ALBERT

Description: Acoustic basement lies at an average of between 6.0 and 6.5 sec two-way time below sea level in the southern Rockall Trough and northern Porcupine Abyssal Plain. The overlying sedimentary succession reaches maximum thicknesses of at least 4.0 sec, and can be divided by 3 regionally-developed seismic reflecting horizons, which are used as a framework to establish an acoustic stratigraphy for the area by selecting three “type” seismic sections. These reflectors are named, in ascending order, Shackleton, Charcot and Challenger. The area is crossed by E—W basement high structures, the Clare Lineament (which may be an easterly extension of the Charlie Gibbs Fracture Zone), that separates the Porcupine Abyssal Plain from the eastern part of southern Rockall Trough. Under the latter, the post-Shackleton acoustic sequence is thickened, as if dammed to the north of the Clare Lineament, whilst a further thickening, above reflector Charcot, occurs along a NE—SW line somewhat farther north into the southern Rockall Trough. This can also be related to shallow-lying acoustic basement features. Pre-Shackleton sediments overlie a very irregular basement topography. The acoustic characters of the various sediment packages are described and it is speculated that major changes in the sedimentary environments took place across reflectors Shackleton and Challenger, the latter probably establishing the modern bottom current circulation patterns. No ages can be unequivocally assigned to the main reflectors, but previously published data suggest a late Eocene—Oligocene age for Challenger. Possible lavas or sills are identified in the succession between reflectors Shackleton and Charcot.

Description: Isopach asymmetry, and sediment component changes in DSDP cores from the SE Atlantic (Orange Basin) support the hypothesis of major drainage system changes in SW Africa during late Cretaceous—Cenozoic time. This involved alternations in the use of the 28°S (modern Orange River) and 31°S (modern Olifants River) exit points across the western escarpment by rivers carrying run-off from the Upper Orange/Vaal catchment areas, as well as radical re-organizations of internal drainage geometry. It is postulated that during late Cretaceous times the 28°S exit was used, with the Middle Orange River following a course in the interior well to the south (up to 150 km) of its modern channel. Sediment discharge rates from this river were relatively high (at least 10 × 106 m3 yr−1), and resulted in rapid advancement of the continental margin sediment prism west of the mouth by large-scale slumping. The Palaeogene Orange/Vaal river exit was via the 31°S escarpment crossing, and during the later part of this period, the Cape Canyon was cut across the continental shelf and slope. A significant reduction in sediment discharge (to 2.0 × 106 m3 yr−1) suggests that the Lower Tertiary climate for SW Africa was drier than that of late Cretaceous times. However, aridity did not commence until late Miocene times, when the Orange/Vaal discharge had switched back to the 28°S exit. Modern sediment discharge rates (6.5 × 106 m3 yr−1) are relatively high and reflect soil erosion caused by agricultural activity. The two major alterations in exit point of the Orange/Vaal (late Cretaceous—early Tertiary, and late Oligocene—early Miocene) are related to periods of low sea level, which promoted river capture adjacent to the western escarpment. An additional factor in the first course change may have been the disruption of the Middle Orange channel by late Cretaceous igneous intrusions. Less important internal reorganizations of the drainage system are postulated in late Miocene—Pleistocene times. Economic implications for offshore diamond distribution are briefly mentioned.

Description: Changes in composition of modern benthic ostracod faunas across the continental margin of southwestern Africa occur at boundaries between and within major water masses: a Mixed Layer-Antarctic Intermediate Water (AAIW) boundary at 200 m, an AAIW salinity minimum zone at 650 m, an AAIW bathyal thermocline at 1000 m, and the AAIW/North Atlantic Deep Water (NADW) boundary at 1500 m. In addition, two population changes occur within the NADW at 1.8–2.0 km and 2.0–3.0 km. The Antarctic Bottom Water assemblage is sparse and poorly preserved.

Description: Ostracod faunas at six locations are compared, and related to distributions in an Atlantic Ocean-wide data base. Five, widely developed, vertical faunal sequences are recognised at particular levels within deep water-masses: Henryhowella Fauna (lower part of Antarctic Intermediate Water); Krithe Fauna (Upper North Atlantic Deep Water); Poseidonamicus-Bosquetina Fauna (upper part of Lower NADW); Dutoitella Fauna (lower part of Lower NADW); Legitimocythere Fauna (Antarctic Bottom Water). These faunas are correlated with previously established deep water benthic foraminiferal assemblages, and their possible palaeo-oceanographic use is discussed.