Abstract: According to the research of tectonic setting, eustatic sea level change, depositional feature et al, two distinct types of marine fandelta facies in Yacheng Formation, Oligocene, Qiongdongnan Basin are identified. The fandelta facies, near mountains closely neighbor upon highland source area, are developed on the steeper slope at downthrow block of basinmargin fault. The sediments deposited in these fan deltas are frequently very coarse in the grain size and poor sorting. The pattern of grainsize probability plot mostly displays arcuate or oblique shapes. Graded and massive bedding resulted from gravity flow is common. These show that depositional hydrodynamics is dominated by the gravitycurrent action accompanied with tractional current action. The fandelta facies, near alluvial fan and characterized by being close neighboring upon subaerial alluvial fan, are usually developed on the relatively gentle slope at downthrow block of basinmargin fault. The sediments deposited in these fan deltas are commonly coarser in the grain size and good sorting. The pattern of grainsize probability graph is dominated by double segment and multisegment shapes. Crossbedding, parallel and ripple bedding resulted from tractional current is common, which shows that depositional hydrodynamics is dominated by the tractional current action accompanied with sediment gravity flow action. The reservoir performance of sand body is worse in fandelta facies near mountains than that in the fandelta facies near alluvial fan. It′s in favor of growing fandelta facies near mountains when there are steeper slope and higher sea level in the basin margin and when there are gentle slope at downthrow block and relatively low sea level, then, it is in favor of growing fandelta facies near alluvial fan. Along with the change of slope gradient and sealevel fluctuation in basin margin, these two kinds of fan delta may have the succession mutually.

Key words: fan delta, depositional characteristics, hydrodynamics, gravity current, tractional current, Qiongdongnan Basin