STRATIGRAPHY OF PERMIAN ROCKS (continued)
BONE SPRING LIMESTONE
The Bone Spring limestone is the oldest formation exposed in the Guadalupe and Delaware Mountains. it forms a bench of varying height along the west-facing escarpment of the mountains, which is fringed on the west by alluvial deposits or outcrops of down-faulted rocks. (For views of typical exposures see pl. 5; for map relations, pl. 3.) The formation passes beneath the surface in the southern Delaware Mountains, south of the area described, but across the Salt Basin to the southwest is extensively exposed and forms the upper three-fourths of the east-facing escarpment of the Sierra Diablo.46
The formation was named by Blanchard and Davis,47 but it had previously been recognized by both Shumard48 and Girty49 as the "basal black limestone" (member 4 of Shumard's section). The type locality is in the lower course of Bone Canyon below Bone Spring, on the west side of the Guadalupe Mountains 1 mile northwest of El Capitan, where there are characteristic exposures of several hundred feet of its upper beds.
The formation is several thousand feet thick, as shown by the sections on plate 8. On the promontory of the Delaware Mountains 18 miles south of El Capitan, 1,500 feet of beds were measured (section 49), and at a point 2 miles north of Bone Spring 1,700 feet (section 7), but at neither place is the base exposed. In the Sierra Diablo, measured sections show a combined thickness for the Bone Spring and underlying Hueco of about 3,000 feet (section 45). This agrees closely with the 3,123 feet recorded in the Updike well near El Capitan (section 47).
In the Delaware Mountains to the south, which in Permian time were a part of the Delaware Basin, the formation is evidently much thicker, for in the Anderson and Prichard well the combined thickness of Bone Spring and Hueco limestones, including the beds exposed above the top of the well, totals 4,540 feet (section 48). According to Adams,50 in this part of the section several faults may have been drilled through, as "chunks of rocks showing slickensides were bailed from the hole." Judgment must be reserved as to whether the possible faults have materially altered the amount of thickness, but they should be kept in mind as a possible source of error.
The Bone Spring is composed almost entirely of limestone beds, as contrasted with the dominantly sandy strata of the Delaware Mountain group which overlies it (plate 7, A). In the Delaware Mountains, and extending as far north as Bone Canyon, the exposed parts of the formation are black, cherty limestone in thin beds, with partings and a few members of shaly limestone and siliceous shale. North of Bone Canyon in the Guadalupe Mountains, the upper part of the black limestone is replaced by a thick-bedded gray limestone, the Victorio Peak gray member, which also forms the capping stratum of the Sierra Diablo. Between the main mass of limestones and the sandstones of the Delaware Mountain group is a small thickness of interbedded limestone and shale, which forms the Cutoff shaly member and its probable equivalents.
SEQUENCE IN THE SOUTH
South of Bone Canyon, the black limestones of the Bone Spring crop out in a bench along the west base of the mountains, forming rounded slopes of a darker color than those carved from the sandstones above. Near United States Highway No. 62 the bench is discontinuous and low, but it rises to the north and south. At the top of the bench in the Delaware Mountains south of the area studied, two cliff-making members of black limestone form steep walls, in places unscalable.
Outcrops of the Bone Spring limestone in the south part of the area are shown on the geologic map, plate 3. A part of the outcrop can be seen in the panorama, plate 5, A, fringing the base of the escarpment below El Capitan and Pine Top Mountain. Stratigraphic sections south of Bone Canyon appear on the right halves of plates 6 (numbers 15-44) and 8 (numbers 48 and 49). The two cliffs referred to form the 460-foot interval in the upper part of the formation in section 49.
BLACK LIMESTONES AND ASSOCIATED ROCKS
In the southern part of the area studied no more than the topmost 500 feet of black limestones is exposed, although more beds come to the surface farther south. These topmost beds are fine-textured, dense, black limestones, in beds a few inches to a foot or more thick. They are in part straight-bedded and in part have lumpy or undulatory bedding surfaces. Black, brown-weathering chert occurs in some of the beds as long, knobby lenses, nodules, and flat sheets. Chert is also common in the Anderson and Prichard well for more than 1,000 feet below the surface,51 suggesting that most of it is original with the deposit. The black limestones are nearly barren of fossils. The known fauna has been collected from discontinuous lenses, generally more granular than the inclosing rock. Ammonoids in some of the lenses not far north of United States Highway No. 62 are filled with free oil, which spills over the rocks when the ammonoids are broken.
The black limestone in most exposures shows no stratification between the bedding planes, but in some exposures it is marked by finer laminations. Limestones marked by closely spaced, light and dark laminae similar to varves are common lower down in the formation (pl. 10, A); they have been observed on the promontory of the Delaware Mountains 18 miles south of El Capitan, in the Sierra Diablo, and in the cores from the Updike well. Some of the limestone beds are separated by partings of shaly black limestone. The strata for several hundred feet beneath the two cliff-making members south of the area studied consist of brown, platy siliceous shale and shaly limestone.
The following analyses of black limestone from the Bone Spring limestone were made. These and subsequent analyses of carbonate rocks in this report were determined by methods described by Hillebrand.52 The only modification was that insoluble residues were caught on Jena glass filtering crucibles, and the organic insoluble determined by the Robinson53 method.
Analyses, in percent, of black limestone from the Bone Spring
Insoluble residues: 1, Dark brownish and carbonaceous, consisting of clay with finely divided quartz particles; 2, similar to No 1; 3, light brown and of fine-grained particles.
At several places, layers as much as 10 feet thick of platy, fine-grained, calcareous sandstone are interbedded with the black limestones. Two specimens of the sandstone, one from a point 2-1/2 miles south-southeast of El Capitan and the other from the mouth of Black Canyon farther south, were studied under the microscope by Ward Smith. The grains have a maximum diameter of 0.2 millimeter and lie in a calcite matrix. They consist chiefly of quartz, with some microcline and plagioclase, and a small but noteworthy amount of zircon, tourmaline, and apatite. These are the more stable minerals of igneous and metamorphic rocks.
