Purpose and plan of paper.—The primary purpose of this paper is to interpret the geologic history of the West Texas region during Permian time, although only the last chapter is actually devoted to the subject. To lay the framework for this interpretation, earlier chapters describe two standard sections of the West Texas Permian, the manner of sedimentation of the Permian rocks, and the correlation of the Permian rocks throughout the West Texas region. The information and conclusions presented in these chapters form the basis of the conclusions regarding geologic history that are given in the final chapter.

Chapter One. Introduction.—For purposes of this paper, the term “West Texas region” is defined as including not only an area in Texas, but also the adjacent southeastern part of New Mexico. In the region, Permian rocks form an important component of the geologic section. In the central part of the region they are buried, but they are exposed on the east in broad plains, and on the west in mountain ranges. Two of the ranges on the west, the Guadalupe Mountains and the Glass Mountains, contain nearly complete sequences of the system, and these serve as type sections. Based largely on these two sections, the later Paleozoic rocks of West Texas have been divided into four series, the Wolfcamp, Leonard, Guadalupe, and Ochoa. This classification is followed in this paper, where the Wolfcamp series is classed as Permian (?) in age, and the remaining three series as Permian in age.

Chapter Two. Guadalupe Mountains section.—The Guadalupe Mountains section is described on the basis of new and hitherto unpublished results by the writer and his associates. The components of the section include beds exposed not only in the Guadalupe Mountains, but also in surrounding areas. They include the Wolfcamp and Leonard series as exposed in the Sierra Diablo, the Guadalupe series as exposed in the Guadalupe Mountains and the Delaware Mountains, and the Ochoa series as represented east of the last two ranges.

The rocks of the Guadalupe Mountains region were deposited near the edge of a feature of Permian time known as the Delaware basin, along whose margin they show complex changes in facies. The rocks laid down outside the basin, in what is here termed the shelf area, are thus very different from contemporaneous basin deposits.

The Wolfcamp series is represented in the shelf area by the Hueco limestone. This lies with marked structural unconformity on Pennsylvanian and older rocks, but its fauna is more or less transitional in character from Pennsylvanian to Permian. In the basin, the series apparently changes into black limestone and shale. In places, it is separated by an unconformity from the Leonard series.

The Leonard series is represented by the Bone Spring limestone. This consists of black, thin-bedded limestone and some shale in the basin, but changes into gray limestone in the marginal and shelf areas. The lower part of the gray limestones along the margin includes reef masses. The upper part is a more continuous sheet that is termed the Victorio Peak gray member. The abundant Leonard fauna has little in common with that of the Pennsylvanian, and is considerably different from that of the Wolfcamp. It is the first characteristically Permian assemblage in the sequence. In the marginal and shelf areas, the series is separated by an unconformity from the Guadalupe series.

The Guadalupe series in the Delaware basin is represented by the Delaware Mountain group, divided into three formations and numerous members. It is a mass of sandstone with some thin limestone beds. Toward the margin of the basin, the lower formation disappears by overlap on the Leonard series, and the middle and upper formations change into reef masses of the Goat Seep and Capitan limestones. In the shelf area, these limestones become thinner-bedded, and finally change into an evaporite facies known as the Chalk Bluff formation. The fauna of the Guadalupe series, like that of the Leonard series, is characteristically Permian, but is of more advanced type. Practically the whole of the fossil record comes to an end at the top of the Guadalupe series.

The Ochoa series is dominantly an evaporite facies, and consists mainly of anhydrite and salt, but with some dolomitic limestone and redbeds. It contains beds of potash salts that are being mined at three places east of Carlsbad, New Mexico. Its lowest formation was deposited only in the Delaware basin, and disappears by overlap on the Guadalupe series along the margin. The succeeding formations spread beyond the basin. The series contains no diagnostic fossils, but the physical relations strongly favor its being a natural part of the Permian system.

Chapter Three. Sedimentation and tectonics in Guadalupe Mountains region.—In this chapter on the conditions of sedimentation in the Guadalupe Mountains region the thesis is advanced that sedimentation was closely related to tectonic features, which produced irregularities on the sea floor; hence, differences in depth. This relation was, however, very complex, and the features that developed resulted from a variety of factors. Many of the irregularities on the sea floor were directly of erosional or depositional origin, although their ultimate origin may have been tectonic.

The Delaware basin was a region in which subsidence was greater than in surrounding areas. The sediments laid down there during Permian time were nearly twice as thick as in the shelf areas. During parts of Permian time, the water in the basin was deep, whereas the water on the shelves remained shallow. Sedimentation was thus nearly continuous in the basin, but was interrupted by unconformities on the shelves.

Along the exposed margins of the basin are a number of monoclinal flexures. These appear to have been in existence during Permian time and bear not only a geometric relation to the basin, but probably also a genetic relation. Several periods of movement before, during, and after Permian time appear to have taken place along them, and to have had an important influence on the distribution of facies in the older Permian deposits, especially those laid down during Leonard time.

The reefs in the younger Guadalupe series are also closely related to the margins of the basin. During growth, they rose steeply from the deep water of the basin, but deposits behind them in the shelves lay at the same level as their tops. Evidence indicates that they owe their location to the shoaling of the water along the edge of the basin, a condition that was partly of tectonic origin, and that growth was maintained by continued greater subsidence of the basin than of the adjacent shelf areas.

