Basal Cambrian microfossils from the Yurtus and Xishanblaq formations (Tarim, north-west China):: Systematic revision and biostratigraphic correlation of Micrhystridium-like acritarchs

Micrhystridium-like acritarchs are widely distributed in basal Cambrian cherts and phosphorites in South China. This paper describes similar acritarchs from the basal Cambrian Yurtus and Xishanblaq formations in Tarim, north-west China. The taxonomy of these acritarchs is revised. The basal Cambrian acritarch assemblage in Tarim and South China is characterized by three genera: Asteridium Moczydlowska, Heliosphaeridium Moczydlowska and Comasphaeridium Staplin, Jansonius and Pocock. This assemblage is named the Asteridium-Heliosphaeridium-Comasphaeridium (AHC) acritarch assemblage. In both South China and Tarim, the AHC acritarch assemblage is associated with the tubular microfossil Megathrix longus Yin L. and the small shelly fossil Kaiyangites novilis Qian and Yin G. This assemblage also occurs in the Lower Tal Formation in the Lesser Himalaya. Correlation with small shelly fossil (SSF) assemblages indicates that the AHC assemblage is restricted to the Meishucunian Stage, and possibly to the lower Meishucunian (Anabarites trisulcatus-Protohertzina anabarica and Siphogonuchites triangularis-Paragloborilus subglobosus SSF assemblages). The AHC assemblage is broadly similar to the Asteridium tornatum-Comasphaeridium velvetum (acritarch) Zone in the East European Platform, which is considered to be Nemakit-Daldynian (and possibly Tommotian) in age.