A mile south of Bone Canyon, several thin conglomerate layers containing black limestone pebbles are interbedded in the black limestone (sec. 17, pl. 13). One of these beds locally attains a thickness of 4 feet and contains boulders several feet across of light-gray, fossiliferous limestone similar to that of the Victorio Peak gray member as developed a few miles to the north. Apparently some erosion of this contemporaneous, light-gray limestone was taking place at the time the black limestones were being deposited.
Near Bone Spring, the upper part of the black limestone contains lenticular masses of poorly bedded, gray, granular limestone as much as 50 feet thick (secs. 15a and 16a, pl. 13). One such mass exposed on the escarpment face not far south of the mouth of Bone Canyon seems to lie in a channel in the underlying black limestone. Other masses have a moundlike upper surface, against which the succeeding beds overlap. They contain the heads of massive bryozoans, and also numerous productids and other brachiopods like those in the Victorio Peak gray member nearby. At least some of these lenticular masses were small reef deposits.
STRUCTURAL FEATURES IN THE BLACK LIMESTONE
The black limestones are thinly and evenly bedded. In the vicinity of Bone Canyon and farther south, however, most of the exposures when viewed as a mass show a great irregularity of stratification, so much so that at nearby points the dip is quite different in direction and amount. This irregularity results from two types of structural features, described below.
The first type is found in the vicinity of Bone Canyon. Here, the black limestone is divided into numerous wedge-shaped and basin-shaped masses as much as 100 feet thick. The strata within each mass are parallel but the masses themselves are separated by sloping planes of contact from other masses of similar lithologic character in which the strata are differently inclined.54 In some places, gently dipping strata overlie more steeply tilted strata, and in other places the overlying strata have the steeper dips. The upper beds are generally parallel to the plane of contact beneath, and the lower beds are cleanly truncated. None of the limestones near the planes of contact is contorted, and none contains any breccia or conglomerate; the overlying limestones rest directly on the underlying. At one or two places, however, the smoothness of the contact is broken by small pockets in the underlying beds, which are filled by limestone like that above and below.
A typical exposure of such features is shown in plate 11, A, in which a pocket like that noted above can be seen on one of the surfaces. The features are shown also on the sections accompanying plate 9, especially in the enlarged sketch on the left, and in figures A and B, accompanying plate 13. The area in which they occur is shown on plate 7, A.
These features are strikingly exposed in Bone Canyon, and in Shumard Canyon,55 the next valley to the north. They are found also for somewhat more than a mile south of Bone Canyon, but are absent beyond. They are absent also north of Shumard Canyon, where the bedding planes in the black limestone are straight and parallel.
In Shumard Canyon, the lower part of the overlying thicker-bedded Victorio Peak gray member contains a few similar structural features, but the angle of divergence between the overlying and the truncated beds is less than that in the beds beneath. In this canyon, the Victorio Peak itself is truncated and overlain by basin-shaped remnants of the Cutoff shaly member (sec. CC', pl. 9).
The second type of structural feature, a remarkable contortion of the black limestone beds, is known only in the area south of El Capitan, where it can be seen in the upper layers of the black limestone, the oldest beds exposed in the district. These features have not been described in previous publications, although they may have been seen by geologists, and confused with the features of the other type near Bone Canyon. A typical exposure of this second type of feature is shown in plate 11, B, and the area in which they occur on plate 7, A.
In many places the canyons that drain across the black limestone bench cut through steep to overturned or recumbent folds, involving 10 to 20 feet of beds. Accompanying the folds are small thrust faults. In places the contorted rocks pass into masses of sheared, wrinkled, and rolled lenses of limestone. The general trend of the folds and thrusts is between east-northeast and west-northwest, but the direction of overturning is either northward or southward. Numerous furrows and slickensides of the same trend as the folds groove the bedding planes, both in the contorted rocks and in rocks not otherwise conspicuously disturbed.
Wherever they are exposed the strata beneath any set of contorted beds are little disturbed. Many of the contorted beds are truncated, and overlain by gently dipping strata. Whether the upper strata lie unconformably on the lower or have been thrust over them cannot be determined with certainty. The contortion has not modified the broader features of the strata, for toward the south the contorted beds stand in cliff-making members that can be traced continuously for long distances.
Both sets of structural features are relatively ancient, for the tilted beds, planes of contact, and thrusts are in many places cut cleanly through by vertical joints of probable Tertiary age, some of which are shown on plate 11, B. The features near Bone Canyon were interpreted by Baker56 as thrust slices. Darton and Reeside57 and later geologists, however, have regarded the truncated surfaces in this neighborhood as local unconformities, and the whole feature as a sort of gigantic cross-beddings58 formed during the time of deposition. This latter interpretation seems best to fit the facts, as the basinlike form of some of the masses and the pockets along some of the planes of contact more closely resemble sedimentary than tectonic features. Further, similar truncated surfaces higher up, which separate the Victorio Peak from the Cutoff member, seem clearly to be local unconformities. Such unconformities do not necessarily mean emergence of the sea bottom; they may have been caused by submarine currents.
The features farther south are certainly the result of some sort of deformation, but I am inclined to believe that they also were formed during or shortly after the time of Bone Spring deposition. The intensity of the contortion and the small thickness of the beds involved suggests that they were deformed under a relatively thin overburden, and that the beds retained a certain plasticity at the time of deformation. They must have been sufficiently consolidated, however, to have been grooved and slickensided. The deformation might have been caused by a sliding of one part of the newly deposited beds over another, causing the beds between to crumple.59 Some of the flat-lying beds that truncate contorted beds may have slid in this manner. (See p. 27.)
CUTOFF SHALY MEMBER
South of El Capitan, the black limestone bench is separated from the first sandstone ledges of the Brushy Canyon formation above by a slope 50 to 150 feet high, carved from shales, sandstones, and thin limestones, of which a typical exposure is shown on plate 14, B. These beds are classed as an upper member of the Bone Spring limestone, and tentatively correlated with the Cutoff shaly member of the Bone Spring, which is found in the northern part of the area studied. Near El Capitan, however, the beds thin out and disappear, so that the actual connection to the north cannot be traced. The Cutoff member of the southern area is well exposed in Brushy Canyon, not far south of United States Highway No. 62 (sec. 36, pl. 6).
