Incised-valley morphologies and sedimentary-fills within the inner shelf of the northern Bay of Biscay

This study is a first synthesis focused on incised-valleys located within the inner shelf of the Bay of Biscay. It is based on previously published results obtained during recent seismic surveys and coring campaigns. The morphology of the valleys appears to be strongly controlled by tectonics and lithology. The Pleistocene sedimentary cover of the shelf is very thin and discontinuous with a maximum thickness ranging between 30 and 40 m in incised-valley fills. Thus the incised bedrock morphology plays a key-role by controlling hydrodynamics and related sediment transport and deposition that explains some variations of those incised-valley fills with respect to the previously published general models.

💡 Research Summary

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This paper presents the first comprehensive synthesis of incised valleys located on the inner shelf of the northern Bay of Biscay, integrating results from recent high‑resolution seismic surveys and coring campaigns. The authors examine ten well‑studied valleys (Leyre, Seudre, Charente, Lay‑Sèvre, Loire, Artimon, Vilaine, Etel, Lorient, and Concarneau) and assess how tectonic framework, bedrock lithology, river discharge, tidal range, and wave climate control valley morphology and sedimentary fill.

The regional tectonic setting is a passive continental margin inherited from Cretaceous opening and subsequent re‑activation of Hercynian faults. Two dominant structural patterns are identified: linear, NW‑SE‑oriented valleys that follow fault scarps, and rectangular networks where valleys intersect faults at oblique angles. Bedrock lithology further influences incision direction, with soft marly strata being preferentially eroded beneath harder carbonate units.

A Late Cenozoic erosional surface, interpreted as a Type 2 sequence boundary, forms the base of all valleys. This surface was likely generated during older lowstands and re‑worked during the last glacial sea‑level fall (≈20–18 ka). Valley fills are generally thin (30–40 ms two‑way travel time, ≈25–35 m) and discontinuous, especially where strong tidal currents or sand‑wave activity have removed sediments. Larger rivers (Gironde, Loire) deliver abundant fine material, producing thicker, more continuous fills, whereas smaller rivers (Leyre, Seudre, etc.) show fragmented deposits.

Four seismic units (U1‑U4) characterize the sedimentary succession:

• U1 (lowstand system tract) consists of fluvial sands and gravels; thickness and continuity vary with river size.
• U2 (transgressive system tract) is dominated by estuarine mud or mixed sand‑mud, displaying sub‑horizontal reflectors and occasional channelised unconformities.
• U3 (upper transgressive system tract) comprises coarse tidal‑channel sands and mouth‑bar deposits, marked by inclined reflectors and channelised features.
• U4 (highstand system tract) forms a sheet‑like drape of mud, shoreface sand, or mixed sand‑mud, representing the modern seafloor.

The authors note a systematic seaward shallowing of incision depth below 40–60 m, linked to the decreasing shelf gradient, mirroring observations on other passive margins. Comparisons with classic models (Zaitlin et al., 1994; Ashley & Sheridan, 1994) reveal that Bay of Biscay valleys deviate markedly: the interplay of structural control, lithology, river discharge, and mixed tide‑wave dynamics generates a more complex, multi‑system fill architecture than the simple single‑cycle models.

The study underscores the importance of integrating seismic stratigraphy with core data to unravel the controls on incised‑valley evolution. It provides a robust framework for reconstructing past sea‑level changes, assessing sediment budgets, and informing coastal management in wave‑dominated shelves. Future work should aim at high‑resolution 3‑D seismic interpretation, refined radiocarbon dating of each system tract, and numerical modelling of hydrodynamic processes to quantify sediment transport pathways within these structurally controlled valleys.