The Amalia Tuff 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 ...

exposed in and around

Questa, New Mexico Questa is a village in Taos County, New Mexico, United States. The population was 1,770 at the 2010 census. The village has trails into the Rio Grande Gorge, trout fishing, and mountain lakes with trails that access the Sangre de Cristo Moun ...

. It has a radiometric age of 25.39 ± 0.04 million years, corresponding to the

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided b ...

Description

The Amalia Tuff is a weakly to densely welded ash-flow

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock co ...

erupted during the formation of the Questa

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcanic eruption. An eruption that ejects large volumes of magma over a short period of time can cause significant detriment to the str ...

25.39 ± 0.04 million years ago ( Ma), which erupted some of

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

. It originally formed an outflow sheet thick that extended as far as to the southwest and to the northeast. Within the caldera the tuff pooled to a depth of . The tuff appears to be a single cooling unit. The tuff is silicic and alkaline, though its

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

content of 77-80% likely reflects some secondary silicification. It contains 10-20%

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

alkali feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocla ...

phenocryst image:montblanc granite phenocrysts.JPG, 300px, Granites often have large feldspar, feldspathic phenocrysts. This granite, from the Switzerland, Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid sh ...

s. Rare occurrences of a vitrophyric tuff at the base of the main bed contain ferrohedenbergite and

fayalite Fayalite (, commonly abbreviated to Fa) is the iron-rich endmember, end-member of the olivine solid solution, solid-solution series. In common with all minerals in the olivine, olivine group, fayalite crystallizes in the orthorhombic system (spac ...

, while the upper beds sometimes preserve sodic

amphibole Amphibole ( ) is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is ...

phenocrysts. The intracaldera tuff contains lenticular megabreccia blocks up to across. The tuff includes a lithic-rich lower

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 ...

that is nonwelded to partially welded and up to thick. This contains up to 5%

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...

and sparse

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 ...

rock fragments. This grades into the main bed of the formation.

Ar-Ar dating Arar or Ar-Ar may refer to: Geography and history * Arar, Saudi Arabia, the capital of Al Hudud ash Shamaliyah (The Northern Border) province ** Arar border crossing, a Saudi-Iraqi border crossing near Arar, Saudi Arabia and Nukhayb, Iraq * Arar ...

of the tuffs and associated

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

s indicates the

that formed the tuff was not generated in place in the upper crust. Isotope evidence suggests the original

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

ic magma assimilated considerable crust material and underwent crystal fractionation at a deeper level of the crust than that of the

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it u ...

responsible for the Questa eruption. File:Amalia Tuff sample from type area.jpg, Sample of Amalia Tuff from the type area, Amalia, New Mexico File:Amalia intracaldera tuff.jpg, Exposure of intracaldera Amalia Tuff at mouth of Cabresto Canyon File:Amalia Tuff.jpg, A fragment of Amalia Tuff from an outcropping near Petaca, New Mexico

History of investigation

The unit was first designated the Amalia Formation in 1956 by P.F. McKinlay. As originally defined, it included basalt flows and sedimentary beds dating from before the Questa eruption. These were removed from the unit definition by Lipman in 1983, who restricted the unit to the densely welded tuff outflow sheet from the Questa eruption and renamed the unit the Amalia Tuff.

Footnotes

References

* * * * * * {{cite journal , last1=Zimmerer , first1=M. J. , last2=McIntosh , first2=W. C. , title=The geochronology of volcanic and plutonic rocks at the Questa caldera: Constraints on the origin of caldera-related silicic magmas , journal=Geological Society of America Bulletin , date=18 May 2012 , volume=124 , issue=7–8 , pages=1394–1408 , doi=10.1130/B30544.1, bibcode=2012GSAB..124.1394Z Paleogene formations of New Mexico Tuff formations of the United States Oligocene volcanism