Sublacustrine hyperpycnal channel-fan system in a large depression basin: A case study of Nen 1 Member, Cretaceous Nenjiang Formation in the Songliao Basin, NE China

Based on the integrated analysis of seismic, drilling and core data, a large channel-fan system of hyperpycnal flow origin was found in the Qijia-Gulong area of the Nen 1 Member of the Cretaceous Nenjiang Formation in the Songliao Basin. The hyperpycnal flow in this area, which originated from the edge of the basin and then passed the northern delta, formed a complete channel-fan system in the deepwater area. The channel-fan system comprises straight channels and meandering channels extending from north to south over a straight distance of more than 80 km with a width of 100-900 m, and distal fan lobes at the channel tip cover a maximum area of 20 km(2). This system, which is dominated by fine-grained deposits, contains massive sandstone, sedimentary structures of flow-water origin, and internal erosion surfaces; it also contains abundant continental organic clasts and exhibits evidence of bed-load and suspended-load transportation mechanisms. The hyperpycnite sequence contains pairs of coarsening-upward lower sequences and fining-upward upper sequences, reflecting the dynamic features of cycles in which floods first strengthen and then weaken. A new sedimentary model has been built for hyperpycnites in the Songliao Basin.