Caballo Mountains

Caballo Mountains (New Mexico) new article content ... Template:Geobox

The Caballo Mountains are a mountain range located in Dona Ana County and Sierra County, New Mexico, United States. The range is located east of the Rio Grande and Caballo Lake, and west of the Jornada del Muerto. The nearest towns are Truth or Consequences and Hatch. Most of the range is 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 Highway 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 Rio Grande Rift is a continental rift zone, which extends approximately 680 miles (~1,100 km) from central Colorado in the north to the state of Chihuahua, Mexico in the south. It is the product of extensional tectonic (or divergent) forces that formed a series of basins between the Colorado Plateau and the North American craton Thin continental crust, normal fault block topography, high regional elevation, and volcanism is found over the course of the rift.

The following five timelines show the geologic time scale to scale. The first shows the entire time from the formation of the Earth to the present, but this gives little space for the most recent eon. The second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, the most recent period is expanded in the fourth timeline, and the most recent epoch is expanded in the fifth timeline.

(Horizontal scale is millions of years for the above timelines; thousands of years for the timeline below)

The history of movement along the rift has been divided into three separate stages: an initial stage during the late Eocene to early Oligocene, a second stage during the late Oligocene to late Miocene, and a third stage beginning in the late Miocene or early Pliocene and continuing to the recent past. Activity in the vicinity of the Caballos during the first stage included explosive rhyolitic eruptions from cauldrons located to the northwest, west, and southeast. The earliest eruptions took place in the Dona Ana Mountains and Organ Mountains between 36.2 and 35.4 Ma. The Emory cauldron, located in the Black Range, was active approximately 34.9 Ma. What is now the San Mateo Mountains (Socorro County) contained at least two cauldrons, the Nogal Canyon cauldron (28.6 Ma) and the Mount Withington cauldron (27.4 Ma). Local subsiding appears to have begun around 34.9 Ma. The Caballo Mountains began to rise in the late Oligocene, and throughout the Miocene. Approximately 9.6 Ma, the eruption of basalts near Fort Selden signaled the renewal of crustal extension and volcanism in the southern part of the rift. Activity along both older and new faults increased the number, but decreased the width of basins along this portion of the rift.

Geologists can assess exposed rock formations in several ways. The relative age of the rock (older or younger) can be determined by comparing the formation's bedding location in comparison with other rocks of a known age. 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 animals fossils that may exist in the formation, it may be possible to determine not only its relative age, but the climatic setting 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.

References

Hawley, J.W. Guidebook to Rio Grande rift in New Mexico and Colorado New Mexico Bureau of Mines and Mineral Resources, Socorro 1978.

Mack, Greg J.,The Geology of Southern New Mexico, University of New Mexico Press, Albuquerque 1997

Seager, William R. and Greg H. Mack, Geology of the Caballo Mountains, New Mexico New Mexico Bureau of Geology and Mineral Resources, Socorro 2003

External links

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