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Text Description: Wrangell Lava. Olivine basalt and andesite flows (Pleistocene). Flows are 2-6 m thick and exhibit conspicuous reddish-brown scoriaceous flow tops and bottoms. Dense flow interiors are medium to dark gray and sparsely porphyritic containing phenocrysts of plagioclase (2-5%, 3-4 mm) and olivine (1-3%, 0.5 - 1 mm) in an intersertal to pilotaxitic groundmass of plagioclase, pyroxene, opaque minerals, and glass. Rarely, flows contains as much as 3% phenocrystic hypersthene. Flows probably erupted from a rift a few kilometers west in the Gulkana B-1 quadrangle and tend to be more silicic than some of the flows included in the map unit in Gulkana B-1 quadrangle (Richter and others, 1989). Major element chemistry for sample 73-ARh-81 (map locality 2) is shown in table 1. Whole rock K-Ar age of sample 73-ARh-87 (map locality 17) is 0.32 +/- 0.09 Ma. (table 2). Aggregate maximum thickness about 360 m. References:  Geologic map of the Nabesna B-6 Quadrangle, south-central Alaska |
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