Depicting architecture and sedimentology of a hypertidal point bar through Lidar and sedimentary-core data

Abstract

International audienceMorphodynamic behaviour of tidal meanders and internal architecture of related sedimentary bodies havereceived scarce attention, although they are ubiquitous features of coastal landscapes. Expansion of tidalmeanders is known to produce a progressive increase of bend sinuosity, along with accretion of point-bardeposits and formation of inclined heterolithic strata. These deposits are considered to be rich in fine-grainedsediments and tend to record tidal rhythmic deposition in the upper part of the bar, being the lower bar depositsdominated by erosional and bypass processes. Although these criteria are widely accepted, faciesmodels for tidal point bars still lack a 3D perspective and overlook the along-bend variability of sedimentaryprocesses. This knowledge gap can have a direct impact on understanding intra-point-bar heterogeneitiesand connectivity, with implications for reservoir production. The present study focuses on a 3 m deep tidalmeandering channel located in the salt marshes of the hypertidal Mont-Saint-Michel Bay (France), and investigatessedimentology of a time-framed bar accretionary package by means of Lidar-topographic data,geomorphological-field surveys and sedimentary cores. The studied accretionary package was accretedalong the bar between 28/03/2012 and 29/11/2012. Integration between Lidar and sedimentary-core datashows that over this time the bar expanded alternating depositional phases along its seaward and landwardside. The maximum thickness of deposits was accumulated in the bar apex zone, and just landwardof it, where the largest amount of mud was also stored. High accretion rate of the bar apex zone endorsedalso a better preservation of rhythmites, which are almost missing from deposits accumulated along thebar sides (i.e. close to riffles). We suggest that alternating depositional loci and high sediment accretion atthe bend apex zone emerge due to a combination of factors, including: i) the spatio-temporal asymmetricnature of tidal currents, which influenced deposition and preservation of flood and ebb deposits along thebend; and ii) the development of low-energy conditions at the apex due to ebb and flood flow configuration,which also promoted mud settling.This study highlights that mud and tidal rhythmites are not uniformly distributed within point-bar deposits,and their occurrence is strongly controlled by the asymmetric and mutually evasive nature of ebb and floodtides