{{good article}} {{Short description|Geological formation in British Columbia, Canada}} {{Infobox Rockunit | name = Big Raven Formation | image = Nahta cone from east june 2006.jpg | caption = [[Nahta Cone]] of the Big Raven Formation | alt = A dark-coloured volcanic cone with a summit crater rising above a sparsely snow-covered rocky plateau. | type = [[Geological formation]]{{sfn|Souther|1992|p=213}} | age = [[Quaternary]]<br/>{{Fossil range|0.020|0}} | period = Quaternary | prilithology = [[Alkali basalt]], [[hawaiite]]{{sfn|Souther|1992|p=213}} | otherlithology = [[Trachyte]]{{sfn|Souther|1992|p=213}} | namedfor = | namedby = [[Jack Souther|Souther]] ''et al.'', 1984<ref name="BRF">{{cite web|url=https://weblex.canada.ca/html/001000/GSCC00053001316.html|title=Big Raven Formation|work=[[Lexicon of Canadian Geologic Units]]|publisher=[[Government of Canada]]|access-date=2023-09-06}}</ref> | region = [[British Columbia]]<ref name="KF">{{cite gvp|vn=320060|title=Edziza: General Information|archive-date=2021-08-10|access-date=2021-09-25}}</ref> | country = Canada<ref name="KF"/> | coordinates = [[Big Raven Plateau]]{{sfn|Souther|1992|p=235}} <br />{{Coord|57|42|59|N|130|45|06|W}}<ref>{{cite bcgnis|id=7967|name=Big Raven Plateau|archive-url=https://web.archive.org/web/20210930220310/https://apps.gov.bc.ca/pub/bcgnws/names/7967.html|archive-date=2021-09-30|access-date=2024-04-03}}</ref> [[Arctic Lake Plateau]]{{sfn|Souther|1992|p=235}} <br />{{Coord|57|17|59|N|130|46|06|W}}<ref>{{cite bcgnis|id=12608|name=Arctic Lake Plateau|archive-url=https://web.archive.org/web/20210925003925/https://apps.gov.bc.ca/pub/bcgnws/names/12608.html|archive-date=2021-09-25|access-date=2024-04-03}}</ref> [[Kitsu Plateau]]{{sfn|Souther|1992|p=234}} <br />{{Coord|57|27|59|N|130|45|06|W}}<ref>{{cite bcgnis|id=4627|name=Kitsu Plateau|archive-url=https://web.archive.org/web/20211001000357/https://apps.gov.bc.ca/pub/bcgnws/names/4627.html|archive-date=2021-10-01|access-date=2024-04-03}}</ref> | unitof = [[Mount Edziza volcanic complex]]<ref name="Edwards">{{cite thesis|last1=Edwards|first1=Benjamin Ralph|degree=PhD|title=Field, kinetic, and thermodynamic studies of magmatic assimilation in the Northern Cordilleran Volcanic Province, northwestern British Columbia|publisher=[[University of British Columbia]]|year=1997|pages=3, 10, 11|isbn=0-612-25005-9}}</ref> | subunits = [[Sheep Track Member]]{{sfn|Souther|1992|p=213}} | overlies = [[Nido Formation]], Spectrum Formation, [[Raspberry Formation]], [[Armadillo Formation]], [[Klastline Formation]], [[Ice Peak Formation]], [[Edziza Formation]], [[Kakiddi Formation]]{{sfn|Souther|1992|p=246}}<ref name="Souther">{{cite map|last1=Souther|first1=J. G.|author-link1=Jack Souther|title=Geology, Mount Edziza Volcanic Complex, British Columbia|map=1623A|scale=1:50,000|type=[[Geological map]]|cartography=M. Sigouin, [[Geological Survey of Canada]]|publisher=[[Energy, Mines and Resources Canada]]|year=1988|language=en|doi=10.4095/133498|doi-access=free}}</ref> | thickness = | extent = | area = | map = Big Raven Formation.png | map_alt = Diagram showing the locations of volcanoes and lava fields of a geological formation. | map_caption = [[Geological map]] of the Big Raven Formation }} The '''Big Raven Formation''' is a [[stratigraphic unit]] of [[Quaternary]] age in northwestern [[British Columbia]], Canada. It is the youngest and least voluminous [[geological formation]] of the [[Mount Edziza volcanic complex]] (MEVC); it overlies at least eight older formations of this volcanic complex. The main [[volcanic rock]]s of the Big Raven Formation are [[alkali basalt]]s and [[hawaiite]]s, although a small volume of [[trachyte]] comprises the [[Sheep Track Member]]. These rocks were deposited by volcanic eruptions in the last 20,000 years during the [[Volcanism of the Mount Edziza volcanic complex#Fifth magmatic cycle|latest magmatic cycle]] of the MEVC. Alkali basalt and hawaiite are in the form of [[lava]] flows and small [[volcanic cone]]s while trachyte of the Sheep Track Member is mainly in the form of volcanic [[ejecta]] which covers an area of about {{Convert|40|km2|mi2|abbr=off}}.
