Sedimentology of a hypertidal point bar (Mont‐Saint‐Michel Bay, north‐western France) revealed by combining lidar time‐series and sedimentary core data

Abstract

Intertidal meanders developed on salt marshes are known to expand and produce inclined heterolithic stratification rich in fine‐grained sediments and to bear evidence for rhythmic deposition in the upper part of the inner meander bend (i.e. the upper part of the point bar). This occurs because the lower point‐bar deposits are washed by strong currents, which remove mud drapes and develop discontinuous record of tidal cycles. Although these criteria are widely accepted, facies models for tidal point bars still lack a three‐dimensional perspective and overlook the along‐bend variability of sediment distribution. The present study focuses on a hypertidal point bar belonging to the upper‐intertidal domain of the Mont‐Saint‐Michel Bay (France), and it analyses the sedimentology of a 3D time‐framed accretionary package formed between 28 March 2012 and 29 November 2012 by means of lidar topographic data, geomorphological field surveys and sedimentological core data. To define the 3D time‐framed accretionary package, data from thirteen lidar (light detection and ranging) topographic surveys were used to create the point‐bar synthetic stratigraphy. Data show that over the study period the point bar expanded alternating deposition along its seaward and landward sides, pointing out the occurrence of depositional patterns more complex than a simple progressive expansion of the bend. The thickest deposits were accumulated in the point‐bar‐apex zone, where the largest amount of mud was also stored. High sediment accretion in the bend‐apex zone is ascribed to the development of low‐energy conditions due to flow and bed configuration. High accretion rate of the point‐bar‐apex zone promoted also a better preservation of rhythmites, which are almost missing from deposits accumulated along the point‐bar sides. This study remarks that preservation of mud and tidal rhythmites within intertidal‐point‐bar deposits is controlled, not only by their elevation with respect to the tidal range, but also by their location along the point bar.

Morpho‐sedimentary evolution of a microtidal meandering channel driven by 130 years of natural and anthropogenic modifications of the Venice Lagoon (Italy)

“…where tidal flat areas are reduced compared to salt marshes), more energetic meso‐ or macrotidal regimes compared to the microtidal setting analysed here (Archer, 2013), and/or different sediment physical properties (e.g. in tidal settings much sandier than the Venice Lagoon) could be much less dependent on the action of wind waves compared to the conditions we illustrated above, thus potentially giving rise to sedimentary features different from those we describe here, especially in term of bedform morphologies and the presence of tidal rhythmites (Bartholdy et al, 2002; Choi, 2010, 2011b; Choi et al, 2013, 2021; Choi & Dalrymple, 2004; Cosma et al, 2022; Dalrymple et al, 2012; Flemming & Bartholoma, 2009; Tessier, 2012; Tessier et al, 1995, 2010). This has potential implications not only for the development of tidal bedforms, but also for the dynamics of meander bends, which are critically influenced by the availability of external sediment supply (Constantine et al, 2014; Horton et al, 2017).…”

Hydrodynamics of Meander Bends in Intertidal Mudflats: A Field Study From the Macrotidal Yangkou Coast, China

“…If, on the one hand, bank collapses driven by seepage flow are also commonly documented in vegetated macrotidal settings (Cosma et al., 2022; K. Zhao et al., 2022) and are therefore more closely linked to sustained tidal oscillations, on the other hand, the abundant erosional gullies (Figure 13) observed at channel banks are most likely specific of unvegetated settings. The formation of such gullies, which can significantly contribute to bank erosion processes, is promoted by strong erosion at the ebb‐bankfull transition and favored by the absence of vegetation cover (Guimond & Tamborski, 2021).…”

“…If, on the one hand, bank collapses driven by seepage flow are also commonly documented in vegetated macrotidal settings (Cosma et al, 2022;K. Zhao et al, 2022) and are therefore more closely linked to sustained tidal oscillations, on the other hand, the abundant erosional gullies (Figure 13) observed at channel banks are most likely specific of unvegetated settings.…”

Evaluating the Impact of Engineering Works in Tidal Flats Using Time-Series Satellite Images – Case of the Mont-Saint-Michel Bay, France

“…Seminal lidar works were carried out for the study of the vegetation of the schorre [31]. More recently, lidar data have been acquired close to the Mont to control the efficiency of the new facilities [32] and to study facies models for tidal point bars lacking a three-dimensional perspective [33].…”