Buchia Faunas and Biostratigraphy of the Jurassic-Cretaceous Boundary Deposits in the Komsomolsk Section (Russian Far East)

Abundant Buchiidae (Bivalvia) from the Upper Volgian-Lower Valanginian sediments of the Northern Sikhote-Alin (Russian Far East) were studied in the key section at right bank of the Amur River opposite to Komsomolsk-on Amur. The analysis of the Buchiidae stratigraphic distribution in the section allowed us to reveal the sequence of Buchia-bearing beds: this is well correlated with the Buchia zonal scales of many Arctic regions. Here, from the base upward, there have been recognized the following: beds with Buchia terebratuloides, beds with B. unschensis and B. terebratuloides, beds with B. volgensis, and beds with B. inflata and B. keyserlingi. The beds with B. unschensis and B. terebratuloides also yielded the Berriasian ammonite Pseudosubplanites? sp. of Tethyan affinity. The obtained paleontological data confirm the occurrence of a large syncline in the studied section. The data also permit one to refine the age spans of the local stratigraphic units.