Research ArticleSedimentological and ichnological characteristics on macrotidal Baeksu tidal shoreface, southwestern coast of Korea
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)
- et al.
Classification of clastic coastal depositional environments
Sediment. Geol.
(1992) - et al.
Tidal flats and subtidal sand bodies
Tidal modulation of storm waves on a macrotidal flat in the Yellow Sea
Estuar. Coast. Shelf Sci.
(2003)- et al.
Characterization of intertidal flat hydrodynamics
Cont. Shelf Res.
(2000) - et al.
Shorefaces
- et al.
Variability of tidal signals in the Brent Delta Front: New observations on the Rannoch Formation, northern North Sea
Sediment. Geol.
(2016) - et al.
Transgressive sedimentation and stratigraphic evolution of a wave-dominated macrotidal coast, western Korea
Mar. Geol.
(2006) - et al.
Autogenic occurrence of Glossifungites Ichnofacies: examples from wave-dominated, macrotidal flats, southwestern coast of Korea
Mar. Geol.
(2009) - et al.
A depositional model for a wave-dominated open-coast tidal flat, based on analyses of the Cambrian–Ordovician Lagarto and Palmares formations, North-Eastern Brazil
Sedimentology
(2012) Deltas