Sedimentology and stratigraphic evolution of the fan delta at the Badaowan formation (lower Jurassic), Southern Junggar Basin, Northwest China

Abstract

A detailed study of sedimentology and stratigraphic evolution of the fan delta at the Sikeshu section in the southern edge of the Junggar basin was conducted. The fan delta of the Badaowan formation is exposed continuously along the cross section, which shows the stratigraphic and geographic distribution of channel complexes, channel conglomerates and coarse sandstones; flood plain mudstones and thin coal seams, and lacustrine mudstones. Eventually, a fan delta sedimentary model has been established based on the reconstruction of sedimentary characteristics. Based on dissection of the section, channel sand bodies can be divided into three types: amalgamated, transitional, and isolated, which correspond to the origin of braided channels, braided channels transforming into distributary channels (transformation part), and distributary channels respectively. The width (W), thickness (T), and width/thickness ratio (W/T) dimensions of the three types of channel sand bodies were measured in the field. The width, thickness, and width/thickness ratio gradually decrease from amalgamated to isolated sand bodies. The characteristics, sedimentology genesis, and variation of sand bodies are controlled by the variation of base-level cycles and their relevant position. The dimensions of channel sand bodies are proportionate to accommodation/sediment supply (A/S) ratio inversely, which the dimensions decrease with the increase in the A/S ratio. Moreover, the proportion, thickness, connectivity, grain size, and dimensions of channel sand bodies decrease in accompany with the rise of base level.