Mount Wrangell two-pyroxene andesite flows (Pleistocene). Extensive, medium- to dark-gray, thin to thick (5-30 m) flows that generally exhibit oxidized and slightly scoriaceous tops and bottoms. In places, unit contains lenses of light-colored silicic airfall tuff. Rock is conspicuously porphyritic, containing phenocrysts of plagioclase (10-24%, 1-10mm), hypersthene (1-8%, 0.5-3 mm), augite (trace to 3%, 1-4 mm), and olivine (trace), in intersertal to pilotaxitic groundmass of plagioclase, mafic minerals, and brown glass. Hypersthene is invariably the dominant phenocrystic pyroxene. * * * Plagioclase from two samples (locs. 6, 9, table 1) yielded K-Ar ages of 0.60+/-0.02 and 0.51 +/-0.02 M, respectively. Most likely source is summit region of Wrangell volcano, but unit may also include flows from Wrangell flank eruptions or from older Chetaslina eruptive center 5 km south of quadrangle, especially in area of Chichokna and Chetaslina Glaciers. * * * Maximum thickness probably more than 400 m.
Geologic map of the Gulkana A-1 Quadrangle, south-central Alaska