The member consists of black, platy, siliceous shale and shaly sandstone, with a few intercalated sandstone beds in the upper part, and many thin beds of compact gray or black limestone. At some localities, the various constituents are very irregularly interbedded. In Brushy Canyon, one of the limestone beds develops locally into a mass 15 feet thick and contains abundant brachiopods, mollusks, and other fossils. The thinner limestones contain little else than fusulinids, and many are unfossiliferous. In some exposures, the shales contain large, spherical, cannon-ball concretions of limestone.
In the lower 25 feet of the member, and resting in places directly on the black limestones beneath, are lenticular beds of conglomerate a few feet thick, composed of round black limestone pebbles set in a calcareous matrix. The upper surface of the black limestones is not channeled, however, and the limestones interbedded in the shales above the contact are identical in appearance with those below. The top of the member is drawn at the base of the lowest prominent sandstone ledge of the Brushy Canyon formation, but this is not a definite boundary, as some similar sandstone is interbedded in the shales below, and shales and platy sandstones are interbedded in the thicker sandstones above.
SEQUENCE IN THE NORTH
Near Bone Canyon the bench of Bone Spring limestone rises to a greater height than farther south. To the north it stands in an imposing line of cliffs that rise 1,000 feet or more above the foothill ridges of downfaulted rocks that flank it on the west. About 4 miles north of Bone Canyon, these downfaulted rocks rise so high that they conceal the Bone Spring beds on the main escarpment. Toward the northwest, however, in the lower ridges near Cutoff Mountain, the formation reappears in places. It and the overlying rocks are much faulted, and some of its limestones form dip slopes that are inclined steeply westward toward the Salt Basin.
Outcrops of the Bone Spring limestone in the northern part of the area are shown on the geologic maps, plates 3 and 9. The whole outcrop in the northern part of the area also can be seen on the panorama, plate 5, B. The part of the outcrop on the main escarpment extends from below El Capitan to the right, northward past points 5738 and 6402 to below the Blue Ridge on the left, where it comes to an end. The outcrops near Cutoff Mountain appear farther to the left, and form the cuestas below point 5443 and elsewhere. Stratigraphic sections of the formation north of Bone Canyon are shown on the left half of plate 6, numbers 1 to 14.
VICTORIO PEAK GRAY MEMBER
The black limestones are exposed for only a few miles north of Bone Canyon and pass from view beyond. Most of the exposed part of the formation in this district belongs to the Victorio Peak gray member, a succession of thick-bedded, gray limestones 800 feet thick, which are the northward equivalent of the upper part of the black limestones.
The member is named for Victorio Peak,60 a high point on the Sierra Diablo escarpment southwest of the Guadalupe Mountains. A correlation of the rocks assigned to the member in the two areas seems assured, because in addition to a similarity of the faunas, the member at the northwest end of the Sierra Diablo is divisible into three parts that are identical with its three divisions in the Guadalupe Mountains. (Compare secs. 46 and 7, pl. 8.) Here, as in the Guadalupe Mountains, it rests on black limestone and is overlain by the Cutoff shaly member.
On the high ridge between Shumard and Shirttail Canyons,61 about a mile north of Bone Spring, two well-marked divisions in the member are recognized. (See sec. 10, pl. 6; for structure of the ridge, see sec. BB', pl. 9.)
The lower division, resting with gradational contact on the black limestone, consists of 350 feet of gray-brown, fine-grained, dolomitic limestone in beds several feet thick. In Shumard Canyon, many layers of thin bedded, hackly limestone are interbedded. Here erosion has carved the limestone of the division into picturesque, serrated walls and pinnacles, which are shown in the lower left-hand part of plate 12, B. The division commonly contains widely spaced, large, subspherical chert nodules, and in many beds fragmental remains of fossils. In Shirttail Canyon, several layers of light-brown, fine-grained sandstone are interbedded in the lower part.
The upper division of the Victorio Peak member on the ridge between Shumard and Shirttail Canyons is a light-gray, nondolomitic, noncherty, thick-bedded calcitic limestone 160 feet thick, which contains various productids and other brachiopods.
The following analyses of limestones from the Victorio Peak gray member were made:
Analyses, in percent, of limestones from the Victorio Peak member
Insoluble residues: 1, Dark brownish, carbonaceous, consisting of clay and finely divided quartz, some of which is perhaps authigenic; 2, dark brown, carbonaceous, with large garnet particles, some of which are well-rounded, and also red tourmaline, quartz, and chalcedony; 3, brown, with quartz, chalcedony, microcline, and coarse garnet.
The two divisions of the Victorio Peak gray member disappear south of Shumard Canyon. The lower division extends as far as a ravine between Shumard and Bone Canyons, where it intergrades abruptly with black limestone, as shown in figure A, plate 13. The upper division is cut off southward by pre-Brushy Canyon (Delaware Mountain) erosion. In the northern branches of Shumard Canyon its beds are truncated by a smooth surface, sloping 15° southeast, against which the sandstones of the Brushy Canyon formation overlap (sec. BB', pl. 9). In the southern branches the upper division extends as a rapidly thinning wedge, which is locally overlain by basin-shaped remnants of the Cutoff shaly member.
The black limestone exposed in Bone Canyon is of the same age as the lower division of the Victorio Peak member a little to the north, and the lenticular masses of gray, granular limestone which it contains are considered as outliers of the Victorio Peak deposits. No equivalent of the upper division is present here. Crude tracing of the ledges suggests, however, that black limestone beds younger than any in Bone Canyon come in beneath the Brushy Canyon formation to the south, as indicated diagrammatically on plate 7, A. They are probably equivalent to the upper division of the Victorio Peak member to the north.
