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Furgerot et al. (2019)

We present a high-resolution 1:15,000 bathymetric map (Main map) of Alderney Race located offshore of northwestern France, with the strongest currents in Europe. We use this map, underwater video transects and Shipek grabs to improve geological maps
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  Full Terms & Conditions of access and use can be found at  Journal of Maps ISSN: (Print) 1744-5647 (Online) Journal homepage: High-resolution bathymetry of the AlderneyRace and its geological and sedimentologicaldescription (Raz Blanchard, northwest France) Lucille Furgerot, Yohann Poprawski, Marc Violet, Emmanuel Poizot, PascalBailly du Bois, Mehdi Morillon & Yann Mear To cite this article:  Lucille Furgerot, Yohann Poprawski, Marc Violet, Emmanuel Poizot, PascalBailly du Bois, Mehdi Morillon & Yann Mear (2019) High-resolution bathymetry of the AlderneyRace and its geological and sedimentological description (Raz Blanchard, northwest France),Journal of Maps, 15:2, 708-718, DOI: 10.1080/17445647.2019.1657510 To link to this article: © 2019 The Author(s). Published by InformaUK Limited, trading as Taylor & FrancisGroup on behalf of Journal of MapsView supplementary material Published online: 10 Sep 2019.Submit your article to this journal View related articles View Crossmark data  Science High-resolution bathymetry of the Alderney Race and its geological andsedimentological description (Raz Blanchard, northwest France) Lucille Furgerot a , Yohann Poprawski b , Marc Violet c , Emmanuel Poizot a,d , Pascal Bailly du Bois e ,Mehdi Morillon e and Yann Mear a,d a LUSAC, Laboratoire Universitaire des Sciences Appliquées de Cherbourg, University of Normandie, Cherbourg-Octeville, France;  b GeologicDi ff  usion, Bordeaux, France;  c Positioning, Hydrographic and Geophysic Services, Montfarville, France;  d Conservatoire National des Arts etMétiers, Intechmer, Cherbourg-en-Cotentin, France;  e IRSN/DEI/SECRE/LRC, Institut de Radioprotection et de Sûreté Nucléaire, Direction del ’ Environnement et de l ’ Intervention, Laboratoire de Radioécologie de Cherbourg-Octeville, France ABSTRACT We present a high-resolution 1:15,000 bathymetric map (Main map) of Alderney Race locatedo ff  shore of northwestern France, with the strongest currents in Europe. We use this map,underwater video transects and Shipek grabs to improve geological maps previouslypublished. We distinguished Proterozoic crystalline rocks, Paleozoic and Cretaceoussedimentary rocks on the present-day sea  fl oor. Some structures as faults and folds are alsomapped. We identi fi ed a Quaternary cover made of pebbles, boulders and blocks interpretedas corestones resulting in di ff  erential erosion and alteration of the substratum. This cover iscommonly encrusted by  fi xed fauna, such as bryozoans and barnacles. Finally, we describethe present-day mobile sediment cover characterized by sand patches and pebble dune fi elds (up to 10 m in height). Our videos show the presence of mobile  fi ne-grained sedimentpatches under the resolution of our map lying between the cobble and pebble cover. Wesummarize our interpretations on a non-exhaustive geological-sedimentary map. ARTICLE HISTORY Received 28 February 2019Revised 2 August 2019Accepted 6 August 2019 KEYWORDS High-resolution bathymetry;Morpho-sedimentologicalcartography; Alderney Race;Shipek grab; submarinecamera 1. Introduction The English Channel is characterized by strong tidalcurrents, reaching up to 5 m.s − 1 in the Alderney Raceand thus is a suitable area for tidal turbines installationfor Marine Renewable Energy creation. Several studieson tidal stream resource estimation and turbine impacton the  fl ow have been carried out in the Alderney Race(Bailly du Bois, Dumas, Solier, & Voiseux, 2012; Coles, Blunden, & Bahaj, 2017; Thiébot, Bailly du Bois, & Guillou, 2015), however only a few studies about sedi-ment transport (Thiébot et al., 2015) and bathymetry have been done. Although deposition of   fi ne particlesis expected to be di ffi cult with strong currents, Foveauand Dauvin (2017) described mobile sediment patches,mainly composed of sand and pebbles. Coarser