Stratigraphy s forming the Big Raven Plateau. The small isolated occurrences east of the plateau are in the neighbouring Kakiddi Valley. The Big Raven Plateau is subdivided into at least 10
geological formations, each representing a distinct period of volcanic activity.
Volcanism during the
first magmatic cycle of the Mount Edziza volcanic complex between 7.5 and 6 million years ago formed the
Raspberry and
Armadillo formations. The overlying
Nido and
Pyramid formations were deposited by volcanic eruptions during the
second magmatic cycle between 6 and 1 million years ago. Between about 1 and 0.8 million years ago, volcanism during the
third magmatic cycle formed the
Ice Peak, Pillow Ridge and
Edziza formations. Volcanic eruptions during the
fourth magmatic cycle between 0.8 and 0.2 million years ago deposited the
Klastline and
Kakiddi formations. The
Big Raven Formation, the youngest forming the Big Raven Plateau, was deposited by eruptions during the
fifth magmatic cycle in the last 20,000 years. Most of the Big Raven Plateau rocks are assigned to the Nido, Ice Peak, Armadillo and Raspberry formations, which are the most voluminous. The Klastline, Kakiddi, Big Raven, Pyramid, Pillow Ridge and Edziza formations are significantly less extensive; the latter three are largely restricted to Mount Edziza. Nearly all of the Big Raven Formation lavas occur on the gently sloping surface of the plateau. However, they also exist on the flanks of Mount Edziza and in the summit area of Ice Peak.
Raspberry Formation The Raspberry Formation is exposed at the base of prominent escarpments on the western, southwestern and southeastern sides of the Big Raven Plateau. It has an elevation of less than along the Mess Creek Escarpment and consists of basaltic lava flows
interbedded with
scoria. More than of Raspberry lava flows are exposed in the Mess Creek Escarpment, most of which issued from a
Late Miocene shield volcano. A smaller volcanic centre northeast of the Raspberry shield was most likely the source of valley-filling lava flows exposed on and around Cartoona Ridge. The timing of Raspberry volcanism has been given a minimum age of 7.4–6.2 million years.
Armadillo Formation on the western side of the Big Raven Plateau Basaltic lava flows of the 6.3-million-year-old Armadillo Formation are exposed in the Kadeya and Sezill canyons, as well as along the Mess Creek Escarpment where they overlie basaltic lava flows of the Raspberry Formation. Armadillo basalt flows form up to thick units at these locations where they are interbedded with air-fall
pumice and
ash flows of
trachytic and
comenditic compositions. The basalt flows were highly fluid and mobile at the time of their eruption as evidenced by their extreme persistence and relatively narrow thicknesses; individual basalt flows are less than thick. The source of these lava flows was probably a cluster of vents further to the east called
Sezill Volcano. In contrast, the air-fall pumice and ash flows probably originated from the more than in diameter
Armadillo Caldera south of the Big Raven Plateau.
Nido Formation The
Tenchen Member is the only
stratigraphic unit of the Nido Formation forming the Big Raven Plateau. It consists of basaltic lava flows and pyroclastic rocks, which were erupted 4.4 million years ago from three major volcanoes and several smaller eruptive centres. Alkali basalt and minor hawaiite of the Alpha Peak, Beta Peak and associated
satellitic eruptive centres are exposed on Sorcery Ridge and Idiji Ridge. They are in the form of lava flows,
agglutinate, flow
breccia,
tuff breccia, pillow breccia and
pillow lava, which are intercalated with gravel. Alkali basalt of the Gamma Peak eruptive centre is in the form of lava flows, tuff breccia and pillow lava. These volcanic deposits occur in the upper canyon of Elwyn Creek and overlie alkali basalt of the lower unit at the southeastern end of the plateau. All of the Tenchen Member eruptive centres have been deeply dissected by erosion, such that only isolated remnants remain. For example, Cartoona Peak and Kaia Bluff both consist of alkali basalt erupted from the Gamma Peak eruptive centre. A lower unit of alkali basalt with minor
hawaiite is exposed at the southern, southeastern and southwestern ends of the plateau. At these locations, the alkali basalt is in the form of lava flows, flow breccia and agglutinate.
