{{Short description|Geological formation in British Columbia, Canada}} {{Infobox Rockunit | name = Nido Formation | image = Edziza042909-- 074-9.jpg | caption = Kaia Bluff in the lower-left corner consists of [[alkali basalt]] of the Nido Formation | type = [[Geological formation]]{{sfn|Government of Canada: Nido Formation}} | age = [[Neogene]]<br/>{{Fossil range|4.4}} | period = Neogene | prilithology = [[Alkali basalt]]{{sfn|Souther|1988}} | otherlithology = [[Hawaiite]], [[picrite]]{{sfn|Souther|1988}} | namedfor = [[Nido Creek]]{{sfn|Souther|1992|p=95}} | namedby = [[Jack Souther|Souther]] ''et al.'', 1984{{sfn|Government of Canada: Nido Formation}} | region = [[British Columbia]]{{sfn|Government of Canada: Nido Formation}} | country = Canada{{sfn|Government of Canada: Nido Formation}} | coordinates = {{coord|57|38|15|N|130|36|10|W|type:mountain_region:CA-BC|format=dms|display=inline,title}}{{sfn|BC Geographical Names: Kaia Bluff}}{{efn|Kaia Bluff consists of alkali basalt of the Nido Formation.{{sfn|Souther|1988}}}} | unitof = [[Mount Edziza volcanic complex]]{{sfn|Souther|1988}} | subunits = Kounugu Member, [[Tenchen Member]]{{sfn|Souther|1988}} | underlies = Spectrum Formation, Arctic Lake Formation, [[Pyramid Formation (British Columbia)|Pyramid Formation]], [[Ice Peak Formation]], [[Edziza Formation]], Pillow Ridge Formation, [[Big Raven Formation]], [[Kakiddi Formation]]{{sfn|Souther|1988}}{{sfn|Souther|1992|pp=93, 98, 100, 246}} | overlies = [[Raspberry Formation]], [[Armadillo Formation]], [[Little Iskut Formation]]{{sfn|Souther|1992|pp=83, 100, 246}}{{sfn|Souther|Armstrong|Harakal|1984|p=340}} | thickness = Up to {{convert|365|m|ft|abbr=on}}{{sfn|Souther|1992|p=106}} | extent = | map = | map_caption = }} The '''Nido Formation''' is a [[stratigraphic unit]] of [[Neogene]] age in northwestern [[British Columbia]], Canada. It is the second most voluminous of 13 [[geological formation]]s comprising the [[Mount Edziza volcanic complex]] (MEVC), which consists of [[volcanic rock]]s of late [[Cenozoic]] age. Underlying the Nido Formation are the [[Raspberry Formation|Raspberry]], [[Little Iskut Formation|Little Iskut]] and [[Armadillo Formation|Armadillo]] formations of the MEVC, which have average ages ranging from 7.4 to 6.3 million years old. The overlying Spectrum, [[Pyramid Formation (British Columbia)|Pyramid]], [[Ice Peak Formation|Ice Peak]], Pillow Ridge, [[Edziza Formation|Edziza]], Arctic Lake and [[Big Raven Formation|Big Raven]] formations, also part of the MEVC, were deposited in the last 3.5 million years. [[Radiometric dating]] of the Nido Formation has yielded ages ranging from 7.8 to 4.4 million years old, but an average age of 4.4 million years has been provided for it.
The Nido Formation is subdivided into two [[member (geology)|members]], each of which is separated by an area of higher terrain. To the north, the [[Tenchen Member]] is exposed in valleys cutting the eastern and western sides of the [[Big Raven Plateau]]. The Kounugu Member to the south is exposed around the periphery of the [[Spectrum Range]] at the southern end of the MEVC. Both members consist mainly of [[alkali basalt]]; [[hawaiite]] and [[picrite]] occur locally at higher elevations. These volcanic rocks were deposited by at least six major and several smaller eruptive centres that were active during the [[Volcanism of the Mount Edziza volcanic complex#Second magmatic cycle|second magmatic cycle]] of the MEVC. The eruptive centres, which are considered to have been [[shield volcano]]es, have since been deeply eroded.