sedi-ments dominated by pebbles and cobbles have beendescribed by  Larsonneur, Bouysse, and Au ff  ret (1982)around the exposed bedrock. Foveau and Dauvin(2017) also described poorly sorted and rounded coarsesediments lacking of   fi xed fauna, re fl ecting theimportant bedload transport in these high-energy environments.In the Alderney Race, the morphology of the bed-rock and the presence of sediments available for trans-port are poorly constrained. However, this knowledgeessential for tidal turbine installation: (i) forimprovement of tidal stream resource estimation withnumerical modeling considering the turbulence gener-ated by bed roughness and depth variations, (ii) for theimpact of the turbines on sediment, and conversely (iii)for possible sediment abrasive impact on the turbines.This paper aims to (1) describe and identify di ff  erentmorphologies from the bathymetric map (Main map)in the Alderney Race area, (2) provide an interpretationfor these di ff  erent bathymetric morphologies using previously published geological and sedimentologicalinformation, new underwater video observations andsediment samples, and (3) show the presence of   fi nesediments potentially mobile with the strong currentof the study area. 2. Regional setting The study area displays Paleo and Neoproterozoic crys-talline rocks and Paleozoic and Cretaceous sedimen-tary rocks (Figure 1(B)). It includes the Icartiangneiss (Paleoproterozoic Pentevrian basement,2.1 Ga) exposed in the Anse du Culeron (located onFigure 2). The Neoproterozoic crystalline rocks consistof two generations of quartz diorite plutons, emplacedfrom 620 to 608 Ma in a back-arc setting and from 570to 540 Ma during the Cadomian orogeny (e.g. © 2019 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of Journal of MapsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License ( ), which permits unrestricteduse, distribution, and reproduction in any medium, provided the srcinal work is properly cited. CONTACT  Lucille Furgerot LUSAC, Laboratoire Universitaire des Sciences Appliquées de Cherbourg, University of Normandie, 145 Chemin de la Crespinière, 50130 Cherbourg-Octeville, France JOURNAL OF MAPS2019, VOL. 15, NO. 2, 708 – 718  Dallmeyer, D ’ Lemos, Strachan, & Mueller, 1991; Inglis,Samson, D ’ Lemos, & Miller, 2005). Cadomian struc-tures in the area consist of SE-directed thrusts striking N45, with a left-lateral component (e.g. Ballèvre, LeGo ff  , & Hébert, 2001; Chantraine et al., 2001). The Paleozoic succession consists of Cambrian toDevonian sedimentary rocks, exposed in the Siouvilleand Jobourg Synclines (Figure 1(B)), striking N110.These synclines are a result of the Variscan Orogeny (Dissler & Gresselin, 1988), that occurred around320 Ma in the area (Ballèvre, Bosse, Ducassou, & Pitra, 2009).The Mesozoic succession is composed of Triassicconglomerates, exposed near the La Pernelle Plateauand Upper Cretaceous  fl int-bearing chalk mapped inthe o ff  shore in the northwest part of our study area(Larsonneur & Walker, 1982). During the Mesozoic,several regression and transgression cycles occurredabove the Variscan basement. Early to Middle Jurassicsedimentary rocks deposited and have been sub-sequently eroded in the Cotentin Peninsula (e.g.Dugué, 2007). During the Early Cretaceous a majorregressive event occurred, evidenced by a weathering and peneplation surface found in Brittany and Nor-mandy (Bessin, Guillocheau, Robin, Schroëtter, & Bauer, 2015). During the Late Cretaceous, previously emerged surfaces were drowned by transgression, asBRGM maps (Bureau des Ressources Géologiques etMinières) show remnants of Upper Cretaceous  fl int-bearing chalk above Proterozoic and Paleozoic rock in Brittany (e.g. Doré, Dupret, Le Gall, & Chalot-Prat, 1977; see Bessin et al., 2015 for more details). During Cenozoic, the Cotentin Peninsula under-went uplift, as shown by four marine terraces andfour Rasas (Pedoja et al., 2018). This regional uplift isrelated to the far- fi eld e ff  ects of the Alpine orogeny that induced the reactivation of some faults, such asthe La Hague fault (e.g. Lagarde et al., 2000, 2003). Quaternary eustatic sea level falls (up to  − 110 mbelow present-day sea level) linked with major climaticchanges induced strong incision, as the Cotentin andthe La Hague deeps (up to 90 m bsl) are interpretedas a  fl uvial paleo-drainage related to tributary riversof the Seine (Antoine et al., 2003) (Figure 1(A)). Figure 1.  (A). Map of the English Channel showing the deeps that coincide with the paleo-Seine and its tributaries (Antoine et al.,2003) and the location of our study area (red rectangle); (B). Geological map (modi fi ed from geological map of France 1/50 000); (C).Sedimentary map (modi fi ed from data.shom); (D). Simpli fi ed bathymetric map showing the maximal tidal current intensities at highand low tides from numerical model MARS2D, resolution of 100 m (method from Bailly du Bois et al., 2012). JOURNAL OF MAPS 709  At present, strong tidal currents prevent signi fi cantdeposition in the deeps, preserving the Quaternary  fl uvial paleo-drainage (Lericolais, 1997). The regionaltwo-dimensional currents model (MARS2D) of  Bailly du Bois et al. (2012) shows high northward/southwardtidal currents west of La Hague Cape (red arrows inFigure 1(D)). North of the Peninsula, the eastward/westward currents align with the La Hague deep.This model has been validated by numerous physicaloceanographic data acquired with conventional tech-niques: bathymetric survey, measurement of variationsin water levels, current measurements, tracking of drif-ters and dispersion of soluble tracers (details in Bailly du Bois et al., 2012). Since, other models and new  fi eld campaigns have been used to improve knowledgeof the hydrodynamics (Thiébot et al., 2015). Currentsgenerally exceed 2 m s − 1 and reach up to 5 m s − 1 during the peak of spring tide and thus are able tomove particles of several cm in diameter according toHjulström (1935). The sedimentary map from theShom (Service Hydrographique et Océanographiquede la Marine) database (Figure 1(C)) shows diversetypes of sediments ranging from sand in the bays tocobbles near the exposed bedrock. Despite the strong currents in this area, patches of mixed sand and peb-bles are described in the Alderney Race area on thismap and in Foveau and Dauvin (2017) (Figure 1(C)). 3. Methods 3.1. Bathymetry  Various bathymetric sources were data used to buildthe bathymetric map come from:(1) the HOMONIM project for the main dataset,acquired using multibeam echosounders (resol-ution up to 1 m) or with a single beam sounder(resolution up to 100 m) (Shom, 2015);(2) the NHDF (Normandie Hauts de France) lidarproject 2016 – 2017 focused on the coastal fringewith a horizontal resolution of 1 m (Shom- Figure 2.  Bathymetric map of the Alderney Race with the location of video transects (red stars, named Tn or Vn with indicating theend of transect). Speci fi c bathymetric morphologies (blue rectangles) are described in detail on the next  fi gures. The yellow trans-ects indicate the bathymetric sections. The dashed brown rectangle represents the high bathymetric map extent. 710 L. FURGEROT ET AL.  ROLNP, 2018) up to 10 m of depth (at least) andsometimes 20 m (depends on the transparency of the water);(3) and the BATHAGUE project 2008 – 2010, for theshallow depths (from 2 or 3 m up to 60 m) andacquired by IRSN (Institut de Radioprotection etde Sûreté Nucléaire) using an interferometricsonar (resolution up to 1 m) (Bailly du Bois, 2008).The  fi nal bathymetric map was created using Uni- versal Transverse Mercator (UTM 30N) conformalprojection and WGS84 reference ellipsoid. The heightreference (0 m elevation) was  fi xed on the low spring tide mean level in Goury (located on bathymetricmap). The di ff  erence between the local Hydrographiczero and the terrestrial zero (IGN69) is 4.827 m (refer-ence: Référénce Altimétriques Maritimes du Shom).This map identi fi ed by a brown rectangle in Figure 2covers the NW of Cotentin Peninsula. However, ourstudy area extends southward, as the southern areasare essential to connect the onshore geology witho ff  shore areas (Figure 1(B)). This southern area is notintegrated into the bathymetric map because of thelack of high-resolution bathymetric data (blanks inFigure 2 and bathymetric map).We used the high-resolution bathymetric map toidentify di ff  erent morphologies, such as tilted Paleozoicbeds, Paleozoic and Proterozoic crystalline rocks, Cre-taceous  fl atbed surfaces and present-day subaqueousdunes. The interpretation of these substrate typescon fi rmed using onshore-o ff  shore correlations fromthe available geological maps and compared with aerialphotographs of the emerged rocky platform. We alsoused the bathymetric map to build bathymetric sec-tions allowing the observation of large structures,such as dunes and the measurement of bedding dips,only for the low-dipping Cretaceous strata. 