Bandelier Tuff

The Bandelier Tuff is a geologic formation exposed in and around the Jemez Mountains of northern New Mexico. It has a radiometric age of 1.85 to 1.25 million years, corresponding to the Pleistocene epoch.

Bandelier Tuff
Stratigraphic range: Pleistocene, 1.85–1.256 Ma
↓
Kwage Mesa, a typical finger mesa eroded out of the Bandelier Tuff in the Pajarito Plateau
TypeGeologic formation
Unit ofTewa Group
Sub-unitsLa Cueva Member, Otowi Member, Tsherige Member
OverliesTschicoma Formation, Paliza Canyon Formation
Thickness330 m (1,080 ft)
Lithology
PrimaryIgnimbrite
OtherPumice
Location
Coordinates35.764°N 106.322°W / 35.764; -106.322
Region New Mexico
Country United States
Type section
Named forBandelier National Monument
Named byHarold T.U. Smith
Year defined1938

Map of Bandelier Tuff exposures

Description

The formation is composed of ignimbrites produced by a series of at least three Quaternary caldera eruptions that culminated in the Valles Caldera eruption 1.256 million years before the present (Mya).[1] The Valles Caldera is the type location for resurgent caldera eruptions,[2] and the Bandelier Tuff was one of the earliest recognized ignimbrites.[3]

The caldera lies on the intersection of the western margin of the Rio Grande Rift and the Jemez Lineament.[4][5] Here magma produced from the fertile rock of an ancient subduction zone has repeatedly found its way to the surface along faults produced by rifting. This has produced a long-lived volcanic field, with the earliest eruptions beginning at least 13 million years ago[6] and continuing almost to the present day[7]

Both upper members of the tuff show compositional zoning, in which the lower pyroclastic flows are more silicic and contain less mafic minerals than the upper flows. This is interpreted as progressive eruption of a gravitationally zoned magma chamber in which volatiles are concentrated at the top of the chamber and mafic minerals have partially settled into the lower, hotter portions of the magma chamber.[8][9]

The tuff contains up to 30% lithic fragments, which in the Otowi Member are estimated to have a total volume of 10 km3 and to be sufficient to quench welding through their cooling effect. The lithic fragments are 90% earlier volcanic rock, 10% Paleozoic sedimentary rock, and only traces of Precambrian rock, implying considerable flaring of the eruption vents. Some of the rock shows indication of contact metamorphism in the magma chamber walls with a magma rich in water and fluorine.[10]

Members

The Bandelier Tuff consists of three members corresponding to at least three distinct caldera eruptions.

The La Cueva Member is an unwelded to poorly welded tuff with phenocrysts of quartz and sanidine and traces of pyroxene and magnetite. It has been divided into two units;[11] the upper unit is nonwelded to slightly welded and contains large pumice clasts, while the lower unit is nonwelded and includes abundant lithic fragments. Separating the two units is a bed of reworked pumice and debris flows. However, the 40Ar/39Ar ages are indistinguishable, at 1.85 ± 0.07 and 1.85 ± 0.04 Ma for the upper and lower units, respectively.[12] The maximum observed thickness is 80 meters (260 feet).

This member was emplaced by the first and smallest (but still enormous) caldera eruption of the Jemez volcanic field. It is exposed in only a few locations, including San Diego Canyon, the southwestern caldera wall, and in scattered locations on the Pajarito Plateau. It is possible that the Toledo Embayment, a structural feature of the northwest rim of the caldera coincident with a gravity low, is the remnant of the La Cueva caldera.[13] On the other hand, the presence of lithic breccia in this member in the La Cueva area suggests the caldera was located to the southwest.[14]

The Otowi Member consists of a basal air fall pumice bed (the Guaje Pumice) and a massive, typically unwelded, ignimbrite.[15] The upper ignimbrite is a poorly to densely welded rhyolitic ash-flow tuff containing abundant phenocrysts of sanidine and quartz, and sparse mafic microphenocrysts. The sanidine may display a blue iridescence (chatoyance). The member contains abundant fragments of country rock. 40Ar/39Ar radiometric ages for the member range from 1.61 ± 0.01[16] to 1.62 ± 0.04 Ma.[17] The maximum exposed thickness is about 120 meters (390 feet).

This member was erupted in the Toledo event, which was named after the Toledo Embayment, a structural feature in the northeast caldera wall which was thought to be the remains of the Toledo caldera.[15] However, more recent work has demonstrated that the Toledo caldera was likely more or less coincident with the Valles caldera.[13] The member is exposed over the entire Jemez area, except within the Valles caldera itself, where it is present only in the subsurface. It is particularly extensively exposed in the Jemez Plateau west of the caldera, but is also exposed across much of the Pajarito Plateau east of the caldera at the bases of its characteristic finger mesas.[18]

  • Guaje Pumice bed north of Los Alamos.
  • Road cut exposing Otowi Member in Pueblo Canyon. Above are cliffs of Tsherige Member.

