The Sheep Track Member is a

stratigraphic unit A stratigraphic unit is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic, lithologic or paleontologic features (facies) that characterize ...

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 six older f ...

, part of the

Mount Edziza volcanic complex The Mount Edziza volcanic complex is a large and potentially active north-south trending complex volcano in Stikine Country, northwestern British Columbia, Canada, located southeast of the small community of Telegraph Creek. It occupies the sout ...

in northwestern

British Columbia British Columbia (commonly abbreviated as BC) is the westernmost Provinces and territories of Canada, province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include ...

, Canada. It consists of

pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular v ...

from an

explosive eruption In volcanology, an explosive eruption is a volcanic eruption of the most violent type. A notable example is the 1980 eruption of Mount St. Helens. Such eruptions result when sufficient gas has dissolved under pressure within a viscous magma s ...

that fell over an area about . The pumice is of

comenditic Comendite is a hard, peralkaline igneous rock, a type of light blue grey rhyolite. Phenocrysts are sodic sanidine with minor albite and bipyramidal quartz.Joseph P. Iddings, Iddings, Joseph Paxson, 1913, ''Igneous rocks: composition, texture and c ...

trachyte Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and ...

composition and reaches a thickness of up to along the western edge of

Tencho Glacier Tencho Glacier is a mountain glacier in northwestern British Columbia, Canada. It is located inside Mount Edziza Provincial Park on the Tahltan Highland, an upland area of the Stikine Plateau. Tencho Glacier is the source of several small streams ...

. The Sheep Track Member was originally defined as the Sheep Track Formation in 1984 and was one of many

geological formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics (lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock expo ...

s comprising the Mount Edziza volcanic complex. However, it was lowered to

member Member may refer to: * Military jury, referred to as "Members" in military jargon * Element (mathematics), an object that belongs to a mathematical set * In object-oriented programming, a member of a class ** Field (computer science), entries in ...

rank in 1988 when it became a unit of the Big Raven Formation. The Sheep Track Member has also been called the Sheep Track Pumice due to the member consisting primarily of pumice.

History

The Sheep Track Member was initially ranked as a geological formation by

Jack Souther Jack Gordon Souther (April 25, 1924 – June 1, 2014) was an American-born Canadian geologist, volcanologist, professor and engineer. He contributed significantly to the early understanding of recent volcanic activity in the Canadian Cordillera ...

Richard Lee Armstrong Richard Lee Armstrong (August 4, 1937 – August 9, 1991) was an American/Canadian scientist who was an expert in the fields of radiogenic isotope geochemistry and geochronology, geochemical evolution of the earth, geology of the American C ...

and J. Harakal in 1984 who grouped it together with the Big Raven Formation in their descriptions and mapping. Formerly called the Sheep Track Formation, it was reassigned as a member of the Big Raven Formation by Jack Souther in 1988; its recognition as a geological formation has since been abandoned. Sheep Track Pumice has been used as an alternative name for the Sheep Track Member by

Natural Resources Canada Natural Resources Canada (NRCan; french: Ressources naturelles Canada; french: RNCan, label=none)Natural Resources Canada is the applied title under the Federal Identity Program; the legal title is Department of Natural Resources (). is the Struc ...

and the

Geological Association of Canada The Geological Association of Canada (GAC) is a learned society that promotes and develops the geological sciences in Canada. The organization holds conferences, meetings and exhibitions for the discussion of geological problems and the exchange ...

. During its time as a geological formation in 1984, it was one of 15 geological formations comprising the Mount Edziza volcanic complex in northwestern British Columbia, Canada. In 1988, the number of geological formations comprising the Mount Edziza volcanic complex dropped to 13 when the Sheep Track and Kounugu formations were reassigned as members of the Big Raven and Nido formations, respectively.

Stratigraphy

Stratigraphically Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostrati ...

, the Sheep Track Member is a minor unit of the Big Raven Formation, covering area of about with a total volume of less than . It was deposited on nearly all Snowshoe Lava Field flows and

cinder cone A cinder cone (or scoria cone) is a steep conical hill A conical hill (also cone or conical mountain) is a landform with a distinctly conical shape. It is usually isolated or rises above other surrounding foothills, and is often of volcanic ...

s of the Big Raven Formation, as well as on sediment of the surrounding Big Raven Plateau. The Sheep Track Member was also deposited on

Koosick Bluff Koosick Bluff is a volcanic bluff in Cassiar Land District (Stikine Country) in northern British Columbia, Canada. It is located in Mount Edziza Provincial Park, just northwest of Cocoa Crater and southwest of Mount Edziza. See also * List of vo ...

of the Ice Peak Formation and on Kakiddi Formation trachyte from Punch Cone on the western flank of

Ice Peak Ice Peak is a stratovolcano, located west of Tatogga and south of Mount Edziza, British Columbia, Canada. It overlaps the 7.5-million-year-old Armadillo Peak. It lies on a large volcanic plateau, which is made of basaltic lava flows from the ...

