Influence des courants profonds sur les faciès sédimentaires du Vema Channel (Atlantique sud) à la fin du Cénozoïque

The sediments of six cores from the Rio Grande Rise side of the Vema Channel reflect major phases in the sedimentary history of this area since Ihe lower Miocene. Cores taken from below the present-day lysocline (approx. 4 000 m) consist essentially of calcareous oozes. The variable degree of preservation of the Foraminifera and Coccoliths in these cores indicates dissolution cycles apparently linked to Antarctic Bottom Water (A.A.B.W.) fluctuations. Cores taken below 4 000 m represent an incomplete and shortened sedimentary sequence parliculary characterized by the presence of red Clay and manganese nodules. The latter have a todorokite core and are associated with relatively indurated phillipsite-rich sediments. In one of the deeper cores an increase, during the Miocene, of transition metals (Mu, Ni, Cu, Co, Li) and a diminution of the montmorillonite content of the sediments accompanies the evidence of intense dissolution of the carbonates. This, in turn, is attributed to a very sharp increase in A.A.B.W. influence together with a corresponding rise of the C.C.D. The age of this increase is not known because of the hiatus problem but it may be mid-Miocene, when there was a general rise of the C.C.D. level in the South Atlantic. While the geochemistry of the sediments reflects water deplh (which influences the dissolution of lime) and oxygen level, their mineralogy depends largely on their provenance (continental erosion). The A.A.B.W. transports minerals from middle high latitude (chlorite, well cristallized illite, montmorillonite, feldspath and quartz). The North Atlantic Deep Water carries clay minerals from lower latitudes (kaolinite, poorly cristallized illite and interstratified chlorite-montmorillonite). [NOT CONTROLLED OCR]