Klyuchi Fm

Widespread throughout Central Sikhote-Alin uplift, Eastern Sikhote-Alin Basin, Pribrezhnoe uplift, and Southern Primorye Basin.

In the Khor-Dalnin zone, the Klyuchi Formation crops out in the tectonic blocks near or at some distance from the Central Sikhote-Alin fault. Near the fault, the formation consists of gravelstones, conglomerates and sandstones. The role of sandstones and silty mudstones in the lithological composition of the formation increases with distance from the fault. The total thickness of the Klyuchi Formation is 1250 m. In the Okrainskoe zone, the Klyuchi Formation is composed of sandstones, siltstones, silty mudstones with conglomerates and gravelstones in the lower part with a total thickness of 1500 m.

In the Koppi-Luzhki zone of Eastern Sikhote-Alin Basin, the Klyuchi Fm comprises siltstones, sandstones, rhythmic alternation of siltstones and sandstones, rarer conglomerates, gravelstones and clayey siliceous rocks (Turbin, 1994; Markevich et al., 2000). The total thickness of the formation is up to 2200 m. Based on the role of sandstones, the formation is subdivided into four units. The first unit is characterized by the domination of sandstones. Its thickness is up to 500 m. The second unit is composed of silty mudstones with rare interbeds of sandstones with a total thickness up to 400 m. The third unit comprises sandstones and alternations of sandstones, siltstones and silty mudstones with the domination of the latter. Its thickness is up to 700 m. The fourth unit is composed mainly by silty mudstones with interbeds of fine-grained sandstones and with a bed of fine-grained sandstones in the lower part. The total thickness of the unit is up to 570 m.

In the Pribrezhnoe zone, the formation consists of sandstones, siltstones and gravelstones with a total thickness of 2100 m.

In sections in the Khor-Dalnin zone, the lower part of the formation yields abundant Buchia keyserlingi, B. inflata, B. pacifica, B. ex gr. uncitoides, B. sibirica (=B. keyserlingi according to Zakharov, 1981) and occasionally B. crassicollis in upper beds (Markevich et al., 2000). The domination of B. inflata and B. keyserlingi in several localities as well as presence of B. pacifica suggests lower Valanginian (Zakharov, 1981; Zakharov et al., 1997; Zakharov and Rogov, 2020). Consequently, the stratigraphic range of the lower part of the Klyuchi Formation is considered as Valanginian. The upper part of the formation contains B. crassicollis, B. keyserlingi and ammonite Thurmanniceras cf. jenkinsi (Markevich et al., 2000) suggesting upper Valanginianelowermost Hauterivian. Thus, the Klyuchi Formation in the Khor-Dalnin zone is considered as Valanginian probably extending to the lowermost Hauterivian. In the Okrainskoe zone, the lower part of the formation yields Buchia keyserlingi, B. inflata, B. bulloides (possibly it is synonym of B. inflata according to Zakharov, 1981), B. crassicollis, B. sublaevis and the upper part yields B. keyserlingi, B. inflata, B. bulloides, B. crassicollis, B. sublaevis, B. solida (possibly it is synonym of B. crassicollis according to Zakharov, 1981) and ammonite Homolsomites sp. (Markevich et al., 2000). The abundance of B. keyserlingi, B. cf. inflata, B. wollossowitschi (=B. keyserlingi according to Zakharov, 1981), B. cf. uncitoides in the lower part suggests lower Valanginian, while abundance of B. crassicollis and B. sublaevis suggests upper Valanginian or even lower Hauterivian for the upper part of the formation. The occurrence of Homolsomites ammonites probably suggests lower Hauterivian for the uppermost part of the formation because this ammonite characterizes the Valanginian/Hauterivian boundary beds and lowermost Hauterivian of Northern Siberia (Zakharov, 1981; Zakharov et al., 1997; Nikitenko et al., 2013; Zakharov and Rogov, 2020).

In the Koppi-Luzhki zone of Eastern Sikhote-Alin Basin, The first unit contains the most abundant fossils. It is characterized by typically Valanginian Buchia keyserlingi, B. inflata, B. wollossowitschi, B. visingensis (the latter two species are included in synonymy of B. keyserlingi by Zakharov, 1981) and more common for the Berriasian B. uncitoides and B. volgensis as well as upper Valanginian B. crassicollis (Markevich et al., 2000). This combination of species suggests a Valanginian range. The second unit is characterized by an increase of B. crassicollis that corresponds to the upper Valanginian (Zakharov, 1981, 1987; Zakharov and Rogov, 2020). The first and the second units also yield fossil plants Coniopteris bureyensis, Cladophlebis ex gr. denticulata, Dictyozamites kawasakii and Nilssonia sp. The third and the fourth units are poorly characterized by fauna. Only finds of B. crassicollis and Homolsomites ammonites are reported for the fourth unit allow to consider the uppermost Valanginian-lower Hauterivian comparing with the Boreal Standard (Zakharov et al., 1997; Nikitenko et al., 2013). Thus, the Klyuchi Formation is Valanginian to possibly lower Hauterivian.

In the Pribrezhnoe zone, the formation yields ammonites Polyptychites sp. and bivalves Buchia crassicollis, B. uncitoides, B. bulloides (possibly it is a synonym of B. inflata according to Zakharov, 1981), B. keyserlingi, B. visingensis (=B. keyserlingi according to Zakharov, 1981), B. solida (=B. crassicollis according to Zakharov, 1981), B. pacifica and B. sublaevis (Markevich et al., 2000). Based on the fossil assemblage the formation is Valanginian to possibly lower Hauterivian.

The Klyuchi Formation is exposed on the southwest of the Partizansk depression on the left bank of the Partizanskaya River south of Molchanovka village (Markevich et al., 2000). The relation of the Klyuchi Formation with underlying deposits as well as the lower beds of the formation is unknown. The Klyuchi Formation consists of (1) fine-grained sandstones with lenses of gravelstones in the lower part with numerous Buchia volgensis, B. keyserlingi, B. inflata, B. wollossowitschi, B. visingensis (the latter two species are regarded as synonyms of B. keyserlingi by Zakharov, 1981), B. uncitoides, B. pacifica, B. crassicollis, B. sublaevis and also Isognomon aff. ricordeana and Corbicula aff. tetoriensis (60 m); (2) alternations of fine-grained sandstones with plant detritus with silty sandstones without fauna (55 m); (3) a shell bed (0.5 m) consisting of shells of Corbicula, Ostrea, Exogyra, Astarte sakawana and Buchia sp. ind. with Exogyra ryosekiensis, Ostrea cf. yoshimoensis, Corbicula aff. tetoriensis, Isognomon aff. ricordeana; (4) alternations of fine- to medium-grained sandstones with silty sandstones with Buchia sp. ind. and plant remains Alsophyllites nipponensis, Onychiopsis psilotoides, Sphenopteris latiboba, Cladophlebis sp., Dictyozamites kawasakii (Markevich et al., 2000). Based on the buchiid assemblage the Klyuchi Formation is considered Valanginian to lower Hauterivian (Zakharov, 1981; Zakharov and Rogov, 2020). V.I. Bodylevsky (1943) noted the similarities between Valanginian fossil assemblages of the Partizansk region andWealdian fossil assemblages of Japan. The difference between these regions is the absence of Buchia in Japan. Thus, the Partizansk region represents an ecotone where Boreal (Buchia) and Tethyan (Exogyra) fauna mixed. The Klyuchi Formation is unconformably overlaid by the upper Hauterivian-middle Albian mainly non-marine coal-bearing Suchan Gr (Turbin, 1994; Markevich et al., 2000).