Characteristics of Late Permian Deep Water Sedimentary Environments: A Case Study of Shaiwa Section, Ziyun County, Guizhou Province, Southwestern China

Sediments of carbonate gravity flows and terrigenous debris turbidites, and normal bathyal deposits were found at the Shaiwa Section, Ziyun County, Guizhou Province, southwestern China. Through grain size analysis of some typical sediments at this section, the changing patterns of the grain parameters and the grain size cumulations were recovered. Results show that the study area was deposited under turbidite control during the Late Permian period, which we also recognized at the outcrop section upon sedimentary characteristics of the sediments. In addition, fossils are abundant in the Upper Permian of the Shaiwa Section, including radiolarians, sponge spicules, bivalves, brachiopods, ammonoids and trace fossils. Radiolarians and siliceous sponge spicules are typical deep water assemblages. Bivalves are dominated by genera of Hunanopecten and Claraia , both showing deep water living characteristics. Ammonoids are composed of planktonic types, showing characteristics of smooth and flat shells. Brachiopods are dominated by a small and thin shelled assemblage, which are commonly flat in shape and usually of slight ornamentations on shells. In addition, trace fossils found at the Shaiwa Section are also common types of deep water facies. Thus, the fossil evidence of the Shaiwa Section also suggests a deep water environment, possibly from the bathyal slope to the basin margin facies, of the studied area during the Late Permian period.

Coupling Relationship between Shelf-Edge Trajectories and Slope Morphology and Its Implications for Deep-Water Oil and Gas Exploration: A Case Study from the Passive Continental Margin, East Africa

Both the shelf-edge trajectories and slope morphology are indicative of deep-water sedimentation, but previous studies are relatively independent from each other in the two dimensions. An integrated investigation can enhance the understanding of deep-water sedimentary systems and enrich reservoir prediction methods. Based on the bathymetry data and seismic data published, this study identified ten slope areas at the continental margin of East Africa and classified the clinoforms into three types: concave-up, sigmoidal and planar. Combined with the distribution of main modern rivers in East Africa, nine modern source-to-sink systems were identified and the catchment area is positively correlated with the size of the shelf-edge delta. It is found that the slope morphology of East Africa is closely related to the geological setting, sediment supply and sediment transport pathway in submarine canyon of passive continental margin. When the sediment supply is stable, the concave-up slopes are dominated by the river-associated and shelf-incising canyons and the sigmoidal slopes are determined by the headless canyons. There exists a strong coupling relationship between the shelf-edge trajectories and slope morphology. In general, concave-up slopes correspond to descending trend, flat and low-angle ascending trend shelf-edge trajectories and high-quality reservoirs developed on the basin floor under the influence of river-associated and shelf-incising canyons which have bright prospects for oil and gas exploration. Additionally, sigmoidal slopes usually correspond to descending trend, flat and low-angle ascending trend shelf-edge trajectories at times of relative sea-level fall and the reservoirs mostly developed on the upper slope under the influence of headless canyons. Moreover, the planar slopes correspond to high-angle ascending trend trajectories which are hardly potential for exploration. The coupling model built in this study will provide an insight for oil and gas exploration in deep-water areas with limited data and low exploration degree.

A unique subseafloor microbiosphere in the Mariana Trench driven by episodic sedimentation

Hadal trenches are characterized by enhanced and infrequent high-rate episodic sedimentation events that likely introduce not only labile organic carbon and key nutrients but also new microbes that significantly alter the subseafloor microbiosphere.Currently,the role of high-rate episodic sedimentation in controlling the composition of the hadal subseafloor microbiosphere is unknown.Here,analyses of carbon isotope composition in a~750 cm long sediment core from the Challenger Deep revealed noncontinuous deposition,with anomalous ^(14)C ages likely caused by seismically driven mass transport and the funneling effect of trench geomorphology.Microbial community composition and diverse enzyme activities in the upper~27 cm differed from those at lower depths,probably due to sudden sediment deposition and differences in redox condition and organic matter availability.At lower depths,microbial population numbers,and composition remained relatively constant,except at some discrete depths with altered enzyme activity and microbial phyla abundance,possibly due to additional sudden sedimentation events of different magnitude.Evidence is provided of a unique role for high-rate episodic sedimentation events in controlling the subsurface microbiosphere in Earth’s deepest ocean floor and highlight the need to perform thorough analysis over a large depth range to characterize hadal benthic populations.Such depositional processes are likely crucial in shaping deep-water geochemical environments and thereby the deep subseafloor biosphere.