The Maesan fan delta, Miocene Pohang Basin, SE Korea: architecture and depositional processes of a high-gradient fan-delta-fed slope system

The Maesan fan-delta-fed slope system in the Miocene Pohang Basin occurs be tween two Gilbert-type fan deltas. Detailed analysis of sedimentary facies and bed geometry reveals that the sequence is represented by 13 sedimentary facies. These facies can be organized into three facies associations, repr esenting distinct depositional environments: alluvial fan (facies associati on I), steep-faced slope (facies association II), and basin plain (facies a ssociation III). Subaerial debris flows and dense, inertia-dominated curren ts were transformed into subaqueous sediment gravity flows in steep-faced s lope environments. Further downslope, these flows were channelized and form ed lobate conglomerate and sandstone bodies at the terminal edge of the cha nnels (or chutes). Interchannel and interlobe areas were dominated by homog eneous mudstone and muddy sandstone, deposited by suspension settling of fi ne-grained materials. Part of the steep-faced slope deposits experienced la rge-scale slides and slumps. The chutes/channels, lobes and splays on the s teep-faced slope of the Maesan system are similar to those in modern subaqu eous coarse-grained fan-delta systems.