North of Shirttail Canyon, the lower division of the Victorio Peak member, which is not widely exposed, is separated from the upper division by a middle division 100 feet thick of slope-making, thin-bedded, light-gray or white limestone, with much buff, fine-grained, calcareous sandstone interbedded. (Shown on secs. 5 and 7, pl. 6.) The upper division is calcitic, light gray, noncherty, and thick-bedded. (See chemical analysis No. 3, above.) Its upper layers contain numerous poorly preserved fusulinids and productid shells.
CUTOFF SHALY MEMBER IN SHUMARD CANYON
In the southern branches of Shumard Canyon, resting unconformably on both the lower and upper divisions of the Victorio Peak member, and overlain unconformably by the Brushy Canyon formation, are small remnants of poorly fossiliferous beds which are probably equivalent to the Cutoff member to the north.
Two divisions are present, separated by an unconformity. The older one, composed of thin-bedded, black, cherty limestone, is exposed at only one place, near the head of the south fork of the canyon. It lies in a steep-sided basin carved in the Victorio Peak limestone, which it fills to a thickness of 90 feet. The younger division crops out somewhat more widely in the branches of the canyon, and consists of thin-bedded black limestone, weathering to ashen-gray, hackly fragments, interbedded with platy siliceous shale. They closely resemble the limestones and shales of the Cutoff member as developed farther north. The younger division is well exposed on the ridge south of the mouth of Shumard Canyon, where it reaches a thickness of 60 feet.62
The outcrops of the two divisions of the Cutoff shaly member in Shumard Canyon are shown on the geologic map, plate 9, and their structure on the accompanying section CC'. The basin-shaped remnant of the lower division stands out prominently on the nearest ridge in the center of the panorama, plate 12, B. The lower division is included in section 12a, and the upper in section 13a of plate 6.
CUTOFF SHALY MEMBER TN NORTH PART OF AREA
In the northern part of the area studied, the Victorio Peak gray member is overlain, apparently conformably, by 230 feet of shales and limestones which crop out on slopes above the limestone cliffs. They form the Cutoff member, which is named for exposures on the west slope of Cutoff Mountain about 1,000 feet below its summit (sec. 1, pl. 6).63
The member consists of thin-bedded, dense limestone of black, buff, or gray color, weathering to dove-gray or ashen, hackly, conchoidal fragments. Some of the lower beds contain irregular masses of black chert. In the upper part, much platy black siliceous shale, brown sandy shale, and soft sandstone is interbedded. The member contains few fossils; some pelecypod imprints were seen in the upper part west of Cutoff Mountain.
About half a mile north of Shirttail Canyon, the southeastward extending outcrop of the Cutoff member comes to an end. At this place an erosion surface slopes southward across the truncated edges of the Cutoff beds, with sandstones of the Brushy Canyon formation overlapping northward against it, as shown diagrammatically on plate 7, A. To the south, the Brushy Canyon beds rest directly on the Victorio Peak member.
Correlation of the typical Cutoff shaly member of the north part of the area with the shales and limestones at the top of the Bone Spring limestone farther south is tentative because only the beds to the south contain fossils in any abundance. The rocks of the different areas are similar lithologically, however, and all are included in the Cutoff shaly member in this report.
BONE SPRING FLEXURE
A study of the region south of El Capitan reveals no unusual features near the Bone Spring-Brushy Canyon contact. The black limestones, which project as a low bench at the base of the mountains, are overlain without apparent break by the interbedded shales, limestones, and sandstones of the Cutoff member. They are followed in turn by the sandstone ledges of the Brushy Canyon formation of the Delaware Mountain group, as in section 36, plate 6. A view to the north along the western side of the mountains, however, shows that the limestone bench rises to a much greater height in this direction, without a similar rise in the overlying sandstone ledges (as shown in pl. 5, A).
At the Bone Spring-Brushy Canyon contact in Bone Canyon a few miles to the north, in the area of higher-standing limestone, the Cutoff member is not found. Instead, the upper surface of the black limestone is channeled and is overlain by coarse conglomerate, which contains fragments derived from the limestone.64 Besides these fragments the conglomerate contains cobbles and boulders of gray limestone unlike any rock exposed here or to the south. The conglomerate grades upward into typical sandstones of the Brushy Canyon formation, as shown in section 15, plate 13.
A view of the relations farther north can be had from the crest of the succeeding ridge (pl. 12, B). Looking down into Shumard Canyon, the next large drainage beyond Bone Canyon, one can see the contact of the limestone and sandstone on the walls of the tributary gorges; it rises from a position beneath the observer to one several hundred feet above him on the farther wall. On the farther wall the black limestones are overlain by gray limestones which stand in a high projecting bench. These gray limestones constitute the Victorio Peak member and are the source of the boulders to the south.65
Brown sandstone ledges of the succeeding Brushy Canyon formation can be traced along the slopes above the limestone, rising less steeply northward than the limestone-sandstone contact. One group of them in middle distance, in the north fork of Shumard Canyon, is seen to overlap abruptly against the sloping surface.
Near the point where the sandstones overlap, one can find innumerable ripple marks on their bedding surfaces, suggesting that the sandstones were laid down near a shore. The shore itself, the sloping surface of the gray limestones, is a smooth face, cut across the edges of gently tilted beds. The sandstones contain no embedded detritus derived from the shore as they do at Bone Canyon to the south. Perhaps this area stood higher on the sea bottom so that the detritus was swept away, and deposited lower down the slope, as at Bone Canyon.
North of El Capitan the Bone Spring limestone is thus flexed into a position much higher than to the south. On the north side of Shumard Canyon the limestone stands 2,000 feet higher than it does south of El Capitan, and 1,000 feet higher than it does in Bone Canyon nearby. This uplift is only mildly shared by the overlying sandstones, and seems to have been largely completed before they were laid down. The upraised limestones were being eroded in early Delaware Mountain time, and the Brushy Canyon formation of that group overlaps their sloping surface. The overlap is so great that 1,000 feet of beds, the entire Brushy Canyon formation, is cut out between Bone Canyon and a point 2 miles to the north. The fold produced by this pre-Delaware Mountain uplift is known as the Bone Spring flexure.
