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Pyramid Formation (British Columbia)

Pyramid Formation
Stratigraphic range: 1.2–0.9 Ma
TypeGeological formation[1]
Unit ofMount Edziza volcanic complex[2]
UnderliesIce Peak Formation, Edziza Formation[1]
OverliesNido Formation[1]
Lithology
PrimaryTrachyte, comendite, pantellerite[1]
OtherBasalt[1]
Location
Coordinates57°30′N 130°36′W / 57.5°N 130.6°W / 57.5; -130.6[2]
RegionBritish Columbia[1]
CountryCanada[1]
Type section
Named forThe Pyramid[1]
Named bySouther et al., 1984[3]

The Pyramid Formation is a stratigraphic unit of Pleistocene age in northwestern British Columbia, Canada.

Naming

The Pyramid Formation takes its name from The Pyramid, a prominent pyramid-shaped lava dome on the northeastern flank of Mount Edziza.[1][4]

Geology

Two subunits comprise the Pyramid Formation. The lower unit is a basal pyroclastic member up to 3 m (9.8 ft) thick. It contains trachytic pumice, sodic pyroxene and alkali feldspar; the latter two are present in the form of lithic clasts and crystals. This unit is rusty brown in colour and was deposited by a pyroclastic surge. The upper unit consists of a basalt member up to 65 m (213 ft) thick. It includes 6 to 10 individual basalt flows that reach thicknesses of 3–20 m (9.8–65.6 ft). They appear to have been deposited almost immediately after the pyroclastic surge was erupted.[1] Several silicic lava domes and minor lava flows of the Pyramid Formation postdate the basalt member. Among the silicic domes are Sphinx Dome, Pharaoh Dome and The Pyramid.[5]

The Pyramid Formation has a volume of 11.4 km3 (2.7 cu mi), making it the eighth most voluminous geological formation of the Mount Edziza volcanic complex after the Little Iskut Formation.[1][2] It overlies the Nido Formation and underlies the Ice Peak and Edziza formations, all of which have formed in the last 8 million years. K–Ar dating of the Pyramid Formation has yielded ages of 1.2 ± 0.4 million years and 1.20 ± 0.03 million years for comenditic glass and 0.94 ± 0.12 million years and 0.94 ± 0.05 million years for trachyte.[1]

See also

References

  1. ^ a b c d e f g h i j k l Souther, J. G. (1992). The Late Cenozoic Mount Edziza Volcanic Complex, British Columbia. Geological Survey of Canada (Report). Memoir 420. Canada Communication Group. pp. 100, 129, 132, 133, 134, 246, 248, 249, 267. doi:10.4095/133497. ISBN 0-660-14407-7.
  2. ^ a b c Edwards, Benjamin Ralph (1997). Field, kinetic, and thermodynamic studies of magmatic assimilation in the Northern Cordilleran Volcanic Province, northwestern British Columbia (PhD thesis). University of British Columbia. p. 10. ISBN 0-612-25005-9.
  3. ^ "Pyramid Formation". Lexicon of Canadian Geologic Units. Government of Canada. Retrieved 2023-08-04.
  4. ^ "The Pyramid". BC Geographical Names. Retrieved 2023-07-02.
  5. ^ Smellie, John L.; Edwards, Benjamin R. (2016). Glaciovolcanism on Earth and Mars: Products, Processes and Palaeoenvironmental Significance. Cambridge University Press. p. 43. ISBN 978-1-107-03739-7.