Cape Basin Continental Rise—Sites 360 and 361

Abstract

A continuous section was penetrated from the Pliocene into the middle Eocene, consisting predominantly of biogenic oozes, chalks, and marly chalks. The deposition is sharply pelagic in nature with episodes of especially high productivity during the late Miocene around 6 m.y., during the middle Miocene at 13 m. y., during the early Miocene at 22 m. y., and during the early Oligocene at 36 m. y. The planktonic foraminifers are for the most part of the cool-temperate type characteristic of the Austral, New Zealand biogeographic province. The earliest major cooling was initiated in the middle part of the late Eocene. Selective carbonate dissolution occurred within the late and middle Miocene and again in the Eocene. An acoustic unit consisting of what may be long-wavelength apparent upslopemigrating sediment dunes correlates to the significantly more marly Eocene section containing thin silty and sandy beds which have been highly bioturbated. The influence of bottom currents on this sediment unit was apparently very subtle, for there are no detectable stratigraphic gaps. The currents promoted the dilution of the biogenic sediments more by accelerating the infusion of terrigenous mud than by the erosional removal of sediment or the condensation of the lithologic column by winnowing. In fact, the stratigraphic intervals which in many other DSDP sites are characterized by hiatuses are here precisely the intervals of peak rates of accumulation. Alternatively, the crumpled internal reflectors may have resulted from slumping, for which there is evidence in the cores. Reflector D of Emery et al. (1975), although not reached by the drill string, has an extrapolated age of earliest Eocene to latest Paleocene. A continuous stratigraphic section was penetrated from the upper Eocene to at least as far down as lower Aptian, the younger Cenozoic having been stripped away by recent erosion. The hole was abandoned following the destruction of the drill bit some 50-100 meters from acoustic basement. Extrapolation of sediment accumulation rates would give this basement, situated on the Mesozoic magnetic anomaly M-4 lineation, a Barremian age. Surficial Eocene mud, calcareous mud, marly nannofossil ooze, and chalk directly overlie Paleocene pelagic clay. The abrupt contact between the carbonate-rich and carbonate-poor strata corresponds to Reflector D. The Maestrichtian through Albian interval is comprised of non-carbonate terrigenous shale with intercalated sandy mudstones and siltstones interpreted as a distal fan turbidite fades deposited in its entirety beneath the carbonate compensation depth. Sediment accumulation rates range from 13 to 20 m/m. y. The Aptian interval is considerably more sandy and highly carbonaceous. The pelagic component throughout is a minutely laminated dark sapropelic shale deposited under euxinic conditions. Thick clastic beds contain wood fragments up to 8 cm in length, much bituminous material, reworked flat pebbles of coaly substances, amber, gas-escape deformation structures, and silicic volcanic sand grains all redeposited in massive quartz-rich turbidites of an interpreted moderately deep proximal fan to fan-valley environment sterile to indigenous benthic life. Many of the massive sands are calcite cemented and have compressional-wave velocities exceeding 4 km/sec. These Aptian-age sandstones correlate with Reflector AH of Emery et al. (1975). Sedimentation rates in the Aptian section exceed 50 m/m.y. Calibration of the Cape Basin magnetic Hneations confirms an initial opening of the South Atlantic during the late Valanginian to early Hauterivian stage of the Early Cretaceous.