194 / 2022-02-28 20:08:58

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

Unidirectionally migrating channels; Contour bottoms; Turbidity flows; Channel-levee complexes; Southern Qiongdongnan basin

In the last decade, the discovery of unidirectionally downstream-migrating turbidite channels (UDMTCs) has aroused the interest of deepwater geologists due to giant gas fields explored in UDMTCs. However, their architecture, controlling factors and evolution history are yet unclear enough. Here, 3D seismic data from Beijiao sag of southern Qiongdongnan basin (QDNB) are used to document previously undescribed characteristics of seven UDMTCs in the lower middle Miocene. The results show that (1) UDMTCs are 5-10 km in length and 0.5 km in width and gradually terminate downslope, whose scales are far less than normal (large-scale) UDMTCs elsewhere; (2) stacking patterns of UDMTCs are compose of multiple channel-levee complexes, whose seismic reflections record unilateral transition from high to low amplitudes responding from channel axis to off-axis positions; (3) Steep and gentle flanks of herein UDMTCs are conversely corresponding to gentle and steep flanks of UDMTCs elsewhere respectively; (4) Dip gradients (angles) of trajectories of UDMTCs are variable alongslope and downslope; (5) These perpendicular-slope channels (perpendicular to the strike of the slope) are characterized by cross-sectional unidirectional migrations, and yet they gradually bend downslope and eventually strike approximately along the strike of the local slope due to the gentle slope, where the parallel-slope channels seismically express vertical aggradation (rather than unidirectional migration) of stacking pattern.

Differing from terrigenous siliceous lithology of large-scale UDMTCs elsewhere, it is inferred that the lithology of herein (small-scale) UDMTCs are 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. Dip gradients are for the first time proposed as an optical parameter for appraising the intensity change of contour currents during the interaction of bottom currents and turbidity flows, in agreement with the previous tank-scale experiment. Differential compaction induces the steep and gentle flanks of herein UDMTCs to be converse to the gentle and steep flanks of UDMTCs elsewhere respectively. Our studies could provide some clarity on the theme of UDMTCs.