==History and etymology== The Nido Formation was first defined by [[Jack Souther]], [[Richard Lee Armstrong]] and J. Harakal in 1984 who grouped it together with the Kounugu Formation in their descriptions and mapping.{{sfn|Government of Canada: Nido Formation}}{{sfn|Souther|Armstrong|Harakal|1984|p=339}} It was mapped as one of 15 [[geological formation]]s of the [[Mount Edziza volcanic complex]] (MEVC), a group of late [[Cenozoic]] [[volcanic rock]]s in northwestern [[British Columbia]], Canada.{{sfn|Souther|Armstrong|Harakal|1984|pp=337, 339}}{{sfn|Edwards|1997|p=3}} In 1988, Jack Souther remapped the MEVC into 13 geological formations; the Kounugu and [[Sheep Track Member|Sheep Track]] formations were reassigned as [[member (geology)|member]]s of the Nido and [[Big Raven Formation|Big Raven]] formations, respectively.{{sfn|Souther|1988}} The Kounugu and Sheep Track members are no longer recognized as formations since that rank for them has been abandoned.{{sfn|Government of Canada: Kounugu Formation}}{{sfn|Government of Canada: Sheep Track Formation}}
The Nido Formation is named after [[Nido Creek]], whose name was adopted 2 January 1980 on the [[National Topographic System]] map 104G/9 after having been submitted by the [[Geological Survey of Canada]].{{sfn|Souther|1992|p=95}}{{sfn|BC Geographical Names: Nido Creek}} ''Nido'' means {{gloss|white man}} in the [[Tahltan language]].{{sfn|BC Geographical Names: Nido Creek}} It is a reference to white men who staked [[mineral claim]]s in the area of the creek before [[Mount Edziza Provincial Park]] was established in 1972.{{sfn|BC Geographical Names: Nido Creek}}{{sfn|BC Parks: Mount Edziza Provincial Park}}{{sfn|Department of Energy, Mines and Resources|1989}}
==Stratigraphy== [[Stratigraphically]], the Nido Formation is subdivided into two units referred to as the Kounugu and [[Tenchen Member|Tenchen]] members. They are separated by the Armadillo Highlands and are not completely underlain by a single [[marker horizon]].{{sfn|Souther|1992|p=93}} South of the Armadillo Highlands, the Kounugu Member is exposed around the periphery of the [[Spectrum Range]] at the southern end of the MEVC.{{sfn|Souther|1992|pp=32, 93, 104}} The Tenchen Member north of the Armadillo Highlands is exposed in steep-walled valleys cutting the eastern and western sides of the [[Big Raven Plateau]] at the northern end of the MEVC.{{sfn|Souther|1988}}{{sfn|Souther|1992|p=93}}
The Nido Formation has a volume of {{convert|127|km3|mi3|abbr=off}}, making it the second most voluminous of the MEVC after the [[Armadillo Formation]]. It is also one of five geological formations of the volcanic complex that contains more than {{convert|70|km3|mi3|abbr=on}} of volcanic material.{{sfn|Souther|1992|p=267}} The Nido Formation is the most widespread geological formation of the MEVC over an area about {{convert|20|km|mi|abbr=on}} wide and {{convert|50|km|mi|abbr=on}} long.{{sfn|Souther|1992|pp=93, 94}} It contains at least 40 [[lava flow]] units and reaches a thickness of {{convert|365|m|ft|abbr=off}} in the Kounugu Member.{{sfn|Souther|1992|pp=104–108}}{{sfn|Souther|Armstrong|Harakal|1984|p=344}}
