described above, as aerial photographs indicate that some of the linear features extend through the locality.
HIGHER FORMATIONS OF OCHOA SERIES
The formations overlying the Castile formation are not exposed in the area studied, but their character is summarized here, on the basis of published descriptions of outcrops and of drill records farther east.
The Salado may be exposed here and there in the Gypsum Plain, near the west base of the Rustler Hills, but most of it is cut out in this region by the unconformity at the base of the Rustler formation. The Salado exhibits its full thickness east of the outcrops.
The formation contains the thickest beds of salt in the west Texas Permian section. They have been referred to as the "upper" or "main" salt in many of the older reports on the region. It contains numerous potash beds, some of which are being mined east of Carlsbad, N. Mex. (fig. 1).21 There are some interbedded layers of anhydrite, and thin ones of dolomitic limestone and red beds. Some lamination is present, which is perhaps comparable to that in the underlying anhydrite of the Castile, but there are no bituminous layers. As indicated by the records of wells drilled east of the outcrops, the maximum thickness of the formation in the Delaware Basin is somewhat more than 2,000 feet. In the shelf areas, north and east of the basin, it is 1,000 feet or less.
Overlying the Salado formation, in places unconformably, is the Rustler formation, which crops out in the low Rustler Hills (fig. 2). On the outcrop, it consists of dolomitic limestones, containing a few poorly preserved fossils, underlain by sandstone and chert pebble conglomerate. Eastward beneath the surface, the dolomitic limestone is overlain by anhydrite, red beds, and salt, which constitute an upper member of the formation. Here, its maximum thickness is nearly 400 feet. The Rustler formation contains the uppermost evaporites in the Permian section. Like the Salado, it was deposited in both the Delaware Basin and the shelf areas beyond.
DEWEY LAKE RED BEDS
Overlying the Rustler formation east of its outcrop are a few hundred feet of red beds, part of which are classed as of Permian and part of Triassic age. According to Adams22 "most of the red beds of the Delaware Basin previously classed as Permian belong in the Triassic Pierce Canyon formation. Uppermost Permian red beds, present in a few localities, are assigned to the Dewey Lake formation."
The Dewey Lake red beds were named by Page and Adams23 and have their type section in the Midland Basin (fig. 3). The Dewey Lake consists of unfossiliferous fine-grained, orange-red sandstones and silts, many of which are cemented by anhydrite. They have a thickness of 250 to 350 feet and "are separated by an unconformity that is commonly marked by a zone of bleaching." In the Delaware Basin, according to Adams,24 "the formation is limited to the structurally low areas along the east and south edges * * *, and no outcrops are known. Apparently pre-Triassic erosion stripped the unconsolidated red beds from the surface of all the higher exposed areas, leaving a Rustler pavement."
In southeastern New Mexico, east of the Pecos River, are many outcrops of red beds that overlie the Rustler formation. They have been penetrated in nearby wells. The beds are termed the Pierce Canyon formation by Lang.25 They occupy the same stratigraphic position as the Dewey Lake red beds and have been correlated with it by some geologists. Lang, however, considers the Pierce Canyon to be unconformable on the underlying Rustler and conformable with the main mass of the Dockum group above, and hence of Triassic age, an interpretation with which Adams26 agrees.
Within the area of this report, and in the Guadalupe Mountains in general, Permian rocks belonging to the Guadalupe and Ochoa series are overlain by Quaternary gravel deposits, and no intervening formations are known. On pages 104-105 and 140 it is deduced that the Permian rocks of the region were at one time peneplaned, and then covered unconformably by Cretaceous rocks. All trace of these Cretaceous rocks has since been removed by erosion in the Guadalupe Mountains, but remnants of the Cretaceous still lie on the Permian farther south in the Rustler Hills and the Apache Mountains (for location, see fig. 1).
East of the Pecos River, older Mesozoic deposits intervene between the Permian and the Cretaceous. They form the Dockum group, of which the Pierce Canyon red beds are classed as the basal unit. The group contains terrestrial fossils in places27 and is classed as of Upper (?) Triassic age by the Geological Survey. As indicated by the work of Page and Adams,28 the Dockum apparently lies unconformably on the Dewey Lake red beds.
