The Research Of Sedimentary Environment And Its Influence On Biological Recovery Of Lower Triassic Helongshan Formation In Northern Chaohu,Anhui Province

Early Triassic is a important period of the transformation from Paleozoic to Mesozoic ecosystem after the end-permian mass extinctions.Therefore, the research of the early Triassic environment and its change is of a great significance for studying biological recovery after the mass extinction.Lower Triassic Helongshan Formation with a thickness of 21.34 meters is exposed clearly in Majiashan north slope, northern Chaohu of Anhui Province. It is the important area of researching early Triassic strata and its sedimentary environment.In this area, Helongshan Formation is mainly composed of nodular limestones, argillaceous limestone and calcareous mudstone, containing rich ammonoids, bivalvia, gastropods, conodonts,ostracods and a lot of calcispheres. The distribution of pyrite in the strata is relatively common. According to the lithology and paleontological characteristics it is concluded that this Formation was deposit in the shallow limited epicontinental sea environment with a low-energy.The observation, statistics and analysis of calcispheres from Helongshan Formation show that algae, represented by calcispheres, were gradually prosperous,and then provided the basic condition of recovery for the bivalvia, gastropods, ostracods and other benthic animals during the sedimentary period of this Formation. It suggests that,in the sedimentary period of the Helongshan Formation, the Marine environment was gradually improved and biological recovery was proceed slowly.The observation and analysis of the pyrites from the sequences of Helongshan Formation indicate that the Formation in north region of Chaohu was deposited in the weak hydrodynamic and anoxic environment. In this environment, the biological recovery slowly. In the late depositional period of Helongshan Formation,the sea water was lack of oxygen and rich of sulphur, which led to the biological recovery falter and delay of the process of biological recovery.Geochemical analysis results of Helongshan Formation manifest that in the north of Chaohu, during the early and middle Helongshan depositional period the marine environment was gradually improved, and until the late period of the Formation, some drastic changes happened, such as dry climate, water temperature rise, hypoxic conditions, which would hinder the process of biological recovery.