Unit Name: Little Iskut Formation
Unit Type: Lithostratigraphic
Rank: Formation
Status: Formal
Usage: Currently in use
Age Interval: Miocene (23.03 - 5.332 ma)
Age Justification: Geochronology. The single sample from the Little Iskut Formation, dated at 7.2 ± 0.3 Ma, is within the range of dates from the underlying Raspberry basalt. The Little Iskut Formation occupies a small part of the area underlain by Raspberry basalt and no erosion is evident between the two piles. Little Iskut activity thus may have been coeval with late Raspberry flows from a center north of the Little Iskut shield, or it may have immediately followed Raspberry eruptions (Souther et al., 1984).
Province/Territory: British Columbia
Originator: Souther et al., 1984.
Distribution:
Spectrum Range and plateau south of Raspberry Pass (Souther, 1988). Little Iskut flows and breccia are confined to a relatively small area about 10 km across, in the northeastern Spectrum Range. They form prominent cliffs around the eastern end of both Artifact and Stewpot ridges, but except for two relatively thin flows in Mess Creek Escarpment, they do not appear in more distal sections. It has a maximum thickness of about 300 m near the centre of Artifact Ridge and thins rapidly from that point, decreasing to only 90 m on Stewpot Ridge (Souther, 1992).
Locality Data:
Thickness(m): Maximum 300.
Lithology:
It is made up of the eroded remnant of a small shield with a proximal thickness of as much as 150 m. The basal unit (15 to 120 m thick) is brecciated and discolored, suggestive of thermal shock fracturing and phreatic alteration associated with water quenching. Above the basal quenched unit, the Little Iskut section is a monotonous succession of thick, irregular, black-weathering, randomly jointed flows and associated blocky, polygonal basal and flow-top breccias. Most Little Iskut rocks are aphyric or sparsely microporphyritic trachybasalts in which the groundmass is a panidiomorphic mosaic of oriented feldspar laths (sodic andesine), ragged interstitial grains of green ferrohedenbergite, and opaques. Phenocrysts, where present, are sodic anothoclase and ferroaugite zoned to rims of ferrohedenbergite (Souther et al., 1984).
Age Determinations:
Method - K/Ar; Material - Whole Rock; Age - 7.2; Err_Minus - 0.3; Err_Plus - 0.3..
Relationship:
The Little Iskut trachybasalt, erupted during the first magmatic cycle, is confined to a relatively small area along the eastern edge of the Mount Edziza Complex, south of Raspberry Pass (Souther et al., 1984). The pile rests conformably on Raspberry basalt and is overlain conformably by Armadillo trachyte and rhyolite (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.
Source: LEXICON_BC
Contributor: Michael Pashulka
Entry Reviewed: No
Name Set: Lithostratigraphic Lexicon
LastChange: 02 Feb 2011