3.2. Nature of the seabed  We used an underwater camera and a Shipek grab tocon fi rm our interpretations of the described mor-phologies from bathymetric map and provide additionaland more accurate information on sedimentary cover(location of points in Figure 2).The underwater video allows the characterization of the sediment cover at the scale of several square meters.However, the camera can only be deployed for 15 minwhen currents decrease during the short slack currentwater and only in good weather conditions. Thedeployment depth is limited by cable length for thehigh de fi nition transmission to the boat (70 m). Ascale of 10 cm  fi xed between the three weighted feetof the camera allows the estimation of the size of sedi-ments when it reaches the bottom. Therefore, the videos provide information about the size of sedimenton the bottom and their location for the lowest current velocity stage. Twenty-three video (covering in totalabout 3 km) transects were conducted for a qualitativedescription of the sedimentary cover.As the Shipek grab is working only for pebbles or fi ner sediments that relatively rare in the area, only 8samples among the 12 collected yielded a su ffi cient volume of sediment for analyses. These samples pro- vide direct information on seabed composition. 4. Results and discussion 4.1. Geology of the deep domain The La Hague deep consists of a deep submarinetrough that coincides with a Quaternary channel of apaleo-tributary of the Seine River incised into theUpper Cretaceous chalk (e.g. Antoine et al., 2003).The Shipek grab samples and videos, mainly showing Cretaceous  fl int rocks pebbles in the area (Figure 3(B,C)) together with the relatively   fl at bedding dips onthe bathymetric map support the Cretaceous age forthe substratum, as proposed by  Larsonneur andWalker (1982). This deep is striking NE – SW west of the La Hague Cape and then E – W north of the LaHague Cape (bathymetric map). At its southern ter-mination, the deep progressively connects with theshallow platform, as the depth gently decreasessouthwestward.West of the La Hague deep, an isolated small deep isdeveloped in Cretaceous chalk (Figure 3(D)). This deepis separated from the La Hague deep by three mainsteps slightly dipping eastward (from 2° up to 5°,pro fi le a-a ′ , Figure 3(F)) and interpreted as slightly dip-ping Cretaceous strata. This is supported by the pres-ence of similar cuesta morphologies located west of the small deep.Across the small deep, the correlation between the fl at strata (see the red star Figure 3(A,D and F)) andthe slightly dipping strata suggests a gentle anticlinalstriking N45°, developed in the Cretaceous rocks.This interpretation is supported by the work of  Benab-delhouahed (2011), who showed several folds withsimilar orientations related with the far- fi eld e ff  ect of the Alpine orogeny in the English Channel. Thesmall deep is opened at the crest of the anticline andincludes di ff  erent depressions, reaching up to 90 mbsl (Figure 3(D)). Similar structures in the EnglishChannel have been attributed to thermokarst devel-oped in the Cretaceous chalk (Lericolais, 1997). There-fore, the small deep may be interpreted as resulting of karstic processes a ff  ecting the Cretaceous chalk andoccurring during Pliocene-Pleistocene lowstands.According to our video, the cuesta morphologies westof the small deep are covered by relatively angular cob-bles and blocks with  fi xed fauna (Figure 3(E)). Thesemaycorrespond either toPliocene-Pleistocene periglacialdeposits, similar to those of the Herquemoulin locality  JOURNAL OF MAPS 711
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