===Overlying units=== [[File:Big Raven Plateau geological map.png|thumb|upright=1.8|alt=A geological map showing the distribution of stratigraphic formations|A [[geological map]] of the [[Big Raven Plateau]] north of the Armadillo Highlands]] The Nido Formation is overlain by seven younger geological formations of the MEVC.{{sfn|Souther|1988}}{{sfn|Souther|1992|pp=93, 98, 100, 246}} The oldest is the 3.1-million-year-old Spectrum Formation, which consists mainly of [[rhyolite]] and [[trachyte]] forming the Spectrum Range.{{sfn|Souther|1988}}{{sfn|Souther|1992|pp=113, 267}} To the north, the Nido Formation is overlain by the 1.1-million-year-old [[Pyramid Formation (British Columbia)|Pyramid Formation]] on [[ridge]]s along the deeply [[volcano#Dissection|dissected]] eastern flank of [[Mount Edziza]].{{sfn|Souther|1988}}{{sfn|Souther|1992|pp=100, 129, 267}} It consists mainly of rhyolite and trachyte, although [[basalt]] overlies a [[pyroclastic surge]] deposit at the base of the formation.{{sfn|Souther|1992|p=129}} The approximately 1-million-year-old [[Ice Peak Formation]] overlies much of the Nido Formation on the Big Raven Plateau.{{sfn|Souther|1988}}{{sfn|Souther|1992|pp=100, 246, 267}} Consisting of [[alkali basalt]], [[hawaiite]], [[mugearite]], [[benmoreite]] and trachyte, it is the most [[petrographically]] complex formation of the MEVC.{{sfn|Souther|1992|p=267}}{{sfn|Souther|Armstrong|Harakal|1984|p=345}}
In the middle of the Big Raven Plateau, the Nido Formation is overlain by the 0.9-million-year-old Pillow Ridge and [[Edziza Formation|Edziza]] formations.{{sfn|Souther|1988}}{{sfn|Souther|Armstrong|Harakal|1984|p=340}}{{sfn|Souther|1992|pp=246, 267}}{{sfn|BC Geographical Names: Mount Edziza}} The Pillow Ridge Formation is limited to the [[Pillow Ridge|Pillow]] and [[Tsekone Ridge|Tsekone]] ridges, which consist of alkali basalt and hawaiite, respectively.{{sfn|Souther|1988}}{{sfn|Souther|1992|p=165}} In contrast, the dominantly trachytic Edziza Formation forms the central stratovolcano of Mount Edziza.{{sfn|Souther|1992|p=165}}{{sfn|Souther|1990|pp=124, 125}} The 0.71-million-year-old Arctic Lake Formation overlies the Nido Formation on the [[Arctic Lake Plateau]] at the southern end of the MEVC.{{sfn|Souther|1988}}{{sfn|Souther|1992|pp=32, 267}} It consists mainly of alkali basalt that issued from at least seven separate eruptive centres on or adjacent to the plateau.{{sfn|Souther|1992|pp=199, 267}} Exposures of the 0.3-million-year-old trachytic [[Kakiddi Formation]] occur on the eastern and southwestern flanks of [[Ice Peak]]. The less than 20,000-year-old [[Big Raven Formation]] overlies Nido [[strata]] comprising the Big Raven and [[Kitsu Plateau|Kitsu]] plateaus.{{sfn|Souther|1988}}{{sfn|Souther|1992|p=267}} It consists mainly of alkali basalt and hawaiite, although it also includes a small volume of [[comenditic]] trachyte [[pumice]] on the Big Raven Plateau.{{sfn|Souther|1988}}{{sfn|Souther|1992|p=213}}
===Underlying units=== [[File:Spectrum Range Area geological map.png|thumb|upright=1.8|alt=A geological map showing the distribution of stratigraphic formations|The extent of the Nido Formation south of the Armadillo Highlands]] Underlying the Nido Formation are three other geological formations of the MEVC.{{sfn|Souther|1988}}{{sfn|Souther|1992|p=246}} Basalt, comendite and trachyte of the 6.3-million-year-old Armadillo Formation are exposed under the Nido Formation in strata of the Big Raven and [[Kitsu Plateau|Kitsu]] plateaus.{{sfn|Souther|1988}}{{sfn|Souther|1992|p=267}} Because only the distal portions of the Kounugu Member overlap with the southern edge of the Armadillo Formation, it is possible that both [[stratigraphic unit]]s are coeval in age. However, it also does not rule out the possibility that the Kounugu Member is older than the Armadillo Formation.{{sfn|Souther|Armstrong|Harakal|1984|p=344}} [[Trachybasalt]] of the 7.2-million-year-old [[Little Iskut Formation]] underlies the Nido Formation in strata of the [[Artifact Ridge|Artifact]] and [[Obsidian Ridge|Obsidian]] ridges south of the Armadillo Highlands.{{sfn|Souther|1988}}{{sfn|Souther|1992|pp=93, 267}} Also exposed under the Nido Formation in strata of the Big Raven and Kitsu plateaus is basalt of the 7.4-million-year-old [[Raspberry Formation]].{{sfn|Souther|1988}}{{sfn|Souther|1992|p=267}}
