Evolution of Reef Ecosystems of the Siberian Platform During the Ordovician

Reef ecosystems of the Siberian Platform have long been the subject of interest for geologists. The most interesting and important object of Early Paleozoic reef investigations in the region was the Cambrian barrier reef belt stretching from the Aldan River to the Yenisei River for about 1,500 km and dividing Cambrian deposits of Siberian palaeocontinent into for-reef, reef and back-reef facies. Originally it was introduced by I.T. Zhuravleva in 1979 under the name “Sakha organogenic belt”. Some researchers believed that it ceased to exist at the end of the Cambrian period, however I.T. Zhuravleva expressed the opinion about the existence of “another and younger zone of organic buildups superimposed on the “Sakha zone.” During recent detailed studies of the Ordovician reefs, it was established that it did indeed continue to exist during the Ordovician period, at least in the north of the Siberian Platform. In this study, we attempted to classify Ordovician reefs according to their biocoenoses. For the Ordovician of the Moyero River section 6 main types of reefs have been identified, changing in time against the background of the Great Ordovician Biodiversification Event: 1) typical stromatolite bioherms and biostromes; 2) microbial-calathium bioherms; 3) bryozoan-moyeronia-angarella bioherms; 4) spherical small stromatolite bioherms; 5) calcimicrobe–stromatoporoid bioherms; 6) calcimicrobe biostroms. The long-term change in biocenoses occurred in parallel with changes in the type of carbonate sedimentation. With the appearance of cool-water carbonates, biocenoses transitioned from microbial-dominated to metazoan-microbial. This can most likely be explained by the oxygenation of the waters of the Siberian paleobasin due to the influence of cold waters brought in by upwelling. The first type of reefs (stomatolite bioherms and biostroms) dominated in the Lower Ordovician. The others are typical for the Middle Ordovician and mark the oncet of cooling and gradual transition from tropical to cool-water carbonates. In the Upper Ordovician typical cool-water carbonates despite of the appearance of corals no reefs have been found yet on the Siberian palaeocontinent.