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

Abstract

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.Unlike terrigenous siliceous lithology of large-scale UUMTCs elsewhere, it is inferred that the lithology of herein (small-scale) UUMTCs is transitional from coarse-grained (carbonate sandstones) debris in the channel axis to fine-grained debris in the channel off-axis positions. Their controlling factors are summarized: carbonate sediment supply, relative sea level fluctuation, alongslope contour currents and local slope configuration. The orientation between turbidity flows and contour currents, controlled by local slope configuration, plays a vital role in the stacking pattern of channels, which are characterized by unidirectional migration of channels perpendicular to the strike of slope and are featured by vertical aggradation of channels parallel to the slope. Dip gradients are for the first time proposed as an optimal parameter for appraising the intensity change of contour currents crossing channels during their interaction, in agreement with the previous flume-tank experiments. Our study could provide some clarity on the formation and evolution of UUMTCs.