The Rinconada 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 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 e ...

of northern

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe, New Mexico, Santa Fe , LargestCity = Albuquerque, New Mexico, Albuquerque , LargestMetro = Albuquerque metropolitan area, Tiguex , Offi ...

. Detrital zircon geochronology establishes a maximum age for the Rinconada Formation of about 1723

Mya Mya may refer to: Brands and product names * Mya (program), an intelligent personal assistant created by Motorola * Mya (TV channel), an Italian Television channel * Midwest Young Artists, a comprehensive youth music program Codes * Burmese ...

, placing it in the Statherian period of the

Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, 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 th ...

Description

The formation consists of gray to buff

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage ...

schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes ...

with some inberbedded

quartzite Quartzite is a hard, non- foliated metamorphic rock which 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 tec ...

and

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 ...

. which fills an inverted

syncline In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimposed ...

in the northern Picuris Mountains. It is missing from the Hondo Group in the

Tusas Mountains The Tusas Mountains are a mountain range in northern New Mexico, extending slightly into southern Colorado. They are considered the southeasternmost part of the San Juan Mountains. Grouse Mesa, , is the highest peak in the range. The mountains a ...

. Detrital zircon geochronology establishes a minimum age for the Rinconada Formation of 1762

. However, this reflects the crystallization age of the source rocks, and the true age is likely closer to 1723 Mya based on zircon ages in the underlying

Ortega Formation The Ortega Formation is a geologic formation that crops out in most of the mountain ranges of northern New Mexico. Detrital zircon geochronology establishes a maximum age for the formation of 1690-1670 million years ( Mya), in the Statherian peri ...

. This indicates a shift in source region between the Ortega Formation and the Rinconada Formation. Weathered out rinconada staurolite

The formation is divided into three informal

members Member may refer to: * Military jury, referred to as "Members" in military jargon * Element (mathematics), an object that belongs to a mathematical set * In object-oriented programming, a member of a class ** Field (computer science), entries in ...

. The first is composed of staurolite

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...

and schist, from to thick, which contains porphyroblasts of

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different ...

and staurolite up to long. Most of the staurolite from these beds is

twinned Twinning (making a twin of) may refer to: * In biology and agriculture, producing two offspring (i.e., twins) at a time, or having a tendency to do so; * Twin towns and sister cities, towns and cities involved in town twinning * Twinning inst ...

, and the large crystals are prized by mineral collectors. Where the schist is heavily micaceous and easily eroded, the ground surface is covered with weathered-out staurolite crystals. This member includes spotty basal andalusite-

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more alum ...

hornfels Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These pr ...

, from to thick, consisting of biotite crystals up to in size in a coarsely felted mass of muscovite and quarts. This includes knots of quartz and andalusite up to in diameter. Staurolite cross

The staurolite likely crystallized out at a temperature of 532 ± 20 °C and about 3,700 bar total pressure under conditions of water undersaturation. The second member is a gray-white quartzite, to thick, with slabby jointing and devoid of

sillimanite Sillimanite 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. Occurrence ...

and kyanite. The third member is a muscovite-rich

phyllite Phyllite ( ) is a type of foliated metamorphic rock created from slate that is further metamorphosed so that very fine grained white mica achieves a preferred orientation.Stephen Marshak ''Essentials of Geology'', 3rd ed. It is primarily compo ...

with a pearly to greenish gray sheen containing small staurolite and garnet crystals. The Rinconada Formation is separated from the overlying

Pilar Formation The Pilar Formation is a geologic formation that crops out in the Picuris Mountains of northern New Mexico. It has a radiometric age of 1488 ± 6 million years, corresponding to the Calymmian period. Description The Pilar Formation consists of ...

by an

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

representing a gap in geologic time of 200 million years. The Rinconada Formation is interpreted as deltaic,

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluv ...

, and shallow marine deposition of the regressive sequence of a

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

. This was likely part of the northern coast of a

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

associated with the Yavapai

orogeny Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An '' orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted ...

, named the Pilar basin. The quartzite at the upper contact with the

then represents renewed transgression.

History of investigation

The beds making up the unit were originally assigned to the Ortega quartzite by Evan Just during his 1937 survey 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 co ...

s in northern New Mexico. Evan gave the informal name, Rinconada schist, to these beds and noted the presence of staurolite

porphyroblasts A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained matrix. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape. The most common porphyrob ...

. Montgomery formalized the designation of the mixed

and

beds as the Rinconada member of the Ortega Formation in 1953. The Rinconada Member was promoted to independent formation status by Bauer and Williams in their sweeping revision of the

stratigraphy of northern New Mexico in 1987.

Footnotes

References

* * * * * * * * {{cite journal , last1=Montgomery , first1=Arthur , title=PreCambrian Geology of the Picuris Range, northcentral New Mexico , journal=State Bureau of Minfluvial to shallow-marine deposition on a southward-deepening siliciclastic shelf.es 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 Schist formations