The Big Raven Formation is widespread throughout the MEVC, occurring on or adjacent to the [[Arctic Lake Plateau|Arctic Lake]], [[Big Raven Plateau|Big Raven]] and [[Kitsu Plateau|Kitsu]] plateaus. Two [[lava field]]s of the Big Raven Formation occur on the Big Raven Plateau which in total contain at least 22 separate vents that issued lava flows. At least four isolated Big Raven vents occur on the eastern flank of [[Mount Edziza]] and on the extreme northern slope of the Big Raven Plateau. The Kitsu Plateau contains a much smaller Big Raven lava field near the edge of an [[escarpment]]; at least three separate vents are in this lava field. Two isolated Big Raven vents on and adjacent to the Arctic Lake Plateau produced lava flows, as did two isolated Big Raven vents in [[Walkout Creek]] valley. Most Big Raven vents are marked by a cone of [[pyroclastic rock]]s.
==Stratigraphy== [[Stratigraphically]], the Big Raven Formation is the youngest unit of the late [[Cenozoic]] Mount Edziza volcanic complex in northwestern British Columbia, Canada.<ref name="Edwards"/> It contains one [[wikt:subunit|sub-unit]] called the Sheep Track Member which is [[lithologically]] distinct from the rest of the Big Raven Formation.{{sfn|Souther|1992|p=213}} Initially, the Big Raven Formation and the Sheep Track Member were recognized as two separate geological formations by [[Jack Souther]], [[Richard Lee Armstrong]] and J. Harakal in 1984; both formations were grouped together in their descriptions and mapping.<ref name="BRF"/><ref>{{cite journal|last1=Souther|first1=J. G.|author-link1=Jack Souther|last2=Armstrong|first2=R. L.|author-link2=Richard Lee Armstrong|last3=Harakal|first3=J.|title=Chronology of the peralkaline, late Cenozoic Mount Edziza Volcanic Complex, northern British Columbia, Canada|journal=[[Geological Society of America Bulletin]]|publisher=[[Geological Society of America]]|page=339|volume=95|issue=3|year=1984|doi=10.1130/0016-7606(1984)95<337:COTPLC>2.0.CO;2|bibcode=1984GSAB...95..337S |issn=0016-7606}}</ref><ref name="STF">{{cite web|url=https://weblex.canada.ca/html/013000/GSCC00053013635.html|archive-url=https://web.archive.org/web/20231215003718/https://weblex.canada.ca/html/013000/GSCC00053013635.html|title=Sheep Track Formation|work=[[Lexicon of Canadian Geologic Units]]|publisher=[[Government of Canada]]|archive-date=2023-12-15|access-date=2024-03-06}}</ref> By 1988, the Sheep Track Formation had been reassigned as a [[member (geology)|member]] of the Big Raven Formation; its recognition as a geological formation has since been abandoned.<ref name="STF"/> The Big Raven Formation is the least voluminous geological formation of the MEVC, consisting of {{convert|1.7|km3|mi3|abbr=on}} of volcanic material.<ref name="Edwards"/>
The Big Raven Formation overlies the [[Klastline Formation|Klastline]], [[Kakiddi Formation|Kakiddi]], [[Edziza Formation|Edziza]], [[Ice Peak Formation|Ice Peak]], Spectrum, [[Nido Formation|Nido]], [[Armadillo Formation|Armadillo]] and [[Raspberry Formation|Raspberry]] formations, all of which are older units of the MEVC.{{sfn|Souther|1992|pp=246, 267}} It is separated from the Klastline Formation by a layer of [[moraine]] that was deposited by retreating glaciers at the end of the [[last glacial period]].{{sfn|Souther|1992|p=216}} The Big Raven Formation overlies the Ice Peak Formation north and south of Mount Edziza.{{sfn|Souther|1992|p=246}} An [[Inliers and outliers (geology)|outlier]] of the Big Raven Formation is separated from the Spectrum Formation by a thick layer of unconsolidated [[felsenmeer]] and [[scree|talus]].{{sfn|Souther|1992|pp=236, 246}} Another outlier of the Big Raven Formation overlies the Nido Formation east of Mount Edziza.{{sfn|Souther|1992|p=246}} Big Raven [[breccia]] on the plateau surface east of [[Mess Lake]] directly overlies trachyte of the Armadillo Formation.{{sfn|Souther|1992|p=235}} At the mouth of [[Tennaya Creek]] valley on the eastern side of the Mount Edziza volcanic complex, the Big Raven Formation overlies trachyte of the Kakiddi Formation.<ref name="Souther"/>
