{{good article}} {{Short description|Geological formation in British Columbia, Canada}} {{Infobox Rockunit | name = Edziza Formation | image = Edziza042909-- 113-16.jpg | caption = The ice-filled summit crater of [[Mount Edziza]] | alt = The flat-topped summit of a snow-covered mountain with a rocky and snow-covered cliff in the foreground. | type = [[Geological formation]]{{sfn|Souther|1992|p=175}} | age = [[Pleistocene]]<br/>{{Fossil range|0.9}} | period = Pleistocene | prilithology = [[Trachyte]]{{sfn|Souther|1992|p=175}} | otherlithology = | namedfor = [[Mount Edziza]]{{sfn|Souther|1992|p=175}} | namedby = [[Jack Souther|Souther]] ''et al.'', 1984<ref name="EF">{{cite web|url=https://weblex.canada.ca/html/004000/GSCC00053004508.html|title=Edziza Formation|work=[[Lexicon of Canadian Geologic Units]]|publisher=[[Government of Canada]]|access-date=2024-03-16|archive-date=2024-03-19|archive-url=https://web.archive.org/web/20240319211400/https://weblex.canada.ca/html/004000/GSCC00053004508.html}}</ref> | region = [[British Columbia]]<ref name="KF">{{cite gvp|vn=320060|title=Edziza: General Information|archive-date=2021-08-10|access-date=2024-03-16}}</ref> | country = Canada<ref name="KF"/> | coordinates = {{coord|57.72|N|130.63|W|type:mountain_region:CA-BC|display=inline,title}}<ref name="KF"/> | 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> | underlies = [[Big Raven Formation]]<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> | overlies = Pillow Ridge Formation, [[Ice Peak Formation]], [[Pyramid Formation (British Columbia)|Pyramid Formation]], [[Nido Formation]]<ref name="Souther"/>{{sfn|Souther|1992|pp=98, 134, 152}} | thickness = | extent = | area = | map = {{Infobox mapframe |wikidata=yes |zoom =8 |coord={{coord|57.72|N|130.63|W}}}} | map_caption = Location in [[Mount Edziza Provincial Park]] }} The '''Edziza Formation''' ({{IPAc-en|ə|d|ˈ|z|aɪ|z|ə}} {{respell|əd|ZY|zə}}) is a [[stratigraphic unit]] of [[Pleistocene]] age in northwestern [[British Columbia]], Canada. First described in 1984, the Edziza Formation was mapped as one of several [[geological formation]]s of the [[Mount Edziza volcanic complex]]. It overlies at least four other geological formations of this volcanic complex that differ in age and composition. The main [[volcanic rock]] comprising the Edziza Formation is [[trachyte]] which was deposited by [[volcanic eruption]]s at the end of the [[Volcanism of the Mount Edziza volcanic complex#Third magmatic cycle|third magmatic cycle]] of the Mount Edziza volcanic complex 0.9 million years ago.
Trachyte of the Edziza Formation is in the form of lava flows and [[pyroclastic rock]]s that comprise the central [[stratovolcano]] of [[Mount Edziza]], as well as [[lava dome]]s on its summit and flanks. At least four [[lava lake]]s ponded inside the summit [[volcanic crater|crater]] which cooled into four rock units that are exposed in the breached eastern crater rim. These lava lakes overlie rock fragments inside the central volcanic conduit which accompanied with collapse of a narrower and higher summit.