Pyramid Formation Exposures of the 1.1-million-year-old Pyramid Formation are limited to the northeastern, eastern and southeastern ends of the Big Raven Plateau; these areas feature rhyolite and trachyte domes, flows and pyroclastic breccia. The Pyramid consists of a roughly high trachyte dome whose base is slightly more than wide. It differs from other domes of the Pyramid Formation in that it has only been moderately eroded and is completely isolated from adjacent rocks. Adjacent to The Pyramid is Sphinx Dome, a partially buried rhyolite dome whose southern edge has been largely destroyed by headward stream erosion. Isolated remnants of the rhyolitic
Pharaoh Dome occur along the eastern and southeastern ends of the plateau between Tennaya Creek in the north and Cartoona Ridge in the south. A series of basaltic lava flows up to thick overlies a trachytic
pyroclastic surge deposit south and east of The Pyramid, both of which are also part of the Pyramid Formation.
Ice Peak Formation and
Koosick bluffs of the Ice Peak Formation The Ice Peak Formation is the most compositionally diverse geological formation of the Big Raven Plateau, having mostly erupted from Ice Peak about one million years ago. Most of the Ice Peak Formation is subdivided into two distinct stratigraphic units: the lower and upper assemblages. The lower assemblage consists of alkali basalt and hawaiite, as well as minor
mugearite,
tristanite and
trachybasalt. These rocks are in the form of subaerial lava flows, pyroclastic breccia,
sideromelane tuff breccia, pillow breccia and pillow lava. The lower assemblage is widespread throughout the plateau, but is largely buried under colluvium. Alkali basalt, trachybasalt, tristanite, mugearite,
benmoreite and trachyte of the upper assemblage compose lava flows, domes and pyroclastic breccia. They form the summit of Ice Peak and are exposed to the south and northeast along the southeastern and eastern sides of the Big Raven Plateau. The Ornostay and Koosick bluffs on the western flank of Ice Peak consist of thick lobes of upper assemblage trachyte. Both bluffs owe their thicknesses from the ponding of molten trachyte lava against glacial ice when the plateau was overlain by a
regional ice sheet. Camp Hill consists of basaltic tuff breccia, pillow breccia, lava flows and
tephra of the upper assemblage. It is considered to be part of the Ice Peak Formation because it is believed to have formed contemporarily with the Ice Peak volcano. Lava from Camp Hill extends to the northwest along the southwestern edge of the Big Raven Plateau, but it is almost completely buried under colluvium deposits. Small exposures of this lava occur along the Mess Creek Escarpment and along Taweh Valley just south of Hoia Bluff.
Pillow Ridge Formation The 0.9-million-year-old Pillow Ridge Formation is limited to the Pillow and Tsekone ridges. Pillow Ridge consists of alkali basalt that is in the form of tuff breccia, pillow breccia and pillow lava. The tuff breccia is crudely bedded and forms much of the lower and central portions of the ridge. Overlying the tuff breccia is an outer mantle of tubular pillow lava, which occurs at the northwestern and southeastern ends of Pillow Ridge. In contrast, Tsekone Ridge consists of a lower and central unit of tuff breccia and an upper unit of fractured, closely
jointed pillows and
lava tubes of hawaiitic composition. All of the Tsekone Ridge units are cut by vertical north-trending
dikes, which fed the tuff breccia and pillow lava forming the ridge. The Pillow and Tsekone ridges are both products of
subglacial volcanism, but they differ in
glass alteration and
vesicular texture; this suggests that they both formed under slightly different conditions.
Edziza Formation The Edziza Formation is a 0.9-million-year-old trachyte formation mainly forming the younger central stratovolcano of Mount Edziza and associated satellitic domes. It overlies the northern flank of the older Ice Peak stratovolcano and consists of pyroclastic breccia, as well as
lahar and ash flow deposits. These rocks are well exposed in high cliffs along the northern side of
Tenchen Glacier where an active cirque has eroded the eastern side of the volcano. Exposed at the head of this cirque is
hydrothermally altered vent breccia of the central conduit. It is overlain by the remains of at least four lava lakes that ponded inside the summit crater. Each lava lake has a thickness of around and are represented by at least four distinct cooling units. Satellitic domes of the Edziza Formation include the Glacier, Nanook and Triangle domes. Glacier Dome on the northern flank of Mount Edziza was the source of a lava flow that travelled along the northern side of Pyramid Valley. Nanook Dome lies on the southwestern rim of the central crater and may have been the source of one or two of the lava lakes. Triangle Dome on the upper western flank of Mount Edziza contains long, slender
columnar joints that have been attributed to subglacial volcanism. An unnamed
pyroclastic cone on the northwestern flank of Mount Edziza, also part of the Edziza Formation, produced two trachytic lava flows. They travelled onto the gently sloping surface of the Big Raven Plateau and are largely buried under ash and colluvium deposits.
