Sedimentology And Geochemistry Of The Early Cambrian Black Rock Series In The Hunan-Guizhou Area, China

The Paleozoic is marked by the occurrence of anomalously organic-carbon-rich laminated shales in large parts of South China and in the world as well. These sediments, enriched in the precious metal elements such as Mo, Ni, PGE, V, Au, Ag and so on, are distributed in widespread strata including the Niutitang Formation of the Lower Cambrian, Wufeng Formation of the Ordovician, Longmaxi Formation of the Silurian, Devonian and Permian. Especially, the black shales of the lower part of Niutitang Formation from the Hunan-Guizhou area were regarded as instantaneous and catastrophic deposition, coincident with the global Oceanic Anoxic Event (OAE). Therefore, this study on the black rock series possesses important theoretic significance and potential economic implication. Two typical sections of Huangjiawan (Guizhou Province) and Ganziping (Hunan Province) have been previously studied on litho- and biostratigraphy, elemental geochemistry, but the temporal and spatial relationships between the Huangjiawan shallow sedimentary systems tract and the Ganziping deep black shales sedimentary systems tract have not been reached unified views. Particularly, the records in organic geochemistry of the Early Cambrian (Tommotian) black shales in South China have received less attention. Based on above-mentioned circumstance, this paper chooses the Early Cambrian black shales in the Hunan-Guizhou area as studying object. Focusing on the sedimentology and geochemistry, especially the organic geochemistry, the present study discusses the genesis of Niutitang Formation black shales and Tommotian oceanic anoxic event, and draws the following conclusions.(1) On the basis of field works in the Hunan-Guizhou area, combining previous achievements, the Early Cambrian stratigraphical systems are built from lower to upper as follows: the Niutitang Formation, Mingxinsi Formation, Jindingshan Formation and Qingxudong Formation with mass occurrences of arthropods, sponges, trilobitas, bacterial colonies, etc. corresponding to Meishuchunian, Qiongzusian, Langcangpuian and Longwangmian, respectively. Compared with the contemporaneous deposits occurred in Siberian, the Early Cambrian in this area corresponds to Tommotian, Atdabanian, Botonian and Toyonian, respectively.(2) Rock types of the Early Cambrian strata consist of carbonates, argillaceous rocks and clastic rocks. The argillaceous rocks, siltstones and sandstones were devoloped in the lower-middle Lower Cambrian with a few intercalated limestones horizons, while the limestones and/or dolomites in the Qingxudong Formation of the upper Lower Cambrian. The Niutitang Formation important black rock series of the Lower Cambrian, are composed of black shales, stone coal, siliceous shales, silicalites, siltstones, phosphorites, baritic rock. Their mineral compositions are mainly quartz, illite and pyrite with subordinate barite, apatite and calcite. According to the bio-, litho- and chronostratigraphy, the synchronous black shale facies also occurred in South and West Asian, European, North Amercian and so on.(3)The Lower Cambrian sedimentary facies in the study area were characterized by clastic...