STRATIGRAPHY OF PERMIAN ROCKS (continued)
SUMMARY OF THE SECTION
The rocks exposed in and near the southern Guadalupe Mountains constitute a great sequence of Permian strata, thicker by far than most other Permian sections in North America. No single set of outcrops, and no single well, extends through the whole system, but its approximate measure is indicated by combining incomplete sections. Thus, the Wolfcamp (Carboniferous or Permian) and Leonard series in the Anderson and Prichard, Borders No. 1 well in the Delaware Mountains are about 4,500 feet thick (pl. 8). The Guadalupe series in the Niehaus et al., Caldwell No. 1 well, down dip to the east of the Delaware Mountains, is about 3,500 feet thick (pl. 6). The Ochoa series in the Pinal Dome Oil Co., Means No. 1 well in Loving County east of the Pecos River, is about 4,200 feet thick.45 Taken together, these figures indicate a thickness for the Permian system (if the Wolfcamp is included) of more than 12,000 feet. The three wells were drilled in the Delaware Basin area, in which maximum thicknesses are expected. In the shelf area to the northwest, outcrop and well sections46 suggest a thickness of 7,000 feet or less.
The rocks comprising this thickness of 7,000 to 12,000 feet include a great number of facies, deposited in varied environments, with the different facies giving place to one another both vertically and horizontally. Most of the section consists of sedimentary rocks of marine origin. These rocks are mainly limestones and sandstones, but include minor amounts of shale, conglomerate, and chert. Many of them contain abundant invertebrate fossils. In the upper part of the section evaporite beds make their appearance, indicating that the area at that time began to be shut off from free access to the sea, perhaps because of the general retreat of the seas from the continent that characterized Permian time. The first evaporites in the Guadalupe Mountains region are found in the Guadalupe series, where they occur only in the shelf areas. In the succeeding Ochoa series they spread over the entire region.
Despite the complexity of the local details of the stratigraphy, and the interfingering of numerous facies, certain broad groupings of the Permian rocks are evident. They are expressed by the subdivision of the sequence into the Wolfcamp, Leonard, Guadalupe, and Ochoa series. These groupings are of both lithologic and faunal significance, and represent distinct cycles of sedimentation.
In the vicinity of the Guadalupe Mountains the four series just named are separated from one another by well-marked changes in sedimentation, and in places by unconformities. A major structural unconformity separates the Wolfcamp series from the beds below, and a minor one, perhaps involving some deformation, lies between the Wolfcamp and Leonard series. Other unconformities are present between the Leonard and Guadalupe series, and between the Guadalupe and Ochoa series. The last two unconformities seem to have been caused not so much by deformation as by widespread withdrawals and readvances of the sea. The unconformities that separate the series are usually poorly marked or absent in the Delaware Basin area, but increase in distinctness in the shelf areas, where the beds above and below are separated by considerable time gaps. Both the structural unconformities and the unconformities caused by withdrawals and readvances of the sea are of more than local significance and are, therefore, of aid in correlating the Guadalupe Mountains section with sections in adjacent parts of the Mid-continent and Cordilleran provinces.
Between some of the series, and at the same levels as the unconformities, there are rather striking changes in sedimentation. They are usually most pronounced in the Delaware Basin area, and less so in the shelf areas where rocks of similar facies were deposited through most of the period. One change in sedimentation took place at the beginning of Guadalupe time, when the basal sands of the Delaware Mountain group spread over the black limestones of the Bone Spring. Another took place at the top of the same series, when deposition of sandstones of the Delaware Mountain group was followed abruptly by deposition of evaporites of the Ochoa series.
TIME SPAN OF GUADALUPE MOUNTAINS SECTION
The thickness of the Permian rocks in the vicinity of the Guadalupe Mountains is impressive, and suggests the possibility that Permian time is more or less completely represented by deposits. Proof of this suggestion, however, must be obtained not from the physical relations, but from the fossils, and especially from those fossil groups whose genera have relatively narrow stratigraphic ranges and are widely distributed throughout the world. In this connection, recent studies of ammonoids and fusulinids are of interest, for they permit comparison of the Guadalupe Mountains section, not only with other parts of North America, but also with the well-known Permian marine sections of Europe and Asia. Such comparisons have recently been made by Miller,47 Dunbar,48 and Miller and Furnish.49
These comparisons suggest that the Guadalupe Mountains section contains a more or less uninterrupted sequence upward from the Pennsylvanian, comparable to that in Russia and other old-world sections. The age of the highest beds is somewhat uncertain, however, because of the general absence of fossils in the Ochoa series. The beds next beneath, in the upper part of the Guadalupe series, contain the zones of Polydiexodina and Timorites, which are evidently of upper Permian age. No fusulinid zones younger than that of Polydiexodia are known elsewhere, but an ammonoid zone, that of Cyclolobus, is considered to be younger than the zone of Timorites. This genus occurs in the highest Permian beds of the Salt Range of India, and also in the Himalayas and Madagascar. Miller and Furnish suggest that the Ochoa series may be equivalent to the beds containing Cyclolobus, If so, the top of the Ochoa series reaches nearly, if not quite, to the top of the Permian as defined.
