Sedimentation on a wave‐dominated, open‐coast tidal flat, south‐western Korea: summer tidal flat – winter shoreface – reply

Abstract

In their Discussion of our recent paper (Yang et al., 2005) on the sedimentology of the open-coast tidal flats of south-western Korea, Chang & Flemming (2006) take exception with several aspects of our analysis. Using provocative language to give the sense of controversy, they claim to propose a ‘different interpretation’ and an alternative model for this hitherto undescribed environmental setting. Careful examination shows, however, that the differences of perspective are small. This is understandable given that the senior author of the Discussion was a student of Chun and a colleague of Yang during the course of the study and was privy to much of our unpublished thinking, some of which is reflected in the Discussion. Their Discussion focuses on two main themes: depositional dynamics of the mud, and the classification of the broader environment. We respond below to these points in turn. We agree with the discussants that water temperature and viscosity are important variables in the deposition of silt- and clay-sized particles, not only because of their direct effect on settling velocity but also because of their influence on the shear stresses imposed on delicate flocs. Even more important in many cases, perhaps, is the affect of temperature on desiccation during exposure: higher temperatures promote greater desiccation and more mud accumulation because of the increased resistance to erosion. We acknowledge these points in the original paper (Yang et al., 2005), even retaining comments on the role of temperature despite the request of a reviewer to remove them. We disagree with the discussants, however, with regard to the relative importance of temperature and wave-energy levels: the discussants imply that temperature changes control mud dynamics more strongly than do changes in energy, whereas we believe that the opposite applies in the present setting. The back-barrier area on which the discussants base their opinion is protected from pronounced changes in wave energy. Thus, it is to be expected that temperature has a larger relative role than is the case on the exposed tidal flats along the Korean coast where wave-energy levels are considerably higher and more variable. In the Korean study area, we observe that the summer mud layer expands upward from the sub-tidal zone as one would expect if the distribution of mud was controlled by decreasing wave energy; there is little or no outward expansion of the inner mudflat, which would have occurred if temperature was the primary control on mud accumulation (because temperature changes affect all parts of the tidal flat equally). Indeed, if temperature-facilitated desiccation was the primary control on mud accumulation, we would have expected the inner mudflat to expand more rapidly than the sub-tidal to lower intertidal mud, but this is not what we see. Therefore, we believe that the discussants are wrong to imply that temperature changes are the most important factor controlling mud deposition in all situations. In cases where energy levels are low and change little, large temperature changes may well be relatively more important; however, in cases such as the exposed Korean tidal flats, the large changes in wave energy appear to be more important than temperature. The discussants are incorrect to apply uncritically the results of their study area to other sites without considering the differences in the environmental conditions. The discussants also take issue with our reporting of sorting values for the summer and winter deposits. While we agree with them about the questionable utility of the values for the summer mud, we believe that those for the winter sands do have value, if only to highlight the limited amount of silt and clay that is incorporated and the scarcity of grains coarser than fine sand. The good sorting of these deposits is a useful parameter for conveying their general character and we would have been justifiably criticized if we had not reported them. In an apparent contradiction of their claim that the textural characteristics of muddy sediments ‘have no meaning whatsoever’, the discussants use them in their fig. 4 to infer that the mud is a high-energy deposit, an interpretation that could well be true as we note in the original paper that much of the mud may be a storm deposit (Yang et al., 2005). Coastal zones are inherently complex, with the morphology and deposits reflecting the interplay of many processes. In an attempt to simplify this complexity, many workers, including Dalrymple et al. (1992), have focused on end-member settings where sedimentation is controlled by a single major process (e.g. waves or tidal currents). However, most coastal environments are mixed-energy settings in which both waves and tides influence the morphology and facies, which will, therefore, differ to some degree from the simplified models. We believe that future research needs to focus on such ‘hybrid’ or ‘transitional’ environments in order to provide a more diverse suite of models for use in the interpretation of ancient deposits, where the application of simplified end-member models might lead to incomplete or even erroneous environmental reconstructions. It is for this reason that we chose to study the tidal flats along the south-western coast of Korea. Because such transitional environments have not been studied in detail before, it is understandable that there will be uncertainty and differences of opinion about which existing terms are most applicable. The Korean tidal flats that we described seemed to us to be unlike any that had been documented before; hence, we were cautious in how we applied existing terms. Before we started our study, the previous work on, or references to, the Korean coastal zone had consistently and universally referred to it as consisting of ‘tidal flats’. We agree that this term seems perfectly apt for the Baeksu study area where broad, very low-gradient areas (flats) are periodically exposed and flooded by the tide, because such characteristics are at the core of the morphological definition of ‘tidal flat’. The fact that the gross sediment distribution locally shows a landward transition from sandy to muddy is also consistent with the general pattern seen in most tidal flats. Therefore, we continue to believe that the term ‘open-coast tidal flat’ is a completely acceptable morphological description of the study area. With regard to the processes operating on these tidal flats, virtually all workers before us had either implied or stated that the Korean tidal flats were tide dominated; the activity of waves was rarely or never mentioned. In this context our use of the terms ‘wave-dominated’ and ‘shoreface’ in the title of our paper represented a dramatic departure from conventional wisdom and was considered to be provocative and controversial. We are pleased that the discussants accept our fundamental position that wave action is very important on these tidal flats! We do not object to the use of the term ‘intertidal shoreface’ for the outer part of the tidal flats because this is a term that we have used informally for several years; in fact, we believe that the discussants have appropriated our own term, without attribution, because we have used this term in their presence as far back as 2000. We did not use this term in our paper, however, because we felt that the term ‘wave-dominated tidal flat’ more accurately described both the morphological and process dynamics than did ‘intertidal shoreface’. Indeed, it remains to be determined in detail to what extent the wave dynamics on these tidal flats are similar to or different than those on true, steeper-gradient shorefaces (c.f. Niedoroda et al., 1985). The transformation of the waves into bores and the widespread presence of depth-limited waves (i.e. the presence of a ‘saturated’ wave spectrum; cf. Le Hir et al., 2000) are, for example, features that are not reported commonly from typical shorefaces. Furthermore, the deposits of the sandy tidal flats at Baeksu, although they contain hummocky cross-stratification (HCS) that is considered to be typical of shorefaces, contain other features that may prove to be different from those of shorefaces; some of these are documented in our paper [e.g. the small maximum size of the HCS (see also Yang et al., 2006a) and abundant landward-directed ripples (Yang et al., 2005)], whereas others are described in a subsequent manuscript that is currently in review. The most important of these consists of something we term a ‘tidally modulated storm deposit’, which consists of a storm deposit that forms over several tidal cycles and contains evidence of wave-energy variations caused by tidally generated changes in water depth. Such deposits truly reflect the mixed-energy nature of this environment. In addition, the summer mud layer also contains evidence of storm sedimentation: in some cases following the passage of a typhoon, several centimeters to decimeters of mud may accumulate in only a few hours to days with fluid muds apparently playing an important role in its accumulation. Observations such as these clearly indicate that the study area is neither a typical tidal flat nor a typical shoreface. We do not agree with the discussants’ proposal that the term ‘tidal flat’ should be restricted to the muddy inner part of the larger tidal-flat environment. However, we agree that, in such areas, there appears to be an inward transition from more wave-dominated conditions on the outer flats to more tide-dominated conditions in the inner part (Fig. 1), because of the progressive landward dissipation of wave energy across the broad, shallow tidal flat. Nevertheless, these inner mudflats contain evidence of storm action because the largest storm waves are not completely dissipated until they reach the high-tide shoreline; therefore, the extent to which they are truly tide dominated is uncertain. The presence of a mudflat that lies gradationally landward of, or vertically above, wave-dominated deposits contrasts starkly with the simple models for both shorefaces/beaches and tidal flats, which suggests that the study area represents a distinct environment with characteristics that are different from those at the ends of the wave-tide spectrum. Schematic diagram showing the along-coast and coast-normal variation in sediment character and depositional processes as a function of tidal-flat width. The right-hand side of the figure corresponds approximately to the Baeksu study area, whereas the left-hand side is represented by the Dongho area located ca 30 km north of Baeksu (see fig. 1B of Chang & Flemming, 2006, for locations). Where the tidal flat is narrow (ca 1–2 km) and relatively steep (slope 0·007 at Dongho), the long-term average wave energy is highest at the high-tide level, creating a beach (cf. Yang et al., 2006b). As the tidal flat becomes wider (ca 3–4 km) and flatter (slope 0·0013 at Baeksu), wave-energy dissipation across the flat causes the zone of highest wave energy to lie seaward of the high-water level and creates an area of low energy near the high-tide shoreline where muddy sediment can accumulate. All parts of the intertidal and sub-tidal zones experience both wave and tidal processes and hence are ‘mixed-energy’; however, the ratio of tide to wave influence is greatest (i.e. the deposits are most likely to reflect tidal influence, or to be tide dominated) in the deeper sub-tidal area (where bottom wave-orbital velocities are small) and in the muddy zone landward of the wide tidal flat. The discussants pose a complex question when they ask the extent to which the presence of the former Baeksu estuary and the subsequent dyking (in 1995) have influenced the present-day situation in the study area. This question is complex because it has several answers, each depending on the time scale under consideration. With regard to the surficial deposits discussed in our paper, the former estuary has no influence because these deposits reflect the present-day processes: all of the sediments recovered in our cancores are probably no more than a few years old. The presence of the dyke might have enhanced the wave domination at the landward end of the YS profile (by reducing the tidal exchange); however, that location was already wave dominated prior to dyking because the dyke is constructed on a pre-existing beach-dune ridge Yang et al. (2006b). Because the surficial sediments are a direct response to the present-day processes, we believe that the inner mudflat, although it may have formed originally (i.e. before dyking) as an outer-estuary mudflat, is in equilibrium with its current open-coast setting and is an integral component of open-coast tidal flats when such flats become very broad, causing wave dissipation (Fig. 1). The presence of the former estuary must be taken into account, however, when examining the stratigraphic succession created by the longer-term evolution of this coast, a subject considered further by Yang et al. (2006b). Such longer time scales are not relevant to the subject of the study under consideration here. The open-coast tidal flats along the south-western Korean coast represent a type of coastal environment that has not been described in detail before. Overall, sedimentation is controlled by waves, but the substantial tidal range and the very low gradient of the tidal flats cause substantial differences in facies from those of a typical shoreface. Therefore, we chose not to use the term ‘intertidal shoreface’, as argued by the discussants, even though this term is one that we have used informally for several years. Because of the hybrid nature of this environment, with characteristics intermediate between those of shorefaces and tidal flats, names taken from other settings with a single dominant process must be applied with caution. The same is true concerning the application of ideas relating to the deposition of mud. We acknowledge that temperature is an important variable influencing mud accumulation and erosion, but we disagree with the discussants’ implication that temperature is more important than wave energy in the case of the exposed Korean tidal flats.