In the canyon of [[Elwyn Creek]] at the northwestern end of the Big Raven Plateau, the Nido Formation is underlain by [[leucogranite]] of the [[Elwyn Creek#Geology|Elwyn Creek Pluton]].{{sfn|Souther|1988}}{{sfn|Souther|1992|pp=43, 241}} This {{convert|5|km|mi|abbr=on}} wide [[pluton]] is of [[Eocene]] age and is part of the Sloko Group, which formed in a [[continental arc]] setting as early as the [[Late Cretaceous]].{{sfn|Souther|1988}}{{sfn|Souther|1992|p=43}}{{sfn|Logan|Koyanagi|1989|p=269}} Peripheral to the Elwyn Creek Pluton is gently tilted [[sedimentary rock|sedimentary]] strata of the [[Sustut Group]].{{sfn|Souther|1992|p=43}} This [[geological group]] consists of [[siltstone]], [[sandstone]], [[shale]], [[arkose]], [[conglomerate (geology)|conglomerate]] and minor [[coal]] that was deposited during the [[Cretaceous]] and [[Paleocene]].{{sfn|Souther|1988}} The formation is also underlain by [[Paleozoic]] and [[Mesozoic]] sedimentary, volcanic or [[metamorphic rock|metamorphic]] rocks of the [[Stikinia terrane]], which [[Accretion (geology)|accreted]] to the [[continental margin]] of North America during the [[Jurassic]].{{sfn|Souther|1988}}{{sfn|Souther|1992|p=39}}{{sfn|Edwards|Russell|2000|pp=1281, 1287}}
==Volcanology== {{see also|Volcanism of the Mount Edziza volcanic complex}} The Nido Formation was deposited by at least six major and several smaller eruptive centres during the [[Volcanism of the Mount Edziza volcanic complex#Second magmatic cycle|second magmatic cycle]] of the MEVC between 6 and 1 million years ago.{{sfn|Souther|1992|pp=8, 267}} Individual eruptive centres are widely separated and formed along the eastern edge of the volcanic complex, mainly in a north–south trending zone.{{sfn|Souther|1992|pp=8, 98}} All of them are the products of a long period of volcanic activity and are now deeply eroded; the remains are considered to be of [[shield volcano]]es.{{sfn|Souther|Armstrong|Harakal|1984|p=344}}{{sfn|Souther|1992|p=8}} The precise sequence of eruptions that formed the Nido Formation remains unknown. However, the geological record suggests that Nido volcanism was characterized by the eruption of highly mobile, fluid lava flows. The lava flowed over lag gravels and outwash from the Armadillo Highlands and created several [[lava-dammed lake]]s where the drainage patterns of streams were disrupted.{{sfn|Souther|1992|p=8}} [[Glacial deposit]]s under the Kounugu and Tenchen members contain [[clast]]s from the Armadillo Highlands, suggesting that both units formed more or less simultaneously.{{sfn|Souther|1992|p=93}}
