Intrastratal flow in the Cretaceous Gyeokpori Formation (SW South Korea)

Abstract

Intrastratal flow is a process that is still poorly understood, rarely described and difficult to interpret in ancient rocks. Sediments in the Cretaceous lacustrine Gyeokpori Formation of southwestern South Korea contain some chaotically deformed sandstone layers with deformed mudstone clasts that are ascribed to this process. The interpretation is based on the fact that these layers cannot be explained as a result of subaqueous debris flows or mass transport, whereas the sedimentary context, including the presence of other soft-sediment deformation structures, indicates that intrastratal flow must have been physically possible. The sedimentary setting was a lake in which mainly siliciclastic rocks were deposited, with some interbedded volcaniclastics. The nearby volcanic activity caused seismic shocks that affected the unstable lake margins resulting in the dominance of gravity-flow deposits, but also in a high sedimentation rate that facilitated soft-sediment deformation partly caused by intrastratal flow. This must have happened fairly frequently during a probably limited time-span, as several layers showing traces of intrastratal flow are present within a succession of only <1 m thick. The combined data on the geological setting and our findings regarding the origin of the various soft-sediment deformation structures may help to recognize the traces left by intrastratal flow elsewhere in the geological record.