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

Intertidal meanders developed on salt marshes are known to expand and produce inclined heterolithic stratification rich in fine-grained sediments and tobear evidence for rhythmic deposition in the upper part of the inner meanderbend (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 anddevelop discontinuous record of tidal cycles. Although these criteria are widelyaccepted, facies models for tidal point bars still lack a three-dimensional perspective and overlook the along-bend variability of sediment distribution. Thepresent study focuses on a hypertidal point bar belonging to the upper-intertidal domain of the Mont-Saint-Michel Bay (France), and it analyses thesedimentology of a 3D time-framed accretionary package formed between 28March 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 andranging) topographic surveys were used to create the point-bar synthetic stratigraphy. Data show that over the study period the point bar expanded alternatingdeposition along its seaward and landward sides, pointing out the occurrenceof depositional patterns more complex than a simple progressive expansion ofthe bend. The thickest deposits were accumulated in the point-bar-apex zone,where the largest amount of mud was also stored. High sediment accretion inthe bend-apex zone is ascribed to the development of low-energy conditionsdue to flow and bed configuration. High accretion rate of the point-bar-apexzone promoted also a better preservation of rhythmites, which are almost missing from deposits accumulated along the point-bar sides. This study remarksthat preservation of mud and tidal rhythmites within intertidal-point-bardeposits is controlled, not only by their elevation with respect to the tidal range,but also by their location along the point bar.