SevernySuchan Fm

Partizansk depression of southern Primorye Basin (S-12; E of Vladivostok). Middle to upper formation of the Suchan Gr

Synonym: Severny Suchan Fm (which is the actual name; but merged as SevernySuchan here to avoid Lexicon software confusing with the Stary Suchan Fm (termed StarySuchan Fm here).

It is composed of siltstones, sandstones and coal beds with a total thickness of up to 800 m. The formation is subdivided into three members based on the presence or absence of coal beds (Markevich et al., 2000). Lower member (170-240 m) consists of fine-grained sandstones, siltstones and mudstones. Middle member (125-260 m) is very similar to the lower member but differs by the presence of up to 15 coal beds. Upper member (Frentsevka Fm) (135-430 m) is composed of fine-grained sandstones with marine fauna, sandstones, siltstones and silty mudstones. Some researchers separate the upper member as the Frentsevka Fm (Krassilov, 1967; Volynets, 2005, 2006). Commercial coal deposits as well as a horizon with marine fauna disappear westward.

Fossil remains were reported throughout the formation. The Early Severny Suchan fossil plants assemblage was identified in the lower member of the Severny Suchan Formation (Volynets, 2005). It includes 33 taxa with domination of ferns (14 taxa) and subdomination of conifers (8 taxa). Common taxa are Osmunda denticulata, Gleichenites porsildii, Anemia dicksoniana, Birisia onychioides, Athrotaxopsis expansa, Elatides asiatica, Nilssonia canadensis. The characteristic feature of this assemblage is the domination of ferns (42%) and conifers (24%) and the significant role of Osmunda, Anemia, Athrotaxopsis and Taxites. The assemblage is considered lower Albian (Volynets, 2005). The Late Severny Suchan fossil plant assemblage was identified in the middle member of the formation (Volynets, 2005). It consists of 46 taxa with domination of ferns (18 taxa) and subdomination of conifers (14 taxa). Common taxa are represented by ferns Osmunda denticulata, Dicksonia concinna, Birisia onychioides, Gleichenites zippei, Anemia dicksoniana, Cladophlebis frigida, conifers Podozamites ex gr. lanceolatus, Elatides asiatica, Athrotaxites berryi, Pseudolarix dorofeevii, Schizolepis neimengensis, and cycadophytes Pterophyllum sutschanense, Nilssonia ex gr. brongniartii. The characteristic feature of the assemblage is the significant role of evolutionary advanced ferns and conifers such as Osmunda, Anemia, Birisia, Arctopteris, Torreyites, Sequoia and the appearance of angiosperms such as Dicotylophyllum sp. The assemblage is considered lower-middle Albian (Volynets, 2005).

The formation also yields an assemblage of non-marine molluscs including Sphaerium [=Lymnocyrena] anderssoni, Protelliptio hamiti, Unio ogamigoensis, Campeloma tani, C. yihsiensis, Viviparus matsumotoi dated as Aptian-Albian (Markevich et al., 2000). Sandstones with marine fauna (called “Nadvelikanovskiy horizon”) are characterized by an assemblage of marine bivalves including Quadratotrigonia (Transitrigonia) fudsinensis, Pterotrigonia hokkaidoana, P. pocilliformis, Ussuritrigonia subpyriformis, “Callista” pseudoplana, Isognomon, Lima and other. This assemblage is considered as middle Albian. This range is also supported by finds of Inoceramus concentricus (Markvich et al., 2000). Such composition of the Suchan Gr is typical only for the Partizanskaya River basin.