The feature was named by Blanchard and Davis,66 who called it the Bone Springs arch. It would seem from their paper that they considered the feature to be anticlinal, and to have a similar, opposing flank to the north. This view was contested at the time by De Ford.67 My work has failed to disclose a north flank to the feature and the term flexure is therefore used instead of arch.
A good general view of the flexure can be seen in the panorama, plate 5, B, which shows the Bone Spring limestone rising from a low position below El Capitan to a high position below Shumard Peak, beyond which the beds flatten out northward. The structure of the beds shown in this view is given in section KK', plate 17. A closer view of the exposures in Shumard Canyon is shown on plate 12, B. The relations of the overlying and underlying beds to the flexure is shown on the map and sections of plate 9, and structure contours on the upraised surface of the Bone Spring limestone on the inset of figure 6.
SOME DETAILS NEAR BONE CANYON
The broader stratigraphic relations of the Bone Spring limestone and Delaware Mountain group are clear, but near Bone and Shumard Canyons local complexities tend to obscure them and deserve further explanation.
The peculiar, cross-bedded structure of the black limestones, and the basins cut into the Victorio Peak gray member and filled by the Cutoff shaly member have already been described. To produce them, uplift and erosion must have taken place on the flexure before Bone Spring time came to an end. The conglomerates interbedded in the black limestone south of Bone Canyon, which are similar to those in the overlying Brushy Canyon formation, lend support to this idea, for they contain fragments not only of black, but also of gray limestone, and thus were not derived entirely from the break-up of the beds next beneath them. Along the unconformity below the Cuttoff member, the Victorio Peak member is deeply eroded, and the break seems more important than those in the black limestones below. In places along Shumard Canyon, this unconformity is more prominently exposed than that between the Cutoff and the sandstones above. This instance is local, however, and the general relations indicate that the younger unconformity is the major one.
The apparent trend of the Bone Spring flexure is east and west, at right angles to the northward trending outcrops, for most of the observable uplift and overlap take place in a northward direction, along the outcrop. Closer scrutiny of the rather narrow belt of outcrop, however, indicates that the actual trend of the flexure is north-northeast. The limestones on each west-projecting ridge rise higher than they do in the heads of the canyons to the east (inset, fig. 6), and a westward overlap of the overlying sandstones and conglomerates can be observed on the walls of Bone and other canyons (pl. 13, fig. B).
Overlying the conglomerates near Bone Spring is a bed of gray-brown, dolomitic limestone which closely resembles the limestones of the lower division of the Victorio Peak member, which lies at about the same altitude to the north. This forms the 28-foot interval in section 15, plate 13. It might be mistaken for a tongue of the lower division projecting into and intergrading with the sandstones of the Brushy Canyon formation were it not that on the south side of the next ravine north of Bone Canyon it can be found overlapping the similar, older, gray-brown limestones (as shown at point 14b, pl. 9, and on fig. A, pl. 13) with the unconformable contact clearly exposed. Moreover, beneath the limestone bed in Bone Canyon, the conglomerate contains fragments of the upper division of the Victorio Peak member as well as of the lower division, thus proving that the bed is much younger than the lower division.
Lloyd 68 considers that "the lower part of the sandstone series [Brushy Canyon] merges laterally with the gray limestone [Victorio Peak] just as the upper part merges into the lower part of the Capitan." His interpretation is based chiefly on the apparent relations of the limestone bed here referred to. This interpretation is not accepted in this report.
RELATIONS NORTH AND SOUTH OF FLEXURE
South of the Bone Spring flexure there appears to be a continuous, gradational sequence from the black limestones of the Bone Spring, through the shales of the Cutoff member, into the sandstones of the Brushy Canyon formation. Deposition probably was nearly continuous from one formation to the other in this region. The gray limestones of the Victorio Peak member are not present between the black limestones and the Cutoff member, but they are not believed to be missing on account of erosion; instead, during Victorio Peak time, black limestone was probably being deposited south of the flexure while the gray limestone was being deposited north of it.
North of the flexure, the unconformity between the Bone Spring limestone and the Delaware Mountain group is not evident, and the strata of the two units lie parallel. The beds next beneath the contact belong to the Cutoff shaly member of the Bone Spring, and those next above the contact to the sandstone tongue of the Cherry Canyon formation. Near the north edge of the flexure, however, the Cutoff member below has been eroded away. Also, on the flexure, a great thickness of beds older than the sandstone tongue wedge in below the Cherry Canyon formation, and constitute the Brushy Canyon formation (pl. 7, A). The absence of the latter north of the flexure indicates that a great, but nonevident break separates the Bone Spring limestone and Delaware Mountain group in that region.
Invertebrate fossils occur in various degrees of abundance in all the members of the Bone Spring limestone. In general, the faunas of all the members are similar, but there are some differences which appear to be related to differences in lithologic facies of the enclosing rocks. Considered as a whole, the fauna is closely related to that in the overlying Guadalupe series, although of slightly more primitive character. It has few resemblances to that of the underlying Hueco, and still fewer resemblances to that of the Pennsylvanian beneath the Hueco.
Some of the fossils from the black limestone beds of the formation were described by Girty69 in 1908, and the general aspect of the fauna of the Victorio Peak member was reviewed by him in 1926.70 Some brachiopods from the formation in the Delaware Mountains and the Sierra Diablo were described by King71 in 1931. The present investigation has furnished much additional information on the fauna, which is summarized below.
In this and succeeding discussions of the fossils of the Guadalupe Mountains section, information on the fusulinids is based on the work of Dunbar and Skinner,72 and that on the cephalopods on the work of Miller and Furnish.73 These studies, which to a great extent were based on collections made during the present survey, have already been published. Information on the other groups of fossils, particularly on the brachiopods, gastropods, and pelecypods, is based on the work of the late G. H. Girty, who was able to complete in manuscript a rather long summary of the collections shortly before his death in 1939. This summary, quoted in this report, is of particular value because it links the pale ontological and stratigraphic ideas of his earlier work, in 1908, with the ideas obtained by other geologists from more detailed subsequent field work and collecting. Throughout his summary, Girty makes frequent comparisons between the faunas as he knew and described them in 1908 and faunas as they are revealed by the present larger collections.
