Pyramid Formation (British Columbia)

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

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	57.5°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

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