Architecture, controlling factors and evolution history of unidirectionally upstream-migrating turbidite channels: A case study from southern Qiongdongnan Basin, northern South China Sea

In the last decade, the discovery of unidirectionally upstream-migrating turbidite channels (UUMTCs) has aroused the interest of deepwater geologists due to giant gas fields explored in UUMTCs. However, their architecture, controlling factors and evolution history are unclear enough. Here, 3D seismic data from the Beijiao sag of the southern Qiongdongnan basin (QDNB) are used to document previously undescribed characteristics of seven UUMTCs in the lower middle Miocene. The results show that (1) UUMTCs are 5–10 km in length and 0.5 km in width and gradually terminate downslope, whose scales are far smaller than normal (relatively large-scale) UDMTCs elsewhere; (2) Stacking patterns of UUMTCs are characterized by unidirectional migration of multiple channel-levee complexes, whose seismic reflections record unilateral transition from high to low amplitudes from channel axis to off-axis positions; (3) Dip gradients (angles) of trajectories of UUMTCs are variable in the alongslope and downslope cross sections; (4) Polygonal faults are absent in the channel axes of UUMTCs and occur in the channel off-axis positions; (5) Channels (C1∼7) perpendicular to the slope are characterized by unidirectional migrations in the relatively steep slope, in contrast coeval channels (C8, 9) approximately parallel to the slope are featured by vertical aggradation in the gentle slope.