Elsevier

Marine Geology

Volume 440, October 2021, 106607
Marine Geology

Research Article
Sedimentological and ichnological characteristics on macrotidal Baeksu tidal shoreface, southwestern coast of Korea

https://doi.org/10.1016/j.margeo.2021.106607Get rights and content

Highlights

  • Tidal-shorefaces are mixed-energy costal settings that have been poorly defined in the current models.

  • The Baeksu tidal shoreface on the southwestern Korean coast has been intensely studied.

  • The resulting deposits conspicuously show similar sedimentology but different ichnologic characters.

  • Our findings can provide distinctive evidences for the tidal-shoreface deposits in the ancient record.

Abstract

Integrated sedimentologic and ichnologic studies along the Baeksu coast of southwestern Korea have revealed that despite the macrotidal conditions, the morphologic characteristics and preserved deposits of the intertidal flats show strong similarities to those commonly reported from wave-dominated shorefaces. Morphologically, a beachface is developed near the high-tide level, which is backed by the high-tide mud flat and fronted by the extensive intertidal shoreface (sensu Dashtgard et al., 2021), where wave-formed swash bars are present. Although sedimentation is strongly influenced by seasonal winds and waves, the preserved deposits conspicuously comprise abundant storm beds that consist mainly of wave-rippled and hummocky cross-stratified sands, because summer muds are typically ephemeral. The storm beds, which are predominant and thick near the low-tide line, become thinner in a landward direction, due to the progressive dissipation of wave-energy by bottom friction. Such physical processes, exerted on tidal-shoreface sedimentation, are also reflected in the ichnofacies distribution, showing that distal expressions of the Skolithos Ichnofacies characterize the lower intertidal zone and are replaced landward by proximal expressions of the Cruziana Ichnofacies. Except for the low-BI beachface (BI 0–1), the intensities of bioturbation generally increase in a landward direction (BI 0–1 to BI 3–4), in correspondence with the physical-energy distribution.

The study area reveals that the sedimentological and ichnological characteristics commonly employed to recognize wave-dominated shoreface settings (even including high-wave-energy tidal-shoreface counterparts) apparently show a reverse trend within the intertidal shoreface, probably overlying the normal-trend, subtidal shoreface. Although these findings have not been broadly tested, we, however, believe that these features are representative of the low-energy end of the full spectrum of wave-dominated coastal settings.

Introduction

Coastal systems and their associated deposits are very diverse and form a significant part of the geological record. Because of their importance in the modern and ancient, they have been the subject of intense investigation (cf. Galloway, 1975; Boyd et al., 1992; Dashtgard et al., 2021). According to our current understanding, coastal zones are distinguishable into two main categories by the relative effects of waves and tidal currents, ranging from exposed, wave-dominated beaches and shorefaces that have relatively steep offshore slopes, to sheltered, tide-dominated tidal flats that have low gradients (Boyd et al., 1992; Dalrymple, 2010; Plint, 2010). As a result, the geologic literature has commonly stated that wave- and tide-dominated settings contain unique sedimentary facies, such as pervasively bioturbated, heterolithic facies in tidal-flat deposits (Gingras et al., 1999; Dalrymple, 2010; Gingras and MacEachern, 2012), and wave and storm-generated deposits, including hummocky cross-stratification (HCS), in beach and shoreface deposits (Clifton, 2006; Plint, 2010).

Yang et al. (2005) have, however, demonstrated that storm deposits including HCS are abundant in the intertidal zone of wave-dominated macrotidal flats (i.e., mixed-energy coastal settings), whereby they have shown that storm and HCS deposits are not restricted to wave-dominated shorefaces that are typically considered to lie entirely in the subtidal zone (see also Yang et al., 2006a, Yang et al., 2008). In other words, wave-dominated (i.e., “classic”) shorefaces cannot be distinguished with confidence on the basis of existing sedimentological knowledge. In addition, a growing number of studies have recently revealed that the generally accepted facies models do not adequately incorporate mixed-energy coastal zones (i.e., ‘tidal shorefaces’; sensu Yang and Chang, 2018; Dashtgard et al., 2021), and there is a growing recognition that mixed-energy deposits are widespread in the Holocene and earlier coastal successions (see Dashtgard et al., 2021, and references therein).

Recognition of such mixed-energy coastal deposits may, in fact, be hindered by the lack of well-defined depositional models for tidal-shoreface successions (Dashtgard et al., 2021). In recent years, integration of sedimentologic and ichnologic data has been emphasized in the recognition and delineation of facies in marginal-marine depositional systems, because they lead to a more comprehensive understanding of how environmental conditions change through the transition between wave- and tide-dominated coastal settings (e.g., Dashtgard et al., 2009; Yang and Chang, 2018). Given this general context, the main purpose of the present study is to describe the integrated sedimentological and ichnological characteristics of the Baeksu macrotidal shoreface, located on the southwestern coast of Korea (Fig. 1a). In particular, the current study mainly focuses on an ichnological analysis of the deposits, because organism abundance and behavior respond sensitively to the very different limiting factors in wave- and tide-dominated environments. Indeed, only a small number of ichnological studies have hitherto been performed in modern, mixed-energy coastal settings. The present study, therefore, has the potential to make a fundamental contribution to our knowledge of such coastal environments and their deposits.

