Sedimentary Characteristics of Lower Ordovician Stromatolites in Songzi Area, Hubei Province

Key words: 
• stromatolites /
• clotted-shaped stromatolites /
• sedimentary characteristics /
• metazoon radiation /
• Lower Ordovician /
• Songzi area

Abstract: The characteristics of Lower Ordovician stromatolites in the Songzi area of Hubei province were studied systematically for the first time, and the stromatolites were distributed in the Lower Ordovician Nanjinguan, Fenxiang and Honghuayuan formations in the study area. They were characterized by decrease of scales, thickness and quantity gradually on the vertical profiles. The stromatolite types were changed laterally from the horizontal bedded, wavy-shaped and gradually into columnar type. According to the geometric classification scheme, the stromatolites in the study area could be divided into the following types, namely, lamellar-shaped, wavy-shaped, columnar-shaped, mound-shaped and clotted-shaped stromatolite, respectively. Among them, the clotted-shaped stromatolite and double cone-shaped stromatolite were discovered for the first time and newly named by the authors. Based on detailed analysis for the sedimentary characteristics of each type of stromatolite, combined with lithology, microphase and microscopic characteristics, the depositional environments were studied for different types of stromatolite, and their sedimentary models were also summarized. According to the corresponding relationship between the increase of metazoon in the early stage of Middle Ordovician period and the rapid reduction of the stromatolites, combined with the phenomenon of a lot of drilling and predatory gastropods Ecculiomphalus fossils inside the stromatolites, the authors believed that the reduction and decline of stromatolites could be related to the increase in abundance of metazoan. Drilling predators not only grazed bacteria algae microorganisms forming stromatolites and resulted in a decrease of stromatolites, but also destroyed the growth of stromatolites, and lead to the formation of clotted-shaped stromatolite. In addition, the author argues that the rapid rise of the sea level could be also one of the reasons for the reduction and disappearance of stromatolites from the beginning of the Middle Ordovician period in the study area.