Because of the fact that this report is primarily a description of the physical stratigraphy of the southern Guadalupe Mountains, because of the large size of the available collections, and because of the preliminary nature of the ideas on many of the fossil groups, it does not seem desirable at this place to include the customary fossil lists. Instead, in the summary written by Dr. Girty, the important features of each fauna are discussed, and only incidental reference is made to specific localities. A similar plan is followed in summarizing the results of Dunbar and Skinner and of Miller and Furnish, although the actual localities of their collections have been given in their publications. Although this method of presentation has some disadvantages, it is believed to have advantages for immediate purposes that outweigh the disadvantages. It is hoped that stratigraphers and paleontologists will find use for the material as it is given.
Although the summary by Dr. Girty quoted herein was completed shortly before his death, he was unable to edit the manuscript in the manner he had contemplated; in its original state it was essentially a rough draft. In order to prepare it for publication, therefore, it was edited by P. B. King and J. S. Williams. King condensed and rearranged certain parts, so that as here given they are not exactly as written by Girty, although the original meaning and style are retained. Williams reviewed the terminology of the genera and species, which were not everywhere consistent in the several parts of the manuscript. Where discrepancies were found an attempt was made to determine the usage actually preferred by Girty at the time of writing. Most of his preferences could be determined from statements in the manuscript itself, but supplementary evidence was obtained by examination of other notes and manuscripts written by Girty that were available to Williams.
Throughout the summary by Girty, the generic assignments given by him are retained, and no attempt has been made to incorporate generic changes that have appeared since Girty's death in 1939. In connection with the generic terminology as used, Girty comments as follows on that of the brachiopods.
The generic names used for the fusulinids are those employed by Dunbar and Skinner in their publication of 1937, and those for the ammonoids are those employed by Miller and Furnish in their publication of 1940.
BLACK LIMESTONE BEDS
In most of the black limestone beds fossils are scarce, being represented by only occasional specimens. In a few layers, which are generally lenticular or nodular, and somewhat more granular than the rest of the rock, they are more abundant, and from these layers most of the known fauna has been obtained. Slight differences exist between the fossil assemblages in the different beds. In some, brachiopods predominate, in others gastropods, pelecypods, and cephalopods. According to Dr. Girty, the differences between the assemblages are not fundamental.
One of the most striking features of the black limestone fauna is the abundance of ammonoids at numerous localities. Nearly all the collections that have been studied, however, came from exposures near or a short distance north of the crossing of the outcrop by United States Highway No. 62 (localities 2920, 2967, 7413, 7691, 7720, and 8596). These ammonoids belong mainly to three species: Paraceltites elegans Girty, Texoceras texanum (Girty), and Peritroehia erebus Girty. At one locality (7720) there is also Agathiceras cf. A. girtyi Bose, and at another (7701), Perrinites hilli tardus Miller and Furnish.74 The genus Perrinites, although rare in the Guadalupe Mountains, is an abundant and characteristic fossil of the type Leonard series in the Glass Mountains, with which the Bone Spring limestone is correlated. According to Miller, a striking feature of the ammonoid specimens collected from the black limestone is that nearly all retain the living chamber, a fragile structure that is usually missing from specimens from other beds and other areas. This suggests that the shells were deposited in unusually quiet water.
Associated with the ammonoids are occasional nautiloids, which were represented in Girty's original collections by Metacoceras shumardianum (Girty). In the later collections Miller and Furnish75 have identified the same species, and in addition, Titainoceras sp., "Orthoceras" sp., and Stearoceras? sp.
By contrast with the ammonoids, fusulinids are nearly absent from the black limestone, although they are abundant in the gray Victorio Peak limestone to the north, where ammonoids are absent (compare fig. 11). Their rarity in the black limestone contrasts with their abundance in most other beds of the Guadalupe Mountains section. Within the area studied they have so far been observed at only one locality in the black limestone (7923)in a canyon a mile south of Bone Canyon. Here the black limestone contains Schwagerina setum Dunbar and Skinner, which is also found in the probably contemporaneous Victorio Peak limestones not far to the north.76 At the point of the Delaware Mountains, 18 miles south of El Capitan, R. E. King in 1928 collected the genus Parafusulina from the black limestone.
Regarding the remaining, much greater part of the fauna, Dr. Girty reports as follows:
VICTORIO PEAK GRAY MEMBER
Fossils are abundant in many beds of the Victorio Peak gray member, but are not always easy to collect, because of the hardness of the rock, and, in places, because of subsequent dolomitization or silicification. The material obtained during the present investigation therefore consists of a relatively small number of collections. Dr. Girty states that many of the specimens in these collections are so fragmentary that they can be identified only by careful comparisons, if at all.
According to Dr. Girty, the faunas of the member closely resemble those of the black limestone beds, and are distinguished more by the absence of forms that are present in the black limestone, than by the introduction of novel or instructive elements. Many of the collections consist entirely of brachiopods, and especially of the larger productids and spiriferoids. The fauna differs notably from that of the black limestone beds in the almost complete absence of cephalopods. No ammonoids have been found, and only one nautiloid (a Tainoceras according to A. K. Miller). The fauna differs from that of the black limestone also in the rather great abundance of fusulinids in certain beds in the upper division (fig. 11, A). They belong to two species, Schwagerina setum Dunbar and Skinner, and Parafusulina fountaini Dunbar and Skinner.
The lower division of the Victorio Peak member is represented by only one collection, made on the south bank of Shumard Canyon at its entrance (locality 7725). For it Dr. Girty gives the following provisional list, with several indeterminate forms omitted.
The upper division of the Victorio Peak gray member is somewhat better represented by collections. The material from each locality is rather scanty, however, and the specific representations are mostly confined to two or three specimens. The largest collections were obtained on the crest of the ridge between Shumard and Shirttail Canyons, whose summit stands at 6,402 feet (locality 7690). Regarding the fauna of the upper division, Dr. Girty writes:
CUTOFF SHALY MEMBER
As will be recalled, the name Cutoff shaly member is given to discontinuous sets of beds at the top of the Bone Spring limestone, which are exposed in three general districts: the northwest part of the area, from which the name is derived; in Shumard Canyon, not far from Bone Spring, where it is separable into two divisions; and along the base of the Delaware Mountains in the southern part of the area. In all of these districts, the member contains some fossils, but the collections which have been made so far are too scanty to furnish much information on the correlation of the beds in the different districts.
