Sedimentary environment and petrological features of organic-rich fine sediments in shallow water overlapping deposits: A case study of Cambrian Yuertus Formation in northwestern Tarim Basin, NW China

Taking the Cambrian Yuertus Formation outcrop profiles in the Aksu-Keping-Wushi areas of northwestern Tarim Basin as examples, the depositional environments of organic rich fine sediment were analyzed by examining the outcrop profiles macroscopically and microscopically. The study reveals that: (1) The lower part of the Yuertus Formation consists of organic-rich fine sediment or thin rhythmic interbeds of organic-rich fine sediment and siliceous sediment, the formation transforms to terrigenous diamictic grain shoal and inverse grading carbonate rocks upward. (2) The thin limestone interbedded with dark shale rhythmically has inverse grading. (3) The thin-bedded siliceous rock has metasomatic residual granular texture, stromatolithic structure and cementation fabric in vugs. (4) There are iron crust layers at the top of the shallowing diamictic grain shoal, beneath which exposed karst signs, such as karrens, dissolved fissures, sack-like vugs, near surface karst (plastic) breccia, breccia inside the karst system and terrigenous elastic fillings, can be seen. (5) Both the outcrops and seismic profiles show that organic-rich fine sediments above the unconformities or exposed surfaces are characterized by overlapping. The organic-rich fine sediment of the Cambrian Yuertus Formation was deposited in the anoxic-suboxidized restricted gulf lagoon environment, and its formation was controlled by high paleoproductivity and poor oxygen exchange jointly, then a shallow-water overlapping sedimentary model has been established. The results will help enrich and improve the sedimentary theory of organic-rich fine sediments.