Section snippets

Study area

The intertidal portion of the tidal shoreface at Baeksu is 4–6 km wide and faces directly onto the Yellow Sea (Fig. 1b). Below the low-tide elevation, the gradient steepens, defining the subtidal portion of the shoreface, the base of which is located at approximately 30 m depth, where the slope flattens onto the floor of the Yellow Sea. It is bordered on its landward side by Cretaceous rocky coastal cliffs or by artificial dykes that have been constructed to reclaim most of the mudflat and all

Data collection

To describe the intertidal-shoreface sedimentology and ichnology, two shore-normal survey lines were established, along which metal poles were installed at 100 m intervals to allow consistent sampling locations. Along the survey lines, topography was measured using a level (Sokkia B21) on a seasonal basis, the results of which were presented in Yang et al. (2005). Approximately 240 small cancores, also known as boxcores (30 cm deep x 18 cm wide x 8 cm thick), were collected seasonally along the

Morphologic characteristics

From extensive field observations and the topographic surveys, the intertidal shoreface is conspicuously characterized by gentle topographic undulations, attributed mainly to the presence of wave-formed swash bars with wavelengths of 100–300 m (Fig. 4). On the lower intertidal shoreface, they are commonly about 0.3 m high, but become larger in a landward direction, reaching 0.5–1 m in height at the upper intertidal zone (see line YS). The swash bars migrate landward, mostly during the winter,

Sedimentology and ichnology

Detailed analysis of the collected cancores and many years of field observations indicate that each morphologic zone (i.e., mud flat, beachface, upper intertidal and lower intertidal shoreface) (see Figs. 1b, 4) is characterized by distinctive sedimentological and ichnological features (Fig. 6). In addition, the encountered burrows in the cancores are generally correlated with the infaunal distribution across the tidal shoreface. Due to the fact that the study area is strongly influenced by the

Discussion

In the geologic literature, wave- (beach and shoreface) (Clifton, 2006; Plint, 2010) to river-dominated (delta) environments are well documented (Bhattacharya, 2010; Hampson and Howell, 2017), whereas depositional systems along the wave-to-tide continuum are relatively poorly understood (Yang et al., 2005; Dalrymple, 2010; Dashtgard et al., 2021). According to end-member models (Fig. 10), coastal systems are commonly defined on the basis of tidal range, so that macrotidal coasts are considered

Conclusions

Although tidal shorefaces are common in modern environments, they are under-represented in the geologic literature. The general paucity of tidal-shoreface identifications may reflect the fact that their sedimentary deposits are very similar to those in conventional wave-dominated shorefaces, since the tidal signature is subtle or largely obscured by the dominance of high-energy wave-generated structures.

An integrated approach incorporating the physical sedimentological characteristics along

Declaration of Competing Interest

The authors declare that there is no conflict of interest with third parties.

Acknowledgements

This study was carried out as part of BCY's Ph.D. thesis, and was supported by research funds from the Natural Science and Engineering Research Council of Canada (#7553-01; RWD) and the National Research Foundation of Korea (TSCHANG, NRF-2017R1D1A1B03035060). Special thanks are due to the Ichnology Research Group in the University of Alberta that kindly supported the ichnofacies analysis. We would like to thank two anonymous reviewers for their constructive criticisms that served to improve the

References (38)

  • R.G. Bromley

    Trace Fossils, Biology and Taphonomy

    (1996)
  • H.E. Clifton

    A reexamination of facies models for clastic shorelines

  • W.R. Dally et al.

    Wave height variation across beaches of arbitrary profile

    J. Geophys. Res.

    (1985)
  • R.W. Dalrymple

    Tidal depositional systems

  • S.E. Dashtgard et al.

    Tidal controls on the morphology and sedimentology of gravel-dominated deltas and beaches: examples from the megatidal Bay of Fundy, Canada

    J. Sediment. Res.

    (2007)
  • S.E. Dashtgard et al.

    Tidally modulated shoreface

    J. Sediment. Res.

    (2009)
  • S.E. Dashtgard et al.

    Wave-dominated to tide-dominated coastal systems: a unifying model for tidal shorefaces and refinement of the coastal-environments classification scheme

    Geosci. Can.

    (2021)
  • D. Fan

    Open-coast tidal flats

  • W.E. Galloway

    Process framework for describing the morphologic and stratigraphic evolution of deltaic depositional systems

  • Cited by (0)

    View full text