==Age and lithology== The exact age of the Big Raven Formation is unknown, but its oldest rocks were probably deposited during the [[Last Glacial Maximum]] at least 20,000 years ago which corresponds with the beginning of the latest magmatic cycle of the MEVC.{{sfn|Souther|1992|p=267}}<ref>{{cite thesis|last1=Hungerford|first1=Jefferson D. G.|url=http://d-scholarship.pitt.edu/id/eprint/20197|degree=PhD|title=The Mechanics of Subglacial Basaltic Lava Flow Emplacement: Inferring Paleo-Ice Conditions|publisher=[[University of Pittsburgh]]|year=2013|page=17|access-date=2024-06-21}}</ref> A Holocene age for its younger rocks has not been wholly determined from geologic dating, but rather from the lack of evidence suggesting that they were overridden by the [[Cordilleran Ice Sheet]] which retreated from the area about 11,000 years ago.<ref name="KF"/><ref name="Wilson">{{cite report|last1=Wilson|first1=Alexander M.|last2=Kelman|first2=Melanie C.|title=Assessing the relative threats from Canadian volcanoes|series=Geological Survey of Canada, Open File 8790|page=16|publisher=[[Natural Resources Canada]]|year=2021|doi=10.4095/328950|doi-access=free}}</ref> [[Tephrochronological]], [[radiocarbon dating|radiocarbon]] and [[fission track dating|fission track]] dating has yielded ages of 6520 BCE ± 200 years, 750 BCE ± 100 years, 610 CE ± 150 years and 950 CE ± 6000 years for some Big Raven volcanic rocks at Mount Edziza.<ref name="KF"/><ref name="OP">{{cite gvp|vn=320060|vtab=Eruptions|title=Edziza: Eruptive History|archive-date=2023-05-20|access-date=2023-06-01}}</ref>
Alkali basalt and hawaiite are the main volcanic rocks comprising the Big Raven Formation, having erupted from at least 30 vents along the entire north–south trending axis of the MEVC.{{sfn|Souther|1992|pp=32, 213, 214}} They are in the form of lava flows and pyroclastic cones which largely comprise two lava fields on the northern and western flanks of Mount Edziza and [[Ice Peak]], respectively.{{sfn|Souther|1992|pp=27, 213, 214}} Isolated pyroclastic cones and lava flows occur at the northern and southern extremities of the Big Raven Formation, as well as on the eastern flank of Mount Edziza and along Walkout Creek.{{sfn|Souther|1992|pp=214, 226}} Trachyte represents a small volume of the Big Raven Formation and mainly comprises [[pyroclastic fall|air-fall]] [[pumice]] of the Sheep Track Member.{{sfn|Souther|1992|p=213}}
==Locations== [[File:Big Raven Plateau.jpg|thumb|right|alt=An overhead view of an oval-shaped, snow-covered plateau with a snow-covered mountain in the middle.|The [[Big Raven Plateau]] contains the largest extent of Big Raven Formation lava flows and cones]] The Big Raven Formation is widespread throughout the MEVC, occurring on or adjacent to the Kitsu, Arctic Lake and Big Raven plateaus.<ref name="Souther"/>{{sfn|Souther|1992|pp=32, 214, 234}} The Big Raven Plateau is the main [[physiographic]] feature at the northern end of the MEVC; its dominant feature is Mount Edziza which rises from the middle of the plateau.{{sfn|Souther|1992|p=32}}<ref>{{cite bcgnis|id=2256|name=Mount Edziza|archive-url=https://web.archive.org/web/20180515043934/https://apps.gov.bc.ca/pub/bcgnws/names/2256.html|archive-date=2018-05-15|access-date=2024-03-06}}</ref> Further south near the middle of the MEVC is the Kitsu Plateau which is bounded on the north by [[Raspberry Pass]], on the west by the [[Mess Creek Escarpment]] and on the south by the [[Spectrum Range]].{{sfn|Souther|1992|p=32}} The Arctic Lake Plateau is a nearly flat [[Upland and lowland|upland]] at the southern end of the MEVC between the Spectrum Range to the east and Mess Creek valley to the west.{{sfn|Souther|1992|pp=22, 32}}