The Tshirege Member has been described as "arguably New Mexico’s most famous rock".[19] It consists of multiple flows of densely welded to nonwelded rhyolitic ash-flow tuff. These contain abundant phenocrysts of sanidine and quartz, sparse microphenocrysts of clinopyroxene and orthopyroxene, and extremely rare microphenocrysts of fayalite. In the more densely welded portions of the member, the sanidine is chatoyant. The member typically contains fragments of country rock, and locally has a thin (less than 2 meters (6.6 foot)) basal pumice and surge deposit bed, the Tsankawi Pumice Bed. This bed contains roughly 1% of hornblende dacite pumice.

The member is exposed throughout the Jemez region and within the Valles Caldera, and has a maximum thickness of over 900 meters (3,000 feet). It was emplaced by the Valles event, which took place 1.256 million years ago[1] and created the Valles caldera.[20]

  • Surge beds in the "E" unit of the Tsherige Member.
  • Close up of "E" bed of the Tsherige Member. U.S. quarter dollar (2.4 cm) for scale.

Much of the material in these deposits now forms the Pajarito Plateau, a scenic region of canyons and mesas on which Los Alamos is situated.

Economic geology

Pumice has been extensively mined from the Guaje Pumice Bed on the east flanks of the Jemez Mountains. Production was high enough in 1994 to help make New Mexico the second largest producer of pumice among the United States. The pumice itself is unconsolidated and easily removed once the overburden (typically Otowi Member ignimbrite) is removed. Much of the pumice was strip mined from public lands before reclamation bonds were required, leaving mining scars that are slowly revegetating.[21]

History of investigation

The formation was given its name by H.T.U. Smith in 1938.[22] The formation was divided into upper and lower units, which were recognized almost at once to correspond to separate caldera eruptions. In 1964, Griggs assigned the formal member names of Otowi Member to the lower unit and Tshirege Member to the upper unit, and gave the name Guaje Pumice to the basal pumice bed of the Otowi Member.[23]. In their paper establishing the stratigraphic framework for the Jemez volcanic field in 1969, Smith, Bailey, and Ross adopted Grigg's unit names and added the name Tsankawi Pumice for the basal pumice bed of the Tsherige Member.[15]

In their 2011 map of the Valles Caldera, Goff et al. formally added the La Cueva Member, informally known until then as the ignimbrite of San Diego Canyon, to the Bandelier Tuff.[13]

Footnotes
  1. Phillips 2004
  2. Smith and Bailey 1968
  3. Ross and Smith 1961
  4. Aldrich 1986
  5. Whitmeyer and Karlstrom 2007
  6. Heiken et al. 1990
  7. Zimmerer et al. 2016
  8. Stix et al. 1988
  9. Boro 2019
  10. Eichelberger and Koch 1979
  11. Spell et al. 1990
  12. Spell et al. 1996
  13. Goff et al. 2011, p.8
  14. Self et al. 1986
  15. Smith et al. 1969
  16. Izett and Obradovich 1994
  17. Spell et al. 1996
  18. Gott et al. 2011
  19. Goff 2010
  20. Goff et al. 2011, p.20
  21. Austin 1994
  22. Smith 1938
  23. Griggs 1964

