Lithofacies Characteristics And Sedimentary Evolution Of The Lower Cambrian Niutitang Shale In The Upper Yangtze Platform

The lower Cambrian Niutitang shale in the upper Yangtze platform was studied in this paper.The major research contents include shale lithofacies types,lithofacies combination patterns,vertical evolution sequences and depositional models.On the basis of the mineral composition,particle size,organic matter content and paleobiology,the Niutitang shale was divided into six types,including silt-containing siliceous shale,silty siliceous shale,silt-containing argillaceous shale,silt-containing calcareous argillaceous shale,silty mixed shale,and silty calcareous mixed shale.On the basis of the sedimentary characteristics,depositional environment,depositional mechanisms of these lithofacies,five lithofacies associations were identified.The associations of silt-containing siliceous shale and silty siliceous shale,silt-containing siliceous shale and silt-containing argillaceous shale and argillaceous siltstone were developed in restricted basin.The association of silt-containing siliceous shale and silty mixed shale was developed in shelf margin.The associations of silty mixed shale and silty calcareous mixed shale,silty siliceous shale and silty mixed shale were developed in intra-shelf low.The vertical and horizontal evolution patterns of the Niutitang shale were summarized.The Niutitang shale was present a prograding sequence,and included three sub-cycles.From the bottom to top,organic matter content decreased and oxic condition strengthened with the water shallowing.In the basin facies,the shale lithofacies was transitioned from siliceous shale to argillaceous shale,and had high content of Mo element,indicating hot water condition existed in the basin.In the shelf facies,the shale lithofacies was transitioned from argillaceous shale to mixed shale,and the Mo element content changed with the variation of TOC content.From shelf to basin,the lithofacies varied from mixed shale to argillaceous shale to siliceous shale,the terrigenous particle sizes became smaller,the TOC contend gradually increased,and the anoxia became stronger.The sedimentary model of the Niutitang shale includes two parts: detention model in the intra shelf low,hot water and anoxia model in the basin.The major controlling factors for the development of Niutitang shale are tectonic movement,paleogeomorphology,sea level variation and source matter type.