| Pyramid Formation | |
| Type: | Geological formation[1] |
| Period: | Pleistocene |
| Prilithology: | Trachyte, comendite, pantellerite |
| Otherlithology: | Basalt |
| Namedfor: | The Pyramid |
| Namedby: | Souther et al., 1984[2] |
| Region: | British Columbia |
| Country: | Canada |
| Coordinates: | 57.5°N -130.6°W[3] |
| Unitof: | Mount Edziza volcanic complex |
| Underlies: | Ice Peak Formation, Edziza Formation |
| Overlies: | Nido Formation |
The Pyramid Formation is a stratigraphic unit of Pleistocene age in northwestern British Columbia, Canada.
The Pyramid Formation takes its name from The Pyramid, a prominent pyramid-shaped lava dome on the northeastern flank of Mount Edziza.[1] [4]
Two subunits comprise the Pyramid Formation. The lower unit is a basal pyroclastic member up to 3m (10feet) 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 65m (213feet) thick. It includes 6 to 10 individual basalt flows that reach thicknesses of 3–. 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 11.4km3, making it the eighth most voluminous geological formation of the Mount Edziza volcanic complex after the Little Iskut Formation.[1] [3] 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]