Abstract: Through flume experiment and investigation of modern deltas, it is found that in front delta, underwater distributary channels are not formed, the distributary channels of delta plain disappear immediately once extending to the lake (sea) level; front delta sand bodies are usually sheet-shaped and fan-shaped, instead of ribbon shaped. These are different with the traditional understandings of them. The study suggests that depositional model of fan deltas differs widely from that of river deltas. On alluvial fans and fan delta plain develop two types of topography units: channel and inter-channel, both of which are sandy-conglomeratic deposits. There is not environment for shale or mudstone deposition. The mudstone deposit interbeded with sand-conglomerate is actually formed at pre-fan and inter-fan during regression of alluvial fan or when the alluvial fan is abandoned. The deposits of front fan deltas are dominated by sandyconglomeratic deposit from channel flow and sheet flow. Sand- conglomerates from channel flow form mouth bars, and those from sheet flow form underwater sheet flow shoal, both of which are connected together to shape sheet-like sandy-conglomeratic deposits, and further connected with those of fan delta plain.

Key words: river delta depositional model, fan delta depositional model, underwater distributary channel, mouth bar, sandbody shape