Abstract:

On the basis of comprehensive analysis of outcrops, cores, thin-section, mineral X-ray diffraction, and drilling material, the lithfacies and sedimentary processes for the Lower Cambrian Niutitang shale in the upper Yangtze Platform are identified. Furthermore, the depositional conditions and sedimentary evolutional patterns for this shale are also analyzed. According to organic matter content, mineral composition, mineral texture and rock fabric, six lithofacies are identified, including organic-rich siliceous shale, silty siliceous shale, argillaceous shale, argillaceous calcareous shale, silty calcareous shale and silty mixed shale. These lithofacies are mainly deposited in the ways of clastic mechanical deposition, carbonate chemical deposition, silica biological deposition, argillaceous flocculation deposition, and bottom current rework deposition. Trace elements analysis reveals that the predominated redox conditions for the Niutitang shale deposition are anoxic to dysoxic environments. The degree of anoxia gradually decreases with the sea level fall upwards. During the lower member shale depositional period, the basinal environment is occupied by siliceous shale, and the shelf region is occupied by calcareous shale. During the upper member shale depositional period, the basinal environment is covered by argillaceous shale, and the shelf region is covered by silty mixed shale. The Niutitang shale generally shows a shallowing-upward prograding depositional sequence. From bottom to top, the organic matter content and anoxic degree decreases largely with the sea level fall. The average content of quartz decreases, but the average content of clay mineral increases.

Key words: marine shale, lithofacies type, sedimentary environment, Niutitang Formation, Upper YangtzePlatform