Organic-walled microfossils from the Tonian Tongjiazhuang Formation of the Tumen Group in western Shandong, North China Craton and their biostratigraphic significance

Neoproterozoic successions in western Shandong of North China Craton contain fine-grained siliciclastic rocks which offer a unique opportunity to explore the Neoproterozoic biosphere. In this study, we have revealed a well-preserved and diversified microfossil assemblage from the Tongjiazhuang Formation, Tumen Group in western Shandong, North China Craton, which is characterized by abundant smooth-walled sphaeromorphic acritarchs and cyanobacterium-like filamentous forms and relatively low abundance of more complex acritarchs. A total of 40 microfossil taxa belonging to 20 morphological genera have been identified, most of which are described for the first time from the Tongjiazhuang Formation of the Tumen Group in western Shandong, including Simia sp., Trachyhystrichosphaera aimika, T. botula, Germinosphaera bispinosa, Unnamed sp. A, Jacutianema solubila, and Pololeptus rugosus. The Tongjiazhuang assemblage revealed in this study is much more diverse than previously reported and well comparable to the Tonian Liulaobei and Gouhou assemblages in the Xuhuai region. Particularly, the occurrence of T. aimika, T. botula, G. bispinosa, J. solubila, and P. rugosus indicates a late Mesoproterozoic to Tonian age for the Tongjiazhuang Formation. In addition, the occurrences of macroscopic carbonaceous compressions, including Chuaria-Tawuia assemblage from the Tongjiazhuang Formation and Tawuia and Sinosabellidites from the Shiwangzhuang Formation, are consistent with geochronological data, suggesting a Tonian age for the Tongjiazhuang, Fulaishan, and Shiwangzhuang formations of the middle and upper Tumen Group, while the Heishanguan and Erqingshan formations of the lower Tumen Group are likely late Mesoproterozoic to Tonian in age. Thus, available biostratigraphic data, lithostratigraphic correlation, and geochronological data suggest that the whole Tumen Group is late Mesoproterozoic to Tonian in age, rather than Cryogenian or Ediacaran. This study significantly improves our knowledge about the diversity of the Tonian biosphere and implies a ∼200–300 million years unconformity (the ‘Great Unconformity’) between the late Mesoprotoerozoic to Tonian Tumen Group and the early Cambrian Liguan Formation in western Shandong.