Tectonic controls on spatio-temporal development of depositional systems and generation of fining-upward basin fills in a strike-slip setting: Kyokpori Formation (Cretaceous), south-west Korea

Abstract

The Kyokpori Formation (Cretaceous), south-west Korea, represents a small-scale lacustrine strike-slip basin and consists of an approximate to 290 m thick siliciclastic succession with abundant volcaniclasts. The succession can be organized into eight facies associations representing distinctive depositional environments: (I) subaqueous talus; (II) delta plain; (III) steep-gradient large-scale delta slope; (IV) base of delta slope to prodelta; (V) small-scale nested Gilbert-type delta; (VI) small-scale delta-lobe system; (VII) subaqueous fan; and (VIII) basin plain. Facies associations I, III and IV together constitute a large-scale steep-sloped delta system. Correlation of the sedimentary succession indicates that the formation comprises two depositional sequences: the lower coarsening- to fining-upward succession (up to 215 m thick) and the upper fining-upward succession (up to 75 m thick). Based on facies distribution, architecture and correlation of depositional sequences, three stages of basin evolution are reconstructed. 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 a half-graben structure for the basin, and the thick accumulation of coarse-grained deposits most likely reflects rapid subsidence of the basin floor 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 readily correlated across the basin margins, indicating a basinwide transgression probably resulting from large-scale dip slip suppressing the lateral slip component on basin-bounding faults. Stage 3 is characterized by gravelly delta-lobe deposits in the upper succession that are smaller in dimension and located more basinward than the deposits of marginal systems of the lower succession. This lakeward shift of depocentre suggests a 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 such 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 at the basin margins.