The Yucca Formation - Early Cretaceous continental and transitional environments, southern Quitman Mountains, Hudspeth County, Texas
Donald H. Campbell
The Quitman Mountains are part of an extensive range, 104 kilometers long, which includes the Malone Mountains to the north in Hudspeth County, Texas, and the Sierra Cieneguilla to the south in the Mexican state of Chihuahua. Structurally the range lies on the eastern side of the Chihuahua Tectonic Belt in which Laramide deformation is typified by overturning and thrust-faulting to the northeast. An inferred high-angle normal fault borders the Quitman Mountains on the southwestern side, intersecting the strike of the Yucca Formation at a low angle. Major normal faulting appears not to have taken place on the northeast side of the Quitmans, where the sedimentary strata dip beneath the bolson deposits. The region is physiograhically placed in the Basin and Range Province (Mexican Section).
Quitman Gap forms a narrow pass between the northern and southern Quitmans and divides the mountains into two lithologically contrasting parts—primarily Tertiary igneous rocks to the north and Cretaceous sedimentary rocks to the south.
The northern Quitmans are composed of silicic, coarsely crystalline plutonic rocks such as granite, monzonite and syenite and slightly older, fine-grained volcanic rocks such as rhyolite and trachyte. The volcanic rocks are believed (Huffington, 1943) to have collapsed into the plutonic mass during its intrusion. A zone of contact metamorphism occurs in the sedimentary rocks adjacent to the intrusive bodies and is observed in prospect pits and arroyos between Quitman Gap and the mountains to the north. Metasedimentary strata include metasandstone and hornfels, with minor amounts of marble. Typical minerals in the zone of contact metamorphism are actinolite, epidote, garnet, wollastonite, scapolite, tremolite, limonite, and quartz. Sulfides and other ore minerals have been reported as replacements in the sedimentary rocks and in fissure veins associated with the metamorphic aureole. Nickel, uranium, and scheelite, as well as silver and gold have also been discovered in small quantities (Huffington, 1943). The age of the volcanic activity, intrusion, and metamorphism is early to mid-Tertiary.
The southern Quitman Mountains comprise Cretaceous sedimentary strata for the most part, including large volumes of sandstone and shale, and smaller amounts of limestone. Structurally the southern Quitmans are a block-faulted, thrust faulted, nearly recumbent anticline and syncline (Jones, 1968; Jones and Reaser, 1970). Jurassic evaporites acted as a decollement zone during much of the Laramide deformation in the Chihuahua Tectonic Belt (Haenggi and Gries, 1970). Evaporites have not been found in the Quitmans but may occur in the subsurface. Reaser (1974) believed that the set of structural-geomorphic features from the Malone Mountains south-southeastward to the Cuchillo Parado on the Rio Conchos forms a major structural element of the eastern Chihuahua Tectonic Belt, similar to and parallel with the La Mula Sierra Blanca Range to the east, reflecting regional Laramide compreslion, tilting, and gravity sliding to the northeast. Regional structural history has been summarized also by Haenggi and Gries (1970).
- Campbell, Donald H., 1980, The Yucca Formation - Early Cretaceous continental and transitional environments, southern Quitman Mountains, Hudspeth County, Texas, in: Trans-Pecos Region, Dickerson, Patricia W.; Hoffer, Jerry M.; Callender, Jonathan F., New Mexico Geological Society, Guidebook, 31st Field Conference, pp. 159-168.