New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

Permian through Eocene sedimentary rocks, and intrusive and extrusive rocks of the Eocene-Oligocene Sierra Blanca volcanic complex and Capitan Pluton are exposed in the northeastern Sacramento Mountains in the vicinity of Ruidoso and Capitan. This region includes the eastern flank of the Laramide-age Sierra Blanca basin. Six contiguous 7.5- minute quadrangles were mapped here with STATEMAP funding from 2003 – 2010 and form the basis for geologic understanding of the area.
The oldest mapped unit is the Permian Yeso Formation, which is well-exposed in rugged terrain north of Ruidoso Downs. Dissolution of evaporites has resulted in intraformational breccias and disharmonic folding within the Yeso, and local collapse of the overlying Permian San Andres Formation. Subdivsions of the San Andres Formation defined in the southeast Sacramento Mountains are not mappable in this area because of the thick forest cover. Karst within the San Andres Formation is indicated by solution cavities and breccias, and terrarossa. Large thickness variations (0 to 135 m) and basal topography of the siltstone and very fine sandstone that comprise the overlying, unconformity-bounded Grayburg Formation suggest that some of the karst is Permian in age. The Triassic Santa Rosa and San Pedro Arroyo Formations are relatively thin and are cut out by the unconformity beneath the Cretaceous Dakota Sandstone in Ruidoso. The same unconformity truncates the Jurassic Morrison Formation near Capitan. Igneous intrusions of all varieties are abundant within the Cretaceous Mancos Shale and Crevasse Canyon Formation sandstones and shales. The Eocene Cub Mountain Formation comprises the Laramide basin fill and occurs in disconnected half-grabens. Syenite of the Bonito stock underlies Monjeau Peak and intrudes dominantly andesite breccias and lavas of the Sierra Blanca volcanic complex. Gravels mantling elongate east- and northeast-trending mesas between Alto and Capitan are western equivalents of the late Miocene to Pliocene Ogallala Formation.
The Ruidoso fault zone forms the eastern boundary of the Sierra Blanca basin in Ruidoso and strikes north to northeast through the mapped area. Maximum throw is at least several thousand feet down-to-the west, near Moon Mountain, where Dakota Sandstone is juxtaposed against San Andres Formation. To the southwest is a complex array of faults near Mescalero Lake. Towards Fort Stanton and Capitan, the fault zone bifurcates into a network of north- to east-striking faults, most of which are buried beneath the gravel-capped mesas, suggesting topographic inversion. Evidence for dextral oblique offset along northeast-striking faults includes sparse shallowly plunging slickenlines, variations in apparent throw along strike, and fold axes nearly perpendicular to adjacent faults. Many of these faults terminate at the Capitan fault, a major east-striking, down-to-the-south normal fault at the south margin of the Capitan Pluton that forms the Capitan Mountains. Fine-grained sedimentary units host abundant igneous intrusions (sills, stocks and laccoliths in the Mancos Shale) and accommodated distributed deformation at fault tip zones (Grayburg Formation). A notable example of the latter occurs along Eagle Creek, where a thick occurrence of Grayburg Formation was deposited in a probable paleokarst depression.