Postulated intracaldera lavas exposed at base of south face of Mount Sanford. Detailed traverse (section C on map sheet; see also, fig. 2) through accessible part of exposure reveals rather uniform series of flat-lying, aphyric to porphyritic olivine basaltic andesite flows in which silica content ranges only from 53.1 to 55.3% (samples 84-ARh-79a to 79e and 79g to 79l). Porphyritic flows predominate; they typically contain phenocrysts of plagioclase (10-25%, as large as 5 mm), olivine (1-6%, 1-2 mm; generally corroded), and hypersthene (trace), in pilotaxitic groundmass. Three whole-rock K-Ar ages on lava flows from lower part of section are 0.73+/-0.08, 0.85+/-0.02, and 0.86+/-0.22 Ma (sample 74-ALe-65);dike (Qsdi) that cuts two oldest flows has K-Ar age of 0.78+/-0.08 Ma (sample 74-ALe-63). These ages are approximately the same as those of some flows that are topographically higher in complex Sanford shields.
Geologic map of the Gulkana A-1 Quadrangle, south-central Alaska