STRATIGRAPHY OF PERMIAN ROCKS (continued)
TERMINOLOGY OF DELAWARE MOUNTAIN GROUP
The great body of sandstone that forms the surface of the Delaware Mountains and parts of the slopes of the Guadalupe Mountains was noted during the first geological exploration of the region. In 1904, Richardson80 named it the Delaware Mountain formation. Richardson's type section was at the south end of the Guadalupe Mountains, where the sandstones are limited above by the Capitan limestone. He included in the formation all the sandstones of the Delaware Mountains, the highest part of which is now known to be younger than the highest sandstones of the type section. As originally defined the unit included the part of the Bone Spring limestone that is exposed at the base of the mountains. At about the same time Girty81 proposed the broader term Guadalupe or Guadalupian series (later classed as a group by the Survey), which included all the exposed fossiliferous rocks of the Guadalupe Mountains.
Subsequent work has resulted in some modifications in usage. The Bone Spring limestone was found to be such a well-marked entity, so different from the beds above, that it has been excluded in redefinitions of both Delaware Mountains82 and Guadalupe.83 In the Delaware Mountains,84 demonstrated that the Delaware Mountain beds above the Bone Spring limestone were divisible into three distinct and nearly equal parts, which later were considered of formation rank. These parts are now termed the Brushy Canyon, Cherry Canyon, and Bell Canyon formations of the Delaware Mountain group.85
In the Delaware Mountains, according to the new definitions, the Delaware Mountain group and the Guadalupe series have the same limits in the sequence, but each term has a different connotation. The name Delaware Mountain is applied to a distinctive facies of dominantly sandy rocks, which projects as tongues between other units of different facies but in part of the same age. The name Guadalupe, on the other hand, is used for a time unit, applied over the whole of the west Texas region to rocks of the same age.
SUBDIVISIONS OF GUADALUPE SERIES
From the standpoint of physical and faunal history, the Guadalupe series can conveniently be divided into three subordinate time units, whose limits correspond to those of the three formations of the Delaware Mountain group. The Guadalupe series is limited below and above by unconformities and abrupt changes in sedimentation, and the three subordinate units are parts of a continuous sequence of sediments lying between. They express more or less perfectly the gradual changes in sedimentation and faunas that took place, by virtue of the passage of time, within a single cycle of sedimentation.
In this report, it is convenient to consider the three subordinate units under separate headingslower, middle, and upper parts of Guadalupe seriesand to describe in turn the various features of each. As a result of this arrangement, it will be noted that the parts of the Delaware Mountain group are separately described under three successive headings, along with the formations with which each is correlative.
BRUSHY CANYON FORMATION
In the Delaware Mountains, the Delaware Mountain group is a mass 2,700 to 3,475 feet thick, whose component formations divide it into approximately equal thirds. The lowest formation was described by Beede as consisting of "thick, yellowish sandstones with rather distant shale partings"; it maintains this character over wide areas. Its present name is derived from Brushy Canyon, which drains westward across the Delaware Mountain escarpment a short distance south of United States Highway No. 62 (pl. 3); along its course the whole thickness of the formation is exposed. The Brushy Canyon formation rests on the Cutoff shaly member of the Bone Spring limestone, and its top is formed by a persistent, massive sandstone ledge that is nearly continuous throughout the area (pl. 7, A). The ledge is prominently developed on the slopes below El Capitan, where it forms a flat projecting bench about halfway up the slope from the black limestone bench to the limestone cliff above (pl. 1).
The Brushy Canyon formation crops out in a broad belt on the west side of the Delaware Mountains, and extends northward along the west slope of the Guadalupe Mountains. North of Bone Canyon, it thins by overlap on the Bone Spring limestone, and its outcrop comes to an end a few miles to the north. The formation is exposed also at many places west of the Delaware Mountains, where it has been downdropped by faulting. In the Delaware Mountains, its outcrop has been cut by many strike faults, so that its full thickness cannot be determined. Below El Capitan, it is about 1,000 feet thick (sec. 18, pl. 6), and in the Niehaus et al., Caldwell No. 1 well, 35 miles east-southeast of El Capitan, it is 1,152 feet thick (pl. 6).
The formation consists largely of sandstone, a part of which, coarser grained than the rest, stands out in massive, yellow or brown ledges or forms the caps of flat-topped mesas (pl. 14, C). Great, rectangular blocks of this sandstone are strewn on the slopes below the ledges. Between the massive sandstones are fine-grained, thin-bedded, or even shaly sandstones, which crop out on slopes.
The formation is easily recognizable on air photographs by its strong ledges, which contrast with the smoothly rounded slopes of the overlying Cherry Canyon formation; and by the abundance on it of cedar and other trees, which give its outcrop a speckled appearance in the photographs.
MASSIVE SANDSTONE BEDS
The massive beds that form the most conspicuous parts of the formation consist of buff or yellowish, medium-grained, friable sandstone, which on some weathered surfaces is coated with a brown crust. Many of the layers contain widely spaced, parallel laminae, and some are cross-bedded. Many of the bedding surfaces are ripple-marked, particularly north of Bone Canyon on the Bone Spring flexure, where the beds overlap the surface of the Bone Spring limestone. Here the general trend is northeastward, parallel to the edge of the flexure, and the same trend is also common farther south (fig. 6). Many of the massive sandstones rest on an undulatory, channeled surface of the thin-bedded sandstones next beneath.
The massive sandstone beds form members from a few feet to more than a hundred feet thick, which alternate with thinner-bedded sandstones. In the south part of the area the beds are thick and closely spaced, but below El Capitan there are only four or five such beds, and for about a mile along the outcrop near Bone Canyon they are absent entirely (secs. 14 and 15, pl. 6). The massive beds thicken and thin rapidly along the strike. On the south slope of El Capitan they are replaced laterally by layers of hard, shaly sandstone. At some localities, lenses of massive sandstone are arranged en echelon, as though a single channel or basin had migrated upward and laterally as sedimentation went on (fig. 4, A and B). A few of the beds are persistent; that at the top of the formation can be traced across nearly the entire area, and some others lower down persist for several miles.
Four specimens of sandstone from the massive beds were studied under the microscope by Ward Smith. The chief minerals are quartz, microcline, and plagioclase; they have a maximum grain size of 0.5 millimeter, and are set in a calcareous matrix. Small amounts of zircon and a few other accessory minerals are present. The grain size is notably coarser than that of other sandstones of the Delaware Mountain group or Bone Spring limestone, in which the maximum diameter is 0.1 to 0.2 millimeter. The only comparable sandstones are in the Goat Seep and Carlsbad formations, in the younger part of the Guadalupe series in the northwest part of the area. In the massive sandstones of the Brushy Canyon formation the accessory minerals are less abundant and varied than in the finer-grained sandstones of the Bone Spring limestone and Delaware Mountain group.
Many of the massive sandstones contain scattered calcareous tests of fusulinids, and in some lenticular beds these tests are so numerous and the sandstone matrix so scant that the rock is more properly called a limestone. Several of the larger of these beds in the Delaware Mountains are separately shown on the geologic map, plate 3. Some of the calcareous lenses contain abraded crinoid stems and brachiopod shells. The fusulinid tests tend in each layer to have a common orientation in some one direction, as shown on plate 19, B, but the direction may differ in different layers. Very commonly the trend is between north and west (fig. 6), or nearly at right angles to the prevailing trend of ripple marks in nearby beds.
The thin-bedded sandstones that lie between the massive beds are generally buff and fine-grained, and are marked by closely set, light and dark laminations, suggestive of varves. In places there are thin, interbedded layers of black, hard, platy, shaly sandstone.
At two localities in Guadalupe Canyon, 250 feet below the top of the formation, there are thin beds of green siliceous shale or chert (in secs. 24 and 27, pl. 6). They may consist of altered volcanic ash like similar rocks in the Manzanita limestone member of the overlying Cherry Canyon formation, but no verification is available because no thin sections were examined.
RELATIONS OF BRUSHY CANYON FORMATION IN BONE CANYON AND NORTHWARD
In Bone Canyon, at the lower end of the Bone Spring flexure, the basal 100 feet of the Brushy Canyon formation consists of conglomerate, limestone, and medium-grained, thin- to thick-bedded sandstone (as shown on pl. 13).
