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Lacustrine beach ridges are often used to reconstruct past lake levels. However, the sedimentary architecture involved in their sedimentary evolution remains unclear, particularly for embayed beaches. To better understand the sedimentary processes, the sedimentary architecture and geomorphology of an embayed beach on the southern of Qinghai Lake were investigated using ground penetrating radar (GPR), trenches, and modern beach observation. The embayed beach's deposits are composed of three major sedimentary facies: washover deposition, fair-weather swash deposition, and lagoonal deposition. According to the variations in sedimentary facies, the formation of the embayed beach can be divided into two stages: lake-level highstand and lake-level lowstand. During lake-level highstand stage, geomorphic changes primarily occur in the cross-shore direction. A single beach ridge with washover lobes and sheets developed along the entire embayed beach. During lake-level lowstand stage, the sediments are mainly transported alongshore within the embayed beach. The northern end shows progradational complex deposits including beach ridge, lagoon and beach system, while only a beach ridge is present at the southern end. Hydrodynamic changes in two stages lead to the heterogeneity of sediment distribution, reflected in the morphology and the sedimentary architecture. The interaction of topography and hydrodynamics controls the formation and evolution of the embayed beach. This study improves the understanding of the sedimentary processes involved in embayed beaches. [ABSTRACT FROM AUTHOR]

In recent years, significant progress in shale gas exploration has been achieved in the Upper Ordovician (Wufeng Formation)-Lower Silurian (Longmaxi Formation) shales in the Upper Yangtze area, South China. Although many studies have been carried out on the Upper Ordovician-Lower Silurian shales, the controlling factors causing organic matter accumulation of these shales remain controversial. This study uses trace-element geochemistry and sedimentological methods to evaluate terrigenous input, redox conditions and primary productivity to explore the mechanisms of organic matter accumulation. The variation of terrigenous fraction elements (Al, Th and Sc) concentrations reflect a mixed influence of sea-level change and weathering. The sea-level of the Upper Yangtze Sea went through two cycles of transgression to regression during the Ordovician-Silurian transition. The Linxiang Formation, Kuanyinchiao Bed and the upper part of Longmaxi Formation developed during the periods of regression, whereas the Wufeng Formation and the lower part of the Longmaxi Formation developed during the periods of transgression. The paleo-productivity indexes of TOC content, ratios of Ba/Al and P/Al, and redox conditions proxies of Mo concentration, ratios of U/Th and V/Cr generally display similar variation patterns with respect to the sea-level changes. High TOC contents and Ba/Al and P/Al ratios indicate the paleo-productivity was high on the sea surface, as shown by relatively good positive correlations between Th vs. TOC, and Sc vs. TOC. This indicates that the paleo-productivity was controlled by the nutrients input through weathering. The good positive correlations between redox conditions indexes (U/Th and V/Cr ratios) with TOC content reflects reductive preservation conditions (anoxic to euxinic), thus implying they were an important controlling factor for organic matter accumulation. Nevertheless, redox conditions were closely associated with sea level change and organic matter decomposition. Therefore, the sea-level change and weathering were the primary controlling factors for organic matter enrichment across the Ordovician to Silurian transition. [ABSTRACT FROM AUTHOR]