GENERAL CHARACTER OF FAUNAS
The Leonard and Guadalupe series of the Guadalupe Mountains contain numerous invertebrate fossils. These fossils have considerable diversity from zone to zone and facies to facies, but enough genera and species extend through the whole to give the faunas of the two series a similar aspect.
The faunas of the two series were collectively referred to as the "Guadalupian fauna" by Girty50 in his monographic report and various shorter papers. This term is not used here because of differences between faunas of the various zones and because of possible confusion between the term and the name Guadalupe series, which is now restricted to a part of the section in the Guadalupe Mountains.
The faunas of the Leonard and Guadalupe series are definitely of later Paleozoic type, and are not like any known Triassic fauna. Here, as in the Mississippian and Pennsylvanian, one finds brachiopods, mollusks, bryozoans, crinoids, and corals belonging to well-known later Paleozoic groups. Among the brachiopods, productids and spiriferoids abound, and the cephalopods belong to the same families as those found in the beds below. As in the Pennsylvanian the most abundant Foraminifera are the fusulinids. The faunas compare favorably with those of the Mississippian and Pennsylvanian rocks in numerical abundance, in the number of invertebrate groups represented, and in the number of genera and species. In these respects they form a notable exception to the general rule that Permian faunas are impoverished and marked by the absence of numerous later Paleozoic families and genera.
Despite the broader resemblances of the Leonard and Guadalupe faunas to other Paleozoic faunas, in detail they differ notably from those elsewhere in North America, either older or contemporaneous. Many genera extend up from the underlying Pennsylvanian, and other genera seem to have developed from Pennsylvanian types, but there are few or no species in common. Some fossils, not clearly related to those below, are probably migrants from other regions. Among the distinctive features of the fauna are fusulinids and ammonoids of larger size and more complex internal structure than those of the Pennsylvanian (such as Parafusulina, Polydiexodina, Perrinites, and Medlicottia). The brachiopods include specialized genera with high cardinal areas (such as Geyerella, Scacchinella, and Prorichthofenia), or with other unusual modifications (such as Leptodus).
RELATION TO OLDER FAUNAS
The change from the Pennsylvanian fauna to the Leonard fauna begins to be evident in the intervening Wolfcamp series, or below the level of the rocks exposed in the Guadalupe Mountains. In the Wolfcamp series, as exposed in trans-Pecos Texas, some of the Leonard and Guadalupe genera and species (such as Squamularia guadalupensis and Camerophoria venusta) make their first appearance in faunas that are dominantly of Pennsylvanian aspect. The Wolfcamp series contains also a distinctive assemblage of fusulinids, including the genus Pseudoschwagerina. Many of the Pennsylvanian genera and most of the species come to an end at the top of the Wolfcamp series, and only a few generalized types (such as various species of Composita) persist. In the succeeding Leonard and Guadalupe beds, many of the genera and most of the species are new.
RELATION TO OTHER PERMIAN FAUNAS OF NORTH AMERICA
Compared with other Permian faunas of the continent, those of the Leonard and Guadalupe series are distinguished by their diversity and novelty. Many of the faunas in other areas contain more species allied to those in the beds below, and some of them differ from those below by the disappearance of certain fossil groups, and the greater abundance of other groups, probably as a result of environmental changes. Most of the faunas in the other areas contain fewer groups than are found in the Leonard and Guadalupe faunas, probably because they were laid down in environments that were less favorable to the majority of invertebrate groups. Thus, faunas in the Cordilleran region (such as the Kaibab and Phosphoria) contain species of brachiopods that are similar to or identical with those in the Leonard and Guadalupe series, and the faunas of the Mid-continent area include ammonoids like those in the Leonard series. Yet many other fossil groups associated with these forms in the Guadalupe Mountains fail to extend into the other two provinces.
