The Jemez Mountains lie on the intersection of the western margin of the Rio Grande Rift and the Jemez Lineament.[1][2] Here magma produced from the fertile rock of an ancient subduction zone[3] 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 in both the northern (Polvadera Group) and southern (Keres Group) portions of the volcanic field.[4] High-silica eruptions of the Tewa Group began about 1.85 million years ago and continued almost to the present day.[5]
The Polvadera Group is a sequence of basalt, andesite, dacite, and rhyoliteflows that underlie part of the central and most of the northern Jemez Mountains. It overlaps the Keres Group to the south and has a maximum thickness of about 1,100 meters (3,600 feet). It overlies the Abiquiu Formation and Santa Fe Group and is overlain by the Tewa Group.[6]
Formations
From oldest to youngest, the formations recognized within the Polvadera Group are the Lobato Formation, the La Grulla Formation, the Tschicoma Formation, and the El Rechuelos Rhyolite.
The Lobato Formation is a sequence of mostly tholeiitic[7] basalt flows that underlies the mesas fringing the northern and northeastern sides of the Jemez Mountains. Major flows include the basalts of Clara Peak, Lobato Mesa, Polvadera Mesa, Escoba Mesa, and Cerro Pedernal. The formation was named for Lobato Mesa (36°07′34″N106°18′32″W / 36.126°N 106.309°W / 36.126; -106.309), where the formation is particularly thick and extensive, by H. T. U. Smith in 1938. The formation has a maximum thickness of 180 meters (590 feet) and is a succession of numerous flows ranging from 6 meters (20 feet) to 15 meters (49 feet) in thickness. The flows are mostly olivine-augite basalts, but hypersthene-, pigeonite-, and titaniferous augite-bearing basalts are also found.[6] The formation was erupted between 14 and 7 million years ago, peaking between 10.8 million and 7.8 Mya.[8][7]
The Lobato Formation is separated from the underlying Abiquiu Formation and Santa Fe Group by an erosional surface locally veneered by boulders and cobbles of Precambrianbasement. The formation in turn is overlain by dacites and quartz latites of the Tschicoma Formation.[6] The earliest flows mapped as Lobato Formation are interbedded with Santa Fe Group sediments and have been dated at 14.1 + 0.3 Mya.[7]
Lobato Mesa
Thin lava and cinder beds of the Lobato Formation exposed in road cut southwest of Clara Peak.
Gabbro from the interior of the Clara Peak eruptive center of the Lobato Formation.
Window Rock Dike, in the badlands between Medanales and Lobato Mesa.
The La Grulla Formation consists of flows underlying the La Grulla Plateau (36°07′34″N106°33′58″W / 36.126°N 106.566°W / 36.126; -106.566) that were originally assigned to the Lobato Formation or the Tschicoma Formation. However, these are distinct in age and geochemistry,[9] and were assigned to the La Grulla Formation by Shari A. Kelley and coinvestigators in 2013. The formation includes low-silica (mafic) lavas erupted from Encino Point between 8.7 and 7.8 Mya and trachyandesite and dacite erupted on the La Grulla Plateau between 7.7 and 7.2 Mya. The contact of the La Grulla Formation with the underlying Paliza Canyon Formation (Keres Group) is described as a sedimentary break.[10]
The Tschicoma Formation is a sequence of mostly dacite and rhyodacite flows, with lesser amounts of andesite, trachyandesite, and rhyolite,[7] that overlies the Lobato Formation and is in turn overlain by Tewa Group flows. Its flows are exposed as the Sierra de los Valles west of the city of Los Alamos (35°53′46″N106°25′01″W / 35.896°N 106.417°W / 35.896; -106.417). It was named by R.L. Griggs in 1964.[11] This formation marked a reduction in tectonic activity in the region. Whereas previous eruptions were vented along active faults, the Tschicoma hybrid magmas formed from coalescence of pockets of mantle- and lower-crust-derived magmas.[12] The eruptions took place between 6.8 and 2.7 Mya, with a peak between 5.0 and 2.7 Mya.[8][13]
The flows were erupted from multiple dome complexes that can be readily distinguished by their geochemistry and petrographic characteristics. The Rhyodacite of Rendija Canyon contains 11-16% phenocrysts of quartz and plagioclase, lesser amounts of sanidine and anorthoclase, and traces (<0.5%) of biotite, clinopyroxene, and hornblende. It is a low-silica rhyolite with an age ranging from 4.98 to 5.36 Ma. The Dacite of Sawyer Dome contains 32-34% phenocrysts of plagioclase, hornblende, and orthopyroxene, and has an 40Ar/39 age between 3.18 and 3.67 Ma. The Dacite of Cerro Grande contains about 21% phenocrysts of plagioclase, hornblende, orthopyroxene, and subordinate clinopyroxene, with an age ranging from 2.88 to 3.35 Ma.