. Both the Ice Peak and Kakiddi formations are older units of the Mount Edziza volcanic complex that were deposited about one million and 0.3 million years ago, respectively. Erosion has largely removed the Sheep Track Member from drainage channels of small intermittent streams on the upper Big Raven Plateau, but deposits as much as thick occur in interfluvial areas.

Lithology

The Sheep Track Member is lithologically distinct from the rest of the Big Raven Formation, consisting mainly of comenditic trachyte that straddles near the

alkaline In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...

–

peralkaline Peralkaline rocks include those igneous rocks which have a deficiency of aluminium such that sodium and potassium are in excess of that needed for feldspar. The presence of aegerine (sodium pyroxene) and riebeckite (sodium amphibole) are indicative ...

boundary. It is in the form of air-fall pumice which reaches a thickness of up to along the western edge of Tencho Glacier where snowball-sized pumice fragments up to wide are distributed over an area of several square kilometres. The pumice rapidly decreases to pea-sized fragments around the margins of the pile, covering a circular area at least in diameter. Thin drifts of wind-blown pumice cover

The Saucer The Saucer is a cinder cone in northern British Columbia, Canada. It is thought to have last erupted in the Holocene epoch.

in the Snowshoe Lava Field which likely postdates the deposition of the Sheep Track Member. Pumice of the Sheep Track Member comprises clear trachytic

glass Glass is a non-Crystallinity, crystalline, often transparency and translucency, transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most ...

with tubular, open

vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like feature ...

;

alkali feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...

is in the form of rare

microphenocryst 300px, Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter 2.3 cm) for scale. A phenocryst is an early forming, relati ...

s. It is compositionally distinct from the

Finlay tephras The Finlay tephras are two tephra deposits in northern British Columbia, Canada. They take their name from the Finley River and were deposited just before 10,220–10,560 years ago. The source for the two tephra deposits is unknown but were l ...

which may have originated from

Level Mountain Level Mountain is a large volcanic complex in the British Columbia Interior, Northern Interior of British Columbia, Canada. It is located north-northwest of Telegraph Creek and west of Dease Lake on the Nahlin Plateau. With a maximum elevatio ...

Hoodoo Mountain Hoodoo Mountain, sometimes referred to as Hoodoo Volcano, is a potentially active stratovolcano in the Northern Interior of British Columbia, Canada. It is located northeast of the Alaska–British Columbia border on the north side of the Isk ...

Heart Peaks Heart Peaks, originally known as the Heart Mountains, is a mountain massif in the Northern Interior of British Columbia, Canada. It is northwest of the small community of Telegraph Creek and just southwest of Callison Ranch. With a maximum eleva ...

or the Mount Edziza volcanic complex.

Source

The Sheep Track Member was deposited by a small but violent VEI-3 eruption from the southwestern flank of Ice Peak near the end of the Big Raven eruptive period. The vent location for this eruption remains unknown but it is probably hidden under Tencho Glacier, the largest glacier of the Mount Edziza volcanic complex. An artifact of the Sheep Track eruption may lie north of Coffee Crater where a deep, circular depression is present on the surface of the glacier. This depression may be caused by subglacial melting brought on by

fumarole A fumarole (or fumerole) is a vent in the surface of the Earth or other rocky planet from which hot volcanic gases and vapors are emitted, without any accompanying liquids or solids. Fumaroles are characteristic of the late stages of volcan ...

hot spring A hot spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow bodies of magma (molten rock) or by c ...

s or heightened ground temperatures adjacent to the vent.

Age

The Sheep Track Member is absent within the

trim line A trim line, also written as trimline, is a clear line on the side of a valley formed by a glacier. The line marks the most recent highest extent of the glacier. The line may be visible due to changes in color to the rock or to changes in vege ...

s of

Mount Edziza Mount Edziza is a stratovolcano in the Stikine Country of northwestern British Columbia, Canada. The volcano and the surrounding area are protected within Mount Edziza Provincial Park and Recreation Area, Mount Edziza Provincial Park. It consist ...

's alpine glaciers, indicating that it was deposited by the Sheep Track eruption during the last glacial advance starting about 2,600 years ago. However, Sheep Track pumice is present on the

subglacial volcano A subglacial volcano, also known as a glaciovolcano, is a volcanic form produced by subglacial eruptions or eruptions beneath the surface of a glacier or ice sheet which is then melted into a lake by the rising lava. Today they are most ...

Tennena Cone Tennena Cone is a subglacial mound in northern British Columbia, Canada, located just southwest of Mount Edziza in Mount Edziza Provincial Park. See also *List of volcanoes in Canada * List of Northern Cordilleran volcanoes * Volcanism of Canad ...

which suggests that glaciers of the last glacial advance had already begun to retreat before deposition.

Fission track dating Fission track dating is a radiometric dating technique based on analyses of the damage trails, or tracks, left by fission fragments in certain uranium-bearing minerals and glasses. Fission-track dating is a relatively simple method of radiometr ...

of pumice on the southwestern flank of Ice Peak has yielded an age of 950 CE ± 6000 years.

History

Stratigraphy

Lithology

Source

Age

See also

References

Sources

Further reading