GEOLOGY OF THE SOUTHERN GUADALUPE MOUNTAINS, TEXAS
By PHILIP B. KING
This report deals with an area of 425 square miles in the western part of Texas, immediately south of the New Mexico line. The area comprises the south end of the Guadalupe Mountains and the adjacent part of the Delaware Mountains; it includes the highest peaks in the State of Texas. The area is a segment of a large mountain mass that extends 50 miles or more northward and southward. The report describes the geology of the area, that is, the nature of its rocks, tectonics, and surface features, and the evidence that they give as to the evolution of the area through geologic time. Incidental reference is made to the geology of surrounding regions in order to place the area in its environment.
Stratigraphy of Permian rocks.The consolidated rocks of the area are all marine sediments of Permian age, whose total exposed thickness is about 4,000 feet. Most of the rocks contain abundant invertebrate fossils, some of which were described by B. F. Shumard in 1858. They were made famous by the classic study of G. H. Girty in 1908. The rocks consist chiefly of sandstones and limestones of various textures and structures, and are notable for their abrupt change from one rock type into another within short distances. This characteristic is believed to have been caused by the rocks being laid down on the margin of the Delaware Basin, a structural feature of Permian time. The margin lay between the more rapidly subsiding basin and a less rapidly subsiding shelf area to the northwest.
The lowest exposed formation is the Bone Spring limestone. Two deep wells indicate that it is underlain by the Hueco limestone (of Carboniferous or Permian age), and this by rocks of Pennsylvanian age. The Bone Spring is predominantly black, thin-bedded limestone to the southeast, in the basin area, but to the northwest this facies changes into gray, thicker-bedded limestone. At the margin of the basin, the formation is raised along the Bone Spring flexure, which was apparently in movement toward the close of Bone Spring time, as the succeeding beds overlap the flexed strata.
Overlying the Bone Spring limestone to the southeast, in the basin area, is the Delaware Mountain group, a mass 2,700 feet thick, consisting largely of sandstone, most of which is fine grained. The group is separable into three formations; in the lower are many beds of coarse-grained sandstone, and in the upper two a number of limestone members.
Northwestward, away from the basin, great changes take place in the rocks of Delaware Mountain age. The lower formation overlaps the older rocks along the Bone Spring flexure and is absent beyond. The lower part of the middle formation persists northwestward as a thin sandstone tongue, but the upper part changes into the Goat Seep limestone. Near its southeast edge this limestone forms a set of massive beds over 1,000 feet thick, whose form suggests that the limestone beds grew as reefs along the edge of the basin area. Farther northwest, the limestone becomes thinner bedded, and contains much interbedded sandstone.
In the same manner, the upper formation of the Delaware Mountain group changes northwestward into the thick mass of the Capitan limestone, which, like the Goat Seep was probably a reef deposit. The Capitan reaches a thickness of nearly 2,000 feet and forms some of the highest peaks and ridges of the Guadalupe Mountains. The formation does not persist far to the northwest, however, and within a few miles is replaced by the thin-bedded Carlsbad limestone. Still farther north, beyond the area studied, these limestones change in turn into the anhydrites, sandstones, and red beds of the Chalk Bluff formation.
The invertebrate fossils of the Delaware Mountain group and its correlatives exhibit considerable variety both laterally and vertically. The lateral changes are interpreted as resulting from differences in environment, and the vertical changes not only to changes in environment, but also to progressive evolution with the passage of time. Differences in environment are suggested by the contrasting nature of contemporaneous deposits; there were probably also differences in the chemistry of the water, its degree of agitation, and its depth. Available evidence indicates that the limestone reefs of the Goat Seep and Capitan formations were laid down in relatively shallow water, and that the equivalent Delaware Mountain deposits to the southeast were laid down in deeper water.
