Distribution:

The Spectrum Formation is confined to the Spectrum Range, which derives its name from the brightly colored rhyolite domes and flows that constitute its high central peaks.

Locality Data:

Thickness(m): Maximum 400.

Lithology:

The formation consists of comendite, pantellerite, and trchyte, of which more than 90% was erupted as lava and less than 10% as pyroclastic flows and pumice. Individual flows are as much as 200 m thick, and proximal sections commonly contain 3 to 10 units, with a combined thickness of as much as 400m. Each flow unit is defined by a vitreous base of black to light bluish0gray or green obsidian that often serves as the only means of distinguishing successive flows. The Spectrum rhyolite includes the same mineral species as the Armadillo comendite, but it is richer in quartz and alkali feldspars relative to the femic constituents. Quartz, commonly pitted and embayed, is a ubiquitous phenocryst phase in the Spectrum rocks, whereas it appears only in the groundmass of most Armadioo comendites. The Spectrum Formation is overlian locally by small remnants of basalt (Kitsu member), from which it is separated by a layer of polymict gravel that includeds clasts of both Spectrum and pre-Spectrum rocks (Souther et al., 1984).

Age Determinations:

Method - K/Ar; Material - Whole Rock; Age - 2.9; Err_Minus - 0.1; Err_Plus - 0.1..

Method - K/Ar; Material - Whole Rock; Age - 3; Err_Minus - 0.1; Err_Plus - 0.1..

Method - K/Ar; Material - Whole Rock; Age - 3.1; Err_Minus - 0.1; Err_Plus - 0.1..

Method - K/Ar; Material - Whole Rock; Age - 3.4; Err_Minus - 0.1; Err_Plus - 0.1..

Method - K/Ar; Material - Whole Rock; Age - 5.9; Err_Minus - 1.1; Err_Plus - 1.1..

Relationship:

The Spectrum Formation belongs to the Mount Edziza Complex and the Stikine Volcanic Belt. Almost the entire Spectrum pile is underlain by Nido basalt. The only exceptions are found along its northern edge where the basalt wedges out against the southern edge of the Armadillo Higlands and overlying Spectrum trachyte rests on Armadillo rhyolite. Also, at the extreme southwestern corner of the pile, Spectrum rhyolite extends beyond the outer limits of the Nido basalt and overlies the old basement surface (Souther, 1992).

Other Citations:

Mihalynuk et al., 1996; Souther, 1988.

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.