Caballo Mountains

The Caballo Mountains, (Spanish: "horse") are a mountain range located in Sierra and Doña Ana Counties, New Mexico, United States. The range is located east of the Rio Grande and Caballo Lake, and west of the Jornada del Muerto; the south of the range extends into northwest Doña Ana County. The nearest towns are Truth or Consequences and Hatch.

Most of the Caballo Mountains are located on land owned by the Bureau of Land Management. The mountains are unusual, and perhaps unique in New Mexico, for the relatively complete geologic history revealed by their rocks. Vehicular access to the range is by Interstate 25 from the west, by New Mexico State Road 51 from the north, and by several dirt roads from the east.

Geography

The Caballo Mountains trend north and south, with a length of approximately 32 miles (50 km). The width of the range varies from 3–6 miles (5–10 km). The highest point on the range, Timber Mountain (7,565 feet), has a topographic prominence of approximately 3,400 feet.

The types, sizes, and profiles of the Caballo watersheds vary significantly according to the part of the range that they drain. No perennial streams flow through the mountains, although the course of the Rio Grande is close to the north, west, and south margins of the range. The northern part of the Caballos drains into the Rio Grande through Ash Canyon and Mescal Canyon. The confluences of those two intermittent streams are just below Elephant Butte Dam. Water falling on the precipitous west face of the range drains directly into the Rio Grande by means of numerous first order canyons with high gradients. All are ephemeral and less than 3 miles in length.

In contrast, the eastern side has only two primary drainages, each with a much lower gradient than those to the north and west. Water falling on the southern half of the east side flows into Barbee Draw. After joining another tributary near Point of Rocks to form Rincon Arroyo, this ephemeral drainage flows southward before joining the Rio Grande near Rincon, New Mexico. The length of this drainage is over 25 miles. The north portion of the east side is drained by Jornada Draw. This drainage, however, is endorheic and has no surface connection to the Rio Grande.

Geology

The geological importance of the Caballo Mountains lies in the variety and scope of their exposed rocks. Geologists can assess exposed rock formations in several ways. Relative dating compares the formation's bedding location in comparison with other formations nearby. This allows not only the identification of the older and younger formations, but conclusions about unusual local conditions, such as folding, a fault displacement, or an unconformity. Absolute dating measures the presence and ratios of chemicals within the rocks to determine the number of years since they were deposited. The conditions existing when the rock was deposited can be determined by examining its type (igneous, metamorphic, or sedimentary), the orientation and size of crystals or sediments within it, and the orientation of the formation within its larger regional setting. By studying plant and animal fossils that may exist in the formation, it may be possible to determine not only its relative age, but the climate at the time of deposition.

Weathering and erosion remove any rock that is exposed at the surface of the Earth. Consequently, there are no sites where all rocks deposited at that location have been preserved. Certain sites, however, may have more complete geological histories because of differences in the timing and circumstances of rock removal. The Caballo Mountains are essentially unique in New Mexico because their section of exposed rocks begins in Precambrian time, and runs through every geological period of the Phanerozoic eon with the exceptions of the Triassic and Jurassic. This provides an unparalleled picture of the geological history of southern New Mexico.

Stratigraphy

The basement rocks of the Caballos include plutons of granite, and metamorphic amphibolite and gneiss. These rocks date from the Early to Middle Proterozoic, and are found along the base of the larger fault blocks.

Throughout the early and middle Paleozoic southern New Mexico alternated between being flooded by shallow tropical seas, and being exposed to erosion. Sedimentary deposits of early and middle Paleozoic age include:
* Bliss Formation of arkosic sandstone and siltstone (Cambrian to Ordovician).
* El Paso Formation of dolomites and limestones (Ordovician)
* Montoya Formation of dolomites and cherty dolomites (Ordovician)
* Fusselman Dolomite (Silurian)
* Percha Shale (Devonian)
* Lake Valley Formation of limestones and cherty limestones ( Mississippian)

An unconformity exists above the Lake Valley Formation, and appears to include the middle Mississippian through the early Pennsylvanian periods. During the Pennsylvanian and Permian periods, a major deformation created a series of uplifts and intermontane basins. This event is often referred to as the Ancestral Rocky Mountains. Associated with this event is the Magdalena Group, which includes deposits of both marine and non-marine sources. The thickness of these strata is greater than that of all earlier Paleozoic rocks. Above the Magdalena Group is the Red House Formation of packstone and shale, the
Nakaye Formation of limestone, the Bar B Formation of interbedded limestone, dolomite, shale, and conglomerate, and the red shale beds of the Abo Formation. All of those strata are Pennsylvanian in age. Above them is the Permian Yeso Formation, which consists of sandstones, siltstone-dolomites, and limestones. The Paleozoic strata are completed by the Meseta Blanca Sandstone Member.

Most of the Mesozoic rocks are missing from the Caballo geological history. A second unconformity lies above the Meseta Blanca Sandstone, and includes all of the Triassic and Jurassic periods. The next oldest rocks are of Cretaceous age and include:
* Dakota Sandstone
* Rio Salado Tongue Member of Mancos Shale
* Tres Hermanos Formation of sandstones
* Cross Tongue Member of Mancos Shale
* Gallup Sandstone
* Crevasse Canyon Formation of sandstones and mudstones
* McRae Formation of sandstones, tuff, and conglomerate

The Mesozoic rocks terminate below another unconformity. Above that is the Love Ranch Formation of conglomerate, mudstone and sandstone, which was deposited in the Paleocene to early Eocene. During the Eocene, a wave of arc volcanism was felt across southern New Mexico. In the Caballo Mountains, this can be seen in the Palm Park Formation, a thick layer of andesitic or tuffaceous mudstones, sandstones, and breccia. Next is the Bell Top Formation, which consists of rhyolite, basalt flows, and tuffaceous sandstones. In the southern part of the Caballo Mountains, there are remnants of a region wide sheet of flood basalts and andesite, which was laid during the Oligocene. Of similar age is the Thurman Formation of white sandstone. The Santa Fe Group (geology) was deposited during the Miocene and Pliocene

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