Fossils are least abundant in the northwestern exposures, from which the member is named, and in the main part of the member only a poorly preserved imprint of a pelecypod was seen (locality 7650). Some of the black limestone beds near the base, however, contain many small brachiopod shells, but they have not been collected or studied. Several miles north of the New Mexico line, the member contains rather abundant specimens of Chonetes (locality 7727).
Only one collection was made in the member in the Shumard Canyon area. This collection was obtained from a lens of massive limestone interbedded in the black limestones of the lower division of the member on the south side of the south fork of Shumard Canyon (locality 7675). Regarding it, Dr. Girty writes:
Fossils are more numerous in the Cutoff shaly member in the southern part of the area, west of the Delaware Mountains. Here, many of the thin limestone beds contain fusulinids, which belong to an undetermined species of Parafusulina, and some contain brachiopods. The largest collection was made on the north side of Brushy Canyon in its lower course, from a limestone bed in the lower part of the member, which has here thickened to the rather unusual amount of 15 feet (locality 7666). On this collection, Dr. Girty reports as follows:
CONDITIONS OF DEPOSITION
The Permian rocks exposed in the Guadalupe and Delaware Mountains, and the Sierra Diablo, were laid down during a well-marked depositional cycle which formed the closing stages of the Paleozoic era. This cycle commenced with the Wolfcamp epoch of Carboniferous or Permian age. By the beginning of Wolfcamp time, the localized mountain-making and the still more widespread crustal unrest that had characterized the preceding Pennsylvanian time in the southwestern United States had largely ceased. Readjustments then began which brought into existence the depositional provinces of Permian time (shown on figure 3). These provinces appear to have been broad, persistent tectonic features, that had a marked influence on sedimentation.
At the opening of the Wolfcamp epoch, deposition began in an advancing sea which spread over a deformed and eroded surface of Pennsylvanian and older rocks. From this epoch to the end of the Permian, a distinctive and characteristic set of deposits was laid down in the west Texas region, and sedimentation was interrupted by only minor pulsations which serve to divide one epoch from the next. In this report, Permian geologic history in west Texas is summarized at the end of the stratigraphic discussion, and on the maps of figures 13 and 14. Under the present heading, only those features that were directly related to the Guadalupe Mountains region are discussed.
In the Guadalupe Mountains region, the deposits of Wolfcamp and Leonard age are not completely revealed by exposures. Additional information is afforded, however, by the two wells already mentioned, and by exposures in the nearby Sierra Diablo. Judging by the thickness of sediments laid down (as suggested by plate 7, B), the Wolfcamp and Leonard epochs were fully as long and as important as the succeeding Guadalupe epoch, whose rocks are more completely exposed in the area of this report.
FACIES AND PROVINCES
During Leonard time (as represented by the Bone Spring limestone), and probably during Wolfcamp time (as represented by the Hueco limestone and other Beds), two unlike facies were deposited in the Guadalupe Mountains region. Deposits of the one are black, petroliferous, shaly limestone, and of the other are light-gray, thick-bedded to massive limestone. The two facies tended to persist in separate areas, which correspond closely to the provinces of Permian time shown on figures 3 and 16, A. Thus, the black limestone facies characterizes the southeast part of the Guadalupe Mountains region, or Delaware Basin of figure 16, A, and the gray limestone facies characterizes the northwest part, or Northwestern Shelf Area of that figure. The basin appears to have been a negative feature, with a marked tendency toward subsidence; the shelf was more positive, and either remained stable or did not subside as much. During Leonard time, the boundary between the provinces lay along the Bone Spring flexure of the Guadalupe Mountains which, it will be recalled, is bent down southeastward toward the basin area.
BLACK LIMESTONE FACIES
In the Delaware Basin conditions throughout the whole of Leonard time were nearly uniform and the black limestones were laid down in successive beds without the admixture of much other material.
Deposits representing this facies consist mainly of calcium carbonate, impregnated with bituminous material which imparts to them their characteristic color. There is also some argillaceous matter and a small amount of primary silica. Parts of the deposit are thinly laminated by light and dark bands in such a manner as to suggest that the amount of organic matter in the sea water fluctuated from seasonal or other causes, and that the water was sufficiently quiet for the material to be laid down in successive layers on the bottom.
Evidently the sea bottom during the time of deposition was not favorable to life, as great thicknesses of strata are nearly unfossiliferous. In many of the fossiliferous lenses, ammonoids are the chief fossils, and these animals were probably free-swimming organisms whose shells dropped to the bottom after death. The associated brachiopods and mollusks, which were certainly bottom-dwellers, are of a relatively few species, and fusulinids are absent. This general impoverishment, however, is not absolute, for some collections within the black limestone contain specimens of productids, spiriferoids, and other brachiopods that are abundant in the gray limestone facies. Further, the trilobites that have been found are not specialized forms but belong to the same species as those found elsewhere in the region in quite different types of deposits. Perhaps the less-specialized animals were occasional migrants into an environment that on the whole was not favorable to them.
