Geological Survey Professional Paper 215 Geology of the Southern Guadalupe Mountains, Texas

HISTORICAL SKETCH⁸

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⁸Under this heading, only the main ideas that have been held in the past regarding the rocks of the Guadalupe Mountains can be mentioned, and not all the papers published on the area are cited. A complete list of papers on the area, with a summary of their conclusions, is given in the annotated bibliography at the end of this report.

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SHUMARD'S DISCOVERY

The first observations on the geology of the Guadalupe Mountains were published during the period of exploration that accompanied the opening up of the western country after the Mexican War, and were an outgrowth of surveys by Army engineers to determine a practicable route for a railroad to the Pacific coast. In 1854, the party of Captain John Pope laid out a route through Guadalupe Pass.⁹ In the following year, when Pope returned to the region to investigate more fully the prospects for artesian water near the route, his party included Dr. G. G. Shumard,¹⁰ a geologist who had gained experience in western explorations as a member of several previous expeditions.

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⁹Pope, John, Report of exploration of route for the Pacific Railroad near the 32nd parallel from the Red River to the Rio Grande: U. S. Pacific Railroad Exploration, 33d Cong. 2d sess., S. Doc. 78, vol. 2, pp. 1-95, 1853.

¹⁰Shumard, G. G., op. cit., pp. 278-282.

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Like Bartlett's party five years before, that of which Shumard was a member approached the mountains from the east. The foot of the Guadalupe Mountains was reached at "the canyon known as the Pinery" (Pine Spring Canyon). This he explored for about a mile, collecting fossils from the white limestone "remarkably rich in organic remains" that formed its rugged sides. Continuing farther, the party descended into Guadalupe Pass, and Shumard saw that the white limestone reposed in heavy beds upon a great thickness of flat-lying sandstones. He found that the section contained the following members in descending order (pl. 1):¹¹

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¹¹Shumard, G. G., op. cit., p. 280.

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Shumard's notes indicate that in the field he regarded the fossils collected from the white limestone and underlying rocks as of Carboniferous (Pennsylvanian) age, but his brother, B. F. Shumard,¹² who later examined the material, was impressed with its dissimilarity to the Carboniferous faunas and observed that many of its brachiopods and other forms closely resembled those of the Permian system that had been established in Europe 17 years before. Moreover, it included the genus Aulosteges "that had not been recognized in formations below the Permian."

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¹²Shumard, B. F., Notice of new fossils from the Permian strata of New Mexico and Texas: St. Louis Acad. Sci. Trans., vol. 1, pp. 290-297, 1858 [1860]; Notice of fossils from the Permian strata of Texas and New Mexico: op. cit., pp. 387-403, 1859 [1860].

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WORK OF GIRTY AND RICHARDSON

Shumard's interesting discovery received little notice for many years. There were few visitors in this region, which had become isolated in the turbulent days that followed the Civil War. Except for Tarr¹³ of the Texas Geological Survey, who made a brief trip to the mountains in 1890, the next geologists to visit and describe the region were G. H. Girty and G. B. Richardson, of the United States Geological Survey, in 1901 and 1903.

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¹³Tarr, R. S., Reconnaissance in the Guadalupe Mountains: Texas Geol. Survey Bull. 3, 1892.

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Girty's collecting trip to the mountains was brief but wonderfully fruitful. Large amounts of fossil material were obtained from the white limestone that forms the slopes of Guadalupe Peak (member 1 of Shumard's section, pl. 1), which was named the Capitan limestone by Richardson.¹⁴ Numerous fossils were collected also from the underlying dark limestone (member 2). The collections were more meager, however, from the underlying sandstone and basal black limestone (members 3 and 4), which together were named the Delaware Mountain formation by Richardson.¹⁵ In his monumental work on the Guadalupian fauna, Girty¹⁶ described the fossils obtained during this visit and those collected by Richardson and others in nearby areas. By his work he expanded Shumard's original assemblage of 54 species to 326 species without, as he says, doing full justice to the richness of the fauna.

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¹⁴Richardson, G. B., Report of a reconnaissance in trans-Pecos Texas north of the Texas and Pacific Railway: Texas Univ. Bull. 23, p. 41, 1904.

¹⁵Idem., p. 38. The Delaware Mountain formation is now classed as a group, but with the basal black limestone separated from it and placed in the Bone Spring limestone.