The Dacite of Pajarito Mountain is divided into two series of lavas. The lower is a two-pyroxene dacite with 10.3-16.4% phenocrysts of plagioclase and subequal orthopyroxene and clinopyroxene, while the upper flows are slightly more silicic, with 23-24% phenocrysts of plagioclase, orthopyroxene, and subordinate clinopyroxene. The age of the upper flows ranges from 2.93 and 3.09 Ma.
The Dacite of Caballo Mountain (New Mexico) is also divided into two sets of flows. The lower flows include to 24% phenocrysts of plagioclase and hornblende with lesser amounts of quartz and biotite and traces of sanidine, orthopyroxene, and clinopyroxene. The summit of Caballo Mountain is underlain by a relatively crystal-poor dacite containing about 2% phenocrysts of plagioclase and hornblende with lesser amounts of clinopyroxene and traces of quartz and orthopyroxene. The lower beds are about 4.66 Ma in age while the summit is 3.06 Ma in age.
Tschicoma Mountain, the highest point in the Jemez Mountains, is underlain by Tschicoma Formation.
Dacite boulder of Rendija Canyon Member of the Tschicoma Formation
Block and ash flow in the Tschicoma Formation exposed at Rendija Canyon
The El Rechuelos Rhyolite is named for El Rechuelos, a small steep drainage on the western side of Polvadera Peak (36°03′50″N106°26′46″W / 36.064°N 106.446°W / 36.064; -106.446). It was originally defined as a set of five small rhyolite domes and a small pumice cone in the northern Jemez Mountains. These rest on dacites of the Tschicoma Formation and are overlain by Bandelier Tuff.[16] Petrological and radiometric age measurements have shown that only two of the domes, plus a vent discovered later, properly belong to the El Rechuelos Rhyolite and have an age of 2 million years. The remaining rhyolite domes are much older and resemble units of the Keres Group to the south.[17][8]
The ages and chemistries point to five distinct batches of magma for the El Rechuelos Rhyolite as originally defined. The Early Rhyolite has an 40Ar/39 age of 7.10 + 0.04 Ma and is characterized by phenocrysts of plagioclase, quartz, biotite, and sanidine; the Intermediate Rhyolite has an age of 7.05 + 0.24 Ma and is characterized by phenocrysts of plagioclase, sanidine, and biotite; the Pumice Ring Rhyolite has an age of 5.61 + 0.48 and is characterized by plagioclase, biotite, quartz, and amphibole phenocrysts in a finely crystalline groundmass; the El Rechuelos Rhyolite (sensu stricto) is 2.23 +0.15 Ma old and is sparsely porphyritic rhyolite glass; and the Young Rhyolite is 1.19 + 0.01 Ma old and has phenocrysts of sanidine, quartz, plagioclase, and sparse fayalite. All have a total rock composition characteristic of rhyolite. The Young Rhyolite appears to be part of the Bandelier Tuff.[18]
The El Rechuelos Rhyolite has geochemistry consistent with an origin in a lower crustal magma chamber generated from a mantle‐derived mafic magma, probably the nearby Lobato Basalt, by fractional crystallization with limited assimilation (less than six percent) of lower crust. This system was likely distinct from the Bandelier Tuff magma system.[17]
History of investigation
The group was first defined by Bailey, Smith, and Ross in 1969 as part of their work establishing the stratigraphy of the Jemez Mountains. The group was named for Polvadera Peak, which is a major eruptive center for the group and the second highest peak in the Jemez Mountains (36°03′36″N106°24′18″W / 36.060°N 106.405°W / 36.060; -106.405). They included in the group the Lobato Basalt, the Tschicoma Formation, and the El Rechuelos Rhyolite.[6] Bruce M. Loeffler and coinvestigators narrowed the definition of the El Rechuelos Rhyolite in 1988,[17] and Shari A. Kelley and coinvestigators defined the La Grulla Formation in 2013.[10]
The division of the precaldera formations of the Jemez Mountains into the Polvadera and Keres Groups, based largely on geography, has been criticized as artificial, and the Polvadera Group itself includes formations of disparate origins and ages.[19][20] Fraser Goff and his coinvestigators abandoned the Polvadera Group in their 2011 mapping of the Valles Caldera, assigning its formations to the Keres Group,[21] as did Shari A. Kelley and coinvestigators in their 2013 study.[10]
Cultural significance
Obsidian obtained from the Polvadera Group has been found in Paleoindian archaeological sites as far east as Oklahoma.[22]
Rhyolite is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral assemblage is predominantly quartz, sanidine, and plagioclase. It is the extrusive equivalent of granite.
