Depositional processes, paleoflow patterns, and evolution of a Miocene gravelly fan-delta system in SE Korea constrained by anisotropy of magnetic susceptibility analysis of interbedded mudrocks

Anisotropy of magnetic susceptibility (AMS) has been used as a proxy of grain fabrics and as a basis of interpretation of depositional processes and paleocurrent directions in sedimentary successions. In this paper, we present the results of the AMS analysis of over 900 core specimens from the sandy to muddy deposits intercalated between the gravelly deposits of a Miocene fan-delta system in SE Korea. Analyses of the mean magnetic susceptibility, thermomagnetic property, and hysteresis show that paramagnetic minerals are the main contributors to the AMS of the deposits. The AMS analysis shows that the deposits have four types of magnetic fabrics, including 1) horizontal and type-A imbricated fabrics, characterized by girdle-distributed to weakly clustered k1 and k2 axes on the bedding plane and subvertical to variably tilted k3 axes, 2) type-B imbricated fabric, characterized by well-clustered k1 and k2 axes and subvertical to tilted k3 axes, and 3) rolling fabric, characterized by streak-distributed k2 and k3 axes along a plane perpendicular to the k1 axes. These fabric types are interpreted to have resulted from suspension settling (horizontal and type-A imbricated fabrics), turbidity currents (type-B imbricated fabric), and debris flows (rolling fabric). Modification of the primary fabrics during sediment compaction on a slope resulted in a wide range of imbrication angles in all fabric types. Paleocurrent directions inferred from the AMS data are generally consistent with those obtained by previous field studies of the gravelly sequences and provide additional information of the temporal change of the paleoflow pattern, which is related to the growth, exhumation, and dissection of the fan-delta system. This study thus suggests that much information can be drawn from the AMS analysis of mudrocks regarding not only the depositional processes and paleoflow pattern but also the evolution of a depositional system.