Quaternary evolution of the southeastern Korean continental shelf, East Sea: Paleo-incised valley and channel systems

Abstract

About 1000 km of high-resolution seismic reflection profiles were used to study the morphology, distribution, nature, and sequence stratigraphy of incised valleys and valley fills within the Quaternary (<ca. 2.0 Ma) sedimentary section of the southeastern Korean continental shelf, East Sea. Seven sequences, S1 to S7 from bottom to top, were analyzed on the modern continental shelf. These sequences comprise a ~300 m thick sedimentary section containing, at minimum, one cycle of sea-level lowstand, transgression, and high stand deposits. The lowstand sections are composed mainly of laterally and vertically accreting fluvial channel deposits (i.e., point bars, channel lags) formed on an alluvial plain during a lowstand in sea level. The transgressive sections consist of confined, low-to moderate-amplitude continuous reflections interpreted as estuarine facies. The high stand sections, in contrast, are comprised of laterally extensive low amplitude reflections interpreted as open marine facies. Each of these sequences rests unconformably on its’ predecessor, particularly at the mid-southeastern edge of the shelf where angular, toplap, and onlapping relationships can be discerned.The sequence boundaries (SB1 to SB7) bounding these sequences are produced by fluvial incision (i.e., valleys and channels) during sea-level fall. SB4, SB5, and SB7 form the most prominent unconformities on the shelf and are incised by extensive paleo-incised valleys and channels. These valleys and distributary channels display simple U- or V-shaped cross sections or complex morphologies; they are ~1–8 km wide and ~10–60 m deep. Incised-valley fills reach a maximum thickness of over 80 m. The largest incised valley fills (~8 km wide, ~60 m thick) are found near the shelf break in the SE, indicating relative sea-level fall on this location (ca. 150 m lower than the present sea level). In the SE, the incised valleys are comprised dominantly of stacked lowstand successions (point bars and channel lags) and a lesser degree of transgressive successions (estuarine mud and levees) that are capped by subsequent high stand deposits (marine mud). The youngest channel above the valley was inferred to have been formed during the Last Glacial Maximum (LGM) on the shelf edge trajectory about 140 m lower than the present. These large incised valleys are thought to have developed through a complex history of repeated avulsion and reoccupation during the lowstand sea level. The most important factors regulating the valley morphology are the duration of sea-level fall, lithologic variations, climate (i.e., fluvial discharge), and the river gradient relative to the shelf gradient. The ancient drainage patterns are more-or-less northeast-southwest orientated, consistent with the trend of the present-day submerged fluvial channel. Superimposed upon this general pattern are the effects of tectonic activity (uplift, folds, and faults), which appear to have modified the courses of the rivers. Our results, therefore, provide a good example of how external factors (i.e., climate, tectonics, sediment supply, and paleogeomorphology) can determine the location and geometry of the valleys and the architecture of the valley-filling deposits of continental shelves.