317 Molluscan paleontology, paleoecology, and North Pacific correlations of the Miocene Tachilni Formation, Alaska Peninsula, Alaska

Molluscan paleontology, paleoecology, and North Pacific correlations of the Miocene Tachilni Formation, Alaska Peninsula, Alaska - BAP #317

The Tachilni Formation, composed mainly of fine- to medium- grained sandstone, crops out near the western end of the Alaska Peninsula, southwestern Alaska, and contains 53 molluscan taxa, including 36 bivalves and 17 gastropods. The high percentage of articulated, closed bivalve shells, including some found in life positions, suggest that Tachilni deposits were largely unaffected by post- mortem transport. The mollusks are inferred to have lived in a cool-temperate, inner shelf habitat.

Many of the mollusks are known in Miocene faunas of Asia (Hokkaido, Sakhalin, Kamchatka) as well as western North America (California, Oregon, Washington). Several of the North American taxa suggest an age of early late Miocene (Wishkahan Stage) for the Tachilni fauna. Reliable correlations may be made with the Etolon Formation of western Kamchatka, middle part of the Bear Lake Formation of the Alaska Peninsula, the lower to middle part of the Yakataga formation of the eastern Gulf of Alaska, and the lower part of the Montesano Formation (type section of the Wishkahan Molluscan Stage) of southwestern Washington. Other possible correlatives of the Tachilni Formation are the Okkopezawa and Togeshita Formations of Hokkaido and the Nutovo Formation of Sakhalin.

Newly described species are Cyclocardia sakamotoi and Lyonsia mooreae.