![]() ![]() The black rock series of the Qiongzhusi Formation is widespread in the Sichuan basin and constitutes a set of hypoxic deposits with an extremely subtle phase transition. The research area is located in the eastern segment of the Meigu-Jinyang depression in the western Yangtze Block (approximately the Sichuan basin) ( Figure 1b). GGF-Guangzhou-Guiyang basement fault HHF-Hetai-Hepu fault (a) the lithofacies paleogeography of the Cambrian Qiongzhusi Formation in the Sichuan basin (b) ( Figure 1b modified after ). The boundary between South China and Indochina follows. Throughout the Paleozoic, the Yangtze Block was a stable passive continental margin that was subject to extensive platform-style marine sedimentation ( Figure 1b).ĭistributions of the Precambrian basement and tectonic divisions of South China. It is generally believed that the Yangtze Block is underlain by an Archean cratonic basement and that the block collided with the Cathaysia block during the Neoproterozoic, together forming part of the Rodinia supercontinent. The Yangtze Block in eastern China is separated from the North China Craton to the north by the Qinling–Dabie orogen, from the Cathaysia block to the southeast by the Jiangnan orogen, and from the Songpan–Ganzi complex to the west by the Longmenshan fault zone ( Figure 1a). Our studies provide basic data for understanding the sedimentary and tectonic evolution along the southwestern margin of the Yangtze Block during the early Cambrian, and thus they provide an essential basis for the comprehensive reconstruction of the early Cambrian sedimentary and tectonic environment of the Yangtze Block. In this paper, we present a systematic analysis on the geochemistry of the black rock series in the lower Cambrian Qiongzhusi Formation to understand the constraints on the provenance of sedimentary rocks and the sedimentary, ecological, and tectonic environment. However, some key issues regarding the early Cambrian marine sedimentary environment along the southwestern margin of the Yangtze Block still remain unclear for example, how was hot, phosphorus-rich water released from the seafloor along the southwestern margin of the early Cambrian Yangtze Block? How did this release affect the enrichment of elements? How did the upwelling current affect the environment? The widespread lower Cambrian black rock series in the Upper Yangtze region was deposited in a shallow-shelf setting and thus provides a very important window for documenting the early Cambrian sedimentary-tectonic-ecological environment of South China. In southwestern China, the Yangtze Block has large amounts of preserved sedimentary rocks that formed over several periods in different environments. Therefore, it is of great geological interest to study the sedimentary and climatic environment of black rock series with oil- and gas-rich beds. This diversification event produced considerable organic matter, leading to the formation of high-quality oil and gas deposits, and this organic matter was preserved in various sedimentary environments and paleoclimates at that time. The early Cambrian was a very important period of geological history because Ediacaran fauna disappeared at the end of the Neoproterozoic and an important biological diversification event called the Cambrian explosion occurred during this epoch, during which skeletal animals emerged. These findings provide an essential basis for the comprehensive reconstruction of the early Cambrian sedimentary environment of the Yangtze Block. Last, the reconstruction of the tectonic environment of the Qiongzhusi Formation indicates that deposition occurred in continental slope and marginal marine environments associated with a continental arc tectonic system. ![]() The combination of upwelling and bottom-water hydrothermal fluids led to environmental changes in the study area, from dry and hot to moist and warm. The reconstruction of the sedimentary environment and weathering intensity shows that P 2O 5 enrichment and water body stratification occurred due to the effects of upwelling ocean currents during the depositional period of the Qiongzhusi Formation. ![]() Furthermore, the strata were weakly influenced by submarine hydrothermal fluids during diagenesis. The basin primarily received continental clastic material with neutral-acidic igneous rocks from a stable source and with a moderate level of maturity during the depositional period of the Qiongzhusi Formation. In this paper, major, trace, and rare earth element data are presented in an attempt to reveal the sediment source during the deposition of the early Cambrian Qiongzhusi Formation and to reconstruct the sedimentary tectonic environment and weathering intensity during that time. The black rock series in the Qiongzhusi Formation contains important geochemical information about the early Cambrian tectonic and ecological environment of the southwestern Yangtze Block. ![]()
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