As a part of the MEVC, the Nido Formation lies within a broad area of volcanoes called the [[Northern Cordilleran Volcanic Province]], which extends from northwestern British Columbia northwards through [[Yukon]] into easternmost [[Alaska]].{{sfn|Souther|1988}}{{sfn|Edwards|Russell|2000|pp=1280, 1281, 1283, 1284}} The dominant rocks comprising these volcanoes are alkali basalts and hawaiites, but [[nephelinite]], [[basanite]] and [[peralkaline rock|peralkaline]]{{efn|''Peralkaline rocks'' are magmatic rocks that have a higher ratio of sodium and potassium to aluminum.{{sfn|McGraw Hill|2003|p=253}}}} [[phonolite]], trachyte and comendite are locally abundant. These rocks were deposited by volcanic eruptions from 20 million years ago to as recently as a few hundred years ago. Volcanism in the Northern Cordilleran Volcanic Province is thought to be due to [[rift]]ing of the [[North American Cordillera]], driven by changes in relative [[plate tectonics|plate motion]] between the [[North American Plate|North American]] and [[Pacific Plate|Pacific]] plates.{{sfn|Edwards|Russell|2000|p=1280}}
===Tenchen Member=== {{main|Tenchen Member}} [[File:Nido Formation.png|thumb|right|upright=0.8|alt=Diagram showing the locations of rock outcrops, prehistoric lakes and the inferred maximum extent of a geological formation.|Paleogeological map of the Nido Formation at the end of the [[Volcanism of the Mount Edziza volcanic complex#Nido eruptive period|Nido eruptive period]]]] At least three major eruptive centres of the Tenchen Member, referred to as the Alpha, Beta and Gamma peaks, flooded an area of more than {{convert|350|km2|mi2|abbr=on}} with lava flows to create a [[lava field]].{{sfn|Souther|1992|p=93}} The northernmost and oldest major eruptive centre, Alpha Peak, is considered to have been adjacent to [[The Pyramid (British Columbia)|The Pyramid]] of the younger Pyramid Formation partially due to the distribution of Nido [[tephra]] in that area.{{sfn|Souther|1992|pp=9, 93, 94, 129}} Cliffs containing 50 to 60% coarse tephra occur along the southern side of [[Pyramid Creek (Kakiddi Creek tributary)|Pyramid Creek]], but the actual vent and conduit system of this eruptive centre remains unexposed. Bright red [[Volcanic bomb#Bomb types|spindle]] and [[cow pie bomb]]s up to {{convert|0.6|m|ft|abbr=on}} wide occur in the tephra. Lava flows [[Interbedding|interlayered]] in the tephra contain rough hackly surfaces and are randomly [[joint (geology)|jointed]] and highly [[vesicular texture|vesicular]].{{sfn|Souther|1992|p=93}} The Alpha shield was at times nearly surrounded by northward-flowing [[outlet glacier]]s of an [[ice cap]] that covered the Armadillo Highlands.{{sfn|Souther|1992|p=10}}
About {{convert|12|km|mi|abbr=on}} to the south, Beta Peak began to form during the late stages of Alpha volcanism when several vents issued thin, coalescing lava flows.{{sfn|Souther|1992|pp=9, 98}} Remnants of this small shield volcano are exposed on the eastern flank of Mount Edziza between the [[Tenchen Creek|Tenchen]] and [[Sorcery Creek|Sorcery]] creeks where flat-lying, [[columnar jointing|columnar-jointed]] lava flows form prominent step-like [[outcrop]]s.{{sfn|Souther|1992|p=98}} Lava flows of the Beta shield on [[Sorcery Ridge]] are overlain by tephra believed to be the remains of a [[pyroclastic cone]].{{sfn|Souther|1988}}{{sfn|Souther|1992|p=98}} It consists of bombs and [[volcanic block|blocks]] that are enclosed by loosely agglutinated [[volcanic spatter|spatter]] and [[lapilli]].{{sfn|Souther|1992|p=98}} At its climax, the pyroclastic cone rose at least {{convert|365|m|ft|abbr=on}} above the surrounding surface of the Beta shield.{{sfn|Souther|1992|pp=9, 98}}