FOSSILS AND AGE
The Ochoa series is nearly unfossiliferous, probably because the waters in which it was deposited were so saturated with dissolved salts that little or no life could exist in them.
The Castile formation contains no fossils, but there is evidence that life existed somewhere in the vicinity during its deposition. The calcareous laminae, intercalated with its anhydrite laminae, are bituminous and this material was doubtless derived from marine plants or animals. They were perhaps forms that swam or floated near the surface of the body of water in which the Castile was deposited, where the concentration of dissolved salts was less than at the bottom; or, the bituminous material may have been swept in from areas of marine water farther south which were poorly connected with the area of Castile deposition.
In the Rustler formation, higher in the section, a few pelecypods and plants were collected by Richardson.29 They are too poorly preserved to afford much evidence as to their age, and paleontologists who have studied them express indecision as to whether they are Paleozoic or Mesozoic forms.
Because of the lack of fossils, there is no paleontological evidence as to the age of the Ochoa series. It is of post-Guadalupe (later Permian) and pre-Dockum (Upper (?) Triassic) age, and may therefore be either late Permian or Lower to Middle Triassic. A Lower Triassic age has been suggested for it by Roth,30 but most geologists consider it to be of late Permian age, and it is so classed by the Geological Survey.
This conclusion is based mainly on physical relations, which suggest that the series is more closely bound to the underlying than to the overlying beds. In places, but not everywhere, it is separated from the underlying Guadalupe series by an unconformity, but the greatest unconformity appears to be at its top, beneath the Dockum group. Moreover, the evaporites, dolomites, and red beds of the series are very similar to the sediments of the Guadalupe and older Permian series in the shelf areas. The deposits in both the Ochoa and older series were apparently laid down under water, in areas that were intermittently connected with the sea. The Dockum group above is likewise of red-bed facies, but it seems to be entirely a terrestrial deposit, laid down on river food plains and in lakes. No evaporites like those in the underlying rocks are known in it.
CONDITIONS OF DEPOSITION
During the Ochoa epoch, the west Texas region was dominantly an area of evaporite deposition, although marine conditions probably persisted farther south. The thickest and most extensive evaporite deposits of the west Texas Permian were laid down at this time. Conditions of deposition during the epoch, especially those concerned with the origin of the evaporites, have been discussed in a number of previous publications.31 Details of the discussions in these papers need not be repeated here, but a description of some of the broader paleogeographic features of the epoch, which were not adequately treated in some of the papers cited is worth while. Especial attention is given to features of the environment under which the first formation, the Castile, was deposited.
BEGINNING OF OCHOA TIME
In the Delaware Basin, the sandstones of the Bell Canyon formation, of Guadalupe age, are succeeded by the anhydrites of the Castile formation, of Ochoa age. The change in sedimentation from the one to the other is one of the most abrupt and striking in the west Texas region, and takes place in an apparently conformable sequence, within a few inches of beds. Marine conditions in the basin then came to an end, and with them, the abundant invertebrate life of the adjacent Capitan reef.
This change has been discussed by Kroenlein,32 who states that "continued excess of evaporation lowered the surface water level and associated reef environment to a point where the accumulated brine killed life on the reef. This caused the death of the reef and closed Capitan time. Further excess of evaporation over marine inflow resulted in concentration sufficient to deposit anhydrite and marks the beginning of Castile deposition."
It seems unlikely, however, that conditions described by Kroenlein could have brought about the change in sedimentation without the aid of other factors. The effects of Kroenlein's conditions would have been gradual, whereas the change is actually abrupt. Moreover, these conditions would not have ended the deposition of sandstone in the basin, whereas deposition of sandstone did end with the beginning of evaporite deposition.
It is therefore probable that the change in sedimentation resulted not only from an excess of evaporation over inflow, but also from a shutting off of the sea from free access to the water of the area, presumably by the growth of a barrier across the southwestern entrance to the Delaware Basin (fig. 14, C and D). South of this barrier, marine conditions probably continued, and over it water still flowed gradually or periodically into the basin, where it evaporated and supplied the great quantity of anhydrite and other evaporites laid down during Castile time.