¹⁶Girty, G. H., The Guadalupian fauna: U. S. Geol. Survey Prof. Paper 58, 1908. The term Guadalupian as used by Girty embraces approximately the Leonard and Guadalupe series of present terminology.

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With this more extensive material before him, Girty was able to confirm Shumard's original opinion as to the unusual quality of the fauna. He was impressed with its dissimilarity to any of those in the Carboniferous of the Mid-continent region, or even elsewhere in North America. Although he emphasized the "very individual facies" of the fauna, like Shumard he found the only closely comparable fossils among those described from the Permian of Europe and Asia.¹⁷

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¹⁷Girty G. H., op. cit., p. 39.

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Richardson's reconnaissance of the northern trans-Pecos area furnished some evidence on the relations of the beds containing the Guadalupian fauna. To the east, they were overlain by unfossiliferous gypsum and red beds.¹⁸ To the west, he found an extensive limestone formation, the Hueco,¹⁹ considered by Girty to be of Pennsylvanian age, which apparently passed beneath the base of the Guadalupian succession, although the actual connection was concealed beneath the unconsolidated deposits of the Salt Basin. Some hint of an extension to the southeast of the beds of the Guadalupe Mountains was given by small fossil collections made by R. T. Hill in Glass Mountains, over a hundred miles away (fig. 1).²⁰ This was confirmed some years later by the important researches of Udden²¹ and Bose.²²

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¹⁸Richardson, G. B., op. cit., pp. 43-45.

¹⁹Richardson, G. B., op. cit., pp. 32-38.

²⁰Girty, G. H., op. cit., pp. 26-27.

²¹Udden, J. A., Notes on the geology of the Glass Mountains: Texas Univ. Bull. 1753, pp. 3-59, 1918.

²²Bose, Emil, The Permo-Carboniferous ammonoids of the Glass Mountains and their stratigraphical significance: Texas Univ. Bull. 1762, 1919.

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To the east, however, beyond the Llano Estacado, red beds and other strata quite unlike those of the Guadalupe Mountains were being assigned to the Permian by various authors, either on account of scanty marine faunas as in Kansas, or because of vertebrate remains as in central Texas. The manner in which these joined or were overlapped by the beds of the Guadalupe Mountains remained a matter for conjecture. Nearer at hand, in the mountains of New Mexico northwest of the Guadalupes, the higher Paleozoic rocks were found to be the red beds and limestones of the Manzano group.²³ Its fossils, although of later Paleozoic age, did not resemble those of the Guadalupe Mountains, and the physical relations between the two groups of strata were unknown.

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²³Lee, W. T., and Girty, G. H., The Manzano group of the Rio Grande valley, New Mexico: U. S. Geol. Survey Bull. 389, 1909.

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The well-marked lithologic units of the section in the southern Guadalupe Mountains seemed to offer no obstacles to the tracing of them into the adjoining, problematical regions, yet many stratigraphic puzzles developed as soon as the beds were followed for any distance away from their type sections. Thus, upon the completion of the Texas work, Richardson²⁴ attempted to trace them northwestward toward the area of the Manzano group and found that the massive Capitan limestone merges along the strike into thin-bedded limestone and sandstone, the limestone element finally disappearing altogether or being represented by thin, local beds. * * * Northward from Guadalupe Point, fossiliferous horizons become rare in the Capitan, and the collections * * * brought in tend to show that with the change in lithology the fauna also changes in character, so that practically nothing of the typical Guadalupian fauna is left.²⁵

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²⁴Richardson, G. B., Stratigraphy of the upper Carboniferous in west Texas and southeast New Mexico: Am. Jour. Sci., 4th ser., vol. 29, pp. 325-357, 1910. See also, Beede, J. W., The correlation of the Guadalupian and Kansas sections: Am. Jour. Sci., 4th ser., vol. 30, pp. 131-140, 1910.

²⁵Girty, G. H., The Guadalupian fauna and new stratigraphic evidence: New York Acad. Sci. Annals, vol. 19, p. 138, 1909.

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As a result of these discoveries, Girty concluded in 1909 that "the evidence is such as to demand consideration, if not adoption, of the hypothesis that the facies of the Guadalupian fauna is a regional matter, denoting not time relations, but geographic relations."²⁶

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²⁶Girty G. H., idem., p. 141.