===Big Raven Plateau=== The Big Raven Plateau contains the [[Desolation Lava Field]] on the northern flank of Mount Edziza which consists of several lava flows that issued from at least 10 separate Big Raven vents.{{sfn|Souther|1992|p=26}} Another area of Big Raven lava flows called the [[Snowshoe Lava Field]] occurs on the western flank of Ice Peak; it issued from at least 12 separate vents.{{sfn|Souther|1992|p=27}} The Sheep Track Member overlies much of the southern end of the Big Raven Plateau, including the Snowshoe Lava Field.{{sfn|Souther|1992|p=228}} At least three Big Raven vents occur on the eastern flank of Mount Edziza and comprise another volcanic zone called the east slope centres.{{sfn|Souther|1992|pp=214, 226}} An isolated Big Raven vent called [[Kana Cone]] occurs on the extreme northern slope of the plateau while two [[cinder cone]]s occur on the southern slope of the plateau in Walkout Creek valley.<ref name="Souther"/>{{sfn|Souther|1992|pp=214, 234}}
====Desolation Lava Field==== Blocky [[basalt]] flows, wind-sculptured [[volcanic ash|ash]] [[bed (geology)|beds]] and cinder cones comprise the more than {{Convert|150|km2|mi2|adj=mid|abbr=off}} Desolation Lava Field.{{sfn|Souther|1992|p=213}} This is the largest lava field of the Big Raven Formation, as well as one of the youngest volcanic features on Mount Edziza.{{sfn|Souther|1992|pp=213, 214}} [[Sleet Cone]] and [[Storm Cone]], the oldest cones in the Desolation Lava Field, were sources of lava flows that directly travelled over [[glacial till]].{{sfn|Souther|1992|pp=216, 218}} The Triplex Cones issued lava that flowed to near the south shore of [[Buckley Lake (British Columbia)|Buckley Lake]] more than {{convert|12|km|mi|abbr=off}} to the northwest. [[Sidas Cone]] and [[Twin Cone]] are complex volcanic piles resulting from simultaneous, multi-vent [[lava fountain]]ing. Both cones issued relatively thin clinkery-surfaced basalt flows that are sparsely [[porphyritic]].{{sfn|Souther|1992|p=219}} [[Moraine Cone]] was the source of a lava flow that travelled northeast into [[Kakiddi Creek|Kakiddi Valley]] near its junction with the [[Klastline River]].{{sfn|Souther|1992|p=222}} The two youngest cinder cones in the Desolation Lava Field, [[Eve Cone]] and [[Williams Cone]], produced basaltic lava that flowed to Buckley Lake and the Klastline River, respectively.{{sfn|Souther|1992|pp=217, 219}} A radiocarbon date of 610 CE ± 150 years has been obtained from [[willow]] twigs preserved in ejecta from Williams Cone.<ref name="OP"/>{{sfn|Souther|1992|p=224}}
[[File:MEVC panorama.jpg|thumb|left|300px|alt=A gently sloping surface rising above vegetated slopes with mountains in the background and foreground.|Panoramic view of the Big Raven Plateau; [[Eve Cone]] is visible as a small dark hill to the right]] Some of the lava flows comprising the Desolation Lava Field issued from vents adjacent to the northern [[trim line]] of Mount Edziza's summit [[ice cap]] where meltwater interacted with the erupting lava to form [[tuff ring]]s. These tuff rings composed of quenched breccia later transitioned into normal subaerial cinder cones as the progressing eruptions displaced ice and meltwater.{{sfn|Souther|1992|p=26}} The more than {{Convert|12|km|mi|abbr=on}} long lava flow from Williams Cone formed a temporary dam across the Klastline River at its distal end.{{sfn|Souther|1992|p=27}} Lava from Moraine Cone temporarily [[volcanic dam#Lava dam|dammed]] Kakiddi Creek and the Klastline River, both of which have since etched new [[channel (geography)|channels]] around or through the lava flow. The presence of [[lacustrine]] [[silt]] in small [[terrace (geology)|terraces]] upstream from the lava flow suggests that both streams were dammed long enough to form temporary lakes.{{sfn|Souther|1992|p=222}}
