黔北下寒武统黑色岩系的沉积环境与地球化学响应

下寒武统黑色岩系在我国华北、华南及塔里木盆地等广泛发育,但富有机质泥页岩的分布特征及其控制因素未有定论。本文对黔北地区下寒武统牛蹄塘组(及同时期地层)不同沉积相带典型剖面中的富有机质泥页岩及其上下层位进行了地球化学分析,识别出寒武纪早期不同沉积相带氧化还原环境的明显差异。台内凹陷相主要岩性为黑色页岩,底部为不等厚的薄层磷块岩和硅质岩或硅磷质结核,沉积的黑色页岩厚度大,有机质丰度高,V、Ni、Mo、U等微量元素显著富集,反映了贫氧-缺氧环境,以及短暂动态的硫化环境,Mo/TOC反映了其为局限盆地特征;上斜坡相为贫氧到氧化的沉积环境,但更多地是表现为氧化的沉积环境,其中牛蹄塘期存在局部的硫化环境,沉积的黑色页岩厚度小,但有机质丰度最高,V、Ni、Mo、U等微量元素的富集程度较低;盆地相为黑色硅质岩与富有机质的黑色硅质页岩间互沉积,同期沉积厚度较小,有机质丰度相对较低,V和U的富集程度大于台内凹陷相,而Ni和Mo的富集程度则低于台内凹陷相,指示了缺氧环境,期间伴随有贫氧和短暂的氧化环境存在,且可能存在局部的硫化环境,Mo/TOC反映了中等的局限程度。总的来说,黔北下寒武统黑色岩系的有机质丰度、厚度以及微量元素富集程度等特征主要受控于不同的氧化还原环境。

The relationship between environment and geochemical characteristics of black rock series of Lower Cambrian in northern Guizhou

The lower Cambrian black rock series are widely distributed in North China, South China, and Tarim basin, but the controlling factors and distribution characteristics of organic- rich mudstone and shale are still being debated. In this study, we collected samples of the Niutitang Formation (or its time- equivalent) from eight typical profiles of different sedimentary environments and carried out integrated geochemical analyses. The results suggest that different sedimentary environments are characterized by different redox conditions during the early Cambrian. Sediment lithologies of the inner platform are mainly black shales with some thinly bedded phosphatic rocks and cherts or phosphate/chert nodules. The black shales are thick and have high abundance of organic matter. Trace elements (V, Ni, Mo and U) of black shale from the inner platform facies are significantly enriched, which indicate sulfidic environment. Mo/TOC reflects a restricted water column. The upper slope settings may have developed under sulfidic- suboxic dynamic water masses. The thickness of black shales is small, but organic matter abundance is the most high. Trace elements (V, Ni, Mo and U) enrichments are low and Mo/TOC data suggest a moderately restricted water column. Basin facies strata are composed of interbedded black chert and black siliceous shale, which are thin and have low organic matter abundance. The enrichment factors of V and U are higher than those in the inner platform, but for Ni and Mo, their EF values are relatively lower. The majority of the basinal strata were deposited in anoxic conditions; however, the fluctuation of Mo/TOC may reflect ephemeral upwelling activities. In conclusion, the thickness, organic matter abundance and enrichment of trace elements of the lower Cambrian black rock series in northern Guizhou are mainly controlled by the belts of different sedimentary environment and different redox states.