Serge Berné

Comptes Rendus Géoscience

C. R. Geoscience 334 (2002) 731, 732 2002 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS. Tous droits réservésS1631-0713(02)01811-4/FLAPERSPECTIVEEvolution of sand banksSerge BernéIfremer, DRO/GM/D, centre de Brest, technopole de Brest-Iroise, BP 70, 29280 Plouzané, FranceThe paper by Chaumillon et al. [7] presents aninteresting case where a sufficiently long record ofhydrographic surveys allows following the evolutionof an estuary sand bank. In contrast with offshore sandbanks that were not precisely positioned on formernautical charts, such coastal shoals, that represent athreat for navigation, were mapped using horizontalsextant with an accuracy of a few metres. Since 1822,the bank experienced changes in deposition and/orerosion rates, possibly in relation with anthropiceffects. The data set also demonstrates that at least theupper part of such sand banks is still in equilibriumwith the present-day hydrodynamic regime, with anaverage progradation rate of about 6 m yr−1 for thesouthwestern flank of the bank. The mechanismsof sand accretion are not known for the ‘Longede Boyard’, but in the southern North Sea, veryprecise repeated bathymetric surveys demonstratedthat deposition on the ‘lee’ side of the MiddelkerkeBank happened during major storms that erodedsediment from the top of the bank [12, 13]. Along theDutch coast, van de Meene [24] was able to date shellsfrom internal clinoforms of shore-connected ridges,and inferred a net migration rate of 0.5 to 1 m yr−1of the ‘steep’ side of the ridges. Using precise radio-electrical positioning, it has also been shown that tidaldunes off the Cherbourg Peninsula, 7–8 m in height,were migrating northward at a rate of 12 m yr−1[1, 3], whereas barkhan-like dunes in the Dover Strait,8–10 m in height, migrate towards the northeast at70 m yr−1 [2].As a matter of fact, the long lasting debate betweenoceanographers considering that all large bedformsare moribund, and those in favour of active processes,has no more reason to persist. There is no doubtthat, in areas where current velocity near the sea-floorreaches value of about 0.6 m s−1 during spring tides,and/or if combined wave and current shear stressis sufficient for transport of any size of sand, largebedforms experience migration that can be detectedthrough successive bathymetric surveys.This does not mean that longer time-scales, suchas the ‘deglacial’ sea-level rise, are inappropriate forunderstanding the origin of sand banks: the seismicdata of Chaumillon et al. [7] clearly show that theLonge de Boyard consists of two distinct units, the up-permost being characterized by relatively steep (5◦?)clinoforms, whereas the lowermost displays high am-plitude, sub-parallel reflections. The authors interpretthese two seismic facies as high-energy and low-energy indicators, similar to the upper- and lower-shoreface facies described by Rabineau et al. [18] onthe wave-dominated shelf of the Gulf of Lions. Theypropose that this internal structure could correspondto a two-step scenario of bank construction, in rela-tion with a stepped sea-level rise during the Holocene.It is however difficult to conclude considering the lackof sampling of the bounding surface between the twounits. In any event, such ‘cored’ sand ridges havebeen reported from contrasted environments includ-ing the southern North Sea [6, 8, 15, 22], the NewJersey and Maryland shelves [10, 17], and the south-ern Yellow Sea [25]. Snedden and Dalrymple [20]proposed that such ‘cores’ represent in fact the ini-tial topographic