The Burned Mountain 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

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 ...

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 U-Pb radiometric age of 1700 Mya, corresponding to 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 Period may refer to: Common uses * Period (punctuation) * Era, a length or span of time *Menstruation, commonly referred to as a "period" Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (o ...

Description

The Burned Mountain Formation as originally defined is a metamorphosed

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...

that appears to have

intruded Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form ''intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: March 27 ...

the Moppin Complex, mostly as sills.Barker 1958, p.55 Portions of the formation are described as quartz-eye schists. The unit was later expanded to include most of the Vadito Group beds found throughout the central Tusas Mountains, representing the upper portion of the Vadito Group in this region and correlating with the Glenwoody Formation 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 ...

.Bauer and Williams 1989, p.49 It is up to thickJones ''et al.'' 2011 and has a uranium-lead radiometric age of 1700 Mya. The metarhyolite making up much of the formation is reddish orange in color, but ranges from brick red to light pink. It includes relict

phenocrysts 300px, Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid shapes when cut through). 1 euro coin (diameter 2.3 cm) for scale. A phenocryst is an early forming, relatively large and usually conspic ...

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 ...

and

microcline Microcline (KAlSi3O8) is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. Microcline typically contains minor amounts of sodium. It is common in granite and pegmatites. Microcline forms during s ...

and most outcrops show drag-folded flow bands. Some outcrops contain relict phenocrysts of

albite Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicat ...

oligoclase Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite ( Na Al Si3 O8) and anorthite ( CaAl2Si2O8). The albite:ano ...

. All of the phenocrysts have been reoriented about axes parallel to that of the drag-folded flow bands. Phenocryst size is from 0.02 to over 5 mm but typically 0.5 mm and show resorption or recrystallization to aggregates of smaller grains. The ground mass has a mosaic texture of 0.02 mm. Normative composition is 34.9% quartz, 28.9%

orthoclase Orthoclase, or orthoclase feldspar ( endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture", because its two cleavage planes are at right angles ...

, 28.8% albite, 3.3%

hypersthene Hypersthene is a common rock-forming inosilicate mineral belonging to the group of orthorhombic pyroxenes. Its chemical formula is . It is found in igneous and some metamorphic rocks as well as in stony and iron meteorites. Many references have f ...

, 3.2%

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...

, 1.7%

anorthite Anorthite (< ''an'' 'not' + ''ortho'' 'straight') is the

calcium

diopside Diopside is a monoclinic pyroxene mineral with composition . It forms complete solid solution series with hedenbergite () and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull ...

, and 0.8%

ilmenite Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printi ...

. The unit is conformable with the surrounding metasedimentary beds, supporting an interpretation that the Burned Mountain Formation beds were originally ash flows. The unit resembles the Tres Piedras Orthogneiss but is slightly older. Several beds of highly

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 ...

ized Burned Mountain Formation were originally assigned to the Petaca Schist,Barker 1958, p.34 but this unit name has since been abandoned.Wobus 1985, p.15

History of investigation

The unit was originally designated as the Vallecitos

by Evan Just in his study 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 the Tusas Mountains.Just 1937, p.44 but this name was already in use, and it was renamed the Burned Mountain metarhyolite by F. Barker in 1958.Barker 1958, p. 54 It was designated as a formation in the

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 ...

by Bauer and Williams in their sweeping revision of northern New Mexico

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 ...

stratigraphy Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...

in 1989.Bauer and Williamns 1989, p.49

Footnotes

References

* * * * * {{cite journal , last1=Wobus , first1=Reinhard A. , title=Changes in the nomenclature and shatigraphy of Proterozoic metamorphic rocks, Tusas Mountains, north-central New Mexico , journal=U.S. Geological Survey Bulletin , date=1985 , volume=1571 , url=https://books.google.com/books?id=Y2lOAQAAMAAJ&dq=+Changes+in+the+nomenclature+and+shatigraphy+of+Proterozoic+metamorphic+rocks,+Tusas+Mountains,+north-central+New+Mexico&pg=PA1 , access-date=21 April 2020 Precambrian formations of New Mexico Rhyolite formations