Depositional processes of submarine debris flows in the Miocene fan deltas, Pohang Basin, SE Korea with special reference to flow transformation

Subaqueous debris hows undergo various flow transformations, involving dilu tion and stripping of surface materials, penetration of ambient water into the flow interior, and detachment and disintegration of hydroplaning flow f ronts. The surface transformation is a self-limiting process because the pr oducts of the process, such as an overriding suspended-sediment cloud or an armor of gravel st the flow front, inhibit effective working of the proces s. The degree of flow transformation therefore depends largely on whether a debris flow hydroplanes or not. For a subaqueous debris flow to hydroplane , its densiometric Froude number should be larger than 0.4, and the time sc ale of pore-pressure decay should be larger than the duration of a debris h ow. In addition, a debris flow should be devoid of an extremely permeable g irth of openwork gravel around the flow head because high pressures cannot be sustained underneath the gravelly material. Detailed sedimentological measurements and estimation of flow properties fo r three debris-how beds in the Miocene fan deltas in SE Korea suggests that only a pebbly debris how with a muddy (impermeable) matrix hydroplaned. On the other hand, bouldery debris flows are interpreted to have not hydropla ned irrespective of the nature of matrix. Nonhydroplaning debris hows were subject mainly to surface transformation and were outrun by surface-transfo rmed suspended-sediment flows and debris-fall blocks after the hows entered a base-of-slope setting. Deposits of nonhydroplaning debris flows therefor e overlie deposits of turbidity currents and debris falls, In the case of a hydroplaning debris how, large chunks of debris could be detached from the flow front repetitively to form a series of small-volume flows that procee ded in front of the host debris how, The preceding hows were promptly dilut ed to produce voluminous suspended-sediment clouds and were outrun by the f aster-moving host debris how, A deposit from a hydroplaning debris flow is therefore associated with thick and extensive turbiditic deposits that may either underlie or overlie the host debris-flow deposit. The turbiditic dep osits associated with a hydroplaning debris flow are distinguished from tho se of a nonhydroplaning debris flow in that the former contain abundant gra vel clasts and chunks of poorly sorted and clast-rich debris that cannot be suspended by the surface transformation process but were more likely deriv ed from the detached fronts of a hydroplaning debris flow. These difference s in sediment volume and grain size of turbiditic deposits and the stacking pattern of related debrites and turbidites provide a clue to the behavior of subaqueous debris flows.