Above the Delaware Mountain group in the basin area are the anhydrites of the Castile formation, also of Permian age, which were laid down after the waters of the region were shut off from free access to the sea. No younger consolidated rocks are exposed in the area. Younger Permian formations are present farther east, however, and a greatly dissected ancient erosion surface on the mountain summits is probably the exhumed surface on which Cretaceous rocks were once deposited.
Tectonic features.The mountain mass of the Guadalupe and Delaware Mountains is a great uplifted block of the earth's crust. Although some earlier movements took place, the movements that raised the block itself took place entirely in Cenozoic time. The structure of the block resembles that of other mountain blocks of the Basin and Range province. The east flank is a gently tilted surface which descends toward the slightly disturbed area of the Pecos valley and Llano Estacado at the east. The west flank is steep and broken by numerous faults, some of which have displacements, of thousands of feet and serve to outline the west side of the mountains. West of the mountains downfaulted rocks are exposed here and there in low foothills, and beyond is a lowland, the Salt Basin, in which the bedrock is greatly depressed and is covered to a thickness of more than 1,000 feet by unconsolidated Cenozoic deposits.
The faults along the west flank of the mountains in general trend parallel to the long axis of the uplift and are either vertical or dip steeply toward the downthrow. The rocks are cut by numerous joints whose dip and trend are similar to those of the faults. The faults appear to be tensional features, but the uplift itself was caused by vertically acting movements, whose ultimate cause may have been compressional force.
Cenozoic deposits and land forms.The present land surface of the Guadalupe and Delaware Mountains closely resembles the structural form of the uplift, but there are actually considerable differences. These differences have resulted from degradation of the uplifted parts and deposition of sediments on the depressed parts by subaerial agencies similar to those now at work in the region. The evolution of the Cenozoic deposits and land forms is thus closely related to the upheaval of the mountain area.
The uplift took place in several stages. After the first uplift, consequent streams formed on the sloping surface of the mountain block, and some of their courses are preserved with little modification today. Material washed from the mountains after the first uplift was deposited in the nearby lower areas and is probably represented by the oldest unconsolidated rocks of the Salt Basin and Llano Estacado. These materials are probably of Pliocene age.
A second period of uplift probably took place in late Pliocene or early Pleistocene time and raised the mountains nearly to their present height. This uplift gave rise in places to new consequent streams, which flowed along fault troughs. It also caused renewed degradation in the mountains. The resistant rocks of the Guadalupe Mountains were incised by deep canyons, and the less resistant rocks of the Delaware Mountains were worn down to a plain of about the same altitude as the present canyon bottoms.
In Pleistocene time, perhaps as a result of fluctuation in climate, a part of this lower country was buried under a sheet of gravel. Deposition of coarse-grained deposits took place west of the mountains also, partly as a result of climatic change but mainly in response to the uplift of the adjacent mountains. During this period the Salt Basin was probably covered by standing water, for the upper surface of the fine-grained deposits that form its floor has a conspicuous levelness, such as could not have been caused by streams or subaerial agencies. Faint beach ridges present in the Salt Basin indicate the existence of a lake in late Pleistocene time.
In late Pleistocene time, the area was again disturbed. Renewed movements of small amount took place along some of the faults on the west flank of the mountains, and some of the previously formed unconsolidated deposits were displaced. The disturbance also caused renewed dissection of the land surfaces. Erosion and sedimentation that followed this time of disturbance have shaped the mountains into their present form.
Economic geology.The main economic interest of the area is indirect. Knowledge of the area is valuable to petroleum geologists because features exposed at the surface here are analogous to features to the east known only from drilling in the oil fields. No oil or gas has been found in the area itself, but the area has not been adequately tested by wells. There is a slight possibility that oil or gas may be discovered in the deeper formations.
The other economic resources of the area are meager. Some building stone, road material, and salt have been produced. In a few places are small mineral deposits, but no ore has been mined from them. The resource most valued by the local residents is ground water, for the region is generally dry and with out permanent streams. Here and there ground water issues as springs, whose intakes are the higher parts of the area, where rainfall is greater than in the lower parts.
Last Updated: 28-Dec-2007