Long term evolution and internal architecture of a high-energy banner ridge from seismic survey of Banc du Four (Western Brittany, France)

Abstract

International audienceThe recent completion of a coupled seismic and swath bathymetric survey, conducted across the sand ridge system of the Banc du Four located on the Atlantic continental shelf of Brittany (Mer d’Iroise, France), provided new data for the study of the long term evolution of deep tidal sand ridges. Five seismic units are distinguished within the ridge, separated by pronounced major bounding surfaces. The basal unit is interpreted to be shoreface deposits forming the core of the ridge. It is overlaid by a succession of marine sand dunes fields forming the upper units. Sandwave climbing, which combines progradation and accretion, is the major process controlling the growth of the ridge. The elevation of the preserved dune foresets reaches values of about 20–30 m within the ridge. The foresets indicate a combination of giant dunes characterized by numerous steep (up to 20°) clinoforms corresponding to a high-energy depositional environment. Moreover, the presence of scour pits linked to the 3D geometries of giant dunes allow the growth of bedforms migrating oblique to the orientation of giant dune crest lines. All of the radiocarbon ages of the biogenic surficial deposits of the Banc du Four range from 10,036 to 2748 cal years B.P. and suggest the Banc du Four has grown during the last sea-level rise. The apparent absence of recent surface deposits could be caused by a change in benthic biogenic productivity or the non-conservation of recent deposits. In contrast, the presence of relatively old sands at the top of the ridge could be explained by the reworking and leakage of the lower units that outcrop locally at the seabed across the ridge. Moreover, the long-term evolution of the ridge appears strongly controlled by the morphology of the igneous basement. The multiphase accretion of the ridge is closely linked to the presence of a residual tidal current eddy, consecutive with the progressive flooding of the coastal promontories and straits that structured the igneous basement.Therefore, the Banc du Four should be thought of as a representative example of a large-scale high-energy banner bank

Tide‐induced bedload transport pathways in a multiple‐sand‐ridge system offshore of Hainan Island in the Beibu Gulf, northwest South China Sea

“…Tidal sand ridges grow slowly and usually have a close relationship with progression. As they can potentially record the history of sediment transport changes during sea level rises, the evolution of tidal sand ridges has drawn considerable attention from many marine geoscientists (Marsset et al, 1999;Reynaud et al, 1999c;Trentesaux et al, 1999;Park et al, 2003;Liu et al, 2007;Scourse et al, 2009;Wu et al, 2010;Snedden et al, 2011;Franzetti et al, 2015). In modern times, the sand ridges in various seas could occur at different life stages (active/moribund); nevertheless, they mostly represent a balance among sediment, bedform, and modern current dynamics (Reynaud et al, 1999b;Schmitt et al, 2007).…”

“…Inside the ridge system, many sand ridges are asymmetrical and can migrate towards the steeper side, suggesting the direction of local net sand transport (modern or historical) (Caston and Stride, 1970;Kenyon et al, 1981;Trentesaux et al, 1999;Liu et al, 2007). En echelon sand ridges could also form along with sea level rises and shoreline retreats, thus leaving relict cores inside their bodies (Reynaud et al, 1999c;Trentesaux et al, 1999;Park et al, 2003;Franzetti et al, 2015). Either a modern origin or origin related to progression are inferred to be reasonable for en echelon sand ridges, which only depend on specific environments (Harris, 1988).…”

Dynamics and stratigraphy of a tidal sand ridge in the Bristol Channel (Nash Sands banner bank) from repeated high‐resolution multibeam echo‐sounder surveys

“…Seismic images collected perpendicular to tidal sand ridges commonly include reflection sequences dipping sub‐parallel to their modern dipping flanks, i.e. clinoforms (Houbolt, 1968; Berné et al ., 1993, 1994; Reynaud et al ., 1999; Trentesaux et al ., 1999; Bastos et al ., 2003; Chaumillon et al ., 2013; Franzetti et al ., 2015). These suggest recent evolution by progressive deposition of sand on the ridge flanks and is observed locally here (Fig.…”

“…The processes operating during the lifetimes of tidal ridges and bars and their ultimate fates have been addressed through a combination of active monitoring (for example, time‐lapse bathymetric data, as here), sampling and geophysical surveys (Berné et al ., 1994, 1998; Fenies & Taslet, 1998; Reynaud et al ., 1999; Trentesaux et al ., 1999; Vecchi et al ., 2013; Franzetti et al ., 2015; Lockhart et al ., 2018), hydrodynamic modelling (De Vriend, 1990; Hulscher et al ., 1993; van Veelen et al ., 2018), and analyses of ancient examples preserved in cored subsurface sections (Folkestad and Satur, 2008; Schwarz et al ., 2011; Messina et al ., 2014; Wei et al ., 2018; Chiarella et al ., 2020) and in the rock record (Tillman & Martinsen, 1984; Gaynor & Swift, 1988; Mellere & Steel, 1995; Martinsen et al ., 1999; Yoshida, 2000; Plink‐Björklund & Steel, 2006; Plink‐Björklund, 2008; Steel et al ., 2008, 2012; Pontén & Plink‐Björklund, 2009; Hampson, 2010; Michaud, 2011; Martinius, 2012; Olariu et al ., 2012b; Scasso et al ., 2012; Chen et al ., 2014; López et al ., 2016; Michaud & Dalrymple, 2016; Sharafi et al ., 2016; Leszczyński & Nemec, 2020; Longhitano et al ., 2021). The latter potentially provide the finest spatial information but, before assessing it, confidence needs to be established that tidal ridges or bars have been correctly identified.…”

Deep‐water dunes on drowned isolated carbonate terraces (Mozambique Channel, south‐west Indian Ocean)

“…). This geometry is very similar to sandbanks (Franzetti et al ., ) and to plastered drifts (muddy contourites), which typically have an oval shape in plan view and a convex shape in cross‐section (Miramontes et al ., ). The resolution of the seismic data used for the present study does not reveal the internal architecture of the sediment bodies, which prevents a more specific classification.…”