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SEARCH FOR OIL IN THE LLANO ESTACADO

The puzzles that developed in correlating the Permian rocks of the Guadalupe Mountains, and in explaining their strange variations in facies, arose in part from the impossibility of deducing what lay beneath the surface in the extensive areas covered by younger deposits. Much light was soon shed on this question by drilling. During, the second and third decades of the century, there was a tremendous expansion in the development of petroleum resources in the southwestern United States. The Llano Estacado area, east of the Guadalupe Mountains and west of the previously discovered oil fields of central Texas, received its share of wildcat drilling. As exploration continued oil was found at many places in beds of Permian age. At about the same time, beds containing potash minerals were discovered in the higher parts of the wells,²⁷ and considerable exploration was begun for this important resource.

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²⁷Udden, J. A., Potash in the Texas Permian: Texas Univ. Bull. 17, 1915.

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When the first wells were drilled, the Paleozoic rocks beneath the Mesozoic and Tertiary cover of the plains were assumed to be warped down in a broad, gentle, and relatively simple synclinorium.²⁸ Thus, east of the plains, the Pennsylvanian and Permian strata were seen to dip westward, and on their western side the Permian strata rose again toward the Guadalupe Mountains and other ranges of the trans-Pecos region. The early drilling in the basin disclosed a sequence of red beds, salt, and anhydrite, which was interbedded below with dolomites. Deeper borings on the east side showed that these beds were underlain by Pennsylvanian rocks.²⁹ To the west, near the Pecos River, deep wells penetrated sandstones of the Delaware Mountain group beneath the salt and anhydrite beds.³⁰

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²⁸Hoots, W. H., Geology of a part of western Texas and southeastern New Mexico, with special reference to salt and potash: U. S. Geol. Survey Bull. 780, pp. 113-114, pl. 17, 1925.

²⁹Udden, J. A., The deep boring at Spur: Texas Univ. Bull. 363, pp. 74-75, 1914.

³⁰Hoots, W. H., op. cit., pl. 16.

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As drilling progressed, it was found that the sequences in different parts of the region were unlike in character, and that the configuration of the synclinorium was far from regular. Thus, in 1926, oil was discovered in the Hendrick field, Winkler County (fig. 1), about 125 miles east of the Guadalupe Mountains, in dolomite that stood high above its anticipated position. It had previously been assumed that this district lay near the axis of downwarping.³¹

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³¹Willis, Robin, Structural development and oil accumulation in Texas Permian: Am. Assoc. Petroleum Geologists Bull., vol. 13, fig. 3, p. 1039, 1929. Ackers, A. L., de Chiccis, R., and Smith, R. H., Hendrick field, Winkler County, Texas: Am. Assoc. Petroleum Geologists Bull., vol. 14, pp. 923-944, 1930.

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A short distance west of the Hendrick oil field, the oil-bearing dolomites were not encountered by the drill. Instead, after passing through salt and anhydrite, the sandstones of the Delaware Mountain group were reached at a much greater depth than the oil-bearing dolomites. East of the field, much anhydrite was interbedded with the dolomites, and no trace of the Delaware Mountain group could be found. Drilling north and south of the new field made it even clearer that the Delaware Mountain sandstones were confined to a relatively restricted area within the major synclinorium, forming a depression now known as the Delaware Basin (fig. 11).³² The higher-standing zone of dolomites that bounded the formation on the east in Winkler County was found to curve westward toward the limestones of the Guadalupe Mountains on the north and the Glass Mountains on the south.

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³²Willis, Robin, op. cit., p. 1034. Cartwright, L. D., Transverse section of Permian basin, west Texas and southeast New Mexico: Am. Assoc. Petroleum Geologists Bull., vol. 14, fig. 1, p. 971, 1930. Originally called the Delaware Mountain Basin, but the shorter term seems preferable, and has come into general use.

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What was the nature of this zone, and what was its relation to the Delaware Mountain group on the one side and to the interbedded dolomite and anhydrite on the other? For answer, the geologists who had been studying the well records turned to the outcrops in the Guadalupe Mountains, for here, lying at the surface, there seemed to be the stratigraphic analog of the oil-bearing beds in Winkler County and elsewhere.