References
  • Aldrich Jr., M. J. (1986). "Tectonics of the Jemez Lineament in the Jemez Mountains and Rio Grande Rift". Journal of Geophysical Research. 91 (B2): 1753–1762. Bibcode:1986JGR....91.1753A. doi:10.1029/JB091iB02p01753.
  • Austin, George S. (February 1994). "Pumice mining and environmental concerns in New Mexico" (PDF). New Mexico Geology. 16 (1). Retrieved 10 June 2020.
  • Boro, Joseph Robert (December 2019). Recharge and Mobilization of Crystal Mush to Produce and Erupt a Zoned Magma Chamber – the Tshirege Member of the Bandelier Tuff, Valles Caldera, New Mexico, USA (dissertation). Washington State University. Retrieved 12 June 2020.
  • Eichelberger, John C.; Koch, Frank G. (February 1979). "Lithic fragments in the Bandelier Tuff, Jemez Mountains, New Mexico". Journal of Volcanology and Geothermal Research. 5 (1–2): 115–134. doi:10.1016/0377-0273(79)90036-2.
  • Goff, Fraser (Winter 2010). "The Valles Caldera: New Mexico's Supervolcano" (PDF). New Mexico Earth Matters. New Mexico Institute of Mining & Technology. Retrieved 19 May 2020.
  • Goff, Fraser; Gardner, Jamie N.; Reneau, Steven L.; Kelley, Shari A.; Kempter, Kirt A.; Lawrence, John R. (2011). "Geologic map of the Valles caldera, Jemez Mountains, New Mexico". New Mexico Bureau of Geology and Mineral Resources Map Series. 79. Retrieved 18 May 2020.
  • Griggs, R. L., 1964, | Geology and ground-water resources of the Los Alamos area,New Mexico: U.S. Geol. Survey Water-Supply Paper 1753,107 p.
  • Heiken, G; Goff, F; Gardner, J N; Baldridge, W S; Hulen, J B; Nielson, D L; Vaniman, D (May 1990). "The Valles/Toledo Caldera Complex, Jemez Volcanic Field, New Mexico". Annual Review of Earth and Planetary Sciences. 18 (1): 27–53. doi:10.1146/annurev.ea.18.050190.000331.
  • Izett, Glen A.; Obradovich, John D. (10 February 1994). "Ar/ Ar age constraints for the Jaramillo Normal Subchron and the Matuyama-Brunhes geomagnetic boundary". Journal of Geophysical Research: Solid Earth. 99 (B2): 2925–2934. doi:10.1029/93JB03085.
  • Kudo, A.M. (1974). "OUTLINE OF THE IGNEOUS GEOLOGY OF THE JEMEZ MOUNTAINS VOLCANIC FIELD". New Mexico Geological Society Field Conference Series (25): 287–289. Retrieved 4 May 2020.
  • Phillips, Erin H. (May 2004). COLLAPSE AND RESURGENCE OF THE VALLES CALDERA, JEMEZ MOUNTAINS, NEW MEXICO: 40 AR / 39 AR AGE CONSTRAINTS ON THE TIMING AND DURATION OF RESURGENCE AND AGES OF MEGABRECCIA BLOCKS (master's thesis). Socorro, New Mexico: New Mexico Institute of Mining and Technology. Retrieved 4 May 2020.
  • Ross, Clarence S.; Smith, Robert L. (1961). "Ash-flow tuffs: Their origin, geologic relations, and identification". USGS Profession Paper Series (366). doi:10.3133/pp366.
  • Self, Stephen; Goff, Fraser; Gardner, Jamie N.; Wright, John V.; Kite, William M. (1986). "Explosive rhyolitic volcanism in the Jemez Mountains: Vent locations, caldera development and relation to regional structure". Journal of Geophysical Research. 91 (B2): 1779. doi:10.1029/JB091iB02p01779.
  • Smith, H.T.U. (1938). "Tertiary geology of the Abiquiu quadrangle, New Mexico". Journal of Geology. 46 (7): 933–965.
  • Smith, R.L.; Bailey, R.A. (1968). "Resurgent cauldrons". Memoir of the Geological Society of America. 116. doi:10.1130/MEM116-p613.
  • Smith, R.L.; Bailey, R.A.; Ross, C.S. (1969). "Stratigraphic Nomenclature of ' Volcanic Rocks in the Jemez Mountains, New Mexico" (PDF). GEOLOGICAL SURVEY BULLETIN (1274-P). Retrieved 4 May 2020.
  • Spell, Terry L.; Mark Harrison, T.; Wolff, John A. (October 1990). "40Ar/39Ar dating of the Bandelier Tuff and San Diego Canyon ignimbrites, Jemez Mountains, New Mexico: Temporal constraints on magmatic evolution". Journal of Volcanology and Geothermal Research. 43 (1–4): 175–193. doi:10.1016/0377-0273(90)90051-G.
  • Spell, Terry L.; McDougall, Ian; Doulgeris, Anthony P. (December 1996). "Cerro Toledo Rhyolite, Jemez Volcanic Field, New Mexico: 40Ar/39Ar geochronology of eruptions between two caldera-forming events". Geological Society of America Bulletin. 108 (12): 1549–1566. doi:10.1130/0016-7606(1996)108<1549:CTRJVF>2.3.CO;2.
  • Stix, John; Goff, Fraser; Gorton, Michael P.; Heiken, Grant; Garcia, Sammy R. (1988). "Restoration of compositional zonation in the Bandelier silicic magma chamber between two caldera-forming eruptions: Geochemistry and origin of the Cerro Toledo Rhyolite, Jemez Mountains, New Mexico". Journal of Geophysical Research. 93 (B6): 6129. doi:10.1029/JB093iB06p06129.
  • Whitmeyer, Steven; Karlstrom, Karl E. (2007). "Tectonic model for the Proterozoic growth of North America". Geosphere. 3 (4): 220. doi:10.1130/GES00055.1. Retrieved 18 April 2020.
  • Zimmerer, Matthew J.; Lafferty, John; Coble, Matthew A. (January 2016). "The eruptive and magmatic history of the youngest pulse of volcanism at the Valles caldera: Implications for successfully dating late Quaternary eruptions". Journal of Volcanology and Geothermal Research. 310: 50–57. doi:10.1016/j.jvolgeores.2015.11.021.
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