THE DISCOVERY OF THE RED RIVER SUBMARINE FAN, NORTHWEST SOUTH CHINA SEA: IMPLICATIONS FOR THE EVOLUTION OF THE TIBETAN PLATEAU AND THE RED RIVER SYSTEM

Based on an integrated analysis of core, well log, and particularly regional seismic data, a large Late Miocene (the Huangliu Stage) submarine fan is discovered at the boundary between the Yinggehai Basin and the Qiongdongnan Basin, northwest South China Sea.

On seismic, the submarine fan show typical oblique, progradational reflection configuration along depositional dip and bilateral downlapping geometry along strike. It covers an area of over 10000km² and has a maximum thickness of about 2000m.

The facies of the fan is verified by 180m continuous core from the YC35-1-2 well. Detailed core description, coupled with analysis of well logs, show that deposits in the fan overall fine upwards and vertically consist of eight 40-160m thick fining-upward cycles. In each of these cycles, sandy sediment gravity flow deposits in the lower part grade upward into muddy hemipelagic facies. Sandstones in the lower part typically are pebbly fine- to coarse-grained feldspathic quartz arenite (quartz 59%, feldspar 10.9%, and lithics 7.1%). Depositional elements identified in the gravity flow deposits include channel, levee-overbank, and proximal lobe. The hemipelagic facies in the upper part of the cycles are dominated by dark mudstones, with very fine-grained thin bedded sandstones. Mudstones are massive with high gamma, density, and low resistivity. The thin bedded sandstones commonly show convoluted bedding, ripple-cross lamination, and graded bedding, interpreted as distal lobe deposits.

Regional seismic profiles (over 250km long) from the northwest of the Yinggehai Basin to the west of the Qiongdongnan Basin shows that the Red River Delta and the submarine fan are genetically related, and the fan, thus, is name as the Red River submarine fan. The large Red River delta-fan system is deposited during 10.5~5.5Ma. This, coupled with preliminary provenance analysis, suggest that sediments for the system are most probably shed from the Tibetan Plateau which underwent rapid uplift in between 13~9Ma. Around 5.5Ma, the Ailao-Red River No. 1 Fault turned into dextral, resulting in the eastward shift of the flowing directions of the Red River tributaries. This dramatically reduced sediments transported into the Yinggehai Basin and the Red River delta, which eventually shot down the growth of the Red River submarine fan.