Rhythmic deposits in the Upper Cretaceous lacustrine deposits at Yeosu area, Korea: Occurrences and origin

Abstract

Diverse modes of rhythmic deposits occur in the Upper Cretaceous lake margin deposits at Sado and Chudo islands at Yeosu Area in Korea. In this study their occurrences are described and the origins were interpreted. The main lithofacies at study area are composed of tuffaceous coarse- to fine-grained sandstone, planar- to cross-laminated fine-grained sandstone to siltstone, tuffaceous siltstone to mudstone, sandstone-dominated interlaminated fine-grained sandstone to siltstone and mudstone, and interlaminated finegrained sandstone to siltstone and mudstone. These lithofacies are interpreted as sheetflood deposits on sandflat to mudflat, and numerous dinosaur tracks are associated with those deposits. Six types of rhythmic sedimentation are recognized at Sado island, viz., alternation of sandstone-dominated interlaminated fine-grained sandstone to siltstone and mudstone and interlaminated finegrained sandstone to siltstone and mudstone in thin beds, alternation of tuffaceous sandstone and tuffaceous siltstone to mudstone in thin to medium beds, alternation of rippled and bioturbated layers within interlaminated fine-graind sandstone to siltstone and mudstone, alternation of planar-laminated and cross-laminated layers within planar- to cross-laminated fine-grained sandstone to siltstone in thin beds, and alternation of tuffaceous beds and non-tuffaceous beds in a few meter scale. Two types of rhythmic sedimentation are recognized at Chudo, viz., alternation of planar-laminated and cross-laminated layers within planar- to cross-laminated fine-grained sandstone and bioturbated, interlaminated fine-grained sandstone to siltstone and mudstone in thin beds. The rhythmic sedimentation at Sado and Chudo is interpreted to have been attributed to the fluctuation o9f sheetflood energy and lake levels due to short-term to long-term climate changes and the non-periodic alternation of active and quiescent stages of volcanic activity. The rhythmic sedimentation records preserved at