Geology of New Mexico

The geology of New Mexico formed beginning over 1.7 billion years ago in the Proterozoic as several poorly understood terranes merged. Five types of igneous and metamorphic crystalline basement rock date to the Precambrian. Throughout the Paleozoic, marine sediments and evaporites formed, followed by a series of major mountain building events and volcanism associated with the subduction of the Farallon Plate. Terrestrial conditions persisted until the late Mesozoic, when a marine transgression flooded the region. Significant volcanic activity including ash falls, lava flows and caldera collapse have defined the Cenozoic in New Mexico, along with the horst and graben rifting of the Basin and Range Province and the formation of the Rio Grande Rift. [1]

Stratigraphy, Tectonics & Geologic History

Precambrian rocks outcrop across approximately five percent of New Mexico and underlie the entire state. The rocks now exposed at the surface were uplifted during the Paleozoic, the early Cenozoic Laramide orogeny as well as block faulting and tilting in the more recent geologic past. For the most part, these rocks are exposed along the Rio Grande rift in the center of New Mexico, except in the Zuni Mountains and Burro Mountains. The total relief of Precambrian rocks is 11 kilometers.

The rocks are 70 percent plutons and 30 percent supracrustal formed between 1.765 and 1.4 billion years ago in the Proterozoic, based on uranium-lead dating. All of the rocks more than 1.65 billion years old show evidence of metamorphism ranging between greenschist and amphibolite grade on the sequence of metamorphic facies. An area in the Cimarron Mountains in the vicinity of Taos reached granulite facies. Geologists debate the extent of different terranes—sections of continental crust—that joined together. Metavolcanic rocks in the Tusas Mountains may be among the oldest, which are intruded by 1.65 billion year old trondhjemite, but display more than one metamorphic fabric.

Precambrian rocks formed volcanogenic polymettalic sulfides, rich in gold, silver and tungsten, kyanite, copper veins and pegmatite with beryllium, lithium, niobium, tantalum and mica. Geologists group New Mexico Precambrian rocks into five groups. Basalt formed between 1.76 and 1.72 billion years ago, followed by rhyolite around 1.7 billion. Meta-pelite and quartzite formed from 1.7 to 1.66 billion years ago, younger rhyolites from 1.66 to 1.65 and granites from 1.5 to 1.4 billion years ago. [2]

Beginning 1.35 billion years ago, coarse pegmatite formed in veins within pink granite as the region experienced magmatic activity associated with a mountain building event.

Paleozoic (541-251 million years ago)

At the end of the Neoproterozoic, shortly before the beginning of the Paleozoic with the proliferation of multi-cellular life, western Laurentia experienced tectonic-related tilting and a marine transgression flooded the region. Sequences of sandstone, siltstone and limestone deposited in the shallow sea and river deltas and floodplains preserved numerous marine fossils. For much of the early Paleozoic, no rocks deposited, or were removed by erosion.

By the late Paleozoic, in the Pennsylvanian and Permian, the Sonoma orogeny uplifted the ancestral Rocky Mountains. Simultaneously, a massive barrier reef developed in southern New Mexico, which developed large deposits of gypsum, potash and salt. [3]

Mesozoic (251-66 million years ago)

New Mexico experienced terrestrial conditions and new mountains were uplifted by the Sevier orogeny and Nevadan orogeny. Highlands shed sand and gravel into alluvial fans, floodplains and deltas, interspersed with volcanic ash.

Volcanism was in-part associated with the breakup of the supercontinent Pangaea as the Farallon Plate subducted beneath North America, generating volcanoes on the surface. Rising mountains in the west cut off moisture, creating a dry desert. By the Cretaceous the Western Interior Seaway flooded the region again. The Laramide orogeny, also powered by Farallon subduction, uplifting the Rocky Mountains and lasting into the Cenozoic. [4]

Cenozoic (66 million years ago-present)

In the Cenozoic, the Mid-Tertiary ignimbrite flare-up deposited significant ash falls across much of New Mexico. During the Oligocene, 30 million years ago, central New Mexico faulted, creating the Rio Grande rift and created isolated plateaus in the northwest.

The San Andreas fault disrupted the movement of the Farallon Plate, reducing the energy propelling it even as it continued to subside into the mantle. As a result, by the Miocene, southern New Mexico and neighboring areas had widespread horst and graben features associated with the Basin and Range Province. Major volcanic eruptions along the western Rio Grande rift showered the area in ash. The Rio Grande River and other waterways did not flow to the sea and with high rainfall, large lakes formed filling with sediment shed from the Rocky Mountains.

The Jemez Caldera formed one million years ago in the Pleistocene, exploding and the collapsing into its magma chamber. Small mountain glaciers formed in the Brazos and Sangre de Cristo Mountains. [5]

Natural resource geology

Under Spanish rule turquoise and lead were mined near Cerillos and copper was found at Santa Rita in the southwest in 1798. Artisanal mining for placer gold took place after an 1821 discovery in the Ortiz Mountains south of Santa Fe. New deposits, along with the reopened Spanish mine in Silver City prompted a boom in copper mining in the late 19th century. Placer gold mining expanded into the Sangre de Cristo Mountains and along the Rio Grande, while lead mining got underway in Las Cruces. Coal was discovered near gold and copper mines and potentially recoverable gold deposits grew more numerous as prospectors traced placer gold back to the veins where it originated.

In 1863, silver was found in Magdalean, west of Socorro, followed by a large find in Grant County. Silver City, White Oaks, Ute Creek, Cerrillos, Elizabethtown, Twining, Chloride, Hondo Canyon, Red River Canyon and Socorro were soon discovered to also have silver. Rising costs and depleted ore bodies have led to widespread abandonment and ghost towns throughout the state, which remain comparatively well-preserved in the dry climate.

Mining is still a cornerstone of the New Mexico economy, although it has largely shifted to open-pit extraction. Coal is mined in the northwest and copper, silver, gold, manganese, zinc and lead are extracted near Silver City. Molybdenum is an important resource in the Sangre de Cristo mountains, including the Questa Mine. Uranium is still mined close to Grants, although production has dropped after high point between the 1950s and the 1970s. Gypsum, limestone, potash and salt are mined out of Pennsylvanian and Permian rocks in the east.

The San Juan Basin in the northwest has active oil and gas production, along with the small extent of the Permian Basin in the southeast. [6]

References

  1. ↑ Chronic, Halka (1987). Roadside geology of New Mexico. Mountain Press Publishing Company. p. 225-228.
  2. ↑ Reed et. al. (1993). Precambrian: Coterminous US. Geological Society of America. p. 228-230.
  3. ↑ Chronic 1987, p. 27-28.
  4. ↑ Chronic 1987, p. 29-30.
  5. ↑ Chronic 1987, p. 30-31.
  6. ↑ Chronic 1987, p. 24-27.
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