RECENT WORK IN THE GUADALUPE MOUNTAINS

Richardson's later observations on the changes in lithologic and faunal facies in the Guadalupe Mountains were amplified by the work of Baker³³ in 1918 and of Darton and Reeside³⁴ in 1925. Baker discovered that the thick succession of sandstones of the Delaware Mountain group (member 3 of Shumard's section, pl. 1), well developed to the south, does not extend far to the north in the Guadalupe Mountains (pl. 7, A). Instead, its lower part passes out by overlap against a surface of unconformity that develops abruptly not far north of El Capitan between it and the underlying black limestone (member 4). The upper part "passes to the north into limestone only a little less massive than the overlying Capitan,"³⁵ the Goat Seep limestone of the present paper. Beyond the point where the sandstone loses both its lower and upper beds, only an inconsequential stratum of sandstone could be found in the middle of a succession of limestones.

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³³Baker, C. L., Contributions to the stratigraphy of eastern New Mexico: Am. Jour. Sci., 4th ser., vol. 49, pp. 112-117, 1920.

³⁴Darton, N. H., and Reeside, J. B., Jr., Guadalupe group: Geol. Soc. America Bull., vol. 37, pp. 413-428, 1926.

³⁵Baker, C. L., op. cit., p. 114.

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The work of the geologists who had sought an answer to subsurface problems by studying the outcrops was directed particularly to the structure of the overlying Capitan limestone (member 1 of Shumard's section, pl. 1) and its relation to adjacent beds. It was found that this formation, like the oil-bearing dolomites to the east, stands at a greater height than do the upper beds of the Delaware Mountain group to the southeast. However, along the Reef Escarpment which bounds the Guadalupe Mountains on the southeast, it was found that the Capitan comes to an abrupt end, with its beds sweeping down in great curves to interfinger with the lower-lying sandstones (as shown in sections on plate 17). Northwestward also, within a few miles, the massive limestones merge with well-bedded limestones, now called the Carlsbad limestone. Farther north, as at Rocky Arroyo in the northeastern Guadalupe Mountains, Baker³⁶ and Darton and Reeside³⁷ observed that the well-bedded limestones interfingered in turn with beds of anyhydrite. The Capitan limestone was thus found to occur only in a narrow belt that followed the northeastward trend of the Reef Escarpment, rising above contemporaneous sandstone deposits to the southeast and forming a barrier between them and the thin-bedded limestones and the anhydrites to the northwest.

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³⁶Baker, G. L., op. cit., p. 115.

³⁷Darton, N. H., and Reeside, J. B., Jr., op. cit., p. 419.

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With these stratigraphic relations in mind, many resemblances became evident between the Capitan limestone and the barrier reefs now being built by corals and other lime-secreting organisms along the coasts of tropical seas. The interpretation of the Capitan limestone as a reef deposit was announced by Lloyd³⁸ in 1929, and was followed in papers by Crandall,³⁹ and Blanchard and Davis,⁴⁰ later in the same year, as well as by Cartwright⁴¹ in 1930. The reef was assumed to extend as a curving barrier around the Delaware Basin from the Guadalupe Mountains through Winkler County to the Glass Mountains (fig. 14B).

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³⁸Lloyd, E. R. Capitan limestone and associated formations: Am. Assoc. Petroleum Geologists Bull., vol. 13, pp. 645-648, 1929.

³⁹Crandall, K. H., Permian stratigraphy of southeastern New Mexico and adjacent parts of western Texas: Am. Assoc. Petroleum Geologists Bull., vol. 13, pp. 936-937, 1929.

⁴⁰Blanchard, W. G., and Davis, M. J., Permian stratigraphy and structure of parts of southeastern New Mexico and southwestern Texas: Am. Assoc. Petroleum Geologists Bull., vol. 13, p. 980, 1929.

⁴¹Cartwright, L. D., op. cit., pp. 977-979.

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It should be noted that these conclusions although now generally accepted, and accepted in this report, were based very largely on the lithologic character of the beds and on their stratigraphic relations to one another, and that in the work done by Lloyd and his contemporaries, little study was made of the fossils. The fossils in the Capitan described by Girty included no corals such as are abundant in modern reefs, and the fauna as a whole did not seem to express a particular specialization to a reef environment. Girty, however, had described a number of massive, lime-secreting sponges from the formation, and Ruedemann,⁴² during a visit to the region in 1927, had found in it and the associated Carlsbad limestone the remains of calcareous algae. One object of the present investigation was to obtain further information on these unsettled problems.