Dacite is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. It is composed predominantly of plagioclase feldspar and quartz.
Trachyte is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and alkali metals. It is the volcanic equivalent of syenite.
Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff. In contrast, intrusive rock refers to rocks formed by magma which cools below the surface.
Medicine Lake Volcano is a large shield volcano in northeastern California about 30 mi (50 km) northeast of Mount Shasta. The volcano is located in a zone of east-west crustal extension east of the main axis of the Cascade Volcanic Arc and the Cascade Range. The 0.6 mi (1 km) thick shield is 22 mi (35 km) from east to west and 28 to 31 mi from north to south, and covers more than 770 sq mi (2,000 km2). The underlying rock has downwarped by 0.3 mi (0.5 km) under the center of the volcano. The volcano is primarily composed of basalt and basaltic andesite lava flows, and has a 4.3 by 7.5 mi caldera at the center.
The Bishop Tuff is a welded tuff that formed 764,800 ± 600 years ago as a rhyolitic pyroclastic flow during the approximately six day eruption that formed the Long Valley Caldera. Large outcrops of the tuff are located in Inyo and Mono Counties, California, United States. Approximately 200 cubic kilometers of ash and tuff erupted outside the caldera.
The Jemez Mountains are a group of mountains in Rio Arriba, Sandoval, and Los Alamos counties, New Mexico, United States.
Rhyodacite is a volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of those plutonic rocks that are intermediate in composition between monzogranite and granodiorite. Rhyodacites form from rapid cooling of lava relatively rich in silica and low in alkali metal oxides.
The Lava Creek Tuff is a voluminous sheet of ash-flow tuff located in Wyoming, Montana and Idaho, United States. It was created during the Lava Creek eruption around 630,000 years ago, which led to the formation of the Yellowstone Caldera. This eruption is considered the climactic event of Yellowstone's third volcanic cycle. The Lava Creek Tuff covers an area of more than 7,500 km2 (2,900 sq mi) centered around the caldera and has an estimated magma volume of 1,000 km3 (240 cu mi).
The Fish Canyon Tuff is the large volcanic ash flow or ignimbrite deposit resulting from one of the largest known explosive eruptions on Earth, estimated at 1,200 cu mi (5,000 km3). (see List of largest volcanic eruptions). The Fish Canyon Tuff eruption was centred at the La Garita Caldera in southwest Colorado; the caldera itself would have formed by collapse, as a result of the eruption. Studies of the tuff show that it all belongs to one eruption due to its uniform bulk-chemical composition (SiO2=bulk 67.5–68.5% (dacite), matrix 75–76% (rhyolite) and consistent phenocryst content (35–50%) and mineralogical composition (plagioclase, sanidine, quartz, biotite, hornblende, sphene, apatite, zircon, Fe-Ti oxides are the primary phenocrysts). This tuff and eruption is part of the larger San Juan volcanic field and the Oligocene Southern Rocky Mountain ignimbrite flare-up.
Calabozos is a Holocene caldera in central Chile's Maule Region. Part of the Chilean Andes' volcanic segment, it is considered a member of the Southern Volcanic Zone (SVZ), one of the three distinct volcanic belts of South America. This most active section of the Andes runs along central Chile's western edge, and includes more than 70 of Chile's stratovolcanoes and volcanic fields. Calabozos lies in an extremely remote area of poorly glaciated mountains.