The black limestones were evidently laid down in quiet water. The bituminous material with which they were impregnated could not have been preserved unless there was little circulation of the water and such a lack of oxygen near the bottom that organic matter was deposited faster than it decayed. These assumed conditions are confirmed by the general poverty of bottom-dwelling organisms in the fauna, and the relative abundance of ammonoids, which swam nearer the surface. Quiet water conditions near the bottom are further indicated by the presence in the ammonoid specimens of the fragile living chamber, which would have been destroyed if the shells had accumulated in agitated water. The conditions just outlined closely resemble those under which the black shales of earlier Paleozoic systems presumably formed.78
Quiet-water conditions during deposition of black shale and limestone deposits do not necessarily indicate the depth of water under which the beds accumulated. There is, however, some evidence to indicate that the beds in the Bone Spring limestone were deposited in deep water. Relations at the Bone Spring flexure, outlined below, suggest that the water was deeper to the southeast, in the black limestone area, than to the northwest, in the gray limestone area. Moreover, the gray limestone deposits seem to have accumulated in agitated water, and it is difficult to see how such differences of deposition could have existed unless there had been also a difference in depth. Further, the Delaware Basin or area of black limestone deposits, received a greater thickness of sediments during Leonard time than the shelf area or area of gray limestone deposits. This greater thickness indicates that the basin area subsided more than the shelf area, and thereby entrapped more sediments. It is possible that subsidence was so rapid that sedimentation did not entirely keep pace with it, and the sea floor stood lower in the basin than on the shelf (sec. a, pl. 7, B).
The black limestone deposits are notably poor in sand and other, coarser, clastics. The few thin, interbedded sandstone layers are very fine grained and consist of the more resistant minerals of igneous and metamorphic rocks. Evidently these sands were transported from a distant source. In its lack of coarser clastic material the black limestone contrasts markedly with the deposits of the Guadalupe series (Delaware Mountain group) that succeeded them, and also with contemporaneous deposits of the Leonard series in the Glass Mountains,79 on the southeast side of the Delaware Basin (fig. 13, B and C). In the Glass Mountains, the deposits include sandstones and conglomerates derived from the erosion of older Paleozoic rocks of the newly uplifted Marathon folded belt. Evidently they were not spread far northwestward into the basin. The few sandstone beds in the black limestone might have been derived from this source, but the fact that similar sandstones are interbedded in the gray limestone toward the northwest suggests that at least some of the sand also probably came from the opposite direction.
In the marginal area, between the Delaware Basin and the northwestern shelf area, deposits of the black limestone and gray limestone facies interfinger. During the last half of Leonard (Bone Spring) time, the gray Victorio Peak member was spread out on the shelf area, extending as far southeastward as the edge of the Delaware Basin, where it apparently intergraded with black limestone. During the first half of Leonard time, black limestones extended for several miles farther northwestward toward the shelf, underneath the gray Victorio Peak beds. In the Guadalupe Mountains, exposures of the black limestone do not extend deeply enough to indicate their relations to the shelf area. In the Sierra Diablo, however, they are replaced near the shelf by limestone reefsa part of the gray limestone facies. They overlap shelfwards on a surface of unconformity that separates the Leonard from the underlying Wolfcamp series.
In the Guadalupe Mountains, the southeastern edge of the gray Victorio Peak limestones follows the upper part of the Bone Spring flexure. This relation of depositional facies to a tectonic feature is probably more than accidental, and implies that the flexure was in existence at the time of deposition. The unconformities in the Bone Spring limestone in Bone and Shumard Canyons suggest contemporaneous movements on the flexure. Possibly also, the small-scale contortion in the black limestone farther southeast was caused by subaqueous gliding of the newly deposited sediments away from the upraised surface of the flexure.
On the Bone Spring flexure, the unconformity at the top of the Bone Spring limestone (between it and the Delaware Mountain group) is clearly much greater than the local unconformities within the Bone Spring. This condition might be taken to indicate that the main movement on the flexure came at the end of Leonard (Bone Spring) time, were it not for opposing evidence. During Leonard time, the water in the basin southeast of the flexure was deep. Further movement on the flexure would either deepen the water in the basin still more, or cause a marked uplift in the shelf area. Neither of these events took place. Actually, as summarized in a later part of this report, the water in the basin during the first part of Guadalupe (Brushy Canyon) time, was probably much shallower than during Leonard time. Also, the shaly, poorly resistant Cutoff member, the last deposit of the Bone Spring limestone, underwent almost no pre-Guadalupe erosion in the shelf area, and its beds lie parallel to those of the succeeding series. These conditions suggest that no uplift took place in the shelf area.
The marked unconformity at the top of the Bone Spring limestone on the flexure thus probably resulted not so much from accentuation of tectonic movements along the edge of the Delaware Basin at the end of Leonard time as from some more widespread phenomenon, such as a general lowering of sea level in the basin, by regional uplift, eustatic change, or other causes.
The Bone Spring flexure, although exposed in only a small area in the Guadalupe Mountains, probably had a wide extent along the northwest edge of the Delaware Basin (fig. 16, A). During late Leonard and early Guadalupe time, it certainly extended southwestward for some distance, as indicated by certain relations at the north end of the Sierra Diablo. Here outliers of the Cherry Canyon, or middle formation of the Delaware Mountain group lie directly on the Bone Spring limestone, just as they do northwest of the flexure in the Guadalupe Mountains (pl. 7, A). The flexure is probably buried under the Salt Basin deposits east of the outliers, for farther east, in the Delaware Mountains, the Cherry Canyon is separated from the Bone Spring limestone by the full thickness of the Brushy Canyon or lower formation of the Delaware Mountain group.
GRAY LIMESTONE FACIES
The gray limestone deposits (Victorio Peak gray member) north of the Bone Spring flexure were probably laid down in shallower, clearer, better aerated water than the black limestones. Their moderately thick beds include layers, traceable for relatively long distances, that were spread out in broad sheets. They are thus unlike the irregularly bedded, massive limestone deposits higher in the section, which have the form of reefs. The Victorio Peak deposits are better designated as limestone banks than as limestone reefs.
The area of gray limestone deposition was a more favorable environment for life than the black limestone area. The many large, thick-shelled productids, spiriferoids, and other brachiopods found in the gray limestone probably found favorable living conditions in clear, shallow waters. The abundance of fusulinids in the gray limestones contrasts with their absence in the black limestones. Conversely, ammonoids which are abundant in the black facies are absent in the gray (fig. 11). It is possible that ammonoids originally lived in both areas, and in the gray limestone area their shells were largely destroyed in the agitated water and were not embedded in the sediments. Support for this suggestion is found in the fact that the nautiloids, whose life habits were similar to those of ammonoids but whose shells were stronger, are represented in the collections from both areas.
Last Updated: 28-Dec-2007