|Abstract:||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