Study on the Main Controlling Factors of the Early Ordovician Stromatolites Development in Liujiachang Area of Songzi City

The Early Ordovician stromatolites in the study area are mainly exposed at the bottom and middle and lower parts of the Second Member of Nanjinguan Formation, the top of the Fourth Member of Nanjinguan Formation, the middle of the Second Member of Fenxiang Formation and the bottom of the First Member of Honghuayuan Formation. In order to find out the controlling factors of Lower Ordovician stromatolites development in the study area, the analysis data of carbon, oxygen isotope samples, macro element samples as well as their variation characteristics are studied and then combined them with previous research results, which is eventually beneficial for reaching the conclusion: sea level change is one of the main factors controlling the formation of stromatolites. There are five sea level change cycles in the Early Ordovician period. The analysis of sea level change and accommodation space variation characteristics show that when the growth rate of the accommodation space is approximately equal to that of carbonate, the circulation of seawater is well, and the amount of light and oxygen is sufficient, so that the cyanobacteria organism can be multiplied in large quantities. The growth and development provide favorable conditions, and favorable conditions are provided for the growth and development of stromatolites; The peak changes in CaO content and CaO/MgO ratio indicate that the formation period of stromatolites is arid climate environment with high water temperature and large evaporation. Prokaryote reproduction is suitable for the environments with high seawater cleanliness. When the amount of land-based debris injected into the sea increases, the turbidity of water will lead to a large number of deaths of algae microorganisms that form stromatolites, and the stromatolitic microbial mats cannot be preserved. In addition, the growth and predation of macro-organisms play a restrictive role in the development of stromatolite-forming micro-organisms blue-green algae. According to the actual situation of the development of the laminated rocks in the study area, what is the dominant function in the formation and decline of stromatolites between the evolution of metazoans and changing environments is discussed in the study.