3D architecture and along-bend sediment distribution of a hypertidal point bar (Mont-Saint-Michel Bay, France)

Marta Cosma - "National Research Council of Italy - CNR Institute of Geosciences and Earth Resources – IGG"

Tidal meandering channels are ubiquitous features of coastal landscapes. Their migration produces point-bar deposits characterized by inclined heterolithic stratification, fining-upward vertical trends, abundance of fine-grained sediments, and tidal rhythmites. Although these criteria are widely accepted, facies models for tidal point bars still lack a 3D perspective and overlook the along-bend variability of sedimentary processes. In this seminar, we will focus on a hypertidal point bar belonging to the upper-intertidal domain of the Mont-Saint-Michel Bay (France), and we will look at the sedimentology of a 3D time-framed accretionary package formed between 28/03/2012 and 29/11/2012. Integration between Lidar topographic time-series data, geomorphological field surveys and sedimentary-core data shows that over this time the bar expanded alternating depositional phases along its seaward and landward sides. The maximum thickness of deposits was accumulated in the bar apex zone, and just landward of it, where the largest amount of mud was also stored. High accretion rate of the bar apex zone endorsed also a better preservation of tidal rhythmites, which are almost missing from deposits accumulated along the bar sides (i.e. close to riffles). We suggest that alternating depositional loci and high sediment accretion at the bend apex zone emerge due to a combination of factors, including: i) the spatio-temporal asymmetric nature of tidal currents, which influenced deposition and preservation of flood and ebb deposits along the bend; and ii) the development of low-energy conditions at the apex due to ebb and flood flow configuration, which also promoted mud settling.