USGS: Geological Survey Bulletin 845 (Itinerary)

The pre-Cambrian schist constitutes the Carrizo Mountains, which lie between the Texas & Pacific and Southern Pacific Railroads west of Van Horn. It is also exposed in a few small areas in the adjacent region. The Millican formation, which contains much red shale and cherty limestone, resembles the Grand Canyon series and Apache group of Arizona. It constitutes an area of foothills northwest of Van Horn. Although undoubtedly it is younger than the schist, the two formations are separated by faults, a feature well exposed a short distance north of Eagle Flat station on the Texas & Pacific Railway, where the limestone of Permian age overlaps both. The presence of schist fragments in the conglomerate and the lesser degree of metamorphism of the Millican beds indicate that they are younger than the schists, doubtless of Algonkian age. The Van Horn sandstone lies on an irregularly eroded surface of the pre-Cambrian rocks. It is several hundred feet thick in the ridges northwest of Van Horn, but it thins out in places, together with the overlying old limestones, so that the limestone of Permian age lies directly on the Millican formation or on schist. In part of the area the Permian limestone lies directly on El Paso limestone, as shown in Plate 15, A. In places the Van Horn beds, especially the lower ones, consist largely of coarse material including fragments of red granite and porphyry of pre-Cambrian age which probably formed a shore near by, now covered by later sediments. Much of the Van Horn material is of red color. The succession of El Paso to Fusselman limestone which largely constitutes Beach Mountain and some of the foothills about the Baylor Mountains, north of Van Horn, is very similar to the formations in the Franklin Mountains north of El Paso and contains the same fossils. Some of them can also be correlated with strata in the Marathon uplift. These limestones thin out to the south and west. They also appear in the foot of the Sierra Diablo, 25 miles north of Van Horn, just west of the road to the Carlsbad Caverns. The Fusselman limestone contains a characteristic Pentamerus; the Montoya contains Columnaria, Halysites, Streptelasma, Rhynchotrema, and Rafinesquina of the Upper Ordovician; and the El Paso limestone contains Piloceras, Eccyliomphalus, Hormotoma, and Ophileta of the Lower Ordovician.

The Permian succession that constitutes the great mountain block of the Sierra. Diablo, the Baylor and Wylie Mountains, and some minor ridges consists mostly of limestones of various kinds presenting considerable variety of fossil assemblages. The thickness remaining in the high eastern front of the Sierra Diablo is 2,700 feet. During Permian time there was nearly continuous subsidence and deposition in the region, uninterrupted by uplift or extensive incursions of coarse sediments. Evidently there were long reefs which persisted during the deposition of thousands of feet of strata. The position and extent of land at that time in the general region are not known. These reefs had a controlling effect on the sedimentation. In the open sea in front of them were deposited materials now represented by flaggy black limestones, siliceous shales, and fine sandstones which contain such Guadalupian fossils as Richthofenia and Leptodus. Behind the reefs were lagoons in which were deposited sediments now represented by thinly stratified dolomite containing fusulinids in extreme abundance. Farther behind, to the west and southwest, there were accumulations of limy sediments with a fauna like that in the Hueco Mountains, including Omphalotrochus, Bellerophon, Productus peruvianus, and Spirifer condor. The reefs consist of very massive limestones or dolomites, built of the remains of algae, bryozoans, sponges, crinoids, and other fossils adapted to reef environment.

The Permian of the Wylie and Baylor Mountains consists of limestones laid down behind the reefs. In the Sierra Diablo a thick body of lagoon limestone constituting the lower third of the series is succeeded by black limestone and other open-sea deposits cropping out in rounded slopes, which are surmounted by great cliffs of the reef limestone. Some of the relations of these strata are shown in Figure 18. In places in the escarpment there is an abrupt transition from black limestone to reef limestone. At the north end of the Sierra Diablo, 40 miles northwest of Van Horn, the reef limestones are overlain by sandstones of the Delaware Mountain formation, a remnant of the thick succession of pre-Capitan strata well developed in the Delaware and Apache Mountains, on the east side of Salt Basin. (P. B. King.)