Observed and modelled tidal bar sedimentology reveals preservation bias against mud in estuarine stratigraphy
Braat, Lisanne; Pierik, Harm Jan; van Dijk, Wout M.; van de Lageweg, Wietse I.; Brückner, Muriel Z.M.; van der Meulen, Bas; Kleinhans, Maarten G.
(2023) Depositional Record, volume 9, issue 2, pp. 380 - 402
(Article)
Abstract
Mud plays a pivotal role in estuarine ecology and morphology. However, field data on the lateral and vertical depositional record of mud are rare. Furthermore, numerical morphodynamic models often ignore mud due to long computational times and simplifications of mixed depositional processes. This study aims to understand the spatial distribution,
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formative conditions and preservation of mud deposits in the intertidal zone of bars in high-energy sand-dominated estuaries, and to elucidate the effects of mud on morphology, ecology and stratigraphic architecture. To meet these objectives, field data (historic bathymetry, bio-morphological maps and sediment cores of the shoal of Walsoorden, Western Scheldt estuary, the Netherlands) were combined with complementary hydro-morphodynamic numerical modelling (Delft3D). Based on the field observations, two types of mud deposits were distinguished: (1) mudflat deposits, which are thick (>10 cm) mud beds at the surface associated with high elevations and low accumulation rates; and (2) mud drapes, which are thin (millimetre to centimetre) buried laminae that form and preserve at a wide range of elevations and energy conditions. Model results show that deposition on mudflats occurs just after high-tide slack water in areas shielded from high flood velocities, suggesting that mud accumulation is mostly controlled by elevation, flow velocity and flow direction. Mud accumulation increases shoal elevation, sometimes to supratidal levels. This reduces flow over the shoal, which in turn reduces chute channel formation, stabilises bar morphology and decreases local tidal prism. These effects further promote mud deposition and vegetation settling. Although observations show that mud cover at the surface is relatively high (20%–40% of the intertidal area), mud constitutes only a small percentage of the total estuary volume (ca 5%) revealing that only a small fraction is preserved in the stratigraphy. Due to this mismatch between surface and subsurface expression of mud, interpretations of estuarine stratigraphy risk underestimating the influence of mud at the surface on morphodynamics and habitats.
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Keywords: estuarine mud deposits, estuary stratigraphy, mud preservation, shoal accretion, tidal bar morphology, Oceanography, Environmental Science (miscellaneous), Geology, Stratigraphy, Palaeontology
ISSN: 2055-4877
Publisher: John Wiley and Sons Inc.
Note: Funding Information: This research was funded by the Domain of Applied and Engineering Sciences TTW (grant Vici 016.140.316/13710 to MK) of the Netherlands Organisation for Scientific Research (NWO) and is part of the PhD project of LB. HJP and MB were funded by the ERC Consolidator project 647570 awarded to MK. The authors thank Rijkswaterstaat for the sharing of data and their support during the fieldwork. In particular Marco Schrijver, Gert‐Jan Liek, Robert Jentink and Edwin Paree. They also thank Rike Wagner‐Cremer, Marcio Boechat Albernaz, Christian Schwarz, Anne Baar and Jasper Leuven for helpful discussions. Diatom screening of the samples was conducted by Koeman and Bijkerk. Joep Storms and an anonymous reviewer are gratefully acknowledged for helpful feedback on the manuscript. They also thank Bram van Prooijen, Ton Hoitink, Ashish Mehta and two anonymous reviewers for comments on an earlier draft. Funding Information: This research was funded by the Domain of Applied and Engineering Sciences TTW (grant Vici 016.140.316/13710 to MK) of the Netherlands Organisation for Scientific Research (NWO) and is part of the PhD project of LB. HJP and MB were funded by the ERC Consolidator project 647570 awarded to MK. The authors thank Rijkswaterstaat for the sharing of data and their support during the fieldwork. In particular Marco Schrijver, Gert-Jan Liek, Robert Jentink and Edwin Paree. They also thank Rike Wagner-Cremer, Marcio Boechat Albernaz, Christian Schwarz, Anne Baar and Jasper Leuven for helpful discussions. Diatom screening of the samples was conducted by Koeman and Bijkerk. Joep Storms and an anonymous reviewer are gratefully acknowledged for helpful feedback on the manuscript. They also thank Bram van Prooijen, Ton Hoitink, Ashish Mehta and two anonymous reviewers for comments on an earlier draft. Publisher Copyright: © 2022 The Authors. The Depositional Record published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists.
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