Gamma Peak, the southernmost and second oldest major eruptive centre, began to form with the eruption of thin lava flows from a vent at the western end of [[Cartoona Ridge]].{{sfn|Souther|1992|pp=9, 94, 95}} These lava flows overlie [[volcanic ash|ash]] of the Armadillo Formation and travelled along the northern side of the Armadillo Highlands as far west as the [[Mess Creek Escarpment]] where they are exposed.{{sfn|Souther|1992|pp=9, 101}} Later, the northward-flowing outlet glaciers originating from the Armadillo Highlands eventually covered the Gamma shield. [[Subglacial eruption|Subglacial volcanism]] under these glaciers formed a meltwater lake above the Gamma shield, whose surface was penetrated by [[explosive eruption|explosions]] resulting from lava-water interactions.{{sfn|Souther|1992|p=10}} After the ice had receded, a fragile pile of [[tuff breccia]] formed by the earlier [[wikt:subaqueous|subaqueous]] activity was buried under [[subaerial]] lava up to {{convert|90|m|ft|abbr=on}} thick from renewed eruptions. Thin lava flows more than {{convert|6.5|km|mi|abbr=on}} long spread to the west.{{sfn|Souther|1992|p=11}} Eroded remnants of the Gamma shield include Kaia Bluff and [[Cartoona Peak]].{{sfn|Souther|1992|pp=99, 101}}
===Kounugu Member=== The Kounugu Member issued from at least four eruptive centres, all of which have been deeply eroded or extensively buried under younger rocks.{{sfn|Souther|1992|p=104}} Swarm Peak formed on the older Little Iskut Formation shield volcano as [[magma]] intruded through swarms of north-trending fractures in the underlying [[basement (geology)|basement]]. Lava from Swarm Peak flowed over [[gravel bar]]s and [[alluvial fan]]s just south of the Armadillo Highlands after descending the southern and western slopes of the Little Iskut shield.{{sfn|Souther|1992|p=11}} Exposed in cliffs along [[Raspberry Pass]], the [[Little Iskut River]] and the [[Artifact Creek|Artifact]] and [[Mess Creek|Mess]] creeks are lava flows believed to have originated from Swarm Peak. Nearly all of the southern edge of the Swarm Peak edifice is buried under lava flows and [[lava dome|domes]] of the younger Spectrum Range.{{sfn|Souther|1992|p=105}}
[[File:Exile Hill cross section.png|thumb|right|alt=Diagram showing the interior of a cone-shaped hill and explaining its geology.|Geological [[cross section (geology)|cross section]] of [[Exile Hill]]]] Most of the lava flows from the Vanished Peak eruptive centre travelled to the north and west.{{sfn|Souther|1992|p=11}} They are exposed adjacent to the southeastern margin of the Spectrum Range where they reach thicknesses of up to {{convert|365|m|ft|abbr=on}}. Lava flows ranging from {{convert|6|to|15|m|ft|adj=mid|abbr=on}} thick form prominent cliffs at the base of the Spectrum Range where they contain well-formed columnar joints. Interbedding these lava flows are {{convert|0.5|to|1.5|m|ft|adj=mid|-thick|abbr=on}} layers of [[scoria]], which is in the form of cinders and blocks. Vanished Peak was most likely active during a period of glacial recession because there is a lack of evidence for lava-water interaction. About {{convert|12|m|ft|abbr=on}} of [[pillow lava]] and [[palagonite]] tuff breccia at the extreme southeastern base of the volcanic pile is the only evidence for lava having interacted with water. Jack Souther proposed that the pillow lava and palagonite tuff breccia formed when lava entered a lake.{{sfn|Souther|1992|p=106}}
