Geology--North Pacific Ocean

The large North Pacific bivalve mollusk index-fossil Fortipecten hallae (Dall, 1921) is present in a well-dated stratigraphic section of the Milky River Formation, Alaska Peninsula, southwestern Alaska. Co-occurring marine diatoms belong to the upper part of the subzone B of the Neodenticula kamtschatica diatom zone of the North Pacific diatom chronostratigraphy, with an age range of 4.8–5.1 Ma (early Pliocene). Based on coeval occurrences in northeastern Kamchatka, Russia, and synchronous changes in the two molluscan assemblages, F. hallae is a useful indicator of early Pliocene climatic warming along the high latitude North Pacific margin.

Fossiliferous rocks of the Snatolskaya and Kovachinskaya formations comprise a Middle and Late Eocene shallow-marine record of the central part of western Kamchatka. Gastropod assemblages of these formations contain taxa that are conspecific with those in Paleogene strata of western North America and Japan, as well as a large percentage of endemic species. Analysis of the latitudinal ranges and worldwide occurrences of gastropod genera from these formations show the presence of three biogeographic components: cosmopolitan, North Pacific, and endemic. No Tethyan, or circumtropical genera are present in these Kamchatkan Middle and Late Eocene gastropod faunas. Changes in the geographic distribution of North Pacific gastropod assemblages through the Middle and Late Eocene indicate that only eastern Pacific Tethyan taxa were subjected to latitudinal range reduction. The distribution of cosmopolitan and North Pacific elements did not significantly change from the Middle to Late Eocene, which suggests a relatively stable environment and climate stability during that time. High-latitude Eocene gastropod assemblages from Western Kamchatka demonstrate a high level of endemism at the species level and a low-level of endemism on the genus level. This pattern is thought to be a result of the unrestricted migration of cosmopolitan taxa northward along the shallow-marine margin of the Pacific rim.