The Pilar 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 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 ...

. It has a radiometric age of 1488 ± 6 million years, corresponding to the

Calymmian The Calymmian Period (from , meaning "cover") is the first geologic period in the Mesoproterozoic Era and lasted from Mya to Mya (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically. The peri ...

period.

Description

The Pilar Formation consists of a dense gray-black to black hard carbonaceous

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

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 ...

with thin white schistose layers that are interpreted as a metatuff. The rock has very irregular slaty cleavage, with tiny

muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula KAl2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage y ...

flakes visible in the hand lens. It contains many

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 ...

veins and some

limonite Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as , although this is not entirely accurate as the ratio of oxide to hydroxide can vary qu ...

augens. Its composition is 50% to 75% quartz and 15% to 30% muscovite with carbonaceous material finely disseminated throughout the quartz and muscovite grains. Montgomery rejected its designation as a

slate Slate is a fine-grained, foliated, homogeneous, metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade, regional metamorphism. It is the finest-grained foliated metamorphic ro ...

, as it has undergone middle-grade metarmophism along with the underlying staurolite-bearing Rinconada Formation. A metamorphosed tuff bed in the Pilar Formation yields an age of 1488 ± 6 Mya, considerably younger than the Rinconada Formation. This suggests that the Pilar Formation and Piedra Lumbre Formation should be removed from the Hondo Group (and placed in the informal Trampas Group) and provides evidence supporting 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 ...

. The base of the formation is 2-3 meters of garnet-bearing quartzite that is blue-black in color. The total thickness is . The contact with the underlying Rinconada Formation is an unconformity representing a 200 million year gap, while the contact with the overlying

Piedra Lumbre Formation The Piedra Lumbre Formation is a geologic formation that crops out in the Picuris Mountains of northern New Mexico. Detrital zircon geochronology yields a maximum age of 1475 million years, corresponding to the Calymmian Period (geology), period. ...

is gradational. Because of its distinctive appearance, the formation is an important structural marker, which provided evidence that the Hondo Group of which it is a part 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 ...

. Its carbon content is one of the earliest traces of life in northern New Mexico.

History of investigation

The formation was originally designated as the Hondo Slate by Evan Just in 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 c ...

s in northern New Mexico. However, this name conflicted with other unit names, and it was renamed the Pilar Phyllite Member of the Ortega Formation by Arthur Montgomery in 1953 Bauer and Williams promoted it to formation rank within the Vadito Group in their sweeping revision of the stratigraphy of northern New Mexico in 1989. With the realization that the formation was much younger than the lower Vadito Group, the formation was reassigned to the Trampas Group by Christopher G. Daniel and coinvestigators in 2013. File:Pilar Formation near Plomo Fault.jpg, Pilar Formation exposures on Arroyo del Plomo File:Pilar Formation outcrop.jpg, Outcrop of Pilar Formation showing metatuff beds (white layers) File:Pilarsample.jpg, Sample of carbonaceous phyllite from the Pilar Formation

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 Schist formations