New-type pliocene channel depositional systems resulting from turbidity flows obliquely interacting with contour currents: A novel case study from the southern Qiongdongnan basin, northern South China Sea

Unlike well-known unidirectionally upstream and downstream migrating channels resulting from turbidity flows perpendicularly interacting with contour currents, the channel depositional system (CDS) resulting from turbidity flows obliquely (not perpendicularly) interacting with contour currents is still in an infancy. 3D seismic data from the southern Qiongdongnan basin (QDNB) are used to document previously undescribed novel characteristics of the channel depositional systems in the upper Pliocene. The novel results show that (1) The CDSs are morphologically divided into the early and late CDSs, and they are not perpendicular (～90°) but oblique (～20°) to the strike of the northern slope of Xisha uplift; (2) Architectural elements of the early CDS are identified: narrow “V”-shaped erosional surface, early channel filled by (sand-rich) turbidite with vertically aggradational high amplitudes, large-scale flake-shaped high-moderate amplitude overbank deposits only on the northwestern side of the channel, and several (four) small-scale fan-shaped high-amplitude overbank deposits only on the southeastern sides of the bends of the channel; (3) Late CDS is subdivided into broad “U”-shaped erosional surface, late channel filled by mass transport deposits (MTDs) with chaotic reflection, and large-scale flake-shaped high-amplitude overbank deposits only on the northwestern side. The “only” distributions are novel and considered to be a result of the advection of upper turbidity flows affected by contour currents. Their controlling factors and depositional and flow process are discussed in detail. The oblique (～20°) orientation of oblique-slope turbidity flows and along-slope contour currents plays an important role in building the architecture of CDSs, expressing vertically aggradational channels rather than unidirectionally migrating channels. The CDCs are a new type, one of the channel systems resulting from the synchronous (oblique) interaction of turbidity flows and contour currents. The CDSs probably holding favorable reservoirs for natural gas hydrate should be emphasized and their four fan-shaped overbank deposits may be filled by upper prolific natural gas hydrate and lower free gas. This study can enrich the knowledge repository for the interaction of turbidity flows and contour currents.