A new type of mixed depositional system: Mixed turbidite‐contourite fans created by the interplay of concordant turbidity‐bottom currents

ABSTRACT We record mixed turbidite‐contourite fans using integrated seismic, borehole, core, zircon U‐Pb geochronological, micropaleontological and petrophysical data. Mixed turbidite‐contourite fans in the Middle Miocene Ledong and Lingshui sags of the Qiongdongnan Basin are markedly different from conventional turbidite fans because of their along‐slope elongation, along‐slope progradation and remarkably high porosity and permeability. Mixed turbidite‐contourite fans are composed of current‐reworked turbidites with good reservoir quality and are closely associated with furrow alignments. They are ascribed to interactions between heterogeneous turbidity‐bottom currents but are markedly different from other well‐documented cases of interplay between turbidity and bottom currents, as these two processes share the same flow direction and pathway. Bottom (contour) currents flowing from SW to NE and involved in the construction of mixed turbidite‐contourite fans are inferred from the occurrence of discrete beds of very fine sand with traction structures and bioturbation, an abundance of planktonic foraminifera, as well as benthic foraminifera indicative of clockwise South China Sea Intermediate Water ( Uvigerina spp. and Pullenia bulloides ). In addition, gravity (turbidity) flows directed from SW to NE and involved in the construction of mixed turbidite‐contourite fans (with a similar flow direction to the bottom currents) are supported by the occurrence of diagnostic zircon U‐Pb age ranges for grain populations derived from Vietnam drainage catchments located to the west of the study area in the Qiongdongnan Basin. Fine‐grained components were likely winnowed away by the persistent action of bottom currents, thereby improving grain‐size sorting and giving rise to excellent reservoirs with high porosity and permeability. To the best of our knowledge, the newly recognised mixed turbidite‐contourite fans represent a previously undocumented system of interaction between concordant turbidity and bottom currents in this specific configuration, creating a new type of deep‐marine reservoir with potential application to many other continental margins and contributing to an improved understanding of deepwater sedimentology and stratigraphy.