====Snowshoe Lava Field==== The Snowshoe Lava Field is a group of volcanic cones and blocky basalt flows similar in age to the Desolation Lava Field. It covers an area of about {{Convert|40|km2|mi2|abbr=off}} and is mostly covered over by air-fall pumice of the Sheep Track Member. As a result, the surface details of most of the upper lava flows in this lava field remain obscured and the cones are mantled with Sheep Track pumice.{{sfn|Souther|1992|p=228}} The Snowshoe Lava Field contains three [[subglacial volcano|subglacial cones]], [[Tennena Cone]] being the only one named.{{sfn|Souther|1992|p=229}} Tennena Cone consists of a pile of [[tuff breccia]] and [[pillow lava|pillow basalt]] and is one of the oldest cones in the Snowshoe Lava Field.{{sfn|Souther|1992|pp=26, 230}} A smaller unnamed subglacial cone {{Convert|1.5|km|mi|abbr=on}} to the south also consists of a pile of pillow basalt and tuff breccia.{{sfn|Souther|1992|p=230}} Further south at the southern terminus of [[Tencho Glacier]] is another unnamed pile of tuff breccia and pillow basalt.{{sfn|Souther|1992|pp=229, 231}} It may be the remains of a tuff ring that has been [[glacial erosion|glacially modified]] after having formed in a meltwater lake when Tencho Glacier existed at lower [[elevation]]s.{{sfn|Souther|1992|p=231}}
[[File:Tennena Cone.jpg|thumb|left|alt=A black cone-shaped mountain rising over glacial ice in the foreground.|[[Tennena Cone]] from the north]] The Snowshoe Lava Field contains five transitional cones,{{efn|''Transitional cones'' are volcanic cones that initially interacted with water during eruption and then later transitioned into normal subaerial volcanic piles as the progressing eruptions displaced ice and meltwater.{{sfn|Souther|1992|pp=26, 182}}}} one of which is named.{{sfn|Souther|1992|pp=229, 231}} Two unnamed transitional cones are on the southwestern edge of Mount Edziza's summit ice cap inside the alpine trim lines, both of which have been reduced to low, [[drumlin]]-like [[ridge]]s as a result of glacial ice overridding them.{{sfn|Souther|1992|p=231}} The three other transitional cones are unglaciated, having formed at lower elevations just below the terminal alpine moraine.{{sfn|Souther|1992|pp=214, 231}} Their inner structures have been exposed by sufficient erosion, but they still retain their central craters and original conical form. One of these cones, [[Coffee Crater]], was the source of a lava flow that extended to the southwest; the northern edge of the terminal lobe of this lava flow forms a prominent, {{Convert|18|m|ft|adj=mid|-high|abbr=off}} escarpment.{{sfn|Souther|1992|p=231}}
Three subaerial cones exist in the Snowshoe Lava Field, two of which are named. The largest subaerial cone, [[Cocoa Crater]], produced a {{Convert|2|km|mi|abbr=on}} wide lava flow that travelled into upper [[Sezill Creek]] canyon.{{sfn|Souther|1992|p=232}} [[Keda Cone]]{{efn|Keda Cone is referred to by the numeronym SLF-9 and is erroneously called Kena Cone in the Catalogue of Canadian volcanoes.<ref>{{cite web|title=Kena Cone (SLF-9)|work=Catalogue of Canadian volcanoes|publisher=[[Natural Resources Canada]]|date=2009-03-10|url=http://cgc.rncan.gc.ca/volcanoes/cat/volcano_e.php?id=svb_kcn_090|access-date=2023-02-19|url-status=dead|archive-url=https://web.archive.org/web/20110706182305/http://cgc.rncan.gc.ca/volcanoes/cat/volcano_e.php?id=svb_kcn_090|archive-date=2011-07-06}}</ref> According to [[BC Geographical Names]], Keda Cone is the proper name.