Alluvial-to-lacustrine systems in a pull-apart margin: Southwestern Eumsung Basin (Cretaceous), Korea

In the pull-apart margin of pull-apart basins, sedimentation patterns are poorly known, compared with those in rift basins. The rhomb-shaped Eumsung Basin (Cretaceous), mid-west Korea, was formed in an overstep of the Kongju fault system and provides significant information on how sedimentary bodies dynamically filled the pull-apart margin. According to a detailed analysis of sedimentary facies, palaeoflow directions, clast compositions and downstream changes in clast size, the southwestern basin fills comprise three facies associations representing distinctive depositional environments: (1) facies association (F.A.) I (volcaniclastics-dominated alluvial fan); (2) F.A. II (alluvial plain); and (3) F.A. III (floodplain/lake). The matrix-supported and clast-supported conglomerate (F.A. I) is indicative of deposition in alluvial fans. The hollow-fill or sheetlike deposits encased in purple siltstone (F.A. II) are suggestive of channel or sheetflood deposition in an alluvial plain. The dominant purple and green mudstone (F.A. III) indicates deposition in floodplain and lacustrine environments. The distribution of facies associations represents a basinward change in depositional environments from alluvial-fan to lacustrine systems. The succession is divisible into three large-scale lateral and partly vertical units: the southernmost Ochang (~1.5 km thick), the medial Baeti (~1 km thick) and the northernmost Kugokri units (~0.5 km thick), on the basis of basinal structure and changes in clast composition and lithofacies. The Ochang-Baeti transition reflects synchronous development of the alluvial-to-lacustrine systems in a pull-apart margin. This succession development is attributable to the basinal faults progressed during the basin development. The Baeti-Kugokri transition recorded the changes in clast composition and lithofacies, which was due to the progressive shifting in basinal setting from a pull-apart margin to a strike-slip margin. Unlike that of the strike-slip margins, clast composition of the pull-apart marginal fills reflects adjacent source-rock lithology. In the pull-apart margin, alluvial-to-lacustrine systems show a transverse-to-margin channel network (060°-135°), converging into the southwestern depocentre. Although the basin is of strike-slip origin, sedimentation patterns in the pull-apart margin are suggestive of relatively synchronous developments of the alluvial-to-lacustrine systems reflecting the lithology of the adjacent source rock and subsurface structure of normal-slip-dominant faults.