Reconstruction of paleoceanic redox conditions of the lower Cambrian Niutitang shales in northern Guizhou, Upper Yangtze region

摘要

The paleoceanic redox conditions and the vertical and spatial distribution of organic-matter-rich lower Cambrian Niutitang shales are varied in the Upper Yangtze region, China. An integrated approach, including organic geochemistry, major and trace elemental geochemistry, and X-ray diffraction (XRD) mineralogy, was used to characterize changes in redox conditions and to determine key controls on organic-matter accumulation during deposition of the lower Cambrian Niutitang shales. A 202-m-long cored well located in Kaiyang County in northern Guizhou, China, was thoroughly characterized by the integrated approach in this study. Vertically, total organic carbon (TOC-) rich siliceous and argillaceous black shales were developed in the Lower and Middle Niutitang Formation, and TOC-lean silty shales were found in the Upper Niutitang Formation. The TOC content in the Lower and Middle Niutitang shales ranges from 0.27% to 6.7%, with an average value of 2.1%, and it decreases from 0.30% to 0.11% in the Upper Niutitang Formation shales. The TOC-rich siliceous shales in the Lower Niutitang Formation were characterized by high total sulfur content, enrichment in redox-sensitive trace elements, high C-org:P ratios, and high authigenic Mo-U covariation, indicating they were deposited under persistent anoxic conditions. Most organic-rich shales from the Middle Niutitang Formation were deposited under anoxic conditions, whereas a few intervals having low TOC contents were deposited in a predominantly oxic depositional environment. In contrast, the Upper Niutitang Formation were deposited under fully oxic conditions. A combination of high primary productivity and enhanced organic preservation under euxinic water column conditions is the key control on organic-matter accumulation in the Lower Niutitang Formation shales. However, a coupling of redox depositional environment and terrestrial mineral dilution plays an important role in organic-matter accumulation and preservation in the Middle Niutitang Formation. Similar organic-matter abundance distribution and carbon isotope excursion of organic matter in the sediments deposited at different depositional facies on the Yangtze Platform suggest regional isotopically homogenous organic carbon input due to the widespread transgressive event with deep basinal waters being transported into the shallow shelf environments at the base of the lower Cambrian. As a result, delta C-13(org) distribution patterns can be used as a stratigraphic correlation marker in the Upper Yangtze region.