The Ortega Formation is a

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

that crops out in most of the mountain ranges of northern

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

Detrital zircon geochronology Detrital zircon geochronology is the science of geochronology, analyzing the age of zircons deposited within a specific sedimentary rock, sedimentary unit by examining their inherent radioisotopes, most commonly the uranium–lead dating, uranium ...

establishes a maximum age for the formation of 1690-1670 million years ( Mya), in the

Statherian The Statherian Period (; , meaning "stable, firm") is the final geologic period in the Paleoproterozoic Era and lasted from Mya to Mya (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically. Th ...

period of the

Precambrian The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of t ...

.Jones ''et al.'' 2011

Description

The Ortega Formation consists of a very clean (98% modal quartzBauer 2004, p.198), typically bluish-white,

quartzite Quartzite is a hard, non- foliated metamorphic rock that was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tecton ...

, with some beds near the base of the formation composed of

metaconglomerate Metaconglomerate is a rock type which originated from conglomerate after undergoing metamorphism. Conglomerate is easily identifiable by the pebbles or larger clasts in a matrix of sand, silt, or clay. Metaconglomerate looks similar to conglome ...

Crossbedding In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The Principle ...

is found throughout the formation, and

aluminosilicate Aluminosilicate refers to materials containing anionic Si-O-Al linkages. Commonly, the associate cations are sodium (Na+), potassium (K+) and protons (H+). Such materials occur as minerals, coal combustion products and as synthetic materials, of ...

minerals are abundant within the formation. These show that its lower beds were buried deeply enough to be metamorphosed to the

sillimanite Sillimanite or fibrolite is an aluminosilicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman (1779–1864). It was first described in 1824 for an occurrence in Chester, Connecticut ...

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or con ...

, at temperatures of over . The Ortega Formation is the principal ridge-forming formation of the

Picuris Mountains The Picuris Mountains are a mountain range in northern New Mexico. They are considered a subrange of the Sangre de Cristo Mountains. Geography The mountains are located to the east of Dixon and surround Picuris Pueblo to the west, north, and ...

and is a uniform thick. The contact between the Ortega Formation and the underlying

Vadito Group The Vadito Group is a group of geologic formations that crops out in most of the Precambrian-cored uplifts of northern New Mexico. Detrital zircon geochronology and radiometric dating give a consistent age of 1700 Mya for the group, correspondin ...

is fairly easy to trace using a regional

manganese Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...

-rich marker bed in the uppermost Vadito Group. This is a ductile

shear zone In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shear ...

associated either with mountain collapse at the end of the

Mazatzal orogeny The Mazatzal orogeny was an orogenic event in what is now the Southwestern United States from 1650 to 1600 Mya in the Statherian Period of the Paleoproterozoic. Preserved in the rocks of New Mexico and Arizona, it is interpreted as the coll ...

or tectonics of the

Picuris orogeny The Picuris orogeny was an orogenic event in what is now the Southwestern United States from 1.43 to 1.3 billion years ago in the Calymmian Period of the Mesoproterozoic. The event is named for the Picuris Mountains in northern New Mexico and ...

that resulted in south-directed displacement of the Ortega Formation over the Glenwoody Formation. Structurally, the Ortega Formation tends to form stiff limbs within which weaker formations are heavily distorted. The Ortega Formation is quite uniform in thickness everywhere but the northern Picuris Mountains, where its thickness appears to have been doubled by tectonic imbrication.Bauer 2004, p.199 The Ortega Formation may

correlate In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistics ...

with the Uncompahgre Formation of

Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...

, the Mazatzal Group in

Arizona Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...

, and other

Proterozoic The Proterozoic ( ) is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...

quartzite successions associated with the

Yavapai The Yavapai ( ) are a Native American tribe in Arizona. Their Yavapai language belongs to the Upland Yuman branch of the proposed Hokan language family. Today Yavapai people are enrolled in the following federally recognized tribes: * Fort ...

and Mazatzal

orogenies Orogeny () is a mountain-building process that takes place at a convergent plate margin when plate motion compresses the margin. An or develops as the compressed plate crumples and is uplifted to form one or more mountain ranges. This involv ...

. These all appear to be first cycle sandstones, in which the individual sand grains have eroded out of igneous or metamorphic rock rather than been recycled from older sedimentary rock. Their remarkable maturity (they were formed from nearly pure quartz grains) may be a result of deep weathering processes acting on the original sediment beds under unusual Proterozoic conditions.Medaris ''et al.'' 2003 The formation is interpreted as the first stage of a

marine transgression A marine transgression is a geologic event where sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling with water ...

on a southward-dipping

siliciclastic Siliciclastic (or ''siliclastic'') rocks are clastic noncarbonate sedimentary rocks that are composed primarily of silicate minerals, such as quartz or clay minerals. Siliciclastic rock types include mudrock, sandstone Sandstone is a Clastic ...

shelf. This was likely part of a

back-arc basin A back-arc basin is a type of geologic Structural basin, basin, found at some convergent boundary, convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found ...

associated with the Yavapai orogeny, named the Pilar basin.Davis ''et al.'' 2011 Tabular

cross-bedding In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The origina ...

permits the orientation of the highly distorted beds to be determined.

History of investigation

The unit was named by Evan Just in 1937 during his investigation of

pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic c ...

s in northern New Mexico. Just included the entire sequence of

and

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...

in the

in his definition, assigning the schist to the Rinconada schist member,Just 1937, p.21-22 and including the feldspathic Petaca Schist in the Tusas Mountains.Just 1937, p.43 Arthur Montgomery recognized the Rinconada schist as well but included the Pilar slate in the Ortega FormationMontgomery 1953, p.1 while assigning some of the schist and conglomerate beds to his Vadito Formation.Montgomery 1953, p.21 During mapping of the Las Tablas area, Barker redefined the Ortega Quartzite to include only the quartzite and basal conglomerate beds.Barker 1958, p.11 In their sweeping revision of the

stratigraphy of Northern New Mexico, Bauer and Williams split the Glenwoody Formation from the Ortega Formation and defined the Ortega Formation to consist only of the massive quartzite and basal conglomerate bedsBauer and Williams 1989, p.50 while reassigning the Petaca Schist mostly to the

( Burned Mountain Formation).Bauer and Williams 1989, p.49

Footnotes

References

* * * * * * * * {{cite journal , last1=Montgomery , first1=Arthur , title=PreCambrian Geology of the Picuris Range, northcentral New Mexico , journal=State Bureau of Mines and Mineral Resources Bulletins , date=1953 , volume=30 , url=https://geoinfo.nmt.edu/publications/monographs/bulletins/downloads/30/Bulletin030.pdf Precambrian formations of New Mexico Quartzite formations