==Etymology== A number of explanations have been made regarding the origin of the name Edziza. A 1927 report by J. Davidson of the [[British Columbia Land Surveyors]] claims that Edziza means "sand" in the [[Tahltan language]], referring to the deep [[volcanic ash]] deposits or [[pumice]]-like sand covering large portions of the [[Big Raven Plateau]] around [[Mount Edziza]]. According to David Stevenson of [[University of Victoria]]'s Anthropology Department, "sand" or "dust" is instead translated as "kutlves" in the [[Tahltan]] language. An explanation listed in the [[BC Parks]] brochure is that Edziza means "cinders" in the Tahltan language. Another explanation proposed by Canadian [[volcanologist]] [[Jack Souther]] is that Edziza is a [[Corruption (linguistics)|corruption]] of Edzerza, the name of a local Tahltan family.<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=2021-09-25}}</ref>
==History== The Edziza Formation was first defined by Jack Souther, [[Richard Lee Armstrong]] and J. Harakal in 1984 who grouped it together with the Pillow Ridge Formation in their descriptions and mapping.<ref name="EF"/><ref name="Souther2">{{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]]|pages=337, 339, 346|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> It was mapped as one of 15 [[geological formation]]s of the [[Mount Edziza volcanic complex]], a group of late [[Cenozoic]] [[volcanic rock]]s in northwestern [[British Columbia]], Canada.<ref name="Edwards"/><ref name="Souther2"/> In 1988, Jack Souther mapped the Edziza and Pillow Ridge formations separately and the number of geological formations comprising the volcanic complex was dropped to 13; the [[Sheep Track Formation|Sheep Track]] and Kounugu formations were reassigned as [[member (geology)|members]] of the [[Big Raven Formation|Big Raven]] and [[Nido Formation|Nido]] formations, respectively, and are no longer recognized.<ref name="Souther"/><ref>{{cite web|url=https://weblex.canada.ca/html/013000/GSCC00053013635.html|title=Sheep Track Formation|work=[[Lexicon of Canadian Geologic Units]]|publisher=[[Government of Canada]]|access-date=2023-12-15|archive-date=2023-12-15|archive-url=https://web.archive.org/web/20231215003718/https://weblex.canada.ca/html/013000/GSCC00053013635.html}}</ref><ref name="Kounugu">{{cite web|url=https://weblex.canada.ca/html/007000/GSCC00053007795.html|title=Kounugu Formation|work=[[Lexicon of Canadian Geologic Units]]|publisher=[[Government of Canada]]|access-date=2023-12-15|archive-date=2023-12-15|archive-url=https://web.archive.org/web/20231215003718/https://weblex.canada.ca/html/007000/GSCC00053007795.html}}</ref>
==Location== Nearly all of the Edziza Formation is located on the east-central edge of the Big Raven Plateau, a [[barren vegetation|barren]] plateau in [[Cassiar Land District]] bounded on the west by [[Mess Creek|Mess Valley]], on the north by [[Klastline River|Klastline Valley]], on the east by [[Kakiddi Creek|Kakiddi Valley]] and on the south by [[Chakima Creek|Chakima]] and [[Walkout Creek|Walkout]] valleys, the latter two of which are separated by mountainous [[terrain]].<ref name="Souther"/><ref name="AK">{{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=2024-04-02}}</ref><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=2021-09-25|archive-date=2021-05-02}}</ref> It lies at the northern end of the Mount Edziza volcanic complex which also includes the smaller [[Arctic Lake Plateau|Arctic Lake]] and [[Kitsu Plateau|Kitsu]] plateaus to the south.{{sfn|Souther|1992|p=32}} This complex of [[shield volcano]]es, [[stratovolcano]]es, [[lava dome]]s, [[caldera]]s and [[cinder cone]]s forms a broad, [[intermontane plateau]] at the eastern edge of the [[Tahltan Highland]], a southeast-trending [[Upland and lowland|upland]] area extending along the western side of the [[Stikine Plateau]].<ref name="HR"/><ref name="LD">{{cite book|editor-last1=Wood|editor-first1=Charles A.|editor-last2=Kienle|editor-first2=Jürgen|last1=Souther|first1=Jack G.|author-link1=Jack Souther|url=https://books.google.com/books?id=eyDRib-FJh4C|title=Volcanoes of North America: United States and Canada|year=1990|publisher=[[Cambridge University Press]]|pages=124, 125|isbn=0-521-43811-X}}</ref><ref name="MS">{{cite report|last=Holland|first=Stuart S.|url=http://cmscontent.nrs.gov.bc.ca/geoscience/PublicationCatalogue/Bulletin/BCGS_B048.pdf|archive-url=https://web.archive.org/web/20181114024907/http://cmscontent.nrs.gov.bc.ca/geoscience/PublicationCatalogue/Bulletin/BCGS_B048.pdf|archive-date=2018-11-14|title=Landforms of British Columbia: A Physiographic Outline|publisher=[[Government of British Columbia]]|pages=49, 50|year=1976|asin=B0006EB676|oclc=601782234}}</ref> Small portions of the Edziza Formation occur north, northwest and southwest of [[Nuttlude Lake]] which is an expansion of Kakiddi Creek.<ref name="Souther"/><ref>{{cite bcgnis|id=17876|name=Nuttlude Lake|access-date=2024-04-02}}</ref>
