Pyramid Formation (British Columbia)

Pyramid Formation
Stratigraphic range: 1.2–0.9  Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
The Pyramid consists of Pyramid Formation trachyte
Type	Geological formation [1]
Unit of	Mount Edziza volcanic complex [2]
Underlies	Ice Peak Formation, Edziza Formation [1]
Overlies	Nido Formation [1]
Lithology
Primary	Trachyte, comendite, pantellerite [1]
Other	Basalt [1]
Location
Coordinates	57°30′N130°36′W / 57.5°N 130.6°W / 57.5; -130.6 [2]
Region	British Columbia [1]
Country	Canada [1]
Type section
Named for	The Pyramid [1]
Named by	Souther et al. [3]
Year defined	1984 [3]
Cross section showing the maximum extent of the Pyramid Formation at the end of Pyramid time

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

Name

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

• Volcanism of the Mount Edziza volcanic complex

References

1. 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. 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.