Geological Quarterly, 2020, 64 (3): 611–625 DOI: http://dx.doi.org/10.7306/gq.1548 Intrastratal flow in the Cretaceous Gyeokpori Formation (SW South Korea) Uk Hwan BYUN 1 , A.J. (Tom) VAN LOON 2 , Yi Kyun KWON 3 and Kyoungtae KO 1, * 1 Korea Institute of Geoscience and Mineral Resources (KIGAM), Geology Division, Daejeon 34132, South Korea 2 Shandong University of Science and Technology, College of Earth Science and Engineering, Qingdao 266590, Shandong, China 3 Kongju National University, Department of Geoenvironmental Sciences, Kongju 314-701, South Korea Byun, U.H., Van Loon, A.J. (Tom), Kwon, Y.K., Ko, K., 2020. Intrastratal flow in the Cretaceous Gyeokpori Formation (SW South Korea). Geological Quarterly, 64 (3): 611–625, doi: 10.7306/gq.1548. Intrastratal flow is a process that is still poorly understood, rarely described and difficult to interpret in ancient rocks. Sedi- ments 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 con- text, 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 re- sulting in the dominance of gravity-flow deposits, but also in a high sedimentation rate that facilitated soft-sediment deforma- tion 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. Key words: intrastratal flow, Gyeokpori Formation, soft-sediment deformation structures, lacustrine environment, Creta- ceous. INTRODUCTION Intrastratal flow in ancient rocks can be interpreted only on the basis of traces left by the process. Such traces consist al- most exclusively of soft-sediment deformation structures (SSDS; this acronym is used here for both singular and plural). This poses a problem as SSDS of numerous types are known to have several possible origins. Moreover, even for sediments accumulated in specific sedimentary environments such as the lake deposits under study here, SSDS with largely different ori- gins have been reported frequently in the past few decades (e.g., Sims, 1973, 1975; Hempton and Dewey, 1983; Alfaro et al., 1997; Rodríguez-Pascua et al., 2000; Moretti and Sabato, 2007; TaêgÍn and Türkmen, 2009; Gibert et al., 2011; TaêgÍn et al., 2011). All these studies emphasize that the deformational processes cannot always be reconstructed with certainty, be- cause small differences in sediment characteristics or depo- sitional processes can lead to entirely different SSDS. To make interpretations of SSDS even more complex, different causes might lead to the development of very similar SSDS. Among the numerous analyses of SSDS, only a few ascribe them to intrastratal flow. This is understandable, because such a process requires specific conditions; these have been de- tailed first by Williams (1960), who mentioned that rapid deposi- tion of silt and fine sand from sediment gravity flows can result in loose packing, a relatively high pore-fluid volume, and relatively low shear strength. Rapid loading of stratigraphically confined layers of loosely packed silt can induce mass flows and can re- duce the original volume of the layer, resulting in increased neu- tral stress (pore-fluid) and decreased effective stress (grain- to-grain contact). Liquefaction can occur if these become equal, at which point the buried intrastratal layer behaves as a concen- trated suspension and flows downslope. Upon cessation of flow, the layer returns to solid state. This explanation by Wil- liams (1960) is still accepted by recent researchers (e.g., Auchter et al., 2016), but they also make clear that only a com- bination of features can make it possible to ascribe deforma- tions in a deformed layer to intrastratal flow. We do so in the present contribution for some layers in the Cretaceous Gyeokpori Formation, which is exposed along the western coast of South Korea. The formation consists of lacus- trine sediments that accumulated on the slope of a subaqueous fan-delta system, a position that is commonly considered most suitable to inducing and preserving SSDS (Gibert et al., 2005; Moretti and Sabato, 2007; Tanner and Lucas, 2007; Ko et al., 2015, 2017; Gladkov et al., 2016; Jiang et al., 2016). Indeed, the formation contains numerous SSDS in addition to those that * Corresponding author, e-mail: kkt@kigam.re.kr Received: October 3, 2019; accepted: November 17, 2019; first published online: August 17, 2020