Hyperpycnal flows are river-derived extrabasinal turbidity currents transporting both sand and clay to lacustrine, coastal, shelf and deep-water sedimentary environments. In recent years, those extrabasinal turbidity current have become hot topic aera in lacustrine sedimentology research. Although experimental research has shown that presence of clay in turbidity currents promotes the development of transient turbulent flows with complex depositional properties, hyperpycnal flows were still interpreted as fully turbulent flows in most previous research. Relatively little is known about the mixed sand–mud bedforms produced by transient turbulent flows deposited in lacustrine hyperpycnal flow deposits. This study provides new interpretation of those mixed sandstone-mudstone bedforms observed in the lacustrine hyperpyncal flow deposits from the Nenjiang Formation, southern Songliao Basin, China. Lithofacies and lithofacies associations are recognized from the cores and interpreted as formed under both turbulent flows and transitional flows. Large current ripples and low-amplitude bed waves were interpreted as typical bedforms of transitional flows. Those interpretation about the mixed sand–mud bedforms could give new insights about the fluid dynamics and flow transformation of the hyperpycnal transporting both sand and clay to lacustrine environment. The mixed sand–mud bedforms can aid the understanding of the depositional processes and the facies distribution of hyperpynal flows in both modern and ancient lacustrine sediments.