<ref>{{cite bcgnis|id=3108|name=Keda Cone|archive-url=https://web.archive.org/web/20211020212226/https://apps.gov.bc.ca/pub/bcgnws/names/3108.html|archive-date=2021-10-20|access-date=2023-08-19}}</ref>}} and an unnamed subaerial cone to the northeast issued the most voluminous lava in the Snowshoe Lava Field, having flowed westward where it engulfed an area more than {{Convert|3|km|mi|abbr=on}} wide and {{Convert|8|km|mi|abbr=on}} long.{{sfn|Souther|1992|pp=214, 233}} This lava then continued to flow westward into upper [[Taweh Creek|Taweh Valley]] where it transformed into a relatively narrow flow that formerly extended to near Mess Creek. A subaerial vent referred to as [[The Saucer]] produced the youngest lava flow in the Snowshoe Lava Field from near the southern edge of Mount Edziza's summit ice cap. Unlike the other three subaerial centres, The Saucer flow appears to have issued without any accompanying lava fountaining, resulting in no cinder cone development.{{sfn|Souther|1992|p=233}}
====Sheep Track Member==== The southwestern flank of Ice Peak and the surrounding Big Raven Plateau are blanketed with loose, air-fall [[tephra]] of the Sheep Track Member.{{sfn|Souther|1992|pp=28, 32, 236}} It covers an area of about {{Convert|40|km2|mi2|abbr=off}} and consists of pumice fragments that range in size from snowball-sized chunks to pea-sized debris.{{sfn|Souther|1992|pp=27, 28, 236}} The largest fragments occur along the western margin of Tencho Glacier while the smallest fragments form a circular area at least {{Convert|10|km|mi|abbr=on}} in diameter.{{sfn|Souther|1992|pp=28, 236}} Erosion has largely removed the Sheep Track pumice from drainage channels of small intermittent streams on the upper Big Raven Plateau, but deposits as much as {{Convert|2|m|ft|abbr=on}} thick occur in [[interfluvial]] areas. Although the Sheep Track pumice was deposited on nearly all of the Snowshoe Lava Field flows and cinder cones, The Saucer is covered by only thin drifts of wind-blown pumice and may postdate the Sheep Track eruption.{{sfn|Souther|1992|p=236}} The source of the Sheep Track pumice remains unknown, but it probably originated from a vent hidden under Tencho Glacier.{{sfn|Souther|1992|p=237}} Fission track dating of Sheep Track pumice on the southwestern flank of Ice Peak has yielded an age of 950 CE ± 6000 years.<ref name="OP"/>
====Walkout Creek valley==== {{Location map|Canada British Columbia|relief=1|lat=57.64|long=-130.67|caption=Location of the Big Raven Formation in [[British Columbia]], Canada|float=right}} Two small Big Raven cones called the [[Walkout Creek centres]] occur on the northern and western sides of Walkout Creek valley, both of which are largely buried and deeply eroded.<ref name="Souther"/>{{sfn|Souther|1992|pp=214, 234}} The larger cone on the northern side of the valley is of geological interest because it was constructed on top of an active, slow moving [[landslide]]. Although both cones produced lava flows, lava from the larger cone was controlled by the underlying landslide which is characterized by [[hummocky]], ridge and trough topography.{{sfn|Souther|1992|pp=234, 235}} As a result, some of this lava was forced to flow into [[depression (geology)|depressions]] paralleling minor landslide scarps.{{sfn|Souther|1992|p=235}}