The conglomerates in the canyon form several beds, as much as 10 feet thick, interbedded with sandstone and composed of pebbles, cobbles, or even boulders up to 4 feet in diameter.86 The smaller fragments are of black limestone like that in the underlying Bone Spring limestone, but many of the cobbles and boulders are of gray limestone or dolomitic limestone, and a few are of calcareous sandstone. The latter can be matched with rocks seen in place in the Victorio Peak gray member of the Bone Spring limestone not far to the north (see pp. 18-19) and contain similar fossils. The conglomerates have a lenticular development along the outcrop for 1-1/2 miles to the south, and boulders of gray limestone occur for a mile south of the canyon. North of the canyon, higher on the flexure, the sandstones of the Brushy Canyon formation rest on the Bone Spring with no intervening conglomerate.
In the vicinity of Bone Canyon, a layer of fine-grained, in part sandy, gray limestone as much as 30 feet thick overlies the basal conglomerates and sandstones. (This forms the 28-foot interval shown in sec. 15, pl. 13.) It overlaps on the Bone Spring limestone in the next ravine north of the canyon (pl. 13, fig. A). Southward it thins out and disappears in the sandstones. Near the point of its disappearance, a mile south of the canyon, another similar limestone bed occurs in the sandstones beneath. (This forms the 18-foot interval shown in sec. 55, pl. 13.)
In Shumard Canyon, north of Bone Canyon, beds of the Brushy Canyon formation that are younger than the conglomerate and limestone just described rest on the Bone Spring limestone. These beds include massive, medium-grained, brown sandstone beds, two groups of which form prominent ledges (secs. 11, 12, and 13, pl. 6). The lower passes out by overlap in the north branch of the canyon, where it has an original dip away from the limestone surface of more than 10 degrees. The upper, at the top of the formation, continues some miles farther but passes out by overlap against the Cutoff shaly member half a mile north of Shirttail Canyon. Apparently no beds of the Brushy Canyon formation were laid down any farther north. In this region, the Bone Spring limestone is overlain directly by higher beds of the Delaware Mountain groupthe sandstone tongue of the Cherry Canyon formation (pl. 7, A).
After the hiatus that intervenes in places between the Leonard and Guadalupe series, deposition apparently went on with little interruption throughout Guadalupe time. At most places the first deposits of the series, the Brushy Canyon formation, extend without break into the succeeding Cherry Canyon formation. Locally, however, the uppermost beds of the Brushy Canyon formation have been cut by channels. One of these channels, shown in figure 4, C, is occupied by fusulinid limestones belonging to the basal Cherry Canyon. These channels seem to be of no more importance than others in the sandstones above and below; they were probably caused by submarine erosion.
Except for fusulinids, fossils are not abundant in the Brushy Canyon formation, perhaps because the sandy facies of the deposits was not favorable for life, or because conditions were not favorable for the preservation of shells. The latter possibility is suggested by the fact that most of the fossils that have been collected are fragmentary and water-worn. The thousand feet of beds in the formation constitutes a conspicuous break in the paleontological sequence.
The great abundance of fusulinid tests in many of the sandstone beds of the formation has been noted in descriptions of the stratigraphy (p. 29, see also fig. 11, A), and was first observed by Shumard.87 The fusulinids all belong to the genus Parafusulina, which occurs also in the Bone Spring limestone below and the Cherry Canyon formation above. The species in the Brushy Canyon are characteristically larger and more highly developed than those in the Bone Spring. They include P. rothi Dunbar and Skinner, P. sellardsi Dunbar and Skinner, P. maleyi Dunbar and Skinner, and P. lineata Dunbar and Skinner.88 The first three of these species have been identified also in the lower part of the succeeding Cherry Canyon formation.
The other fossil groups are found only in occasional lenticular calcareous beds, and though considerable material has been obtained from some of the localities, Dr. Girty observes that "the preservation of the specimens is, in every instance, so poor as to hamper close identification." The largest collection was obtained on the southeast side of a gravel-capped butte 3 miles south-southeast of El Capitan and half a mile southwest of bench mark 4733 (locality 7656). A collection containing many of the same species and from nearly the same place (locality 2919) was described by Girty89 in 1908.