==Stratigraphy== [[File:Mount Edziza from Mount Glenora2.jpg|thumb|right|alt=A glaciated, flat-topped, gently-sloping mountain shadowed by clouds with a much lower barren mountain in the right foreground.|Mount Edziza as seen from Mount Glenora in the northwest]] [[Stratigraphically]], the Edziza Formation is the fifth youngest unit of the Mount Edziza volcanic complex, having been deposited over the Nido, [[Pyramid Formation (British Columbia)|Pyramid]], [[Ice Peak Formation|Ice Peak]] and Pillow Ridge formations.<ref name="Edwards"/><ref name="Souther"/>{{sfn|Souther|1992|pp=98, 134, 152}} It has a volume of {{convert|18|km3|mi3|abbr=off}}, making it the sixth most voluminous geological formation of the Mount Edziza volcanic complex after the Ice Peak Formation. The Edziza Formation is also the youngest geological formation of the Mount Edziza volcanic complex involving more than {{convert|10|km3|mi3|abbr=on}} of volcanic material.<ref name="Edwards"/>
[[Basalt]] flows of the 4.4-million-year-old Nido Formation are overlain by the Edziza Formation on the eastern flank of Mount Edziza between [[Tenchen Creek]] and [[The Pyramid (British Columbia)|The Pyramid]] where they overlie a [[ridge]] of [[Mesozoic]] [[basement (geology)|basement]] rocks.<ref name="Edwards"/>{{sfn|Souther|1992|pp=93, 98}} The Edziza Formation overlies the westernmost portion of [[Sphinx Dome]], a [[rhyolite]] dome of the 1.1-million-year-old Pyramid Formation; two thick remnants of the Edziza Formation also occur on the steep northern and southern flanks of this dome.<ref name="Edwards"/>{{sfn|Souther|1992|p=134}}
Basalt flows of the roughly 1-million-year-old Ice Peak Formation are overlain by the Edziza Formation which overlaps with the northern flank of Ice Peak.<ref name="Edwards"/><ref name="Souther2"/>{{sfn|Souther|1992|p=152}} The 0.9-million-year-old Pillow Ridge Formation is sparsely overlain by the Edziza Formation, but much larger portions of the Edziza Formation overlap with the western and northern edges of [[Pillow Ridge|Pillow]] and [[Tsekone Ridge|Tsekone]] ridges, respectively.<ref name="Edwards"/><ref name="Souther"/>{{sfn|Souther|1992|p=171}} [[Hawaiite]] and patches of [[pyroclastic fall|air-fall]] [[tephra]] assigned to the [[Holocene]] Big Raven Formation overlie the northern end of the Edziza Formation where they are part of the [[Desolation Lava Field]].<ref name="Souther"/><ref name="KF"/>
==Lithology== The Edziza Formation consists mainly of [[trachyte]] that was erupted 0.9 million years ago at the end of the [[Volcanism of the Mount Edziza volcanic complex#Third magmatic cycle|third magmatic cycle]] of the Mount Edziza volcanic complex.{{sfn|Souther|1992|p=267}} Its composition straddles near the [[pantelleritic]] trachyte/[[comenditic]] trachyte boundary and is in the form of [[explosive eruption|explosion]] [[breccia]]s, lava flows and lava domes. The trachyte contains a [[matrix (geology)|matrix]] of [[sodic]] plagioclase [[alkali feldspar]], [[wikt:katophorite|katophorite]], [[arfvedsonite]], [[aenigmatite]] and [[opaque]] oxides that is embedded with sparse [[phenocryst]]s of [[anorthoclase]] and sodic ferrohedenbergite. Edziza trachyte is [[mineralogy|mineralogically]] similar to trachyte of the [[Kakiddi Formation]] which was erupted 0.3 million years ago during the final stages of the [[Volcanism of the Mount Edziza volcanic complex#Fourth magmatic cycle|fourth magmatic cycle]]. However, the Kakiddi trachyte flows travelled along gently sloping valleys for at least {{convert|10|km|mi|abbr=on}}, suggesting that they were extruded more fluidly than those of the Edziza Formation.<ref name="Souther2"/>
