Abstract: Geology of South Augustine Island, Lower Cook Inlet, Alaska

R. T. Buffler

Mesozoic sedimentary rocks mapped in stream gulleys and along the beach on the south flank of Augustine Island, Lower Cook Inlet, Alaska, consist mostly of Upper Jurassic Naknek Formation dipping 10-15° away from Augustine Volcano. This 300-ft (91 m) section was subdivided into three mappable lithologic units: (1) a lower microcross-bedded sandstone unit (50 ft or 15 m) cut by a large sandstone channel interpreted to be sandy tidal-flat and tidal-channel deposits; (2) a middle marine siltstone unit (150 ft or 46 m) probably deposited on a restricted shallow-marine shelf or in a protected bay or estuary; and (3) an upper coarse-grained marine-sandstone unit (100 ft or 30 m) deposited in a high energy beach and/or shallow marine-shelf environment. Cretaceous marine san stones apparently in fault contact with the Jurassic rocks crop out in gulleys along the western edge of the map area. Jumbled marine sandstones along the beach probably represent a large landslide derived from an upper Naknek (unit 3) outcrop area farther up the flank of Augustine Volcano. No Tertiary sedimentary rocks are present in the study area, suggesting that the Cook Inlet Tertiary basin does not extend southeastward as far as Augustine Island.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA