Distribution:

Mount Edziza and northern plateau; Ice Peak, Armadillo Peak and central plateau (Souther, 1988). Rhyolite and trachyte flows, domes and thick piles of primary pyroclastic breccia are exposed in prominent, light-coloured cliffs and narrow-crested ridges along the deeply dissected eastern flank of Mount Edziza (Souther, 1992).

Lithology:

It includes two distinct facies, each the product of a separate pulse of activity. The first to erupt was a symmetrical lava dome of highly porphyritic trachyte approximately 2 km in diameter and 1,000 m high. The rock contains abundant, 1- to 2-cm phenocrysts of sanidine and anorthoclase and sparse 1- to 2-mm phenocrysts of sodic hedenbergite in a holocrystalline matrix of quartz, feldspar, sodic pyroxene, and opaques. Gravels of this unique rock are present beneath subsequent rhyolite flows of the upper Pyramid Formation. At the extreme northern and southern ends of the outcrop area, the upper Pyramid consists of thick rhyolite flows. These are characterized by basal obsidian layers from 3 to 10 m thick, much thicker than beneath flows of similar size in the older salic formations. The crystalline rhyolite contains sparse phenocrysts of quartz, sanidine, and sodic hedenbergite in a fine-grained, holocrystalline matrix of intergrown quartz and alkali feldpars plus finely disseminated opaques, aenigmatite, and acmite. Between these two areas of flows, the upper Pyramid Formation is primarily clastic and at least partly water-laid. About 100 m of stratified, poorly sorted beds of lithic and vitric rhyolite slasts is interlayered with thicker and more uniform beds of pumice and fine ash. These are associated with contorted and fragmented obsidian-rich flows and layers of subangular obsidian cobbles. Locally, the formation rests on polymict till, suggesting that the chaotic, vitroclastic assemblage is an ice-contact deposit (Souther et al., 1984).

Age Determinations:

Method - K/Ar; Material - Whole Rock; Age - 1.2; Err_Minus - 0.4; Err_Plus - 0.4..

Method - K/Ar; Material - Whole Rock; Age - 1.2; Err_Minus - 0.03; Err_Plus - 0.03..

Method - K/Ar; Material - Whole Rock; Age - 0.94; Err_Minus - 0.12; Err_Plus - 0.12..

Method - K/Ar; Material - Feldspar; Age - 0.94; Err_Minus - 0.05; Err_Plus - 0.05..

Relationship:

The Pyramid Formation, erupted at the end of the second magmatic cycle, is confined to the eastern margin of the Mount Edziza Complex, north of Raspberry Pass (Souther et al., 1984). It is underlain by Nido basalt and either overlain or surrounded by Ice Mountain flows. Those parts of the Pyramid pile that lay beyond, or projected above the level flooded by Ice Mountain lavas, have been mostly covered by younger flows of the Edziza or Big Raven formations (Souther, 1992).

Other Citations:

Mihalynuk et al., 1996.

References:

Mihalynuk, M., Bellefontaine, K., Brown, D., Logan, J., Nelson, J., Legun, A. and Diakow, L., 1996. Digital Geology, NW British Columbia (94/E, L, M; 104/F, G, H, I, J, K, L, M, N, O, P; 114/I, O, P); Ministry of Energy, Mines and Petroleum Resources, Open File 1996-11.

Souther, J.G., 1988. Geology, Mount Edziza volcanic complex, British Columbia; Geological Survey of Canada, "A" Series Map no. 1623A, 2 sheets.

Souther, J.G., 1992. The Late Cenozoic Mount Edziza Volcanic Complex, British Columbia; Geological Survey of Canada, Memoir 420, 329 p.

Souther, J.G., Armstrong, R.L., and Harakal, J., 1984. Chronology of the peralkaline, late Cenozoic Mount Edziza Volcanic Complex, northern British Columbia, Canada; Geological Society of America Bulletin, March 1984, Vol. 95, Issue 3, pp. 337-349.