Late Ordovician microbial reefs in the Lianglitag Formation (Bachu, Tarim, NW China)

摘要

Reefs in the Late Ordovician are usually built up by metazoans, whereas in the Early and Middle Ordovician microbial reefs are dominating. In the Late Ordovician (Katian) Lianglitag Formation, at Bachu, Tarim, NW China, however, three distinct stages of thrombolithic microbial reefs are exposed. The lower and upper reef intervals are characterized by widespread microbial carbonates with almost constant thicknesses (biostromes). The middle reef interval exposes dome-like mounds, and shows a higher diversity of reef-building and reef-dwelling organisms. Calcareous algae such as Vermiporella are abundant, but also other components such as fragments of Halysis, brachiopods, molluscs, echinoderms, bryozoans, and trilobites have been found in the microbial reef units. The purpose of this study is to describe for the first time the composition and microfacies of the reefs in this remote area. Especially the question of whether or not these Late Ordovician reefs represent remnants of the Early/Middle Ordovician microbial-dominated reef type just lacking Calathium and lithistid sponge is addressed. The results indicate that the local conditions on the leeward side of the carbonate platform, where waters are less well agitated and thus less well oxygenated, and probably also characterized by temporarily elevated water temperatures hampering the growth of metazoan reefs, were responsible for the proliferation of the Late Ordovician microbial reefs in the Bachu area.