Middle Cretaceous Sediments from the Eastern Part of Walvis Ridge ,THE Walvis Ridge Is One of the Inost Consl~Icuoiisfeatures of T,He South Atlantic

Art. NO 205 Contribution COB No 229

(Reprinted from Nature, Vol. 248. No. 5448. pp. 495-496, April5, 1974)

Middle Cretaceous sediments from the eastern part of Walvis Ridge ,THE Walvis Ridge is one of the inost consl~icuoiisfeatures of t,he South Atlantic. It extends from ncar Tristan da Cunha on the Mid-Atlantic Ridge to tlie African continental margin and has three main segments. The western scgmcnt is oriental SI$T-NE, the central one N-S and tlie rastern one SSW-NNE. The Walvis Ridge may have kept Pace nith thc opening of the South Atlantic which started in the Early Cretareousl-6, either hy transform feiilt mech:inisms7~8or by a mnntle hot spot and pl~me~-~~.The eastern segment is thought to have been biiilt throiighout the LIiddle Cretace- ous, that is, betwwn 120 m.y. ago and 80 m.y. ago13-'? IWe report information obtainrd froin an investigation of the age and depositional conditions of sediincntary rock recovered diiring t,he Walda cruise of the RV .Jean Charcot (April- Aiieiist 1971). - White chalk fragments containing niirneroiis fossils wcre FIG.2 Seismic reflection profile across the northern scarp recovered from a dcptli of 2,700 m by a tlrwige (CH 18 DR of the caslcrn segment of Walvis Ridge and location of the 04) on the northern flank of the Walvis Ridge in the centre dredged rocks. a, Site of dredge. of the eastern segment (19' 33's; 09" OI'E) (Fig. 1). Smear slides of the calcareoi~smatrix show main157 coccoliths and micritic calcite. Glauroptiane and biotite are also present. The fossils indicate an Albian-Senoniaii agr. The nannofossils most abundant macrofossils sr? gastropods, remains of consist mainl y of Lithastrinu? jîornlis St ratincr. Cricolithus rchinoderms (regular and irregiilar iirrhiiis: Holasteroida, multirarliatus Kamptner, C. pemmntoidvvs Deflaridre, Lozo- Spatangoida) and bivalves (including Ostrridae, Arcacea, lithus armilla (Black) and Parhabdolithus embergeri (Noël). Heterodonta and Neithea). One specimen of N~itlleacoiild be These are typical Alhian-Cenomanian species. The sediment Neithea cf. shawi Pcrvinquière (= N. coquandi Péron). is therefore tentatively dated lliddlc Cretaceous, that is, The distrihiition of N. shaiai is Albian-Cenomanian in the about 100 m.y.sP. This agrees with ages proposed by other Angola Basin biit mostly Crnomanian in northern Africa16. au th or^^"^^. This specimen is, however, poorly prcscrved and coiild belong hiacr0fossils arc not common in a coccolith ooze. Hypoth- to the species N. reg~darisSrkilothrim. In Eiirope the distri- cses for a depositional rnvironment of the sdiments, which bution of N. regdaris Schlotheim ranges from Turonian to might explain the observcd association inrliide (1) the rocco- Senonian (Freines, persona1 communication). Ttiiis the macro- liths accumulated in deep water and the macrofossils are thanatococnoses transportrd from shallow drpths by tiir- bidity ciirrcnts or some othrr procrss; (2) both the cocroliths and macrofossils wrre dcr)ositrd in shall«w mater. IVe favour the second hypothcsis, tentativrly rejecting a rriixed origin for the sedimrnts brcausr of the distance from the African coast and the absciice of local sramoiints. A shallow water environmcnt, moiild comply with the ecological needs of the macrofossils. NPith~a,for example, is cornnionly found in shallow watcr Albo-Cenomanian deposits in the Angola Basin16. Fiirt hermore, coc,rolit