The biostromal community structure and biostratigraphic correlation of the Mesozoic Suowa Formation in Bandaohu area, Qiangtang depression, Tibet

The Lower part of Suowa Formation in the north Qiangtang depression of Tibet has been correlated to the Late Jurassic. However, dating evidence of the upper part of the formation remains insufficient. Three reefal units from Suowa Formation in Bandaohu area are gentle morphologically and composed of corals and stromatoporoids frameworks. The community structures show that benthic metazoanl taxa are mainly of Pachythecopora pachytheca Deng, Actinostromaria motianlingensis Dong et Wang and Spongiomorpha robusta Yang et Wang, and subordinately of Latusastraea xigazeensis Liao et Xia, Preverastraea sp., Ptychochaetes cf. globosus Koechlin and Spongiomorpha asiatica xainzaensis Deng. The pollen assemblages marked by Dicheiropollis etruscus from the upper part of the Suowa Formation are initial elements of the Early Cretaceous in the surrounding areas of the Tethys. Moreover, Senegalosporites and Steevesipollenites have never been recorded from horizons in advance of the Cretaceous, and they are common in the pollen assemblages of the late Early Cretaceous and even later. Therefore, coral and pollen assemblages indicate that the main part of the Suowa Formation is correlated to the Upper Jurassic, and the upper part of the formation is reasonably assigned to the Lower Cretaceous.