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⁴²Ruedemann, Rudolf, cited in King, P. B., and King, A. E., The Pennsylvanian and Permian stratigraphy of the Glass Mountains: Texas Univ. Bull. 2801, p. 139, 1928. Ruedemann, Rudolf, Coralline algae, Guadalupe Mountains: Am. Assoc. Petroleum Geologists Bull., vol. 13, pp. 1079-1080, 1929.

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GENERAL FEATURES OF STRATIGRAPHY

Previous geologic studies in the Guadalupe Mountains, as summarized in the preceding section, have indicated that the strata change greatly in character from southeast to northwest across the region. In the southeast, resting on the basal limestones (member 4 of Shumard's section, pl. 1), that are now called the Bone Spring limestone, is a great thickness of sandstone—the Delaware Mountain group. Northwestward, the sandstone thins nearly to disappearance, partly by overlap of the lower beds on the upraised surface of the Bone Spring limestone, and partly by intergradation of the higher beds with different limestone masses, including those of the Capitan limestone. The Capitan itself has been shown to occupy a zone only a few miles wide, northwest of which it is replaced by thinner-bedded limestone anhydrite, and other rocks. These relations have suggested that the Capitan limestone is a reef deposit comparable to modern barrier reef deposits.

The present investigation has confirmed and amplified these observations. The complex stratigraphy of the southern Guadalupe Mountains was studied by detailed mapping, by measuring numerous stratigraphic sections, and by making fossil collections. The stratigraphic sections were spaced closely enough to trace the rock units involved through successive sections across the area.

The areal relations are shown on the geologic map, plate 3. The stratigraphic sequences in the northwest and southeast parts of the area are so different that it is necessary to explain them in two separate columns on the map. Basic stratigraphic data are also shown on the sheet of correlated stratigraphic sections, plate 6. Other basic data are presented on the structure sections through the limestone mass of the Guadalupe Mountains (pl. 17). On these structure sections only the rocks that can be seen on escarpments and canyon walls are shown, and their hypothetical underground extensions are omitted.

As shown on section K—K', plate 17, the deepest exposures—which also give the most complete idea of the stratigraphic changes—are those on the escarpments at the western side of the mountains. The other sections shown on plate 17 lie farther northeast and show only parts of the upper beds. The long stratigraphic sections shown on plate 6 were measured on this western escarpment, and the shorter sections elsewhere in the area.

These basic stratigraphic data are assembled, summarized, and interpreted on plate 7. Plate 7, A is a stratigraphic diagram extending from northwest to southeast across the area, on which the structure of the rocks of the area is shown as it is assumed to have existed at the close of Permian sedimentation. Plate 7, B is a group of similar diagrams, each for a successive stage of the Permian, which show the manner in which the structure of the rocks is assumed to have developed.

The stratigraphic features shown on plate 7, A are in a vertical plane, and therefore are two-dimensional. A part of the stratigraphic information on the area must be of this two-dimensional sort, as it is obtainable only on the west-facing escarpment of the mountains. For the higher beds, however, exposures in the canyons east of the escarpment, and in downfaulted areas west of the escarpment, are so numerous that one can express their stratigraphic features in a horizontal, as well as a vertical plane. For them, three-dimensional stratigraphic information is therefore available, This is summarized in three maps, figures 6, 8, and 10, for successive stages of the higher beds. On these maps, the boundaries of the different facies are shown by lines. Note that the information is least complete for the oldest beds (fig. 6) and most complete for the youngest (fig. 10).

PLATE 1.—EL CAPITAN FROM SOUTH. The cliff of El Capitan lies near the center, with Guadalupe Peak concealed behind it. Numbers refer to original section by Shumard. 1, White limestone (Capitan); 2, upper dark limestone (Pinery); 3, yellow sandstone (Delaware Mountain); 4, basal black limestone (Bone Spring). Letters refer to Quaternary deposits. a, Older slope deposits; b, younger slope deposits. Aerial photograph by U. S. Army Air Corps.