====East slope centres==== The heavily eroded eastern flank of Mount Edziza contains at least three satellitic centres of the Big Raven Formation. [[Cinder Cliff]] consists of thin, slaggy basalt flows that ponded against an ice dam in the upper part of [[Tenchen Creek|Tenchen Valley]]. Tuff breccia and ash, as well as glacial and [[fluvial]] gravel, form mixed deposits at the base of Cinder Cliff.{{sfn|Souther|1992|p=226}} [[Icefall Cone]] on the northeastern spur of Tennaya Cirque and [[Ridge Cone]] on the western rim of Idiji Cirque are remnants of two Big Raven eruptive centres that have been greatly modified by [[mass wasting]] and alpine glaciation. They both consist of agglutinated spatter, [[volcanic bomb|bombs]], [[scoria|cinders]] and slaggy basalt flows; the lava flows are almost completely buried under talus, moraine, fluvial gravel and glacial ice. An intermittently exposed basalt flow extending to near [[Kakiddi Lake]] may have issued from Icefall Cone, Ridge Cone or an undiscovered vent inside a narrow, wedge-shaped notch on the eastern flank of Mount Edziza.{{sfn|Souther|1992|p=228}}
====Kana Cone==== The northernmost Big Raven vent and the northernmost vent of the entire MEVC is marked by the nested Kana Cone which rises about {{convert|60|m|ft|abbr=on}} above the surrounding [[terrain]].{{sfn|Souther|1992|pp=214, 224, 225}} Its summit contains a roughly {{Convert|20|m|ft|abbr=on}} deep crater that is breached to the north, exposing oxidated bombs and spatter. Lava from the crater breach extends downslope into Klastline Valley where it temporarily dammed the Klastline River; the river has since etched a new channel along the northern valley wall. The lava continued to flow downstream through Klastline Valley and possibly reached the [[Stikine River]] where Big Raven lava flows overlie about {{convert|100|m|ft|abbr=on}} of sediment at the mouths of the Klastline and [[Tahltan River|Tahltan]] rivers.{{sfn|Souther|1992|pp=224, 225}}<ref>{{cite journal|last1=Spooner|first1=Ian S.|last2=Osborn|first2=Gerald D.|last3=Barendregt|first3=R.|last4=Irving|first4=E.|url=http://www.acadiau.ca/~ispooner/pdfs_of_papers/Stikine%20River%20PAleomag%20Paper%20CJES.pdf|title=A Middle Pleistocene (isotope stage 10) glacial sequence in the Stikine River valley, British Columbia|journal=[[Canadian Journal of Earth Sciences]]|publisher=[[NRC Research Press]]|page=1429|volume=33|year=1996|issue=10 |doi=10.1139/e96-107 |bibcode=1996CaJES..33.1428S |access-date=2024-03-08}}</ref>
===Arctic Lake Plateau=== [[File:Nahta cone from southeast june 2006.jpg|thumb|right|alt=A sparsely snow-covered rocky landscape with a dark volcanic cone in the foreground and a cloud-covered mountain ridge in the background.|Nahta Cone and lava flow]] Two isolated occurrences of the Big Raven Formation occur on the Arctic Lake Plateau, [[Nahta Cone]] near its northern edge being the southernmost vent of Big Raven age.{{sfn|Souther|1992|pp=214, 235}} This cone overlies a glacially-scoured [[limestone]] hill and contains a crater that is breached to the east.{{sfn|Souther|1992|pp=26, 235}} At least five small conelets comprise Nahta Cone, the largest of which has a [[topographic]] relief of approximately {{convert|60|m|ft|abbr=on}}.{{sfn|Souther|1992|p=235}} The breach in the eastern crater rim served as a passageway for at least two highly fluid lava flows that travelled north along a drainage system.{{sfn|Souther|1992|p=235}}<ref name="Logan">{{cite report|last1=Logan|first1=J. M.|last2=Drobe|first2=J. R.|url=https://cmscontent.nrs.gov.bc.ca/geoscience/publicationcatalogue/Paper/BCGS_P1993-01-09_Logan.pdf|archive-url=https://web.archive.org/web/20220322174232/http://cmscontent.nrs.gov.bc.ca/geoscience/PublicationCatalogue/Paper/BCGS_P1993-01-09_Logan.pdf|archive-date=2022-03-22|title=Geology and Mineral Occurrences of the Mess Lake Area (104G/7W)|series=Geological Fieldwork 1992, Paper 1993-1|publisher=British Columbia Geological Survey|year=1993|page=141|access-date=2024-03-03}}</ref> Extending {{convert|700|m|ft|abbr=on}} north and {{convert|500|m|ft|abbr=on}} west of Nahta Cone are two tephra deposits, suggesting it was active twice during different wind directions.<ref name="Logan"/>
