ACRITARCHS FROM THE EARLY-MIDDLE CAMBRIAN KAILI FORMATION IN THE WULIU-ZENGJIAYA SECTION,EAST GUIZHOU PROVINCE,CHINA

As a proposed GSSP for Stage 5(Middle Cambrian boundary),the Wuliu-Zengjiaya section of the Kaili Formation,China,has yielded well-preserved macroinvertebrate fossils,especially trilobites.A more detailed study of acritarch biostratigraphy of the section is therefore necessary to provide an accurate record of biotic and environmental changes at the boundary.Acritarchs of the Kaili Formation in the Wuliu-Zengjiaya section can be recognized as two distinct assemblages:the Leiomarginata simplex-Fimbriaglomerella membranacea assemblage(0—52 m from the base of the Kaili Formation);and the Cristallinium cambriense-Heliosphaeridium nodosum-Globosphaeri-dium cerinum assemblage(52—140 m).From 140 m to 214 m,only a few acritarch specimens have been found.This is probably because the limestones that dominate the upper part of the Kaili Formation are not suitable for preserving organic-walled microfossils.A significant change of the acritarch assemblage occurs in a bed located approximately 52 m above the base of the Kaili Formation.Undoubtedly,this change in the acritarch assemblages provides important data for interpretation of the depositional environment.The boundary between the two acritarch assemblages(52.3—52.7 m)is slightly below the potential GSSP boundary of Stage 5 marked by the first appearance of the trilobite Oryctocephalus indicus.