Late Proterozoic and Early Cambrian microfossils and biostratigraphy, northern Anhui and Jiangsu, central-eastern China

Abstract

The Upper Proterozoic in northern Anhui and Jiangsu Provinces, central-eastern China, is in a region of complex structure on the southern margin of the North China Platform. It consists of the Huainan Group to the south, and Huaibei Group to the north of the Huaihe River. The Huainan Group underlies glacial diamictite of the Fengtai Formation. Both groups yield abundant microfossils, carbonaceous megafossils, stromatolites and putative non-skeletal metazoan fossils. Recently this region has been investigated intensively, in the expectation that it will provide the evidence needed to correlate the sequences of the North and South China Platforms. This study is directed at that problem. There is considerable interest in the Liulaobei Formation of the Huainan Group, because it contains worm-like possible metazoan fossils. It has been considered by various workers to be pre-Sinian, Sinian or Early Cambrian. The greatest abundance and diversity of microfossils occurs in the lower part of the formation, which is interpreted as having been deposited in relatively deep water. The Chuaria-Tawuia assemblage which occurs in the Liulaobei Formation has been considered biostratigraphically useful, but current evidence is that the assemblage is long-ranging with limited biostratigraphic value. The Liulaobei acritarch assemblage is dominated by spheroidal forms but there are some complex forms of more use in biostratigraphy; the formation is most likely to be Sinian. Acritarchs have also been recovered from other units. The Gouhou Formation was previously considered to be late Sinian, but here is interpreted as Early Cambrian. The microfossil assemblages from the shale of the upper Sinian Shijia Formation and the silty carbonates of the upper Sinian Zhaowei and Jiayuan Formations have also been investigated. Evidence for the ages of the Groups is not definitive, but on balance suggests that all but the uppermost formations are Sinian. Substantial differences between the groups are most easily explained as the result of deposition in separate basins. Seven major lithofacies can be recognized in this region. Outcrop observations combined with a study of thin sections, stable isotope analyses and microfossils indicate a range of marine shelfal environments from relatively deep and below wave base across a shallow inner shelf to peritidal and fluviatile environments. Palaeoecological study suggests the abundance and diversity of microfossils increase from the nearshore to relatively offshore environments on the shallow marine shelf. Thirty-one spheroidal acritarchs and nine filamentous microfossls are described; four of these are emended or combined. They are Leiosphaeridia epikoila Zang comb. sp. nov., Leiosphaeridia timofeevi (Vidal), Zang, comb. nov., Leiosphaeridia visingsa Zang comb. sp. nov., and Sinianella uniplicata Yin, emend. Zang.