Abstract:

It is the first time that Pelmatozoan reefs was discovered from early Tremadocian strata (lower Nanjinguan Formation) of early Ordovician at Liujiachang in Songzi area of Hubei Province. Pelmatozoan not only forms individually bafflestone reef, but also combines together with low microbial algae to form microbial reefs, namely baffling and binding reefs. Baffling reefs can be divided into Pelmatozoan bafflestone reefs and stalked echinodermsconcentric circles cyanobacteriabindingbaffling reef, Binding reefs can be divided into the siliconized spongestalked echinodermscyanobacteria bafflingbinding reef and stalked echinodermscyanobacteria binding reef. Accordingly, it can be divided into 4 biomes, namely, Pelmatozoan community, stalked echinodermsconcentric circles cyanobacteria community, siliconized spongestalked echinodermscyanobacteria community and stalked echinodermscyanobacteria community. Composition, structure, function and ecologic environment of each community are expounded and analysed. In the reefs discovered, the diameter of pelmatozoan is much smaller than that of pelmatozoan during late Tremadocian, this phenomena shows the evolution rule of organism from small to large during geological time. In the baffling reefs found, the content of pelmatozoan is lesser than that of late Tremadocian pelmatozoan, this phenomena reveals that reproductive ability and reefbuilding function of organism is weaker during early geological time. In the discussion of the relationship between filter feeding of pelmatozoan and destruction of the stromatolites lamina in the study area, this paper argues that the baffling of pelmatozoan may be the main reason blocking off part of the lamina growth of stromatolites, while filter feeding of pelmatozoan may be secondary reason, As the pelmatozoan is not a wandering animal or boring animal, the predation and disturbing ability may be poorer than that of other boring animals. The discovery of organic reefs demonstrates that the earliest reefbuilding age of stalked echinoderms in the world is early Tremadocian in the study area, at the same time, it also confirmed that early Tremadocian is also the earliest reefbuilding age during Ordovician. Therefore, the discovery of early Tremadocian Pelmatozoan reefs is not only of important significance theoretically for the evolution studying of stalked echinoderms reefs and Ordovician reef, but also of certain practical significance for the evaluation of oil and gas for Jianghan Plain and other fields of the economy.

Key words: Pelmatozoan, organic reefs, microbialite, Early Tremadocian, Songzi Area