Chapter Four. Glass Mountains section.—The sequence in the Glass Mountains has been described in various earlier publications, so that it is only briefly treated here. Correlations with the Guadalupe Mountains section are discussed, and several new interpretations are presented. Among these is the suggestion that an unconformity exists in the upper part of the Leonard series in the eastern Glass Mountains, and that the Word formation of the Guadalupe series includes thick reef masses (Vidrio member) in the same district. These reef masses had previously been considered to be a part of the younger Capitan limestone.

Chapter Five. Regional correlations.—Correlation of the sections in the Guadalupe Mountains and the Glass Mountains with sections in other parts of the West Texas region presents many problems, because of great changes that take place in lithologic and faunal facies. All available methods of correlation, including various physical and paleontological techniques, are therefore used.

The Wolfcamp series and its equivalents contain marine fossils throughout most of the West Texas region. These include the genera Pseudoschwagerina and Properrinites, as well as other distinctive fusulinids, ammonoids, and other invertebrates. The fusulinids are especially useful in differentiating beds of Wolfcamp age in subsurface. In central Texas, beds of Wolfcamp age probably extend from the Harpersville formation of the Cisco group to a horizon in the Admiral formation of the Wichita group, but the precise limits are not certainly known. In New Mexico, the Wolfcamp equivalent includes the upper part of the Magdalena group, and perhaps the lower part of the Abo sandstone, as those units are used in mapping.

In the Leonard series and its equivalents, the marine facies is more restricted than in the Wolfcamp series, although some thin beds with marine fossils extend far out into the shelf areas. The series contains the genus Perrinites, the older species of the genus Parafusulina, and various other distinctive invertebrates. In central Texas, the Leonard equivalent apparently includes the upper part of the Wichita group, the Clear Fork group, and the San Angelo, Blaine, and Dog Creek formations. Its top is limited by the unconformity at the base of the Whitehorse group. In New Mexico, the Leonard equivalent includes the Yeso and San Andres formations, and perhaps most of the Abo sandstone beneath. In the intervening Llano Estacado, where the section is known only from subsurface work, considerable uncertainty exists about the upper boundary. Some of the limestones in this district that have been assigned to the San Andres formation may be of Guadalupe age.

The Guadalupe series in the Delaware basin is characterized by the later species of the zone of Parafusulina (part of which may be termed the Parafusulina rothi assemblage), by the more advanced fusulinid genus Polydiexodina, and by the ammonoids Waagenoceras and Timorites. Very few of these and other distinctive invertebrates extend outside the basin area, due to marked restriction of the area of marine environment. East and northeast of the Delaware basin is the Whitehorse group, which has been correlated with the Guadalupe series by physical methods. Beneath the Whitehorse in the Llano Estacado, but not extending to the surface in central Texas, are beds containing the Parafusulina rothi assemblage. These beds are probably also Guadalupe in age.

The Ochoa series is believed to be confined to the West Texas region, and not to reach the outcrops in central Texas or Oklahoma.

Chapter Six. Paleogeography and geologic history.—Before Wolfcamp time, the rocks of the West Texas region were considerably deformed. To the south, a geosynclinal area was strongly folded and faulted to form the Marathon folded belt. The foreland to the north, before Permian time, was more gently folded and several uplifts were raised and deeply eroded.

During Permian time, the foreland area was divided into a number of irregularly shaped provinces which received different types of deposits, and which were probably tectonically unlike. Some were basin areas, like the Delaware basin. Others were shelf areas. Akin to the shelves were several narrow masses, or platforms, lying between the basins.

Following the conclusions reached in Chapter Three, the provinces are believed to have been of tectonic origin, the basins being areas of greater subsidence, the platforms and shelves areas of less subsidence. The provinces appear to have been inherited from the pre-Wolfcamp foreland features, and each platform seems to be underlain by one of the more important pre-Wolfcamp uplifts. The Permian tectonic features may have been formed during a time of dominant crustal tension, following the pre-Wolfcamp time of dominant crustal compression.

These tectonic features were at least partly responsible for the stratigraphic features found in the Permian rocks. The basins were centers of accumulation of clastic rocks—first black shales and later sandstones—and the total thickness of beds deposited in them was greater than elsewhere. Limestone tended to form over all the higher-standing areas. Landward, because of climatic conditions that favored evaporation, evaporites were laid down in the fringing seas. On the margins of these seas, redbeds were deposited that were derived from the bordering lands.

In Wolfcamp time the seas spread over the whole of the West Texas region, but from Leonard time onward they became progressively more restricted, so that the belts of redbeds, evaporites, and limestones encroached farther toward the Delaware basin. One notable encroachment of evaporite took place in later Guadalupe time, when most of the area north and northeast of the Delaware basin was covered by it. Another took place in Ochoa time, when the Delaware basin itself was covered.

The general and gradual retreat of the seas was complicated by several lesser fluctuations, which from time to time caused marked restrictions and readvances. The most notable of these were at the end of Leonard time and the end of Guadalupe time, when the seas appear to have been nearly restricted to the Delaware basin. Following each restriction, the area of deposit and the seas gradually spread out again. These two restrictions may have arisen, not from local causes, but from a widespread change in sea-level with respect to the land, due either to a broad uplift of the continent, or to some sort of eustatic change.

The final event of Permian time was the disappearance of evaporite-depositing waters from the West Texas region, and the spreading over it of the last formation of the Ochoa series, a thin sheet of redbeds. After this, deposition ceased in the area until later Triassic time.

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