Sokolovka Fm

Okrainskoe zone of central Sikhote-Alin uplift

It is composed of basal conglomerates and mainly of middle- to fine-grained sandstones intercalated with silty sandstones and siltstones with a total thickness of about 1500 m.

The lower part of the formation contains abundant fossils, among them the most stratigraphically important are ammonites Phyllopachiceras cf. chitinanum, Kosmatella cf. cappsi, Puzosia alaskana, Parasilezites cf. bullatus, Hulenites ex gr. reesidei and H. ex gr. jimboi and bivalves Aucellina caucasica, A. aptiensis, A. ucturiensis, A. anthulai, Entolium utukokense (Markevich et al., 2000). This fossil assemblage indicates Albian and the ammonite Parasilesites cf. bullatus is a lower Albian index species (Markevich et al., 2000). Thus, the range of the lower part of the formation is dated as lower Albian. The upper part of the formation also contains a lower Albian bivalve assemblage including Entolium utukokense, Aucellina caucasica, A. aptiensis, A. anthulai and Pleuromya kelleri (Markevich et al., 2000). However, the uppermost part of the formation is lithologically very similar to the middle Albian upper member of the Assikaevka Fm of the Western Sikhote-Alin zone and to the middle Albian Svetlovodnaya Fm of the Koppi-Luzhki zone and yields only Aucellina caucasica. Based on these similarities, the middle Albian range is supposed for the uppermost part of the Sokolovka Formation (Markevich et al., 2000).