irregularities that are necessary forsand bank initiation in the Huthnance [14] numeri-cal model. In other situations, Berné et al. [5] con-sidered that the inner parts of sand banks, which dis-play very different lithologies and seismic facies com-pared to the classical clinoforms, represent remnantsof former deposits that have been ‘cannibalised’. Sucha process may happen when substrate is erodible andsediment flux limited, and can be compared to the mi-gration with a negative angle of climb of transversebedforms [19].Time-scales of processes responsible for forma-tion and evolution of sand bodies are in relation withdimensions of these bed forms. Small ripples arethe product of almost instantaneous hydrodynamicregime and may reverse their asymmetry during eachsingle semi-diurnal tidal cycle, small dunes (megarip-ples) generally represent peak current velocities dur-ing spring tides, whereas the asymmetry of largedunes (sand waves) is mainly controlled by springtidal currents or seasonal changes of net bedloadE-mail address: Serge.Berne@ifremer.fr (S. Berné).731PERSPECTIVES. Berné / C. R. Geoscience 334 (2002) 731, 732transport [4, 16, 21]. Large (tidal) sand banks, such asthe Longe de Boyard, result in fact of the combinationof these different processes, together with longer-termchanges due to ‘deglacial’ sea-level rise and decreasein sediment supply.Surprisingly, more than 30 years after the pioneer-ing work of Houbolt [11], very few comprehensivedescription of the internal lithology of modern sandbanks are found in the literature, making the interpre-tation of bank genesis rather speculative. Attempts todrill with geotechnical vessels such formations werelargely unsuccessful [9], best results being obtainedwith high frequency vibrocorers [23], whose penetra-tion is unfortunately limited to about 5 m. Consider-ing that some sand banks are more than 50 m thick,drilling and coring should be considered as the mostimportant objective for fully understanding sand bankgenesis.References[1] Berné S., J.-P. Auffret, P. Walker, Internal structure of subtidalsand waves revealed by high-resolution seismic reflection, Sedimen-tology 35 (1988) 5–20.[2] Berné S., G. Allen, J.-P. Auffret, H. Chamley, J. Durand,O. Weber, Essai de synthèse sur les dunes hydrauliques géantestidales actuelles, Bull. Soc. géol. France 6 (1989) 1145–1160.[3] Berné S., J.-F. Bourillet, J. Durand, G. Lericolais, H. Ondreas,Les dunes subtidales géantes de Surtainville (Manche Ouest), Bull.Centres Rech. Explor. Prod. Elf Aquitaine 13 (2) (1989) 395–415.[4] Berné S., P. Castaing, E. Le Drezen, G. Lericolais, Morphol-ogy, internal structure and reversal of asymmetry of large subtidaldunes in the Gironde lower estuary (France), J. Sediment. Petrol. 63(1993) 780–793.[5] Berné S., G. Lericolais, T. Marsset, J.-F. Bourillet, M. deBatist, Erosional shelf sand ridges and lowstand shorefaces: examplesfrom tide- and wave-dominated environments of France, J. Sediment.Res. 68 (4) (1998) 540–555.[6] Berné S., A. Trentesaux, A. Stolk, T. Missiaen, M. De Batist,Architecture and long-term evolution of a tidal sandbank: the Mid-delkerke Bank, Southern North Sea, Mar. Geol. 121 (1994) 57–72.[7] Chaumillon E., H. Gillet, N. Weber, M. Tesson, Évolution tem-porelle et architecture interne d’un banc sableux estuarien : la Longede Boyard (littoral atlantique, France), C. R. Geoscience 334 (2)(2002) 119–126.[8] D’Olier B., Sedimentary events during Flandrian sea-levelrise in the south-west corner of the North Sea, in: E.D. Nio,R.T.E. Schüttenhelm, T.C.E. Van Veering (Eds.), Holocene MarineSedimentation in the North Sea Basin, Spec. Publ. Int. Assoc.Sedimentologists, Blackwell, Oxford, 1981, pp. 221–227.