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The Brittany region of France is located in a low seismicity intraplate zone. Most of the instrumented earthquakes are limited to a shallow crustal depth without surface rupture. A paleoseismological analysis was performed on deposits on the Crozon Peninsula and in the Elorn estuary. We highlight hydroplastic deformations induced by liquefaction leading to clay diapirism, which were likely triggered by past earthquakes. This diapirism seems to be frequent in continental nonconsolidated sediments and to develop on the inherited tectonic structures, when a shallow water table and confining layers exist. Timing of deformation is dated using paleoenvironmental data, and electron spin resonance and infrared-stimulated luminescence dating methods. Two seismic periods were identified in western Europe during early Marine Oxygen Isotope Stage (MIS) 10 (~380 ka) and early MIS 8 (~280–265 ka). The lack of similar deformations affecting the Holocene tidal deposits in the Bay of Brest suggests that the magnitude of the triggering paleoearthquakes is probably higher (M w ~6) than the recent events (M w 5.4). These unusual intraplate major paleoearthquakes need specific factors affecting the far-field crustal stress loading to be triggered, such as a brief acceleration of the Africa-Eurasia lithospheric plate convergence, glacio-isostatic stress perturbations associated with the onset of major glaciations in northern Europe, or other processes induced by orbital forcing. [ABSTRACT FROM AUTHOR]

The 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. [ABSTRACT FROM AUTHOR]

Our study aims to understand the recurring climatic conditions prevailing during the largest storms reaching NW France (Brittany). These storms are responsible for the breaching of coastal barriers and major flooding of lowlands. In a first part of our work, we examine the morphological impact and stratigraphic record of storm events along Western Brittany rocky coasts, with a special focus on the southern coast of the Bay of Audierne, the most exposed coast of the region. In a second paper (‘Middle- to Late-Holocene Storminess in Brittany (NW France): Part II’), we shall focus on the chronology of storm events and their climate forcing conditions. Drilling transects and stratigraphic analyses were first undertaken to constrain chronology, strength and wind direction during the main Holocene storm events. New dates, observations and a relative sea-level (RSL) curve were then used to inform discussion of the necessary climatic and morphologic conditions leading to destructive storm events. Most recorded events appear to be linked with cooling episodes of the Holocene and a RSL close to present. Some storms are clearly responsible for breaching and dune building or remobilisation. We demonstrate that storm frequency and intensity appear to rise in a stepwise manner during the late Holocene. Maximum efficiency is reached during the ‘Little Ice Age’ with clustered events probably lasting several days, but major storms also occurred immediately prior to the ‘Medieval Warm Period’. We suggest that recent coastal dune building from c.ad 1100 until now, despite a sea level close to present and continuously rising, may be a direct consequence of the restoration of beaches after periods of recurrent storminess. This building activity often occurred during dry negative North Atlantic Oscillation (NAO) events, in connection with the available sedimentary supply. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this paper is to improve the knowledge of spatio-temporal variability of very large to giant dunes in deep tide dominated environment. Their growth mechanisms and dynamic evolution still remain in debate and constitute a significant scientific challenge in regard to difficulties to achieve accurate measures of migrations. This motivated the realisation of three recent swath bathymetry surveys across the wide dune field of the Banc du Four located offshore the western Brittany. The Banc du Four field is composed of more than 500 large dunes exhibiting a great diversity of morphologies ranging from 2D to 3D shapes and reaching the largest sizes of such sedimentary structure ever described (wavelength over 1000m and height over 30m). The analysis of the entire ensemble of dune parameters offers the opportunity to discuss the relative influence of forcings and the validity of the equations predicting the bedform geometry in shallow waters. Our results show a good height-spacing power law correlation but do not strike the usual statistical regression observed in previous studies. The steeper positive slope of the equation and the outstanding values of the height parameter reveal the complexity of local hydrodynamic regime, interacting with bed load transport and sediment supply. No clear relationship has been observed between asymmetry and size parameters. Water depth cannot be considered as a major factor controlling the size of dunes. Otherwise, the vertical sedimentary variation suggests the action of storm waves on dune crests and the horizontal migration rates of dunes in the eastern field of the Banc du Four were found to range from 3 to 20m·yr−1. Such velocities were only mentioned before by Van Landeghem et al. (2012) on deep continental shelves (>70m deep) and attest of the still present morphodynamical equilibrium of the large dunes. Furthermore, as was previously found for similar dunes in the Irish Sea, no power–law relationship between dune migration rates and dune size can be deduced. Similarly, no strong correlation is established between migration rates and the degree of asymmetry. Nevertheless, the dune asymmetry can be used to predict the migration direction. As for dimension parameters, the evolution asymmetry depends on the study area and migration rates. More precisely, when the crest lines of dunes form straight lines and their migration rates remain below 8–10m·yr−1, the degree of dune asymmetry increases with the migration rate. This trend is reversed for the dunes with higher migration velocities and crests which are sinuous. These results relativize the migratory predictions based on punctual geometrical observations. [Copyright &y& Elsevier]

Sediment structures including submarine banks and dune fields are ubiquitous on tide-dominated continental inner shelves such as in the Iroise Sea. These are of current interest to human activities in several respects: they constitute an obstacle to navigation, they are dredged for beach nourishment or exploited for marine aggregates. In addition, the morphodynamic characterising of these sedimentary structures improves the knowledge of the sediment transfers that occur in coastal areas. This study documents a submarine sand dune field located along the northern flank of the Goulet channel connecting the Bay of Brest to the Iroise Sea and the Bay of Biscay. Subject to a macrotidal regime with strong currents, and to large waves during storm events, this sedimentary system features large dunes with very high migration rates. The analysis of six bathymetric datasets (from March 2013 to October 2019) allows specifying the morphodynamic characteristics of this small dune field about 3.5 km in length and 500 m in width. These dunes have heights on the order of 0.5 m–3.7 m with migration rates that can vary significantly within the range from 10 m/yr to 70 m/yr. The results highlight that the ebb tidal current and slope of the channel are the main factors controlling the evolution of these biogenic sandy structures migrating offshore (SW). Furthermore, seasonal variations in coastal hydrodynamics forcing, driven by tidal currents, appear to affect the temporal and spatial evolution of the dunes at this shorter time scale. This paper proposes a model of sediment transport patterns at the mouth of the bay of Brest according dune field characteristic, strong ebb current and residual tidal gyre. • Channelised tidal currents in macrotidal environment induce large variation in dune migration rates across a dune field at the outlet of the Bay of Brest. • Tidal gyre in the shallower nearshore terrace contributes to the recirculation of sediment from dune field through external pathways. • The steep slopes of the channel impact dune migration rates, with reduced rates on the negative slopes and increased rates on the positive slopes. [ABSTRACT FROM AUTHOR]