Most of the identifiable material from this and other localities consists of brachiopods, although the presence of other groups is suggested by occasional specimens. Girty's original collection contains the bryozoan Fistulipora grandis guadalupensis Girty. The more recent collections from station 7656 include some fragmentary cephalopod shells, mostly unidentifiable, but according to A. K. Miller probably including the nautiloid Coloceras. In addition, H. C. Fountain has noted the presence of abundant crinoid stems, and poorly preserved cup corals, pelecypods, and gastropods. Dr. Girty comments as follows on the brachiopod assemblage:
CONDITIONS OF DEPOSITION
After the close of Leonard time, at the beginning of Guadalupe time, a marked change in sedimentation took place in the Guadalupe Mountains region. The preceding deposits were spread across the whole area, whereas those of the Brushy Canyon formation were restricted to the southeastern part, or Delaware Basin. The preceding deposits were limestones or very fine clastics, whereas the early Guadalupe (Brushy Canyon) deposits were dominantly sandstone, in part moderately coarse grained. The preceding deposits in the Delaware Basin (black limestone facies) show evidence of having been deposited in quiet and perhaps deep water, whereas many beds of the succeeding Brushy Canyon formation in the same area were laid down in agitated water, and the whole formation is probably a shallow-water deposit.
Some of the causes of this change in sedimentation have already been considered (p. 27). It was concluded that at the beginning of Guadalupe time the Delaware Basin became an area of shallow water, and the adjacent shelf areas were emergent, but did not stand high.
Because of this condition, sediments could be washed into the basin from almost any direction, and transportation of coarse material to it was probably less impeded than at any other time in the Permian. The occurrence of relatively coarse-grained sandstone in the Brushy Canyon deposits of the Delaware Basin thus does not necessarily indicate renewed uplift in the lands that supplied sediments to the region.
The coarser sands continue to the top of the Brushy Canyon formation, where they come to an end in a single, persistent layer; in the Delaware Basin no similar beds are seen in the higher Permian beds. Sands equally coarse, however, are found northwest of the basin in the younger Goat Seep and Carlsbad formations. These relations suggest that the source of the sands lay somewhere to the northwest, and that erosion of the source area continued after the close of lower Guadalupe time. Later on, southeastward transportation of the material into the basin was probably hindered by the development of limestone-reef barriers of middle and upper Guadalupe age (Goat Seep and Capitan limestones) and coarser sands could be laid down only in the shelf area northwest of the basin.
The different types of sediment in the Brushy Canyon formation alternate in rude cycles, as shown on section 33, figure 5. Each massive sandstone generally rests on a channeled surface which records a time of maximum current action. They themselves contain ripple marks, cross beds, and oriented fusulinids, which indicate that they were laid down rapidly in agitated water, within reach of effective wave action. The massive beds are succeeded by thin-bedded, fine-grained sandstone, with varvelike laminae, which record slower, quieter deposition. Toward the top of each cycle are intercalations of dark, shaly sandstone, probably with a considerable bituminous content, which suggest an approach to the stagnant bottom conditions of the older black-limestone deposition. Each cycle is brought to an end by another period of channeling and deposition of coarser sandstone.
These rude cyclical units cannot be traced far along the outcrops, and it is questionable whether any one is of more than local extent. They indicate, however, a regular fluctuation in conditions of sedimentation from agitated to quiet water but probably with no accompanying changes in depth.
The ripple marks in the massive sandstones have nearly the same northeastward trend as the Bone Spring flexure, which formed the shore in lower Guadalupe time (fig. 6). They were evidently shaped by movements of the water oriented at right angles to the shore. These movements might have been undertow currents, caused by the return along the bottom of water that had previously been piled up on the shore by the waves. Or they might have been the to-and-fro oscillation of water within the waves themselves. Movements of the first sort would form current ripples, and of the second sort oscillation ripples.90 The marks in the Brushy Canyon formation appear to have a symmetrical cross section, which indicates they are oscillation rather than current ripple marks. However, no secondary crests are found, such as occur in many oscillation ripples. Current movements are indicated by the channeling of the associated deposits.
The fusulinid tests, which are commonly strung out in a northwestward direction, at right angles to the trend of the ripples, were probably placed in this position by the same oscillation movements of the water that produced the ripples. After the death of the animals, their many-chambered tests probably had considerable buoyancy, and were easily turned in the direction of least resistance to water motion; that is, elongate parallel to the movement.
The sea bottom during lower Guadalupe time was probably inhospitable to many forms of life, because of its sandy surface, and the probable agitation and turbidity of the overlying water. Shells of whatever bottom fauna existed were largely broken up before they could be fossilized. Whatever the conditions of life for most of the fauna, the lower Guadalupe sea was definitely favorable to the existence of fusulinids and the preservation of their tests as indicated by the enormous numbers of the tests that were enclosed in the sediments.
Last Updated: 28-Dec-2007