ACRITARCH ASSEMBLAGES FROM THE EARLY - MIDDLE CAMBRIAN KAILI FORMATION OF EAST GUIZHOU PROVINCE AND BIOSTRATIGRAPHIC IMPLICATION

Acritarch fossils from the Early Middle Cambrian Kaili Formation at both the Balang and Pingzhai sections, Guizhou Province, have been studied. seventeen genera and 35 species of acritarchs from the Kaili Formation of the Balang section have been identified, in which Leiosphaeridia, Synsphaeridium, Cymatiosphaera, Pterospermella are abundant. Three acritarch assemblages near the Early and the Middle Cambrian boundary have been established as follows (in ascending order): Cymatiosphaera cf. cristata Fimbriaglomerella membrancea assemblage; Cristallinium Micrhystridium Pterospermella assemblage; Dictyotidium Granomarginata assemblage. thirteen genera and 21 species of acritarchs from the Pingzhai section have been identified, and three acritarch assemblages have been established near the Early and the Middle Cambrian boundary, which are (in ascending order): Leiosphaeridia Tasmanites assemblage; Retisphaeridium Micrhystridium tentatium assemblage; Baltisphaeridium Bubomorpha hunjiangensis assemblage. A detailed study, concerning the relative abundance of major genera and the variation of acritarch abundance in each acritarch assemblage, has been conducted. The acritarch assemblages of the Kaili Formation are very well correlated with those from Lower and Middle Cambrian elsewhere in the world. It suggests that the boundary between the Early and the Middle Cambrian should be placed at the bottom of layer 9 2 in the Balang section in Taijiang. This boundary defined by the acritarch assemblages is consistent with the boundary marked by the appearance of trilobite Oryctocephalus indicus or the extinction of large number of species of Bathynotus, Redlichia, and Protoryctocephalus. Present study suggests that acritarch assemblages are of particular importance for the biostratigraphy correlation of Cambrian stratigraphy in the area where other index fossils are absent.