Kyungsik Choi

Processes, Morphodynamics, and Facies of Tide-Dominated Estuaries

As defined in this chapter, an estuary forms during a shoreline transgression and then fills during a progradational phase that is transitional to a delta. The spatial distribution of processes, grain sizes and facies within tide-dominated estuaries is predictable in general terms. Tidal currents dominate sedimentation along the axis, with wave-dominated sedimentation occurring along the flanks of the estuary in its outer part. Tidal energy increases into the estuary but then decreases toward the tidal limit, with a gradual transition to river-dominated sedimentation at its head. The interaction of the tidal wave with the morphology of the estuary, and with river currents, causes the outer estuary to be flood-dominant, with a net landward movement of sand. By contrast, the inner estuary is ebb-dominant, creating a bedload convergence within the estuary. The axial sandy deposits are typically finest at this location. In transgressive-phase estuaries, the main channel shows a low—high—low pattern of sinuosity, with the tightest bends (sinuosity ≥ 2.5) occurring at the bedload convergence. These bends experience neck cutoff in the transition to the progradational phase of estuary filling. The estuary-mouth region is characterized by cross-bedded sands deposited on elongate sand bars, although wave-generated structures can be important in some cases. Estuaries that are down-drift of major rivers have anomalously muddy outer estuarine deposits. Further landward, upper-flow-regime parallel lamination can be widespread. The margins of the inner estuary are flanked by muddy salt-marsh and tidal-flat deposits that can contain well-developed tidal rhythmites and evidence of seasonal variations in river discharge.

Spherulitic siderites in the Holocene coastal deposits of Korea (eastern Yellow Sea): elemental and isotopic composition and depositional environment

Abstract Siderite concretions are recovered from both freshwater (Unit II) and tidal (Unit I) deposits in the Holocene macrotidal flat of Kyunggi Bay, west coast of Korea. Siderites from both units show a spherulitic and well-rounded texture and well-developed equant rhombs on the external surfaces. The size of the concretions in Unit II (50–150 μm in diameter) is normally larger than that in Unit I (10–20 μm). The siderites in Unit II are chemically pure and highly enriched in Fe, whereas those of Unit I are relatively impure and distinctly zoned, demarcating high Mn (up to 69.1 mol%) in the core and high Fe (up to 75.9 mol%) in the margin. Within siderites of Unit I, substitution for Fe by Ca and Mg reaches up to 20.3 mol% (mean 15.1 mol%) and 16.1 mol% (mean 12.6 mol%), respectively. Bulk δ 18 O values (−0.7 to +1.3‰ PeeDee Belemnite (PDB)) for siderites in Unit I are relatively higher than those (−7.1 to +0.5‰ PDB) in Unit II. Regardless of units, siderite δ 13 C values (−21.3 to −11.2‰ PDB) are persistently low, indicating microbially-mediated precipitation at shallow burial depth. Extensive Mn, Ca and Mg substitution coupled with enrichment of 18 O in the siderites of Unit I seems to reflect marine influence during the Holocene transgression. The occurrence of early diagenetic siderites provides supporting evidence on the depositional environment (marine vs. non-marine) of the Holocene muddy coastal deposits of Korea (eastern Yellow Sea).

Chapter 3 – Data

The main database for this study consists of short cores, long cores, and seismic data collected from the subaqueous delta platform and shelf during the 1990 and 2000s. Previous work on the open coast tidal flats and deltaic distributary channels is also incorporated. The nature of the database is tabulated here.

Near-Surface Sediment

The four geomorphic zones that make up the large tidal bar investigated herein (see Chapter 4 ) have distinct near-surface facies: (1) The open-coast tidal flats that make up the inner part of the large tidal bar are covered by dune cross-stratified sand subtidally, heterolithic sediment intertidally, and bioturbated mud near the high tidemark. Most stratification is generated by tidal currents, with waves having a subordinate influence. (2) Sandbars and swatchways are covered by dune cross-stratified sand, both intertidally and subtidally. The largest dunes occur at the base of channels, a common theme in channels throughout the delta. (3) The mid-outer large tidal bar is largely dune-free and is covered by interlaminated mud and sand, which is variably bioturbated. (4) The distal end of the large tidal bar, which forms part of the prograding deltaic clinoform, is covered by a veneer of fine sand. However, the older, underlying clinoform deposits consist primarily of interlaminated mud and sand, as revealed by long cores (see Chapters 7 and 8 Chapter 7 Chapter 8 ).

Discussion and Concluding Remarks

The Han, like all tide-dominated and strongly tide-influenced deltas studied to date, exhibits the following key traits: a large subaqueous delta whose clinoform is the main subtidal locus of mud deposition in the system; gently inclined (<1°) heterolithic clinoform deposits that record deltaic progradation; and a high degree of channelization in the proximal, shallow subaqueous delta topset, an area that functions as the mouth bar zone, where most fluvial bedload is trapped and where channels commonly migrate laterally, generating sharp-based, upward-fining, channel–bar successions.