Lost Peak is a subaerial and subaqueous dome of [[pyroclastic rock]]s near the Spectrum Range that rises to a prominent pyramid-shaped cone.{{sfn|Souther|1992|pp=11, 12, 106}} The western flank consists of a more than {{convert|300|m|ft|abbr=on}} thick sequence of waterlain tuff breccia, pillow lava and quenched lava toes.{{sfn|Souther|1992|pp=12, 106}} These volcanic deposits probably formed when lava ponded against a thick lobe of glacial ice that advanced from the [[Coast Mountains]] onto the surrounding plateau.{{sfn|Souther|1992|p=108}} In contrast, the volcanic deposits forming the eastern flank do not show evidence of having interacted with water. They consist mainly of thin lava flows that are interbedded with layers of thick scoria. Underlying the Lost Peak rocks are lava flows that issued from the Vanished Peak eruptive centre, as well as a thick layer of coarse gravel that was deposited by streams.{{sfn|Souther|1992|p=106}}
[[Exile Hill]] is the remains of a small pyroclastic cone that formed at the northern end of the [[Arctic Lake Plateau]].{{sfn|Department of Energy, Mines and Resources|1989}}{{sfn|Souther|1992|pp=11, 108}} It was almost completely inundated by younger lava flows from the south and east, but thick deposits of bombs and agglutinated spatter are exposed on the southeastern flank.{{sfn|Souther|1992|pp=11, 108}} A series of lava flows up to {{convert|180|m|ft|abbr=on}} thick, believed to have originated from a vent adjacent to Exile Hill, are exposed in isolated [[butte]]s and in cliffs along the upper Mess Creek valley. The lowermost lava flow forming the base of Exile Hill is more than {{convert|30|m|ft|abbr=on}} thick whereas the uppermost rubbly lava flows are only a few metres in thickness. Characterizing the lowermost lava flows are long, curving columns that occur in sheath-like clusters. In contrast, the upper lava flows contain random, blocky columnar jointing or stout, spheroidally weathered columns.{{sfn|Souther|1992|p=108}} These lava flows are overlain by a thick layer of gravel that consists mainly of pebbles and [[cobble (geology)|cobblestones]]. Overlying the gravel is a roughly {{convert|100|m|ft|abbr=on}} thick series of Spectrum Formation flows capping the summit of Exile Hill.{{sfn|Souther|1992|p=122}}
==Lithology== [[File:TAS-Diagramm.svg|thumb|right|alt=|Depiction of the [[total alkali-silica diagram]] with field outlines and labels. [[Alkali basalt]] is generally located in region B whereas [[hawaiite]] and [[picrite]] are located in regions S1 and Pc, respectively.]] The Nido Formation resembles the Raspberry Formation both [[mineralogically]] and [[geomorphologically]].{{sfn|Souther|Armstrong|Harakal|1984|p=344}} It is remarkably uniform in composition despite having been issued from several separate eruptive centres over a long period.{{sfn|Souther|1992|pp=93, 108}} Alkali basalt is the most widely distributed volcanic rock, occurring in both the Tenchen and Kounugu members.{{sfn|Souther|1988}} Its [[lithology]] ranges from [[wikt:aphyric|aphyric]] to slightly [[phyric]]; [[phenocryst]]s consist of [[feldspar]] and [[pyroxene]].{{sfn|Souther|Armstrong|Harakal|1984|p=344}} It predominates the lower portions of the Nido Formation and exhibits a great variety of depositional forms.{{sfn|Souther|1988}}{{sfn|Souther|Armstrong|Harakal|1984|p=344}} These include subaerial lava flows and subaqueous pillow lava, sideromelane tuff breccia and ice-contact [[facies]].{{sfn|Souther|1988}}