Evidently a marine environment persisted in the Guadalupe Mountains region that was as congenial to life as were the seas in other parts of the continent during preceding periods. Life continued here as before, developing into new forms that impart a distinctive aspect to the fauna. Elsewhere in the continent more rigorous conditions set in and species, genera, and even whole groups disappeared, leaving behind only those forms that were able to withstand the new environment.
DISTRIBUTION OF THE FAUNAS OUTSIDE THE GUADALUPE MOUNTAINS
The faunas of the Leonard and Guadalupe series have been recognized in several mountain areas of the trans-Pecos region south of the Guadalupe Mountains. They can be traced southward through the Delaware and Apache Mountains, which form the topographic continuation of the Guadalupe Mountains on the south (fig. 1), and are found again in the Sierra Diablo across the Salt Basin to the west. Farther southeast, in the Glass Mountains, where the Permian rocks rise again from beneath the Mesozoic and Cenozoic rocks, fossils of the Leonard and Guadalupe faunas occur abundantly. Occasional fossils of the same type have been recovered from well cores farther east. All these occurrences are in or closely adjacent to the Delaware Basin (fig. 3), which was a structural and depositional feature extending across part of the west Texas region during Permian time. The basin appears to have constituted the head of an embayment extending northward into the continent from the open sea.
The faunas of the Leonard and Guadalupe series show both vertical and lateral changes in character, which have been pointed out in the discussion of the individual faunas. Many of these changes are caused by differences in facies from place to place and time to time. Some of the vertical differences, however, appear to result from gradual evolutionary changes in the character of the organisms with the passage of time. In all the fossil groups, one finds a record of the appearance, development, and disappearance of genera and species in successive beds. Further, such groups as the ammonoids and fusulinids seem to have more complex shells in the higher beds than the lower ones.
Some of these features are illustrated by figure 11, in which the known occurrences of some fusulinid and ammonoid genera are plotted on stratigraphic diagrams. Equally interesting diagrams no doubt could be prepared for other invertebrate groups were sufficient data available.
Ammonoids appear to be strongly influenced by the facies and, except for a few occurrences in the Capitan limestone, are confined to the Delaware Basin area (right half of fig. 11, B). By contrast, the fusulinids seem to be fairly common in all types of rock (fig. 11, A).
Many of the genera selected have been interpreted as closely related to one another, and some of the younger ones are thought to have developed from the older ones. Thus, Dunbar and Skinner51 suggest that Parafusulina developed from Schwagerina and gave rise in turn to Polydiexodina. Also, Miller and Furnish52 suggest that Timorites developed from Waagenoceras. The ranges of some of these genera overlap. Thus, Schwagerina and Parafusulina both occur in the upper part of the Bone Spring limestone (fig. 11, A), and Waagenoceras and Timorites both occur in the Bell Canyon formation (fig. 11, B). By contrast, so far as known, Parafusulina and Polydiexodia nowhere occur together; one is common in the Cherry Canyon formation, the other in the overlying Bell Canyon formation.
The lowest fossil zone of the Guadalupe Mountains section is in the upper part of the Bone Spring limestone of the Leonard series. Its characteristic fossils include the fusulinid genus Schwagerina, which first appears in the underlying Wolfcamp series, and does not extend into the overlying Guadalupe series. With it are older species of Parafusulina, which are smaller and less highly developed than species of the same genus in the Guadalupe series above. Ammonoids are represented by a characteristic group of species of the genera Peritrochia, Texoceras, and Paraceltites; and by the genus Perrinites. The latter is rare in the Guadalupe Mountains, but very common in other west Texas sections; neither here nor elsewhere does it occur in beds younger than the Leonard. The brachiopods include a number of species not found in lower or higher beds, such as Productus ivesi Newberry, P. leonardensis King, and Enteletes liumbonus King. The collections suggest that many of the characteristic brachiopods disappear in the highest part of the zone, or Cutoff shaly member.
The second fossil zone lies in the lower part of the Guadalupe series, or Brushy Canyon formation. The information yielded by this unit is disappointing because many invertebrate groups are poorly represented or absent. Little is therefore known of the nature of the transition from the Leonard fauna to the Guadalupe fauna. So far as is known, however, the fauna of this zone more closely resembles those of higher rather than of lower zones. Thus, the fusulinids are all large, highly developed species of the genus Parafusulina, quite distinct from those in the Leonard series but identical with those in the lower part of the middle of the Guadalupe series. Also, the small brachiopod fauna contains none of the characteristic Leonard forms but contains instead such species as Chonetes subliratus Girty, Productus capitanensis Girty, P. wordensis King, P. indentatus Girty, and P. popei opimus Girty, that characterize the higher parts of the Guadalupe series.