The nature and location of this barrier is uncertain. According to Adams33 "a sand dune ridge, perhaps made up of calcareous sands and protected by organic reefs, would be a logical type of barrier to shut off migration [of sea water] through such channels. Breaches could be produced by storm waves and sealed off by normal wind action." The writer34 has suggested that the barrier lay near the south end of the basin, in the vicinity of Hovey, not far northwest of the present Glass Mountains. Adams35 notes that the Castile in the Seven Heart Gap area of the southern Delaware Mountains contains a greater quantity of limestone than elsewhere, and suggests that an entrance to the basin may have existed in that vicinity.
GENERAL ENVIRONMENT OF CASTILE TIME
The primary relief on the sea floor during Castile time was inherited from Guadalupe time, when the Capitan reef was built high above the bottom of the nearby Delaware Basin. This relief may have been accentuated by still further subsidence in the basin area during Castile time. The Carlsbad limestone in the southern Guadalupe Mountains dips southeast, probably as a result of tilting toward the basin in late Permian time (pp. 85-86); similar tilting is reported near Carlsbad Cavern.36 Published cross sections of the Ochoa series in the Delaware Basin37 indicate that the Castile and Salado formations as a whole, and each of their individual members, increase in thickness toward the center of the basin. This increase may in part have been brought about by continued subsidence of the basin area.
The Castile deposits are chemical precipitates, which could be derived only from water whose concentration of dissolved salts was greater than that in the open sea. The concentration was probably caused by evaporation of water which was prevented from free communication with the sea by a barrier. Adams38 outlined the conditions of deposition of the Castile as follows:
ORIGIN OF LAMINATIONS
The laminated structure of the Castile formation is of much interest, and has been discussed by Udden39 and various subsequent authors. The laminae are structures original in the deposit. In the succeeding Salado formation there is evidence of extensive replacement and reconstitution of the originally deposited minerals, so that much of the original structure of the beds has been destroyed.40 If such processes acted on the Castile deposits, they were not extensive enough to destroy the lamination.
Adams41 suggests the following process for the formation of the laminations:
LATER PARTS OF OCHOA TIME
By the end of Castile time, the deep depression of the Delaware Basin had been largely filled by deposits, and the succeeding Salado formation was probably deposited in shallower water.43 With the filling of the basin, the area of deposition began to spread beyond the basin into the surrounding areas (fig. 14, D). This general spread was partly because of the general submergence of the region. However, Kroenlein44 states that the Salado deposits pass into a near-shore facies near the Pecos River, considerably east of the west edge of the Delaware Basin. He concludes that the area of deposition was tilted eastward near the beginning of Salado time, causing the eastern edge of the area to spread farther east beyond the margin of the Delaware Basin.
Salado time closed with a period of movement, by which the formation was tilted, uplifted, and eroded. In places along the thinned edge of the deposit, erosion cut deeply enough to remove the whole formation and lay bare the Castile or other pre-Salado beds. After this the Rustler and Dewey Lake formations were laid down. Rustler and Dewey Lake time was probably shorter than preceding Castile and Salado time, for it is not represented by as great a thickness of deposits.
The Rustler formation, which overlapped the eroded surface of the older beds, is also dominantly of evaporite facies, and its deposits were laid down over much the same area as those of the Salado. During Rustler time parts of the area were covered by dolomitic limestones. They formed under conditions that permitted the existence of an impoverished pelecypod fauna. The waters in which they were laid down were probably of more normal salinity than elsewhere. Conditions were not favorable enough for a rich invertebrate life, like that of Guadalupe time, to return to the region.
At the end of Rustler time, the supersaline waters in which the formation was deposited disappeared from the west Texas region, and whatever access there had been to the basin from the open sea came to an end. The fine-grained red clastics of the Dewey Lake red-beds were then spread over the preceding evaporite deposits. They were probably washed in from the surrounding marginal lands, most of which by this time had been worn down to low relief. Dewey Lake time was relatively brief, and when deposition ceased, the Ochoa epoch came to an end, and with it the Permian period.
Last Updated: 28-Dec-2007