Oujiang Fm

Depressions north of Taiwan. The naming section is located in Lingfeng No. 1 well in the Oujiang depression, East China Sea continental shelf basin. Named by the Shanghai Marine Geological Party, Ministry of Geology and Mineral Resources in 1985.

The formation is divided into the upper, middle and lower members: Lower member, light, pebbly, medium or coarse sandstone with silty and fine sandstone, sandy dolostone and mudstone; Middle member, light gray, bioclastic siltstone with bioclastic limestone and mudstone; Upper member, greenish gray, muddy, silty sandstone with mudstone, the base of which is light gray, lime sandstone and pebbly sandstone. The lithology is uniform and may be well correlated. The thickness is 444–992.5 m. According to the geological data of four wells in different structural sites, the Oudong deep depression is thicker and has more complete strata than the Ouxi deep depression. For example, Wenzhou No. 6-1-1 well in the Oudong deep depression reveals a thickness of 992.5 m, while well Shimentan No. 1 in the Ouxi deep depression only encounters 444 m and well Mingyuefeng No. 1 is 697.5 m.

This formation has abundant fossils. The planktic foraminifera belong to zones P7-P11, and the main zonal fossils are Subbotina bakeri, Pseudohastigerina wilcoxensis and Globorotalia (A.) interposita etc., and the benthic foraminifera are characterized by the development of large ones, including Nummulites nuttali, N. donghaiensis, Marginulinopsis marshalli, Gyroidina orbicularis obliquata, Cibicidoides spp. and Haplophragmoides spp. etc.; the ostracods are represented by the Paijenborchella-Loxoconcha assemblage; the calcareous nannofossils include those of zones P13-P17, the representative types being Discoaster sublodoensis, Rhabdosphaera inflata, Neochiastozygus rosenkrantzii, Chiasmolithus grandis and Calcidiscus simplex; dinoflagellates include five assemblages: Homotryblium pallidum-Fibrocysta mutabilis, Areoligera cf. senonensis-Thalassiphora delicata, Wetzeliella articulata-Kisselovia coleothrypta, Rhombonium draco-Schematophora speciosa and Thalassiphora eocenica-Areosphaeridium arcuatum; the sporopollen is the Cupuliferoipollenites-Quercoidites assemblage.

The Oujiang Formation is littoral-neritic deposits