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

Marta Cosma (1) , Dimitri Lague (2) , Andrea D’alpaos (1) , Jérôme Leroux (2) , Baptiste Feldmann (3) , Massimiliano Ghinassi (1)

1 Unipd - Università degli Studi di Padova = University of Padua
2 GR - Géosciences Rennes
3 OSUR - Observatoire des Sciences de l'Univers de Rennes

Morphodynamic behaviour of tidal meanders and internal architecture of related sedimentary bodies have received scarce attention, although they are ubiquitous features of coastal landscapes. Expansion of tidal meanders is known to produce a progressive increase of bend sinuosity, along with accretion of point-bar deposits and formation of inclined heterolithic strata. These deposits are considered to be rich in fine-grained sediments and tend to record tidal rhythmic deposition in the upper part of the bar, being the lower bar deposits dominated by erosional and bypass processes. 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. This knowledge gap can have a direct impact on understanding intra-point-bar heterogeneities and connectivity, with implications for reservoir production. The present study focuses on a 3 m deep tidal meandering channel located in the salt marshes of the hypertidal Mont-Saint-Michel Bay (France), and investigates sedimentology of a time-framed bar accretionary package by means of Lidar-topographic data, geomorphological-field surveys and sedimentary cores. The studied accretionary package was accreted along the bar between 28/03/2012 and 29/11/2012. Integration between Lidar and sedimentary-core data shows that over this time the bar expanded alternating depositional phases along its seaward and landward side. 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 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. 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 flood tides.

Domains: Geomorphology

Marta Cosma, Dimitri Lague, Andrea D’alpaos, Jérôme Leroux, Baptiste Feldmann, et al.. Depicting architecture and sedimentology of a hypertidal point bar through Lidar and sedimentary-core data. 35th IAS Meeting of Sedimentology, Jun 2021, Virtual, Czech Republic. pp.118. ⟨insu-03373470⟩