[[Picrite]] and hawaiite occur locally in the uppermost part of the Nido Formation and represent minor [[porphyritic]] phases of Nido volcanism.{{sfn|Souther|Armstrong|Harakal|1984|p=344}}{{sfn|Souther|1992|pp=93, 101}} Both rocks are also present in the Tenchen and Kounugu members, the latter of which contains phenocrysts of feldspar.{{sfn|Souther|1988}}{{sfn|Souther|Armstrong|Harakal|1984|p=344}}{{sfn|Souther|1992|p=101}} The presence of hawaitte suggests that some alkali basalt underwent partial [[crystal fractionation]] and feldspar accumulation in subterranean [[magma chamber]]s prior to eruption.{{sfn|Souther|Hickson|1984|p=79}} Picrite is limited to the Vanished Peak and Gamma Peak eruptive centres where it forms lava flows in the upper parts of both edifices.{{sfn|Souther|1992|pp=101, 102, 106}}
Essential mineral constituents in the Nido Formation include [[diopside|salite]], [[olivine]], [[plagioclase]] and [[ilmenite]]. Phenocrysts composed of plagioclase are the most common; they are normally in the form of {{convert|2|to|5|mm|in|-long|adj=mid|abbr=off}} tabular laths. The laths are usually [[euhedral]] and complexly twinned, rarely comprising more than 5% of Nido alkali basalt. Hawaiite containing up to 50% plagioclase phenocrysts forms the upper lava flows of the Beta Peak eruptive centre. Olivine and [[clinopyroxene]] phenocrysts are the second and third most common phenocrysts, respectively. Phenocrysts composed of [[opaque]] oxides are rare.{{sfn|Souther|1992|p=108}}
==Age== At least four [[potassium–argon date]]s have been obtained from samples of the Nido Formation.{{sfn|Souther|Armstrong|Harakal|1984|p=344}}{{sfn|Souther|1992|p=249}} The two youngest dates, 4.4 ± 0.5 million years and 4.5 ± 0.3 million years, are from the Tenchen Member.{{sfn|Souther|1992|p=93}}{{sfn|Souther|Armstrong|Harakal|1984|pp=341, 344}} Analytical precision of these two dates gives an average age of 4.4 million years.{{sfn|Souther|Armstrong|Harakal|1984|pp=341, 344}} This age places the Tenchen Member in the [[Pliocene]] and has also been used as the average age of the Nido Formation.{{sfn|Souther|1992|p=267}}{{sfn|Souther|1988}} The third youngest date, 5.5 ± 1.6 million years, is also from the Tenchen Member but it has a large error.{{sfn|Souther|1992|p=93}}{{sfn|Souther|Armstrong|Harakal|1984|pp=341, 344}} As a result, this date may be anomalously old. Rocks with high atmospheric [[argon]] content, such as the sample this date was obtained from, are susceptible of producing erroneous ages. The average age of the Tenchen Member increases only to 4.5 million years if the third youngest date is included.{{sfn|Souther|Armstrong|Harakal|1984|p=344}}
The oldest date of 7.8 ± 0.3 million years is from a thick section of Kounugu Member basalt southeast of the Spectrum Range.{{sfn|Souther|1992|p=249}}{{sfn|Souther|Armstrong|Harakal|1984|pp=341, 344}} If this date is correct, it suggests that the overall age of the Kounugu Member may span those of the Raspberry and Armadillo formations.{{sfn|Souther|Armstrong|Harakal|1984|p=344}} It also suggests that Nido volcanism began during the [[Miocene]] rather than the Pliocene, both of which are epochs of the [[Neogene]] period.{{sfn|Souther|1988}}{{sfn|Raffi et al.|2020|p=1142}} The location where the oldest date was obtained is beyond the southern extent of the Armadillo Formation where the Kounugu Member overlies [[Mesozoic]] or [[Paleozoic]] basement.{{sfn|Souther|1988}}{{sfn|Souther|Armstrong|Harakal|1984|p=344}}
==Notes== {{notelist}}
==References== {{reflist}}
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{{Mount Edziza volcanic complex}}
[[Category:Geologic formations of the Mount Edziza volcanic complex]] [[Category:Neogene British Columbia]] [[Category:Neogene volcanism]] [[Category:Basalt formations]] [[Category:Neogene shield volcanoes]] [[Category:Shield volcanoes of Canada]] [[Category:Tahltan place names]]