hs are epipe1:igic org:inisms and cannot, be usrd as prrcise depth indicators17. Although they are typiral of open sea sedirrientation, their occurrence does not, necessarily imply deep nater. If the second hyliothcsis is correct, 100 m.y. ago the eastern segment of the Walvis Ridge would have been sitiiated in an elevated position. Subsidence would have occurred probably by the latc Cenomanian as seems to have been t,he case for sevcral basiiis of the continental margin of West ilfrirala. The total subsidence is cstimated to be :iboiit 2.5 km. The rate of subsidcnce is 25 m every million years, which is in good agreement with rates estimated by Fox et al.19 If oiir calculatioiis are correct, this rate is similar to that of a port ion of cooling lithosphcre of similar age and sitiiatrd at the same distance from a mid-oceanic ridgeZ0. Tlic results prcsented Iirre suggest that the siibsidence of thc eastern segment of the IValvis Iiidge occurred after Albian-Cenomanian time (100 m.y.s~). We tliaiik tlic scientific party and the crcw of RV Jean Frc. 1 Rnthymet,ric map of the eastern segment of the Charcot diiring Walda eriiisc for tlicir liclp. We acknowledge Wtdvis Ridge showing the location of tlie seisrriic refltsct,ion himc Frcinrx, LI. Brehion, l1. Roman from the Museum profile shown in Fig. 2, and the locations of the sediment samples. a, Site of dredge. Depths in corrected rrietres. Cori- National d'Histoire Naturelle and M. Deres from Société tours at intervals of 200 m. Nationale des Pktroles d'Aquitaine for advice concerning fossil identifications. Helpful discussions were held with J. M. J. E., Saito, T., Percival, S. E., Milow, jun, E. D., and Aiizcnde, X. Le Pichon, H. D. Needham and G. Pautot. Boyce, R. E., SÇiace, 168, (1970). L. AST TOURET ?Le Pichon, X., and Hayes, D. E., J. geophys. Res., 76, (1971). Francheteau. J.. and Le Pichon., X.., Bull. Am. Ass. Petrol. Centre Océanologique de Bretagne, Geo., 56, (1972). BP 337, 9 Wilson, J. T., Nature, 207, (1965). 29273 Brest, France 10 Dietz, R. S., and Holden, J. C., J. geophys. Res., 75, (1970). '1 Morgan, W. J., Nature, 230, (1971). J. GOSLIN 12 Morgan, W. J., Mem. geol. Soc. Am., 132, (1972). Institut de Physique de Globe, l-add, J. W., Dickson, G. O., and Pitman, W. C., in The age 4. Avenue de Neptune, of the South Atlantic (Nairn and Stehli, New York, 1973). 94100 St-Maur-des-Fosses, France 14Sclaîer, J. G., and McKenzie, D. P., Bull. geol. Soc. Am., 84, 11472) 15 Goslin, J., Mascle, J., Sibuet, J. C., and Hoskins, H., Bull. Received January 16, 1974. geol. Soc. Am. (in the press). 18 Freinex, S., in Sedimentary bcrsim of the Afriean coasts (Asso- 1 Creer, K. M., Miller, J. A., and Smith, A. G., Nature, 207, ciation of African Geological Surveys, 1966). (1965). "Noël, D., Coccolithes crétacés, Centre National de la ZAmaral, G., Gordani, U. G., Kawashita, K., and Reynolds, Recherhe Scientifique, Pans (1970). J. H., Geochim. eosmochim. Acta, 30, (1966). 1s Brognon, G., and Verrier, G., in Sedimentary basins of the 3 McDougall, I., and Ruegg, N. R., Geochim. cosmochim. Acta, African coasts, (Asociation of African Geological Surveys, 30, (1966). 1966). 4Sait0, T., Ewing, M., and Burckle, L. H., Sckce, 151, (1966). 19 Fox, P. J., Heezen, B. C., and Johnson, G. L., Science, 170, SSiedner, G., and Miller, J. A., Earth pht. Sci. Lett., 4, ,*".",.11071>\ (19â8). ZOSclater, J. G., and Francheteau, J., Geophys. J. R. astr. Soc., &Maxwell, A. E., Von Herzen, R. P., Hsü, K. H. Andrews, 20, (1970).

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