Abstract: The lower Ordovician microbialites in the Songzi area, Hubei Province are studied systematically for the first time. The microbialites are distributed in the Nanjinguan, Fenxiang and Honghuayuan formations of the Lower Ordovician in the study area. In this paper, microbial rocks are divided into two categories: in-situ and non-in-situ growth types. The former includes stromatolite and thrombolites, and the latter is mainly oncolite. According to geometric classification scheme, stromatolites in the study area are characterized as lamellar-, wavy-, columnar- and mound-shaped types; in addition, we discovered and named a new type, the double cone-shaped stromatolite. Likewise, the thrombolites are divided into three types: porphyritic, reticular and striped clotted types. Fossil record of drilling predation on mollusks Ecculiomphalus and traces of bioturbation were found in leopard porphyritic and striped thrombolites, both, we suggest, are bioturbated thrombolites. The discovery of bioturbated thrombolites filled a gap in domestic and international Ordovician research contributing new knowledge towards understanding of thrombolites genesis. Based on analysis of sedimentary characteristics, depositional environments and their distribution patterns are described for various types of microbiological rocks. And the relationship between microbialites evolution and resuscitation and radiation of metazoan during the early to middle Ordovician is discussed. We believe that the Ordovician microbialites are decreasing with time, and certain relationship exists between the decline of microbiological rocks and increase of metazoan abundance. We also believe that grazers not only chewed up the stromatolite-forming bacteria and algae microbes to cause continuing stromatolite reduction, but also destroyed the growth condition for stromatolites, resulting in bioturbation thrombolites with unique characteristics.

Key words:  microbialites, stromatolites, thrombolites, oncolite, Lower Ordovician, Songzi area