Study On Contourites Of Southern Middle Yangtze Platform In Lower Ordovician

Deep-water contourites research has lasted for 50 years, though some achievements have been made, research on formation mechanism of contourites and sedimentary environment is less. In the Early Ordovician, the sedimentary pattern of the southern Middle Yangtze Platform is stable. And the zoning of sedimentary facies is obvious, which are divided into shallow-water carbonate platform, deep-water slope and deep sea basin from northwest to southeast. Thus the study area belongs to the mature environment in the marginal zone of South China Continent. Not only deep-water gravity flow deposits but also contour current deposits are developed in the area. The study on contourites of the Lower Ordovician in southern Middle Yangtze Platform enriches and promotes contour current deposits study. The study has great significance to contourites formation mechanism, and the re-recognition of the ancient geography, climate and tectonics when the contourites formed. Additionally, the study is significant for prospecting sedimentary mineral resources, especially oil and gas.The article explores the circulation characteristics forming contourites, and the differences of environment forming various types of contourites in research area, based on the measured field section, as well as the detailed description and analyses of carbon and oxygen isotope characteristics. Based on field geological work, with the guidance of Modern Sedimentology theory and contourite-related theory in Oceanography, and by using the method including indoor macro and micro study, routine analysis and advanced tests, the contourite drifts in the study area are focused. The types, characteristics, sedimentary sequences and horizontal and vertical distribution rules of the contourites are studied deeply. And by analyzing the test result of the contourites geochemical indicators characteristics, the formation mechanism of the contourites, the paleotopography, paleoclimate, palaeosalinity and other environmental conditions are studied.By measuring and researching the section in detail, we discover that the Lower Ordovician Panjiazui Formation in south of Mid-Yangtze Platform is composed of deep-water carbonate rocks. The rocks are mainly gray micrites, calcisihitic, calcarenitic and bioclastic limestones. Three types of sediments are developed, namely, vertical landing sediments, gravity flow sediments and contourites. According to lithological characters, we divide the contourites into five types. They are respectively calcilutitic contourite, calcisihitic contourite, calcarenitic contourite, fine calcisiltitic contourite and bioclastic contourite. Among them, calcilutitic contourite is well developed, followed by calcarenitic contourite and bioclastic contourite. We also summarize the development characteristics, Carbon and Oxygen isotope characteristics and the vertical sequence. Our study show that there are five types of incomplete contourite sequences in the study area, namely, a single calcilutitic contourite sequence, a single bioclastic contourite sequence, a single calcarenitic contourite sequence, a calcilutitic calcarenitic contourite sequence, and a bioclastic calcarenitic contourite sequence. In addition, there are very few intact contourite sequences in the study area. In the section, the Panjiazui Formation is divided into three parts according to the type and extent of development. From Part Ⅰ to Part Ⅲ, the proportion of contourite thickness tends to increase. Considering the regional sedimentary background, we think that on the whole, the contour current strength of this area varied from weak to strong during the sedimentary period of Early Ordovician Panjiazui Formation.In order to study the ancient environment forming contourites, we sampled different types of contourites and sediment in situ in research area systematacially for carbon and oxygen isotope analyses. Results show that, in Panjiaju Formation, the Lower Ordovician,δ13C values of contourites are between -0.83‰ and 2.31‰,δ18O values are between -10.93‰ and -9.54%o, Z values are between 120.82 and 127.14, T values are between 16.98 and 24.81℃. Compared with autochthonous deposits,δ13C、Z and T values of contourites are higher. Besides, with increase in particle sizes of contourites, values of δ13C、Z and T rise gradually. It shows that the sedimentary environment of contourites is totally different from that of deposits in situ. Contourites mainly form in relatively deep water environment with high salinity. As the salinity and temperature increase, high-energy contourites, such as caleilutitie contourite and bioclastic contourite, are more likely to form. The formation cycles of contourite sequences show that the ancient salinity, water temperature, and sea level firstly increase and then decrease.