Alchan Fm

Western Sikhote-Alin.

The Alchan Formation is dominated by volcanogenic rocks, sharply different from the underlying formation dominated by marine deposits. It is subdivided into two members. The lower member is mainly composed of dacites and, to a lesser degree, rhyodacites and rhyolites with a total thickness of up to 385 m. The upper member is composed of andesites and tuffs with a total thickness of 600-900 m.

Lower Member: It does not contain any marine fossils but yields rich fossil plant assemblages. Early Alchan flora assemblage is identified in the lower member (Volynets, 2005). Most common taxa are ferns Osmunda, Anemia, Dicksonia, Birisia, Arctopteris, conifers Podozamites ex gr. lanceolatus, Elatides asiatica, Torreyites cf. dicksonioides, Sequoia reichenbachi, Athrotaxites berryi, Athrotaxites expansa, cycadophytes Pterophyllum sutschanense, Taeniopteris bikinensis, Nilssonia ex gr. brongniartii, Nilssonia densinervis. The characteristic feature of the assemblage is domination of ferns (37%) and conifers (27%) and absence of angiosperms. Plant remains suggest a middle-lower upper Albian range for the lower member of the Alchan Formation (Volynets, 2005). In addition to fossil plants, non-marine bivalves Sphaerium [=Limnocyrena] anderssoni and S. cyclica and non-marine gastropod Viviparus sp. were reported from the lower member.

Upper Member: It yields numerous plant megafossils, spores and pollen grains. The Late Alchan flora assemblage is identified in the upper member between the Alchan, Bikin and Marevka rivers (Volynets, 2005). It is characterized by high taxonomic diversity (more than 150 taxa) with domination of ferns (41 taxa) and conifers (41 taxa) and significant role of angiosperms (26 taxa) and cycadophytes (26 taxa). Common taxa are represented by Osmunda, Gleichenites, Arctopteris, Sapindopsis, Quercophyllum, Araliaephyllum, Laurophyllum, Dicotylophyllum, Birisia, Coniopteris, Eogymnocarpium, Pseudolarix, Pityophyllum, Pityocladus, Pityospermum, Pityostrobus, Pityolepis, “Picea”, Elatides, Athrotaxites, Athrotaxopsis, Sequoia, Cinnamomoides, Vityphyllum, Cissites, Celastrophyllum, Lindera, Menispermites, Magnoliaephyllum, Dalembia, Sassafras, Araliaephyllum. This assemblage is similar to the upper Albian flora from the upper part of the Severny Suchan Fm (SevernySuchan Fm) and the Kangauz Fm (Partizansk Basin) and from the Galenki Fm (Razdolny Basin) of Southern Primorye (Krassilov, 1967; Amelchenko et al., 2001; Volynets, 2006). Based on the fossil plants assemblages the Alchan Formation is considered as middle-upper Albian (Amelchenko et al., 2001; Volynets, 2005).