Klastline Formation The Klastline Formation consists of thick alkali basalt flows that issued from at least three vents on the northern and eastern sides of the Big Raven Plateau 0.62 million years ago. A lobe of alkali basalt extends eastward from the northern end of the plateau to Buckley Lake as a series of scattered outcrops; much of the lobe is buried under colluvium deposits and younger lava flows. The source of this lobe was probably a pyroclastic cone that has been eroded to a low grassy hill in elevation. Klastline Cone on the eastern side of the plateau near the head of Pyramid Creek was the main source of Klastline Formation lava flows in the Klastline Valley. Erosion has since reduced these lava flows to small
buttes and buttresses along the Klastline River.
Kakiddi Formation The 0.3-million-year-old Kakiddi Formation east and west of Tencho Glacier on the southern flank of Ice Peak consists mainly of trachytic pyroclastic rocks and lava flows. Punch Cone on the southwestern flank of Ice Peak is a roughly long, steep-sided ridge composed of agglutinated Kakiddi spatter and breccia. It was the source of a largely buried lava flow that extends westward onto the gently sloping surface of the Big Raven Plateau. Erosional remnants of thick Kakiddi lava flows and pyroclastic rocks to the east occur on Ice Peak and in valleys on the eastern side of the plateau. The remnants on Ice Peak originated from a vent near its summit whereas those in the adjacent valleys originated from unknown vents. However, Nanook Dome on the summit of Mount Edziza may have been a major source; it consists of similar Edziza Formation trachyte. Trachytic lava flows of the Kakiddi Formation erupted more fluidly than those of the Edziza Formation as evidenced by their greater extant on gently sloping
terrain.
Big Raven Formation in the Snowshoe Lava Field from the northwest Alkali basalt and hawaiite of the Big Raven Formation is widespread at the northern and southern ends of the Big Raven Plateau. The largest occurrences of these rocks are the Desolation and Snowshoe lava fields at the northern and southern ends of the plateau.
Radiocarbon dating of
willow twigs preserved within tephra from Williams Cone in the Desolation Lava Field has yielded an age of 1340 ± 130 years. Kana Cone on the extreme northern slope of the plateau is the northernmost occurrence of Big Raven Formation alkali basalt and hawaiite. It was the source of lava flows that forced the Klastline River against the northern valley wall. The
Walkout Creek centres on the extreme southern slope of the Big Raven Plateau are also part of the Big Raven Formation. They consist of the remains of two small cinder cones and associated alkali basaltic lava flows.
Icefall Cone,
Cinder Cliff and
Ridge Cone on the eastern flank of Mount Edziza are three isolated Big Raven Formation basaltic eruptive centres. The Big Raven Formation also includes the
Sheep Track Member, a
pyroclastic fall deposit of comenditic trachyte pumice. It obscurs much of the surficial details of the Snowshoe Lava Field.
Basement As a part of the Mount Edziza volcanic complex, the Big Raven Plateau is underlain by the
Stikinia terrane. This is a
suite of
Paleozoic and
Mesozoic volcanic,
sedimentary and
metamorphic rocks that
accreted to the
continental margin of North America during the
Jurassic. These rocks are exposed along the western side of the plateau in Mess Valley and along the southern side in the Walkout and Chakima valleys. They also occur along the eastern side of the plateau where they form the lower units of Idiji Ridge, Sorcery Ridge and other neighbouring ridges. At the extreme northern end of the plateau, these rocks form
inliers. Also underlying the Big Raven Plateau are
Cretaceous and
Paleocene rocks of the
Sustut Group. This
geological group consists of
conglomerates,
arkoses,
siltstones,
sandstones,
shales and minor
coal; they are exposed along the western side of the plateau.
Leucogranite of the
Eocene-age
Elwyn Creek Pluton is exposed in Elwyn Canyon, as are Eocene rhyolite,
dacite and
andesite pyroclastic breccias. Lava domes and flows of Eocene age, also exposed in Elwyn Canyon, are mainly andesitic in composition. All of these Eocene rocks are part of the Sloko Group, which formed in a
continental arc setting as early as the
Late Cretaceous.