Chivinar is a 5,125 metres (16,814 ft) high Late Miocene volcano in Northwest Argentina. The volcano is noted for having erupted rhyolites containing topaz in its early stage of evolution, the only known occurrence of such in South America. This topaz did form during the crystallization of the magma in a fluorine-rich phase thereof at low temperatures and pressures. The topazes reach sizes of 8 centimetres (3.1 in). The topaz bearing rocks cover a surface area of 8 by 4 kilometres that is interrupted by a lava flow. Much older granitoid bodies also lie in the area and are 502-425 million years old.
Cerro Chascon-Runtu Jarita is a complex of lava domes located inside, but probably unrelated to, the Pastos Grandes caldera. It is part of the more recent phase of activity of the Altiplano-Puna volcanic complex.
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. The tuff was erupted in a series of at least three caldera eruptions in the central Jemez Mountains.
The Keres Group is a group of geologic formations exposed in and around the Jemez Mountains of northern New Mexico. Radiometric dating gives it an age of 13 to 6 million years, corresponding to the Miocene epoch.
The Tewa Group is a group of geologic formations exposed in and around the Jemez Mountains of northern New Mexico. Radiometric dating gives it an age of 1.85 million to 72 thousand years, corresponding to the Pleistocene epoch.
The Latir volcanic field is a large volcanic field near Questa, New Mexico, that was active during the late Oligocene to early Miocene, 28 to 22 million years ago (Ma). It includes the Questa caldera, in whose deeply eroded interior is located the Molycorp Questa molybdenum mine.
East Diamante is a submarine volcano located 43 km (27 mi) south of Anatahan in the Northern Mariana Islands of the northwestern Pacific Ocean. It forms part of the Izu–Bonin–Mariana Arc and is hydrothermally active, containing a complex, elongated caldera at its summit.
The Bearwallow Mountain Andesite or Bearwallow Mountain Formation is a geologic formation exposed in and around the Mogollon Mountains of southwest New Mexico. It has a radiometric age of 27 to 23 million years, corresponding to the late Oligocene to early Miocene epochs.
Bement, Leland C.; Larrick, Dakota; Hughes, Richard E.; Carlson, Kristen (31 January 2020). "Evidence for Late Paleoindian Scavenging of Early Paleoindian Obsidian, Oklahoma Panhandle". PaleoAmerica. 6 (2): 194–198. doi:10.1080/20555563.2019.1709323. S2CID213251095.
Gardner, Jamie N.; Goff, Fraser; Garcia, Sammy; Hagan, Roland C. (1986). "Stratigraphic relations and lithologic variations in the Jemez Volcanic Field, New Mexico". Journal of Geophysical Research. 91 (B2): 1763. Bibcode:1986JGR....91.1763G. doi:10.1029/JB091iB02p01763.
Griggs, R. L. (1964). "Geology and ground-water resources of the Los Alamos area, New Mexico". U.S. Geol. Survey Water-Supply Paper. 1753. CiteSeerX10.1.1.939.251.
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. Bibcode:1990AREPS..18...27H. doi:10.1146/annurev.ea.18.050190.000331.
Loeffler, Bruce M.; Vaniman, David T.; Baldridge, W. Scott; Shafiqullah, Muhammed (1988). "Neogene rhyolites of the northern Jemez volcanic field, New Mexico". Journal of Geophysical Research. 93 (B6): 6157. Bibcode:1988JGR....93.6157L. doi:10.1029/JB093iB06p06157.
Singer, Bradley S.; Kudo, Albert M. (November 1986). "Assimilation-fractional crystallization of Polvadera Group rocks in the northwestern Jemez Volcanic Field, New Mexico". Contributions to Mineralogy and Petrology. 94 (3): 374–386. Bibcode:1986CoMP...94..374S. doi:10.1007/BF00371445. S2CID129818680.
Turbeville, B.N; Waresback, Damon B; Self, Stephen (February 1989). "Lava-dome growth and explosive volcanism in the Jemez Mountains, New Mexico: Evidence from the plio-pleistocene puye alluvial fan". Journal of Volcanology and Geothermal Research. 36 (4): 267–291. Bibcode:1989JVGR...36..267T. doi:10.1016/0377-0273(89)90074-7.
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