[9] Davis R.A., P.B. Balson, Stratigraphy of a North Sea tidal sandridge, J. Sediment. Petrol. 62 (1) (1992) 116–121.[10] Field M.E., Sand bodies on coastal plain shelves: Holocenerecord of the US Atlantic Inner shelf off Maryland, J. Sediment.Petrol. 50 (1980) 505–528.[11] Houbolt J.J.H.C., Recent sediments in the Southern Bight ofthe North Sea, Geol. Mijnb. 47 (4) (1968) 245–273.[12] Houthuys R., Chronosequential depth measurements in veryshallow waters by overcraft: recording and processing, in: I. Heyse,G. de Moor (Eds.), Sediment transport and bedform mobility in asandy shelf environment (Starfish), University of Gent, Belgium,1996, pp. 5/1–5/25.[13] Houthuys R., A. Trentesaux, P. De Wolf, Storm influences ona tidal sandbank’s surface (Middelkerke Bank, Southern North Sea),Mar. Geol. 121 (1994) 23–41.[14] Huthnance J.M., On the formation of sand banks of finiteextent, Estuar. Coast. Shelf Sci. 15 (1982) 277–299.[15] Laban C., R.T.E. Schüttenhelm, Some new evidence on theorigin of the Zealand ridges, in: E.D. Nio, R.T.E. Schüttenhelm,T.C.E. Van Veering (Eds.), Holocene Marine Sedimentation in theNorth Sea Basin, Spec. Publ. No. 5, Int. Assoc. Sedimentologists,Blackwell, Oxford, 1981, pp. 239–245.[16] Le Bot S., J.-P. Herman, A. Trentesaux, T. Garlan, S. Berné,H. Chamley, Influence des tempêtes sur la mobilité des dunes tidalesdans le détroit du Pas-de-Calais, Oceanol. Acta 23 (2) (2000) 129–141.[17] McKinney T.F., W.L. Stubblefield, D.J.P. Swift, Large-scalecurrent lineations on the central New Jersey shelf: investigations byside-scan sonar, Mar. Geol. 17 (1974) 79–102.[18] Rabineau M., S. Berné, E. Le Drezen, G. Lericolais, T. Mars-set, M. Rotunno, 3D architecture of lowstand and transgressiveQuaternary sand bodies on the outer shelf of the Gulf of Lion, France,Mar. Petrol. Geol. 15 (5) (1998) 439–452.[19] Rubin D.M., Cross-bedding, bedforms, and palaeocurrents,SEPM Concepts in Sedimentology and Paleontology series No. 3,Tulsa, Oklahoma, 1987, 187 p.[20] Snedden J.W., R.W. Dalrymple, Modern shelf sand ridges:from historical perspective to a unified hydrodynamic and evolution-ary model, in: K.M. Bergman, J.W. Snedden (Eds.), Isolated Shal-low Marine Sand Bodies: Sequence Stratigraphic Analysis and Sed-imentologic Interpretation, SEPM Spec. Publ. Soc. Sediment. Geol.(SEPM), Tulsa, USA, 1999, pp. 13–28.[21] Thauront F., S. Berné, P. Cirac, Évolution saisonnière desdunes tidales dans le bassin d’Arcachon, France, C. R. Acad. Sci.Paris, Ser. IIa 323 (1996) 411–418.[22] Trentesaux A., S. Berné, M. De Batist, H. Chamley, Architec-ture interne d’un banc sableux de la Mer du Nord méridionale, C. R.Acad. Sci. Paris, Ser. II 316 (1993) 99–106.[23] Trentesaux A., S. Berné, A. Stolk, Sedimentology and stratig-raphy of a tidal sand bank in the southern North Sea, Mar. Geol. 159(1999) 253–272.[24] van de Meene J.W.H., The shoreface-connected ridges alongthe central Dutch coast, PhD thesis, Nederlandse GeografischeStudies 174, University of Utrecht, The Netherlands, 1994, 222 p.[25] Yang C.S., J.S. Sun, Tidal sand ridges on the East ChinaSea shelf, in: P.L. de Boer, A. van Gelder, S.D. Nio (Eds.), Tide-influenced sedimentary environments and facies, Sedimentology andPetroleum Geology, D. Reidel, Dordrecht, The Netherlands, 1988,pp. 23–38.732

Evolution of sand banks

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Evolution of sand banks

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