Basin-fill sequences and evolution model of the Cretaceous Kyokpo Basin, Southwest Korea

Abstract

Based on facies distribution, architecture and correlation of depositional sequences, three stages of basin evolution are reconstructed (Fig. 1). Stage 1 is represented by thick coarse-grained deposits in the lower succession that form subaqueous breccia talus and steep-sloped gravelly delta systems along the northern and southern basin margins, respectively, and a sandy subaqueous fan system inside the basin, abutting against a basement high. This asymmetric facies distribution suggests half-graben structure of the basin and the thick accumulation of coarse-grained deposits most likely reflects a rapid subsidence of the basin floor due to fault movements in the basin margins during the transtensional opening of the basin. Stage 2 is marked by sandy black-shale deposits in the upper part of the lower succession. The black shale is well correlated across the basin margins, indicating a basinwide transgression probably due to large-scale dip slip suppressing lateral slip component on the basin-bounding faults. Stage 3 is characterized by retrogradationally stacked, gravelly delta-lobe deposits and sandy black shales in the upper succession. The delta-lobe deposits are smaller in dimension and located more basinward than the deposits of marginal systems of the lower succession. This lakeward shift of depocentre is suggestive of loss of accommodation in the basin margins and quiescence of fault movements. This basin evolution model suggests that the rate of dip-slip displacement on basin-margin faults can be regarded as the prime control for determining stacking patterns of basin fills. The resultant basinwide fining-upward sequences deviate from the coarsening-upward cycles of other transtensional basins and reveal the variety of stratigraphic architecture in strike-slip basins controlled by the changes in relative sense and magnitude of fault movements in the basin margins.