PLATE 4.—PANORAMIC VIEWS OF REEF ESCARPMENT ON SOUTHEAST SIDE OF GUADALUPE MOUNTAINS. For locations see plate 2. The escarpment marks the southern edge of the Capitan limestone. In front are plains cut on the Cherry Canyon and Bell Canyon formations of the Delaware Mountain group, which are mantled by Quaternary gravels. Qoa, Older alluvial deposits; Pcb, Carlsbad limestone; Pc, Capitan limestone; Pdb, Bell Canyon formation (8, Lamar limestone member, 7, flaggy limestone beds, 6, Rader limestone member, 5, Pinery limestone member, 4, Hegler limestone member); Pdc, Cherry Canyon formation (3, Manzanita limestone member, 2, South Wells limestone member). F, Fault. A, McKittrick Canyon to Pine Top Mountain. B, Rader Ridger to El Capitan. (click on image for a PDF version)

PLATE 5.—PANORAMIC VIEWS OF WESTWARD-FACING ESCARPMENT OF GUADALUPE MOUNTAINS. For locations, see plate 2. The escarpment is related to faults, whose traces extend along its base. On the escarpment nearly the whole Permian succession of the mountains is exposed in horizontal position. To the west the same rocks, tilted and downfaulted, project here and there in foothills but are in part concealed by Quaternary alluvial deposits. Qya, Younger alluvial deposits; Pcb, Carlsbad limestone; Pc, Capitan limestone; Pdb, Bell Canyon formation (6, Rader limestone member, 5, Pinery limestone member); Pg, Goat Sheep limestone; Pdc, Cherry Canyon formation (1, Getaway limestone member); Pd, Sandstone tongue of Cherry Canyon formation; Pdy, Brushy Canyon formation; Pbc, Cutoff shaly member of Bone Spring limestone, Pbv, Victorio Peak gray member, Pbl, black limestone beds. F, Fault. A, Looking north from near the fork of the Van Horn and El Paso roads. B, View west from Lewis Well. (click on image for a PDF version)

TERMINOLOGY

The complex stratigraphic relations of the Permian rocks of the Guadalupe Mountains are difficult to express in a workable scheme of terminology. Such terminology must take into account, not only the rock units, which interfinger with one another in a complex manner and are likely to be of small geographic extent, but also time units, which from place to place include dissimilar rock units of the same age. The terminology as now worked out attempts to make use of both time and rock classifications.

The first subdivision of the section into rock units was made by Richardson in 1904 and although his original names still remain, later authors have redefined them and have introduced many new ones. The newly named units are subdivisions of the original rock units, or are rock units that were not known at the time the original classification was made. Some of the more important changes that have been made since Richardson's time are indicated in the table below. The publications cited therein are by no means all that have appeared on the area; they are selected because they are representative of a particular stage in the geologic study of the mountains—a task that has been carried on by many geologists. Not all of the new names that appear in each column were proposed by the particular author cited; many have originated in contemporaneous writings of other geologists.

Most of the units listed in the following tables are of lithologic significance, and have only an incidental time value. The west Texas Permian, however, is now divided by Adams and others into the four series shown in column 5. These units are dominantly of time significance, and are applied across the region to beds of the same age, regardless of their local rock or faunal facies.

The writer's terminology in the southern Guadalupe Mountains, shown in column 4 of the table below, is given in greater detail in the table on p. 12, and diagrammatically in plate 7, A.

In the descriptions of the stratigraphy that follow, the beds are divided into five local time units. The first of these units corresponds to the Leonard series, the next three to the lower, middle, and upper parts of the Guadalupe series, and the last to the lower part of the Ochoa series. They are treated in turn, from oldest to youngest. The outcrops of each, and the lithologic changes that take place in them, are followed across the area from the southeast to the northwest.

ROCKS NOT EXPOSED

The oldest rocks exposed in the southern Guadalupe Mountains belong to the Bone Spring limestone, of Permian age. The rocks beneath it do not come to view, but they have been penetrated in two wells that have been put down in the region. Some deductions as to the character of the underlying beds can be made from the data of the wells and also from study of pre-Bone Spring rocks exposed in nearby mountain ranges.

History and general classification of the geologic terms used in the Guadalupe Mountains

Detailed classification of the formations in the Guadalupe Mountains as used in this report

The two wells are the N. B. Updike, Williams No. 1, put down with diamond-drill tools in 1921 and 1922 at a point 3 miles south of El Capitan, and the Anderson and Prichard, Borders No. 1, put down with cable tools in 1934 and 1935 at a point about 14 miles south of El Capitan. The first well started at or a little above the top of the Bone Spring limestone, and was drilled to a depth of 3,400 feet (section 47, pl. 8). The second started 590 feet below the top of the formation, and was drilled to a depth of 4,728 feet (section 48, pl. 8).⁴³

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⁴³Information on the N. B. Updike well is obtained from the driller's log, from notes taken by J. W. Beede, who visited the well when it was being drilled, and from examination by H. C. Fountain and the writer of the cores themselves, which were lying unmarked on the ground at the head of the well. Information on the Anderson and Prichard well is based on microscopic examination of cuttings by Max Littlefield and Charles Rynicker of the Gypsy Oil Co.

