关键词: 上扬子地区, 牛蹄塘组, 沉积环境, 热液流体, 有机质碳同位素, 金属元素

Abstract: Sampling of two quarry sections occurred at Zunyi, Guizhou, and Youyang, southeastern Chongqing. These samples were systematically collected from organic-rich, polymetal-enriched shales at the base of the Lower Cambrian Niutitang Formation. Analysis encompassed organic geochemistry, mineralogy, and major and trace element compositions. Despite being approximately 300 kilometers apart, both sections are situated within the Upper Yangtze Platform, facilitating comparative study. Vertical variations in TOC, δ¹³Corg, and major and trace element abundances suggest that these sections occupy distinct positions on the continental shelf slope. Compared to the Songlin section, samples from the Danquan section indicate proximity to the source area, characterized by higher clastic input fluxes. Although multiple depositional environment changes occurred within less than a 9-meter range from the Niutitang Formation’s base upwards in both sections, the depositional environments tended to converge with rising sea levels. The relationship between shale organic matter abundance (TOC), organic carbon isotopes (δ¹³Corg), and certain trace elements associated with hydrothermal activity (e.g., As, Ni, Sb) in both sections indicates different mechanisms for trace element enrichment. In the Songlin section, organic matter adsorption primarily drives trace element enrichment at the Niutitang Formation’s base, whereas in the Danquan section, mixing with codepositional hydrothermal fluids is the primary mechanism.

Key words: Upper Yangtze region, Niutitang Formation, sedimentary environment, hydrothermal fluids, δ¹³Corg, metal elements