The third fossil zone occupies the middle part of the Guadalupe series, or the Cherry Canyon and Goat Seep formations and associated beds. Here facies are so well differentiated that faunas in the different members of the Cherry Canyon formation differ notably from one another and also from the fauna of the Goat Seep limestone. However, all the faunas taken together constitute a relatively well characterized assemblage that differs notably from that of the Leonard series, and has many differences from that of the upper part of the Guadalupe series.
The Cherry Canyon formation contains the youngest representatives of the fusulinid Parafusulina, the species in the lower part being the same as those in the underlying Brushy Canyon formation. It contains ammonoids at several horizons, but only the South Wells limestone member in the upper part contains diagnostic genera, such as Waagenoceras. The Getaway limestone member and Goat Seep limestone contain considerable numbers of sponges, which foreshadow their still greater development in the overlying Capitan limestone.
The brachiopods, although similar to the few that are known in the underlying Brushy Canyon formation, are notably different from those in the Leonard series beneath but have many resemblances to those in the upper part of the Guadalupe series. There are, for example, species of Productus, Aulosteges, and Spirifer that are absent from lower horizons (Productus pileolus Shumard, P. popei Shumard, Aulosteges guadalupensis Shumard, Spirifer sulcifer Shumard, S. pseudocameratus Girty and Spiriferina billingsi Shumard). Similar relations are found in the rhynchonellid group. The greater abundance and diversity of the terebratuloids foreshadows the conditions of later Guadalupe time. An exception to the general rule is the genus Enteletes, which is an abundant and characteristic genus in the underlying Leonard, Wolfcamp, and older faunas. It occurs in the Goat Seep limestone but is apparently near the upper limit of its range, for it is absent from the higher beds. In his report on the faunas of the middle part of the Guadalupe series, Dr. Girty compares many of the brachiopod and mollusk species with species in Permian formations of the Cordilleran region.
The fourth fossil zone in the Guadalupe Mountains section occupies the upper part of the Guadalupe series, or Bell Canyon, Capitan, and Carlsbad formations. Here, as in the third fossil zone, faunal facies are marked, but here again, the sum of the faunas is a distinctive and characteristic assemblage.
The faunas of the zone all contain the complex, highly developed fusulinid genus Polydiexodina, and are thus readily distinguishable from those of the middle part of the Guadalupe series, which contain Parafusulina. The ammonoids are less distinctive. The most abundant genus is Waagenoceras, which is also characteristic of the underlying South Wells fauna. With it, however, at one locality, is Timorites, a genus that is believed to have developed out of Waagenoceras. Xenaspis is encountered for the first time, and is of interest because it also occurs high in the Permian sections in Asia.
The brachiopods include many species in common with those in the middle part of the Guadalupe series, but some are more abundant here than below. There are, however, some new species, such as Chonetes permianus Shumard, Productus latidorsatus Girty, P. pileolus Shumard, and P. limbatus Girty. Terebratuloids are very common, especially in the Capitan and Carlsbad formations, but Enteletes is no longer present. It may be significant that in his reports on the faunas of the zone Dr. Girty makes no comparisons with species of other regions, as he did for the middle Guadalupe faunas. The upper Guadalupe faunas may be younger than the Permian faunas of other regions, or there may have been no longer any marine connection between them.
With the upper Guadalupe faunas, the definite fossil record of the Permian of the area comes to an end. The few fossils and evidences of life in the overlying Ochoa series are too meager to permit one to trace the development of invertebrate life through them.
Faunal facies are a marked feature of the Permian faunas of the Guadalupe Mountains. The most important differentiations in facies depend on the position of the faunas in relation to the Delaware Basin. Faunas in the basin differ from those in the reef deposits on the margin. In upper Guadalupe time, a third facies may be recognized in the back-reef Carlsbad beds deposited in the shelf area. Another type of facies differentiation is found in successive beds in the Delaware Basin sequence and probably depends on changes in the depth of water that took place there from time to time.
Differences in facies are expressed mainly by variations in the abundance of certain groups, genera, and species. Some groups are exceedingly abundant in one facies, and entirely absent in others. By contrast, a few groups, such as the fusulinids, appear to continue unchanged in number through nearly all types of deposits in the area.
