Late Quaternary evolution of macrotidal Kimpo tidal flat, Kyonggi Bay, west coast of Korea

Abstract

Up to 32 m thick late Quaternary deposits are developed in the Kimpo tidal flat, Kyonggi Bay, west coast of Korea. The deposits consist of six lithofacies units that are grouped into three unconformity-bounded sequences, which formed in response to late Quaternary sea-level fluctuations. The lowermost sequence (S3) consists of late Pleistocene fluvial deposits (Unit VI) and overlying tidal deposits (Unit V). Unit VI is interpreted to have formed in a meandering channel and associated floodplain environment. Weak pedoturbation characterizing the upper part of Unit VI suggests that pedogenesis occurred prior to initial transgression. Well-developed tidal rhythmicities in Unit V indicate that tidal sedimentation prevailed during the first marine inundation. Intermediate sequence (S2) is composed of late Pleistocene fluvial deposits (Unit IV) and overlying tidal deposits (Unit III). Depositional environments of Unit IV and Unit III are similar to those of Unit VI and Unit V, respectively. Upper parts of Unit IV and Unit III are variably indurated and pedoturbated, suggesting prolonged subaerial exposure. The uppermost sequence (S1) consists of early Holocene sphaerosiderite-rich marsh deposits (Unit II) and overlying middle to late Holocene tidal deposits (Unit I). Pervasive occurrence of similar tidal rhythmites in both Holocene and late Pleistocene tidal deposits indicates that tidal environment recurred over several sea-level cycles. Age dates and stratigraphic position indicate that Holocene tidal deposits were formed since 7–8 ka BP. Given the validity of age dates, however, timing of late Pleistocene tidal deposits is controversial in the light of eustatic sea-level curves. Considering tectonically stable condition of study area, the elevation of the late Pleistocene tidal deposits is too high to fit the curves. Elucidation of the exact timing of the tidal deposits requires more independent age controls as well as further understanding on the tectonic regime of the west coast of Korea.