===Stratovolcano=== The main feature consisting of Edziza Formation trachyte is the central stratovolcano of Mount Edziza.{{sfn|Souther|1992|p=175}} It comprises explosion breccias, [[volcanic landslide|landslide]] or [[lahar]] deposits and thick, steeply-dipping flows that were erupted as highly viscous lava.{{sfn|Souther|1992|pp=177, 182}} These rocks are well-exposed in {{Convert|850|m|ft|adj=mid|-high|abbr=off}} cliffs on the eastern flank of Mount Edziza where they occur along the north side of Tenchen Valley. This section of the stratovolcano contains a lower assemblage of chaotic explosion breccias and an upper assemblage of lava that overlies the breccias.{{sfn|Souther|1992|pp=175, 177}} Lava of the upper assemblage is highly irregular or [[lens (geology)|lenticular]] in [[cross section (geology)|cross section]], but it contains individual cooling units as much as {{convert|150|m|ft|abbr=on}} thick.{{sfn|Souther|1992|p=177}}
[[File:Mount Edziza summit.jpg|thumb|right|alt=Black and white photo of a flat-topped, snow-covered mountain summit.|The summit of Mount Edziza]] The lower assemblage encloses up to {{Convert|120|m|ft|abbr=on}} long and {{Convert|15|m|ft|abbr=on}} thick lavas that occur as relatively small, crudely jointed, irregular masses and discontinuous lenses.{{sfn|Souther|1992|p=175}} Extremely coarse breccias occur in the western proximity and contain massive, up to {{Convert|7.5|m|ft|abbr=on}} wide [[volcanic block]]s that were probably deposited directly from the central conduit during eruption.{{sfn|Souther|1992|pp=175, 177}} At the eastern end of Tenchen Valley are more distal breccias that rarely contain volcanic blocks more than {{convert|1.5|m|ft|abbr=on}} wide. Local rounding and [[stratum|stratification]] of these breccias is indicative of lahar or landslide deposition.{{sfn|Souther|1992|p=177}}
Exposed in the steep Tenchen [[cirque]] [[headwall]] are large volcanic blocks and small breccia fragments within the central conduit that accompanied with collapse of a narrower summit with a much smaller [[volcanic crater|crater]].{{sfn|Souther|1992|p=177}} The collapse may have been caused by a violent, climactic eruption, effusion of the Kakiddi lava flows or the outpouring of voluminous lava during dome formation.{{sfn|Souther|1992|pp=21, 177}} Prior to collapse, the summit of Mount Edziza was at least {{convert|610|m|ft|abbr=on}} higher than its current [[elevation]] of {{convert|2786|m|ft|abbr=on}}.<ref name="KF"/>{{sfn|Souther|1992|p=21}}
====Summit crater==== The broad, nearly flat summit of Mount Edziza is truncated by a {{convert|3|km|mi|abbr=on}} wide crater that formed after collapse of the original summit.{{sfn|Souther|1992|p=177}} A circular ridge surrounding this ice-filled crater is partially exposed above the [[ice cap]] as a discontinuous series of spires and serrated [[nunatak]]s. Spires forming the southern end of the ridge are the highest and consist of greenish grey, sparsely [[porphyritic]] trachyte. They comprise well-formed, small diameter rock columns that rise nearly vertically for more than {{Convert|90|m|ft|abbr=on}} above the ice cap. Nunataks elsewhere on the summit ridge are more subdued, consisting of [[pyroclastic rock|pyroclastic]] debris that has been glacially reworked.{{sfn|Souther|1992|p=175}}
The remnants of several [[lava lake]]s are exposed inside the crater where the eastern side of the summit ridge has been breached by active cirques.{{sfn|Souther|1992|p=175}}<ref name="LD"/> They overlie breccia of the central conduit and are in the form of at least four cooling units. Trachyte with well-developed [[columnar jointing]] forms the lower two units, each of which is about {{convert|30|m|ft|abbr=on}} thick. The two upper units consist of lighter green trachyte; the lower unit has a thickness of nearly {{convert|90|m|ft|abbr=on}}. Well-developed, vertical and rectangularly jointed trachyte forms this unit and comprises a shear cliff across the full width of the Tenchen cirque headwall. The uppermost unit is largely buried under overhanging ice, but it appears to consist of agglutinated volcanic blocks.{{sfn|Souther|1992|p=185}}