Northeast of Nahta Cone on the south flank of [[Kuno Peak]] in the Spectrum Range are the remains of a Big Raven pyroclastic cone.{{sfn|Souther|1992|pp=214, 236}} The remains consist of steeply dipping beds of coarse agglutinated pyroclastic rocks and basaltic lava flows that overlie thick felsenmeer and talus deposits. Much of this cone has been destroyed by repeated landslides and [[solifluction]] on Kuno Peak, the former of which also buried lava flows on the Arctic Lake Plateau that originated from this cone.{{sfn|Souther|1992|p=236}}
===Kitsu Plateau=== [[File:Mess Lake Lava Field.jpg|thumb|right|alt=An overhead view of the edge of a cliff-bounded plateau.|False colour image of [[pyroclastic fall|air-fall]] [[tephra]] from [[The Ash Pit]]]] The [[Mess Lake Lava Field]] is an area of lava flows and tephra deposits between [[Raspberry Creek (British Columbia)|Raspberry Creek]] in the north, [[Nagha Creek]] in the south and Mess Lake in the east.{{sfn|Souther|1992|p=235}}<ref name="HR">{{cite map|url=https://volcano.si.edu/maps/GVAlaskaCanada/G910509-006.jpg|archive-url=https://web.archive.org/web/20210502071600/https://volcano.si.edu/maps/GVAlaskaCanada/G910509-006.jpg|title=Telegraph Creek, Cassiar Land District, British Columbia|scale=1:250,000|series=104 G|map=A 502|edition=3|type=Topographic map|publisher=[[Department of Energy, Mines and Resources]]|year=1989|language=en,fr|access-date=2024-03-07|archive-date=2021-05-02}}</ref> It covers an area of about {{convert|18|km2|mi2|abbr=off}} and contains three separate vents of Big Raven age. The two oldest vents are marked by slightly eroded pyroclastic cones whose craters and cone-shaped structures are still apparent despite being somewhat rounded. Lava flows from both cones travelled westward to the edge of the Mess Creek Escarpment where they most likely cascaded into Mess Creek valley, but no evidence of this phenomenon has been found on or below the escarpment.{{sfn|Souther|1992|p=235}}
At the southern end of the Mess Lake Lava Field on the steep, south-facing side of Nagha Creek is the third separate vent called [[The Ash Pit]]. This {{Convert|60|m|ft|abbr=on}} deep and {{Convert|0.4|km|mi|abbr=on}} in diameter conical depression is the youngest vent of the Mess Lake Lava Field; it may also be the youngest vent of the entire MEVC.{{sfn|Souther|1992|p=235}} The Ash Pit was the source of a {{Convert|2.5|km|mi|abbr=on}} wide and {{Convert|6.5|km|mi|abbr=on}} long tephra deposit on the Kitsu Plateau that extends to the northeast.{{sfn|Souther|1992|pp=234, 235}} It was also the source of a lava flow that travelled down Nagha Creek valley towards the eastern side of Mess Lake, portions of which are buried under stream gravel, outwash and [[braided channel]] deposits.<ref name="Souther"/>
==See also== *[[Geology of British Columbia]]
==Notes== {{notelist}}
==References== {{reflist}}
===Sources=== {{refbegin}} *{{Cite report|last1=Souther|first1=J. G.|author-link1=Jack Souther|title=The Late Cenozoic Mount Edziza Volcanic Complex, British Columbia| work=[[Geological Survey of Canada]]|publisher=Canada Communication Group|series=Memoir 420|year=1992|isbn=0-660-14407-7|doi=10.4095/133497|doi-access=free}} {{refend}}
==Further reading== *{{cite book|last1=Smellie|first1=John L.|last2=Edwards|first2=Benjamin R.|title=Glaciovolcanism on Earth and Mars: Products, Processes and Palaeoenvironmental Significance|publisher=[[Cambridge University Press]]|year=2016|pages=44, 45|isbn=978-1-107-03739-7}}
{{Mount Edziza volcanic complex}}
[[Category:Big Raven Formation| ]] [[Category:Holocene volcanism]] [[Category:Trachyte formations]] [[Category:Basalt formations]]