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. This thick layer consists of the Hayner Ranch Formation (conglomerate, sandstone and shale), the Rincon Valley Formation (conglomerate, sandstone, and mudstone), the Selden basalts, and the Camp Rice/Palomas Formations (ash fall deposits, basalts, and pumice-clast conglomerates). Quaternary and Holocene deposits consist of alluvium. On the west side of the range along the Rio Grande, the alluvial fill consists of unconsolidated fluvial deposits of boulders, cobble, and gravel. In the closed basin on the east side of the range, there are relatively few fluvial deposits. Instead, the sediment includes a flat, thin veneer of sand and gravel.^[1]

Tectonics

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.^[2] 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^[3] Thin continental crust, normal fault block topography, high regional elevation, and volcanism is found over the course of the rift.^[1]

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 (above timelines) / Thousands of years (below timeline)

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.^[1]

Economic Geology

The Caballo Mountains have few large intrusives, and as a result, hydrothermal mineralization is not extensive. Nevertheless, prospecting in the range began in about 1883, and several mines opened in the first decade of the 20th century. Production has been concentrated in three areas:

• Northern Red Hills - fluorspar
• Palomas Gap - copper, vanadium, lead, and fluorspar
• Southern Caballos - gold, fluorspar, barite, and manganese

It has been estimated that the total value of copper, silver, gold, and lead produced from the Caballos before 1952 did not exceed $200,000. At present, more fluorspar is produced than any other mineral. The largest fluorspar deposits are found in Precambrian granite, and jasperized Fusselman Dolomite.^[1]