New progress on the reefs of Late Ordovician in Bachu area, Tarim Block, NW China

A series of extrinsic (geological) and intrinsic (biological) factors such as water depth, temperature, nutrients, salinity, CO₂ and O₂ content of the atmosphere, ocean circulation, sea-level development, and interactions between the biota are discussed as causal factors for the shift in overall reef composition. Reefs in the Late Ordovician were commonly built up by sessile metazoans with increasing diversity and abundance of metazoan reef－builders such as bryozoans, corals, stromatoporoids, and calcareous algae, while in the Early and Middle Ordovician microbial mud mounds were predominantly which were consisting of stromatolites and thrombolites. In contrast to most Late Ordovician reefs, the reefs in Bachu, Tarim Block, NW China were dominated of microbial mud mounds and appear “old-fashioned” as they resemble Early Ordovician reefs from other areas in the world. Thus, the control factors of the microbial reefs in Bachu area are interesting aspects to discuss.

After years of study and cooperative investigation with colleagues from other universities, professor LI Yue from Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences and his team reported the latest progress of reefs in Bachu, Tarim Block, NW China.

The research has indicated that the reefs of the Late Ordovician Lianglitag Formation, Bachu area, Northwest of China deviated significantly from other Late Ordovician reefs because they were mainly built up of thrombolitic microbial carbonates and calcareous algae, whereas this time is characterised by metazoan reefs in other areas of the world. The main organisms of this reef type are Girvanella，Renalcis，Vermiporella，Halysis，Subtifloria，Wetheredella and Rothpletzella, in which Vermiporella and Halysis are abundant.

In Ordovician, the Tarim Block was positioned in low latitudes, probably south of the equator prevailling southeast trades and the northwest part of the Tazhong Uplift faced the open ocean. In this respect, it could be assumed as that the microbial reefs in the Bachu area were simply the palaeogeographic position on the shallow, gently dipping, leeward side of the carbonate platform. The water on the windward, steeper side of the platform was favourable for metazoan reef growth in Tazhong because (a) it was well-oxygenated, (b) has no strong temperature changes because it faces the open ocean, and (c) it was mesotrophic (eutrophic conditions would probably prevent metazoan reef growth). On the less-steep leeward side of the shallow platform, the water was less turbulent and warmer because it was heated while it flew over the platform. Because in warmer water less gas can be dissolved these water masses were depleted in both oxygen and CO₂, the latter resulting in increasing (super) saturation of calcium carbonate, which favoured the growth of microbial carbonates in the Bachu area.

Related informations of this paper: Zhang Yuanyuan, Li Yue, Axel Munnecke. Late Ordovician microbial reefs in the Lianglitag Formation (Bachu, Tarim, NW China).Facies, 2014, 60(2)：663－684.