Pyramid Formation (British Columbia)
The Pyramid Formation is a stratigraphic unit of Pleistocene age in northwestern British Columbia, Canada.
| Pyramid Formation | |
|---|---|
| Stratigraphic range: | |
| 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 | 57.5°N 130.6°W[2] |
| Region | British Columbia[1] |
| Country | Canada[1] |
| Type section | |
| Named for | The Pyramid[1] |
| Named by | Souther et al., 1984[3] |
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
- Souther, J. G. (1992). The Late Cenozoic Mount Edziza Volcanic Complex, British Columbia. Geological Survey of Canada. pp. 100, 129, 132, 133, 134, 246, 248, 249, 267. doi:10.4095/133497. ISBN 0-660-14407-7.
- Edwards, Benjamin Ralph (1997). Field, kinetic, and thermodynamic studies of magmatic assimilation in the Northern Cordilleran Volcanic Province, northwestern British Columbia (PhD). University of British Columbia. p. 10. ISBN 0-612-25005-9.
- "Pyramid Formation". Lexicon of Canadian Geologic Units. Government of Canada. Retrieved 2023-08-04.
- "The Pyramid". BC Geographical Names. Retrieved 2023-07-02.
- 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.