====Subvolcanic intrusions==== Exposed in the underlying basement rocks on the deeply eroded eastern flank of Mount Edziza are [[subvolcanic]] trachyte [[cupola (geology)|cupolas]], [[sill (geology)|sills]], [[dike (geology)|dikes]] and irregular [[intrusive rock|intrusive]] masses linked to the [[magma]] plumbing system. Dikes are present as [[dike swarm|swarms]] and are usually vertical or subvertical, having been distributed in a crudely radial pattern towards the summit crater. They individually range in thickness from a few centimetres to nearly {{convert|2|km|mi|abbr=on}} and occur both within the stratovolcano and in the underlying basement rocks. Cupolas occur throughout Tenchen Valley as {{convert|3|to|9|m|ft|abbr=on}} wide projections through thick pyroclastic deposits and resemble inverted [[sand casting]]s up to {{convert|12|m|ft|abbr=on}} high where the breccia has eroded away.{{sfn|Souther|1992|p=184}}
====Hydrothermal alteration==== [[File:Mt Glenora Stikine Valley 20.jpg|thumb|right|alt=A glaciated, flat-topped, gently-sloping mountain with a much lower barren mountain in the right foreground.|Mount Edziza from the northwest]] The Tenchen cirque headwall exposes dikes, sills and breccias within the central conduit that have been intensely [[hydrothermal alteration|altered]] by [[hydrothermal]] solutions, resulting in the rocks being bright yellow and [[ochre]]-weathered. Complete alteration occurs in the middle of the altered zone where rock has been reconstituted to a pure white, amorphous material; the only original mineral present is alkali feldspar which comprises small tabular phenocrysts.{{sfn|Souther|1992|p=184}}
Permeating the completely altered material are [[chalcedony]] veinlets less than {{convert|1|mm|in|abbr=off}} thick which are cut by more extensively distributed [[vein (geology)|veins]] {{convert|1|to|5|cm|in|abbr=off}} wide. Tiny cubes of [[pyrite]] occupy many of the larger veins while the surrounding [[wall rock]] contains disseminated pyrite and [[marcasite]]. The conduit breccia and surrounding rocks were likely altered by intense [[fumarolic]] activity that occurred on a prolonged basis.{{sfn|Souther|1992|p=184}}
====Subfeatures==== At least three trachyte domes of the Edziza Formation occur on Mount Edziza, all of which were created by the rapid effusion of viscous trachytic lava. The nearly circular [[Nanook Dome]] forms the southeastern buttress of the summit crater and is the largest of the three lava domes with a diameter of about three-quarters of a kilometre.{{sfn|Souther|1992|p=179}} It may have been the source for the two upper lava lakes inside the summit crater as they consist of trachyte similar to that of Nanook Dome.{{sfn|Souther|1992|pp=179, 185}} [[Glacier Dome]] and [[Triangle Dome]] are two elliptical masses of trachyte with concentric flow layering on the northwestern and western flanks of Mount Edziza, respectively.{{sfn|Souther|1992|p=181}} Triangle Dome displays a pattern of columnar cooling joints that indicates it was formed by volcanic activity in a subglacial environment.<ref>{{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|page=275|isbn=978-1-107-03739-7}}</ref> A {{convert|24|m|ft|abbr=on}} thick trachyte flow that may have issued during an early, fluid phase of the Glacier Dome eruption is exposed for {{convert|2|km|mi|abbr=on}} north of Pyramid Creek; its distal end thins to about {{convert|15|m|ft|abbr=on}}.{{sfn|Souther|1992|p=182}}
South of Pillow Ridge on the northwestern flank of Mount Edziza is a rounded [[pyroclastic cone]] with a small central crater that probably formed during the latter stages of Edziza Formation [[volcanism]].{{sfn|Souther|1992|p=177}} This pile of agglutinated trachyte spatter, pumice and [[volcanic bomb|bombs]] was the source of at least two separate lobes of lava that flowed onto the adjacent plateau surface.{{sfn|Souther|1992|pp=177, 179}} The northwesterly lobe partially encircles Tsekone Ridge and borders as well as overlies breccia of Pillow Ridge. Extending west of the pyroclastic cone is the second lobe of lava which, together with the northwesterly lobe, is largely overlain by volcanic ash and [[colluvial material]].{{sfn|Souther|1992|p=179}}
==See also== *[[Geology of British Columbia]]
==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}}
{{Mount Edziza volcanic complex}}
[[Category:Geologic formations of the Mount Edziza volcanic complex]] [[Category:Mount Edziza]] [[Category:Pleistocene British Columbia]] [[Category:Pleistocene volcanism]] [[Category:Trachyte formations]] [[Category:Tahltan place names]]