Part I. Sinsudo Island

The Gyeongsang Basin, the largest Cretaceous basin in the Korean peninsula, is a backarc basin formed by extension associated with subduction of the Izanagi plate. The study area, Sinsudo Island, is located at the southwestern margin of the basin with various sedimentary rocks (Haman Formation, Jindong Formation, and Sinsudo Diamictite) and pyroclastic rock (Kusandong Tuff). The sedimentary rocks of Sinsudo Island have lithofacies or lithology that are distinguished from the corrrelative sedimentary formations in the rest of the basin. This study aims to understand the relationships between tectonics, volcanic activity, and depositional processes in sedimentary basins adjacent to volcanic arcs. The deposits of the Sinsudo Island are divided into fourteen sedimentary facies based on grain size, color, texture, and internal sedimentary structures. These can be grouped into three facies associations on the basis of constituent sedimentary facies and bed geometry: (1) Lacustrine facies association (FA I) composed mainly of alternating layers of dark gray mudstone and fine to medium sandstones, and rarely of massive mudstones with dinosaur footprints, biogenic structures and desiccation cracks, (2) Mouth bar or delta front facies association (FA II) comprising mainly tabular medium to very coarse sandstone beds with thinly intercalated dark gray mudstones and showing coarsening-upward trend, and (3) Fluvial facies association (FA III) composed of purple mudstones, medium to very coarse sandstones, and conglomerate showing fining-upward trends.
The deposits of the study area can be divided into lower and upper parts based on the distribution of facies association and occurrence of the FA III. Cyclic depositional trends of the lacustrine deposits (FA I) and the mouth bar or delta front deposits (FA II) are recognized in the lower part. The upper part consists of lacustrine deposits, mouth bar or delta front deposits, fluvial deposits, Sinsudo Diamictite and Kusandong Tuff. The cyclicity within the lower part suggests changes in lake level affected by precipitation, which is regulated by climate. The deposition of FA I is interpreted to have formed by suspension settling and turbidity current during the high lake level. During the lake level fall, the lake experienced contraction due to decreasing fluvial discharge. During the climate shift to more humid conditions, the lake level rises rapidly due to heavy rainfall and flood events. At this time, FA II developed near the lake shorelines as delta lobes. In this way, it is interpreted that the lower part deposits of the study area were formed by changes in the lake level. These cyclic deposits are overlain by the Sinsudo Diamictite, a massive chaotic sedimentary rock composed of meter-long blocks of conglomerate, sandstone, mudstone (purple to gray), and calcrete nodules. The diamictite is interpreted to have been deposited by a large-scale landslide from a nearby volcanic highland. FA III, which directly overlies the diamictite, suggests abruptreduction of the accommodation space due to deposition of the Sinsudo Diamictite. The Kusandong Tuff is an ignimbrite deposited by exceptionally powerful pyroclastic density currents derived from multiple eruptive centers in the volcanic arc. After the deposition of the Kusandong Tuff, the depositional environment of the Sinsudo area changed from lacustrine to fluvial. Overall, the upper part deposits of the study area are interpreted to have formed by the progradation of the fluvial systems associated with basin extension and eastward ? migration of the depocenter.
Paleocurrent vectors measured from ripple cross-laminations change from the ESE-ward in the lower part to the ENE-ward in the middle part of the studied deposits. These results indicate that the sediment source directions changed from the west to the south. During the deposition of the sedimentary successions in the study area, the Gyeongsang Volcanic Arc played two important roles by creating accommodation for sediment accumulation and acting as sediment sources. Based on these observations, the depositional environment of the study area is interpreted to have changed from shallow lacustrine to fluvial under the allogenic controls of both tectonics and climate.