Soft-sediment deformation structures in the Cretaceous Gyeokpori Formation of the Buan area, Korea: Structural characteristics, reconstruction of paleoslope and triggering mechanism of slump

This study aims at revealing the structural characteristics and triggering mechanism of the soft-sediment deformation (SSD; up to 2 m-thick) structures that occurred continuously (>200 m-wide) within the black mudstone strata in Dalgibong area of the Cretaceous Gyeokpo Basin (or Kyokpori Basin). The highly deformed black mudstones indicate that the deformation is generated by gravity-driven slumping on a basin plain environment with low gradient less than <1 degrees. The SSD structures, consisting of fold, thrust and normal fault, show different geometric characteristics at three sites (site 1 to 3 from east to west). The folding at site 1 predominantly occurs in the middle to upper part (from 1 to 2 m-high) of the deformed layer together with syn-deformational faults (thrust and normal fault), whereas folding observed in the whole layer (up to 2 m-thick) at site 2 and 3 without the faults. Fold vergence at site 1 and 2 is toward east, but is variable at site 3. Scattered directions of the fold hinge and axial plane of the folds together with diverse vergence directions of the folds indicate that the folds can be classified into a non-cylindrical fold. The inferred paleoslope direction on the basis of the Fold Hinge Azimuth and Interlimb Angle Method (HIM) is ESE (120 degrees), and it is toward the Buan Volcanics locating the east side of the Gyeokpo Basin and is inconsistent with paleoflow direction inferred from a previous study (north-to northwest-ward; Kim et al., 2003). This envisages that the triggering mechanism of the SSD is closely related to volcanogenic-slumping, which was caused by volcanic-ward tilting of the basin floor during and/or shortly after volcanic eruptions.