Raspberry Formation

Unit Name: Raspberry Formation
Unit Type: Lithostratigraphic
Rank: Formation
Status: Formal
Usage: Currently in use
Age Interval: Miocene (23.03 - 5.332 ma)
Age Justification: Geochronology. Four samples of Raspberry basalt yielded K-Ar dates ranging from 5.5 to 11.4 Ma. The large spead is probably due to pervasive carbonate alteration and relatively large atmospheric contents, which are common to all of the Raspberry rocks. The 11.4 ± 1.5 Ma age, a whole-rock analysis, has the greatest error and the highest content of atmospheric argon. The Ar has probably fractionated, so that the determined date is anomalously old. A feldspar separate from the same sample yielded an age of 6.4 ± 0.3 Ma. This and the other 3 whole-rock ages have smaller errors and relatively high radiogenic argon (>19%). The spread in these 4 better dates (5.5 to 8.4 Ma) is probably still greater than the actual duration of Raspberry volcanism, which, from the absence of intraformational erosion surfaces, appears to have been relatively short-lived. One approximation of the age is a weighted average of the four better dates, which places the Raspberry event at 6.2 Ma. Another would be an average of the two feldspar dates, 7.4 Ma. The samples are for the most part altered, more highly feldspar-phyric rocks near the top of the formation. The 6.2 to 7.4 Ma age is thus a minimum (Souther et al., 1984).
Province/Territory: British Columbia

Originator: Souther et al., 1984.

Distribution:

Mount Edziza and northern plateau; Ice Peak, Armadillo Peak and central plateau; Spectrum Range and plateau south of Raspberry Pass (Souther, 1988). Flat-lying, rusty, brown weathering flows of Raspberry basalt and interlayered reddish-brown to yellow or orange scoria are exposed at the base of prominent escarpments on the east and west sides of both Mount Edziza and Spectrum Range. The elevation of the base of the pile varies from 5700 ft. (1740 m) above sea level on the east, near Armadillo Peak, to less than 4300 ft. (1310 m) above sea level along Mess Creek Escarpment. Thicknesses vary from more than 300 m in the southeastern part of the pile to only a few metres along its northern and eastern margins where narrow, valley-filling flows are exposed in cross-section (Souther, 1992).

Lithology:

Proximal sections of Raspberry basalt include as many as 25 flows. Individual flow units are 1 to 30 m thick and constitute a central core of crudely columnar, redish-brown-weathering basalt; a relatively thin basal breccia of ropey clinker; and a variable thickness of flow-top breccia. Intraflow fluvial layers are uncommon, lenticular, and of local origin, suggesting that Raspberry activity was not interrupted by long periods of dormancy. Raspberry basalt includes both alkali olivine basalt and hawaiite. The alkali olivine basalt is commonly aphyric or contains sparse phenocrysts of plagioclase and locally of titanaugite in an ophitic to subophitic groundmass of calcic plagioclase, titaniferous augite, intergranular olivine, and opaques. The hawaiites, which are coarsely and abundantly feldsparphyric, are most common in the upper part of the Raspberry pile. Most Raspberry basalts contain carbonate-filled amygdules and locally the groundmass shows carbonate alteration (Souther et al., 1984).

Age Determinations:

Method - K/Ar; Material - Whole Rock; Age - 5.5; Err_Minus - 0.1; Err_Plus - 0.1..

Method - K/Ar; Material - Whole Rock; Age - 6.1; Err_Minus - 0.4; Err_Plus - 0.4..

Method - K/Ar; Material - Whole Rock; Age - 11.4; Err_Minus - 1.5; Err_Plus - 1.5..

Method - K/Ar; Material - Feldspar; Age - 6.4; Err_Minus - 0.3; Err_Plus - 0.3..

Method - K/Ar; Material - Feldspar; Age - 8.4; Err_Minus - 0.4; Err_Plus - 0.4..

Relationship:

Raspberry basalt is the basal formation of the Mount Edziza Complex and the initial material of the fist magmatic cycle. It rests on a late Miocene erosion surface that slopes westward into the ancestral valley of Mess Creek. The lava flowed west and ponded in the lowlands, where more than 180 m of flows is exposed in the Mess Creek escarpment. North of Raspberry Pass, the eastern edge of the Raspberry pile laps out against the basement surface and there only a few valley-filling flows are preserved (Souther et al., 1984). The Raspberry Formation unconformably overlies the Eocene Sloko Group (Souther, 1988). South of Bourgeaux Creek, basalt of the Raspberry Formation is overlain by trachybasalt of the Little Iskut Formation, deposited as a small shield that originally covered only the extreme southeastern corner of the Raspberry pile. Elsewhere, the Raspberry Formation is separated from overlying Armadillo and younger members by an erosional surface (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: GSC file of geological names; T.E. Bolton and J. Dougherty (compiler)
Contributor: Michael Pashulka
Entry Reviewed: No
Name Set: Lithostratigraphic Lexicon
LastChange: 02 Feb 2011