A sedimentary environment analysis of the first member of the Quemo Co Formation inWoruo Mountain area of the North Qiangtang Basin

The first member of the Quemo Co Formation in the Woruo Mountain area of the North Qiangtang Basin is mainly composed of purplish red pebbly sandstone, medium to fine sandstone, and argillaceous siltstone, deposited in an oxidation environment. The lithologic characters, sedimentary structures, probability cumulative grain size curves, Sahu formula, grain- size parameters scattergram, and C-M pattern of the sandstones of the first member of the Quemo Co Formation all show that the sandstones are of bad sorting, and the frequency histogram is bimodal or multimodal, the probability cumulative curve of the sandstones mostly represents two-stage and a few three-stage, the percentage of the traction populations is high, and the percentage of the saltation and suspension populations is relatively low. In the Sk-σi diagram, the sandstones of the first member of the Quemo Co Formation fall into the river sand area. Its C-M pattern has the typical feature of fluvial deposit, with the PQ section richly developed but the QR section poorly developed. The deposition of the Quemo Co Formation started with fluvial-alluvial facies, and then these strata were overlain by delta and restricted platform deposition, thus overall displaying a deepening-upward transgressive sequence.