The Nido Formation is a stratigraphic unit of Neogene age in northwestern British Columbia, Canada. It is the second most voluminous of 13 geological formations composing the Mount Edziza volcanic complex (MEVC), which consists of volcanic rocks of late Cenozoic age. Underlying the Nido Formation are the Raspberry, Little Iskut and Armadillo formations of the MEVC, which have average ages ranging from 7.4 to 6.3 million years old. The overlying Spectrum, Pyramid, Ice Peak, Pillow Ridge, Edziza, Arctic Lake and 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 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 second magmatic cycle of the MEVC. The eruptive centres, which are considered to have been shield volcanoes, 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. It was mapped as one of 15 geological formations of the Mount Edziza volcanic complex (MEVC), a group of late Cenozoic volcanic rocks in northwestern British Columbia, Canada. In 1988, Jack Souther remapped the MEVC into 13 geological formations; the Kounugu and Sheep Track formations were reassigned as members of the Nido and Big Raven formations, respectively. The Kounugu and Sheep Track members are no longer recognized as formations since that rank for them has been abandoned.
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. Nido means in the Tahltan language. It is a reference to white men who staked mineral claims in the area of the creek before Mount Edziza Provincial Park was established in 1972.
Stratigraphy Stratigraphically, the Nido Formation is subdivided into two units referred to as the Kounugu and Tenchen members. They are separated by the Armadillo Highlands and are not completely underlain by a single marker horizon. South of the Armadillo Highlands, the Kounugu Member is exposed around the periphery of the Spectrum Range at the southern end of the MEVC. 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.
The Nido Formation has a volume of , 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 of volcanic material. The Nido Formation is the most widespread geological formation of the MEVC over an area about wide and long. It contains at least 40 lava flow units and reaches a thickness of in the Kounugu Member.
Overlying units
The Nido Formation is overlain by seven younger geological formations of the MEVC. The oldest is the 3.1-million-year-old Spectrum Formation, which consists mainly of rhyolite and trachyte forming the Spectrum Range. To the north, the Nido Formation is overlain by the 1.1-million-year-old Pyramid Formation on ridges along the deeply dissected eastern flank of Mount Edziza. It consists mainly of rhyolite and trachyte, although basalt overlies a pyroclastic surge deposit at the base of the formation. The approximately 1-million-year-old Ice Peak Formation overlies much of the Nido Formation on the Big Raven Plateau. Consisting of alkali basalt, hawaiite, mugearite, trachyte and benmoreite, it is the most petrographically complex formation of the MEVC.
In the middle of the Big Raven Plateau, the Nido Formation is overlain by the 0.9-million-year-old Pillow Ridge and Edziza formations. The Pillow Ridge Formation is limited to the Pillow and Tsekone ridges, which consist of alkali basalt and hawaiite, respectively. In contrast, the dominantly trachytic Edziza Formation forms the central stratovolcano of Mount Edziza. 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. It consists mainly of alkali basalt that issued from at least seven separate eruptive centres on or adjacent to the plateau. 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 composing the Big Raven and Kitsu plateaus. It consists mainly of alkali basalt and hawaiite, although it also includes a small volume of comenditic trachyte pumice on the Big Raven Plateau.
Underlying units
Underlying the Nido Formation are three other geological formations of the MEVC. 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 plateaus. Because only the distal portions of the Kounugu Member overlap with the southern edge of the Armadillo Formation, it is possible that both stratigraphic units are coeval in age. However, it also does not rule out the possibility that the Kounugu Member is older than the Armadillo Formation. Trachybasalt of the 7.2-million-year-old Little Iskut Formation underlies the Nido Formation in strata of the Artifact and Obsidian ridges south of the Armadillo Highlands. 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.
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 Pluton. This 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. Peripheral to the Elwyn Creek Pluton is gently tilted sedimentary strata of the Sustut Group. This geological group consists of siltstone, sandstone, shale, arkose, conglomerate and minor coal that was deposited during the Cretaceous and Paleocene. The formation is also underlain by Paleozoic and Mesozoic sedimentary, volcanic or metamorphic rocks of the Stikinia terrane, which accreted to the continental margin of North America during the Jurassic.
Volcanology
The Nido Formation was deposited by at least six major and several smaller eruptive centres during the second magmatic cycle of the MEVC between 6 and 1 million years ago. Individual eruptive centres are widely separated and formed along the eastern edge of the volcanic complex, mainly in a north–south trending zone. 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 volcanoes. 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 lakes where the drainage patterns of streams were disrupted. Glacial deposits under the Kounugu and Tenchen members contain clasts from the Armadillo Highlands, suggesting that both units formed more or less simultaneously.
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. The dominant rocks that make up these volcanoes are alkali basalts and hawaiites, but nephelinites, basanites and peralkaline phonolites, trachytes and comendites 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 rifting of the North American Cordillera, driven by changes in relative plate motion between the North American and Pacific plates.
Tenchen Member
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 with lava flows to create a lava field. The northernmost and oldest major eruptive centre, Alpha Peak, is considered to have been adjacent to The Pyramid of the younger Pyramid Formation; this is partially due to the distribution of Nido tephra in that area. Cliffs containing 50 to 60% coarse tephra occur along the southern side of Pyramid Creek, but the actual vent and conduit system of this eruptive centre remains unexposed. Bright red spindle and cow pie bombs up to wide occur in the tephra. Lava flows interlayered in the tephra contain rough hackly surfaces and are randomly jointed and highly vesicular. The Alpha shield was at times nearly surrounded by northward-flowing outlet glaciers of an ice cap that covered the Armadillo Highlands.
About to the south, Beta Peak began to form during the late stages of Alpha volcanism when several vents issued thin, coalescing lava flows. Remnants of this small shield volcano are exposed on the eastern flank of Mount Edziza between the Tenchen and Sorcery creeks where flat-lying, columnar-jointed lava flows form prominent step-like outcrops. Lava flows of the Beta shield on Sorcery Ridge are overlain by tephra believed to be the remains of a pyroclastic cone. It consists of bombs and blocks that are enclosed by loosely agglutinated spatter and lapilli. At its climax, the pyroclastic cone rose at least above the surrounding surface of the Beta shield.
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. These lava flows overlie 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. Later, the northward-flowing outlet glaciers originating from the Armadillo Highlands eventually covered the Gamma shield. Subglacial volcanism under these glaciers formed a meltwater lake above the Gamma shield, whose surface was penetrated by explosions resulting from lava-water interactions. After the ice had receded, a fragile pile of tuff breccia formed by the earlier subaqueous activity was buried under subaerial lava up to thick from renewed eruptions. Thin lava flows more than long spread to the west. Eroded remnants of the Gamma shield include Kaia Bluff and Cartoona Peak.
Kounugu Member