| Text Description: |
Wrangell Lava. Andesite flows, flow breccias, and volcaniclastic rocks, undifferentiated. Light to dark-gray hypersthene andesite flows, commonly showing red scoriaceous tops and bottoms. The rocks are holocrystalline, often with labradorite phenocrysts in a trachyitc groundmass of plagioclase, minor clinopyroxene, and traces of olivine. Flows are often highly vesicular and locally amygdaloidal, with chalcedony the most common vesicle filling. Although individual flows are rarely over 5 m in thickness, the entire unit may exceed 1000 m in places. Massive lenses of andesite breccia, up to several hundred meters thick, including some vent debris and thinner lenses of volcaniclastic sand, gravel, and boulder deposits, as much as several meters thick, occur scattered through the unit. A sample of a flow from high in the section at the extreme northern extent of unit (sec. 17, T. 8 N., R. 13 E.) yielded a K-Ar (whole rock) age of 2.77 +/- 0.10 m.y. (M. Lanphere, written commun., 1977). Similar rocks from the adjacent Nabesna C-5 quadrangle yielded an age of 1.97 +/- 0.15 m.y. (Richter and Schmoll, 1973). |