Geothermal activity The Elwyn and Sezill canyons contain
geothermal springs with water temperatures of , respectively. Their discharge may be linked to shallow
hydrothermal systems driven by residual magmatic heat as they are near recently active volcanic centres on the Big Raven Plateau. Geothermal springs in volcanic areas are rich in minerals; this is because the heated spring water dissolves the surrounding host rocks as it rises to the surface. Flow rate, flow path and the length of time the water is in contact with the surrounding rocks influence the amount of dissolved minerals present in volcanic springs.
Elwyn Creek springs About 10 small hot and warm springs are present along the banks of Elwyn Creek at elevations of . They occur adjacent to the base of the plateau where basalt of the Nido Formation overlies leucogranite comprising the Elwyn Creek Pluton. The Elwyn Hot Springs are above the
timberline and contain clear and odourless water. However, because the water is highly concentrated with dissolved minerals such as
sodium,
calcium and
bicarbonate, it has a slight mineral taste. The springs have varying discharge rates of per second. They are west of a group of recently active cinder cones along the southern edge of the Desolation Lava Field. Thick
tufa deposits formed by the precipitation of
calcite and
aragonite from active and inactive vents occur on the southwest side of Elwyn Creek where they host a series of warm ponds. A small vent midway between Elwyn Creek and the easternmost pool has the highest recorded water temperature of . Discharge on the opposite side of Elwyn Creek has resulted in the creation of a few warm springs.
Sezill Creek springs About upstream from the Taweh Creek junction, the banks of Sezill Creek are lined with small warm pools and about eight hot and warm springs for more than . They are adjacent to the base of the plateau where basalt of the Armadillo Formation overlies
basement rocks. The springs contain odourless water that is highly concentrated with mainly
silica, sodium, bicarbonate and calcium. Therefore, the water discharging from the Sezill Creek springs has a pronounced soda-mineral taste. A discharge rate of about per second has been recorded for the principal vent, which has a water temperature of . The springs are about northwest of the edge of the Snowshoe Lava Field, which also contains recently active cinder cones. They have been referred to as the Taweh Hot Springs in literature, but according to
BC Geographical Names, their official name is the Sezill Hot Springs.
Sezill has its origins from a word meaning "it is hot" in the
Tahltan language. Tufa deposits associated with these springs are present on both sides of Sezill Creek. However, the most extensive deposits occur on the north bank of Sezill Creek where the principal vent lies within a couple of metres from the stream. Also on the north side of Sezill Creek is a mushroom-shaped, tufa deposit topped by a small pool fed by a jet of carbonated water. The tufa is characterized by multiple hues of grey, orange, brown and white. About downstream from this deposit is another vent discharging water on the south side of Sezill Creek.
Volcanic hazards lava flow descending the northeastern side of the Big Raven Plateau Renewed
effusive eruptions in the Desolation and Snowshoe lava fields could produce highly fluid basaltic lava flows that may enter local streams to form
lava dams. The fluidity of these lava flows is due to their low silica content, such that they are capable of travelling more than from their source. Evidence for past damming is present in the valleys of Kakiddi Creek and the Klastline River; both streams have etched new channels around or through lava flows to expose beds of
lake silt further upstream. The ignition of forest fires by lava eruptions is also a possibility since the lower slopes of the Big Raven Plateau are vegetated. More silica-rich trachytic and rhyolitic lavas have issued from Mount Edziza, which are associated with
explosive eruptions due to their high viscosities. The eruption of highly viscous lava can result in the formation of domes; these bulbous mounds can collapse to create
pyroclastic flows. The possibility of renewed explosive activity at Mount Edziza is highlighted by the Sheep Track Member, which was deposited by an explosive eruption around 950
CE. If explosive volcanism were to resume, an
ash column could be released into the atmosphere which would affect parts of northwestern Canada. Ash columns can drift for thousands of kilometres downwind and often become increasingly spread out over a larger area with increasing distance from an erupting vent. Mount Edziza lies under a major
air route from
Vancouver, British Columbia to
Whitehorse, Yukon, suggesting that the volcano poses a potential threat to air traffic. Volcanic ash reduces visibility and can cause jet engine failure, as well as damage to other aircraft systems. ==Name and etymology==