Assikaevka Fm

Western Sikhote-Alin. "The Assikaevka Formation is very widespread and well exposed in the Assikaevka River basin, in the northeastern side of the Bikin River and in the vicinity of the Vyazemsk railway station, extending northeastwards."

Lower member mainly consists of sandstones with basal conglomerates and beds of sandstones alternating with siltstones and mudstones as well as interbeds of dacite tuffs with a total

thickness up to 780 m. Middle member conformably overlays the lower member and is composed of siltstones and silty mudstones with beds of sandstones alternating with siltstones, andesites, andesite-basalts and tuffs with a total thickness of up to 870 m. Upper member conformably overlies the middle member and mainly consists of sandstones with a total thickness of up to 870 m.

Lower Member: The lower part of the member yields marine bivalves Nuculopsis sp. ind., Entolium sp. ind., Entolium cf. utukokense, Eumorphotis sp. and Pleuromya sp. (Markevich et al., 2000; Amelchenko et al., 2001). The upper part of the member contains bivalves Nucula sp. ind., Isognomon sp., Trapezium? sp., Gresslya? sp., Thracia sp. ind. and ammonite Hulenites sp. (Markevich et al., 2000; Amelchenko et al., 2001). The member also contains a fossil plants assemblage identified as the Early Assikaevka flora assemblage (Volynets, 2005). The assemblage is characterized by low taxonomic diversity (18 taxa) with domination of cycadophytes (6 taxa) and ferns (5 taxa). The common species are cycadophytes Pterophyllum sutschanense, Nilssonia ex gr. brongniartii, ferns of the genera Birisia, Onychiopsis, Polypodites, Coniopteris (Dicksonia), Cladophlebis, conifers Elatides asiatica, Athrotaxites berryi, Pityophyllum sp. and Podozamites sp. The assemblage of marine molluscs from the lower memberwas considered as Aptian-Albian (Markevich et al., 2000; Amelchenko et al., 2001). However, this assemblage does not contain any Aptian-Albian index fossils, e.g. bivalve Aucellina. The fossil plants assemblage suggests an age not older than Aptian (Amelchenko et al., 2001; Volynets, 2005). Nevertheless, the lower member roughly corresponds to the Peide Fm and Qihulin Fm of the Longzhaogou Gr. The Qihulin Fm yields Barremian or possibly lower Aptian ammonites (Futakami et al., 1995; Li and Bengtson, 2018, 2019) that suggests that the Peide Fm is probably Barremian. Therefore, the lower part of the lower member of the Assikaevka Formation is also possibly Barremian.

Middle Member: The member yields marine bivalves Aucellina caucasica, A. aptiensis, A. anadyrensis, A. ucturiensis, Entolium utukokense, Yoldia ex gr. kissoumi, Thracia cf. kissoumi, Astarte ex gr. semicostata, Protocardia sp., Nucula ex gr. pectinata, Chlamys? sp. nov., Camptonectes ex gr. dattermani, Inoceramus ex gr. anglicus and ammonites Puzosia sp. and Anagaudriceras? sp. ind. (Markevich et al., 2000; Amelchenko et al., 2001). The assemblage of marine bivalves is typical for the Aptian-Albian (Markevich et al., 2000; Sey et al., 2004) but Inoceramus ex gr. anglicus is limited to the upper lower-upper Albian (Zonova and Yazykova, 2001, 2004; Sey et al., 2004) suggesting that the upper part of the middle member is not older than the lower Albian. The range of the middle member is probably Aptian-lower Albian. This stratigraphic range does not contradict with the range based on fossil plants from the lower (Aptian) and upper (middle Albian) members. The absence of fossil plant persists in the middle member as well as the presence of marine fauna throughout the member indicates that the accumulation of the middle member occurred in fully marine conditions.

Upper Member: Marine fauna occurs throughout the member in the axial part of the Alchan basin, while in the eastern part of the basin it occurs together with plant remains only in the lower part of the member (Amelchenko et al., 2001). The lower part of the member yields marine bivalves Thracia ex gr. sanctae-crucis, Entolium sp. ind. and Aucellina caucasica, the middle part contains Entolium utukokense, Mactromya sp., Pleuromya cf. kelleri and the upper part contains Leda sp., Entolium utukokense, Aucellina caucasica, Inoceramus ex gr. altifluminis, Mactromya sp., Homomya sp. ind., Thracia cf. stelcki and Gresslya? sp. (Markevich et al., 2000; Amelchenko et al., 2001). Based on the presence of Inoceramus ex gr. altifluminis, the upper part of the upper member is dated as middle Albian (Markevich et al., 2000; Amelchenko et al., 2001). The lower and upper parts of the upper member contain a fossil plants assemblage identified as the Late Assikaevka flora assemblage (Volynets, 2005). The assemblage is characterized by high taxonomic diversity (54 taxa) with a significant quantity of ferns, conifers and cycadophytes. Common taxa are: ferns Onychiopsis psilotoides, Anemia dicksoniana, Gleichenites porsildii, Birisia onychioides, Birisia alata, representatives of the genera Ruffordia, Dicksonia, Osmunda, Adiantopteris, Alsophilites, Coniopteris, Arctopteris, Lobifolia, Cladophlebis, conifers Elatides asiatica, Podozamites ex gr. lanceolatus, Athrotaxites berryi, Sequoia reichenbachi, cycadophyte Nilssonia ex gr. brongniartii. The characteristic feature of the assemblage is the significant role of “young” genera such as Anemia, Osmunda, Birisia, Arctopteris, Sequoia, Sphenolepis, Taxites and the first appearance of angiosperms such as Sapindopsis, Laurophyllum, Dicotyophyllum, Quercophyllum, Nissidium and Onoana. The flora assemblage suggests a middle Albian range of the member (Amelchenko et al., 2001). Similar plant assemblages are known from the upper beds of the Suchan Gr and Nikan Gr of Southern Primorye (Amelchenko et al., 2001).