The facies that developed in the Delaware Basin, in its extreme form, is characterized by an abundance of ammonoids and rhynchonellid brachiopods, and by the absence of most other groups. Some of the forms were bottom-dwellers that were able to persist in an environment of deep, stagnant water; others were swimmers or floaters whose shells sank to the bottom after death. Faunas of this type characterize the Delaware Basin area during Leonard time, during part of middle Guadalupe time (South Wells member), and during upper Guadalupe time. During lower Guadalupe time, and part of middle Guadalupe time, however, the facies are somewhat different, owing perhaps to decrease in depth and increase in agitation of the water in the basin area. During this interval, the faunas are much more diversified in the basin; there is a great increase in the numbers of pelecypods and gastropods, and many more groups of brachiopods are represented.
The reef or marginal facies is characterized by a general abundance of brachiopods, pelecypods, and gastropods, of a sort that thrive in clear, shallow waters. In this respect it resembles the normal neritic facies of other later Paleozoic systems. This condition appears to be the cause of the occurrence of Enteletes in the Goat Seep limestone, but not in the contemporaneous Cherry Canyon formation;53 and of Meekella in the Capitan limestone but not in the contemporaneous Bell Canyon formation. The same condition applies to the productids and spirifers that are numerically so much more abundant in the Victorio Peak gray member of the Bone Spring limestone than in the equivalent black limestone, and to the spiriferoids Martinia and Squamularia that are common in the Capitan. In addition to these features, some of the deposits (especially the Capitan) contain true reef-building forms, such as sponges and algae. In the field, their numerical abundance is far more impressive than is the diversity of genera and species.
In the upper part of the Guadalupe series, a more or less distinct subfacies may be recognized in the limestone members of the Bell Canyon formation along the base of the Capitan reef. This subfacies differs markedly from the facies in the Delaware Basin that characterizes the same limestone members farther southeast and is much more like the Capitan reef facies. It differs from the Capitan reef facies, however, in the greater abundance of bryozoans, in the presence of brachiopod and mollusk species not found in the Capitan, in its lack of sponges, and in the absence of some Capitan species belonging to other groups.
One of the most striking changes in faunal facies in the area takes place between the Capitan reef and the back-reef or Carlsbad deposits. In the back-reef facies gastropods greatly increase in numbers, but sponges nearly disappear and brachiopods are greatly reduced in numbers. Moreover, numerous important brachiopod families and genera (such as the productids and spiriferoids) that characterize most other later Paleozoic faunas are absent from the Carlsbad, although they occur in the Capitan deposits only a few miles away.
CORRELATION OF GUADALUPE MOUNTAINS SECTION WITH PERMIAN ROCKS OF OTHER REGIONS
Correlation of the Permian rocks of the Guadalupe Mountains section with the Permian rocks of other regions involves many difficulties because of the extreme variations in lithologic features and faunas in the Permian rocks. Correlation of the Guadalupe Mountains section with other sections in the Delaware and Marfa Basins is more successful than with sections in other regions because fossils of the same general facies are abundant throughout the two basins, and because the physical histories of the different parts were similar.
Correlation of the Guadalupe Mountains section, and others in the Delaware and Marfa Basins, with sections to the northwest and northeast is more difficult, because many fossil groups drop out, and many changes in lithologic character take place in these directions. For such purposes, however, the Guadalupe Mountains section is strategically located. Toward the northeast the formations of the Guadalupe Mountains pass beneath the surface but can be recognized in many wells. From such wells, their correlatives can be traced northeastward by subsurface methods into the outcrops of the Mid-continent area. Toward the northwest, beds can be traced on the surface into the central New Mexico sequence.
Questions of correlation have been discussed in some detail in another paper54 and need not be repeated here. The conclusions given in that paper are summarized on the correlation chart, figure 12. It should be recognized that any correlation chart of the Permian prepared at this stage can be only tentative, and can represent only one of a number of possible interpretations. This chart is no exception. Various other correlations have been proposed, both recently and in the past.55 To the writer, the correlations given on the chart appear to be the most satisfactory, but they are subject to modification as new evidence is obtained.
The geologic history of Permian time in the southern Guadalupe Mountains has been described and interpreted in earlier parts of the text (pp. 24-95). The geologic record in this area is probably of more than local significance, and may resemble that of considerable areas in the Delaware Basin and its margins. However, in other parts of the region of Permian deposition in the southwestern United States different conditions prevailed. The general paleogeography of this large region of Permian deposition has been discussed in other papers,56 and is not repeated here. It is summarized in the paleogeographic maps on figures 13 and 14, which illustrate the relations during Permian time of the southern Guadalupe Mountains area to the larger region of which it formed a part.
Last Updated: 28-Dec-2007