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ROCKS OF PENNSYLVANIAN AGE

From a depth of 3,183 to 3,400 feet in the Updike well, and a depth of 3,950 to 4,728 feet in the Anderson and Prichard well, there are black shales and dark limestones which are probably of Pennsylvanian age. That they are of this age is suggested by some fragmentary fossil evidence. In the Anderson and Prichard well, between the depths mentioned, Rynicker has identified Triticites. In the Updike well, in cores from an unknown depth, Fountain has broken out fossils, including fusulinids on which Dunbar⁴⁴ comments as follows:

Before completing the Texas volume [in 1937] we worked these small pieces of the core for all they were worth and got eleven rather well-oriented sections. * * * The two species of Triticites closely resemble two that I have from the Home Creek limestone of central Texas. The single specimen of Dunbarinella is probably juvenile. The type species of that genus was described by Thompson from the Deer Creek limestone, which would be up in the middle of the Cisco. It is possible that the large species of Triticites is the form described by Needham from the upper part of the Magdalena limestone, as Triticites ventricosus sacramentoensis. * * * In short, upon restudying this collection after a lapse of several years, I am still convinced that it presents a horizon in the Pennsylvanian, though possibly it is a little higher than the top of the Canyon.

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⁴⁴Dunbar, C. O., letter of July 1945. For an earlier statement, see Dunbar, C. O., and Skinner, J. W., Permian Fusulinidae of Texas: Texas Univ. Bull. 3701, p. 592, 1987.

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Brachiopods, pelecypods, and gastropods included in the same lot (No. 7714) were studied by G. H. Girty, who reported that "nothing in the fauna definitely points to a geologic age older than the Bone Spring limestone, and it has no affinities to the Hueco fauna." In view of the fusulinid evidence, this lot evidently contains specimens broken from cores of different depths.

The lower strata penetrated by the wells noted are thus probably of upper Pennsylvanian age. They are younger than the lower Pennsylvanian rocks which underlie the Hueco limestone in the northern Sierra Diablo, not far to the southwest.

WOLPCAMP SERIES OF CARBONIFEROUS OR PERMIAN AGE

Above the depth of 3,183 feet in the Updike well and the depth of 3,950 feet in the Anderson and Prichard well, most of the sequence consists of black limestones and shales like those forming a part of the Bone Spring limestone at the surface. However, from a depth of 2,912 to 3,183 feet in the first well, and of 3,660 to 3,950 feet in the second, there are clastic beds. In the Updike well these clastic beds consist of conglomerate composed of rounded limestone pebbles in a limestone matrix, interbedded with layers of gray limestone and black shale. In the Anderson and Prichard well, they consist of dark limestones, in which are embedded clastic fragments of quartz and feldspar. At the base are granite and porphyry pebbles as much as 4 millimeters in diameter. Despite certain dissimilarities, the clastic beds in the two wells are probably of the same age. They are probably correlatives of clastic beds exposed elsewhere in trans-Pecos Texas, which lie at the base of the Wolfcamp series, on a surface of unconformity which cuts across Pennsylvanian and older rocks.⁴⁵

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⁴⁵King, P. B., Permian stratigraphy of trans-Pecos Texas: Geol. Soc. America Bull., vol. 45, pp. 716-717, 1934.

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Further evidence that the rocks in this part of the two wells are of Wolfcamp rather than of Leonard (Bone Spring) age is afforded by the occurrence in rocks in the Anderson and Prichard well, as reported by Rynicker, of the fusulinid genus Pseudoschwagerina. This is a characteristic fossil of the Wolfcamp series. In the well, it occurs in black shale and dark limestone identical with the Bone Spring beds above and of a facies unlike that seen in rocks containing it in the Sierra Diablo and other ranges to the west. In those ranges it occurs in the gray thick-bedded Hueco limestone, the local representative of the Wolfcamp series. In the Updike well, the limestones for several hundred feet above the conglomerate are gray and thus more like the outcrops of the Hueco limestone, but no diagnostic fossils have been reported from them.