Site 515: Brazil Basin

Abstract

.10Depth sub-bottom (m): 636.4 Nature: Zeolite-rich calcareous mudstone Age: early Eocene Measured velocity (km/s): 1.87 to 2.14 Principal results: Hole 515 was rotary drilled and washed to 55 m, but the drilling was terminated when the pressure core barrel jammed in the bottom-hole assembly. In Hole 515A, the hydraulic piston corer (HPC) was used to core to 107.9 m, with 89% recovery. Hole 515B was washed to 94.9 m, rotary drilled to 636.4 m, with 79% recovery. The lithologic section is divided into three lithologic units. 351 m of dark greenish gray biosiliceous mud and mudstone of middle Miocene to late Oligocene age with occasional nannoplankton-rich and rare foraminifer-rich horizons. Subunit 2b contains 84 m of dark greenish gray terrigenous mudstone of late Oligocene age, with only traces of siliceous micro fossils and evidence of silica diagenesis. Unit 3 is 20 m (minimum) of lower Eocene greenish gray calcareous zeolitic mudstone.Site 515 was chosen to study the onset and variability of the flow of Antarctic Bottom Water (AABW) through the Vema Channel into the Brazil Basin. The 22 Ma hiatus between Subunits 2b and 3 includes the Eocene/Oligocene boundary, which might correspond with the onset of AABW production. The lithologic boundary between Subunit 2b and Unit 3 corresponds precisely to a sharp and strongly discordant acoustic reflector, which extends throughout the Brazil Basin. The overlying sediments of Units 1 and 2 are fine grained.Early Eocene. Pelagic carbonates accumulated at normal rates of approximately 4 m/Ma during the early Eocene (Cores 515B-56 and 515B-57). Core 515B-56 grades upward from a pelagic carbonate to a dissolution facies consisting of a zeolitic clay with detrital mineral grains. The core contains thin laminations and cross laminations and a weak magnetic grain fabric aligned in a northnortheast direction, all evidence of active bottom currents beginning in the early Eocene.Early Oligocene to middle Miocene. Biosiliceous and terrigenous mudstones accumulated rapidly (approximately 40 m/Ma). Considerable amounts of detrital minerals and intermittent laminated intervals indicate persistent bottom-current activity. The cessation of siliceous sedimentation during the middle Miocene is an oceanwide event in the tropical and temperate Atlantic and reflects a major threshold event of paleocirculation. That event is probably related to the widening of Drake Passage and consequent deepening of the eastward circumpolar flow, which may in turn have shifted the South Atlantic circulation from a pattern of meridional flow to one of zonal flow. The initiation of strong eastward circumpolar flow may have blocked the entry of cold and productive Antarctic and sub-Antarctic waters into the South and central Atlantic, and siliceous sedimentation abruptly ceased in the lower and middle latitudes.Middle Miocene to middle Pleistocene. Terrigenous mudstones and intermittent layers of biogenic carbonate accumulated at rates of about 20 m/Ma. Absence of significant quantities of displaced Antarcti...

The lower Eocene-lower Oligocene ostracodes from DSDP Site 515B, Brazil Basin, Southwestern Atlantic Ocean

“…The area is very close to the present CCD, which is at nearly 4,500-4,600 m water depth, and has changed only slightly vertically since the Eocene (Barker et al, 1983).…”

“…A hiatus between these subunits represents the Eocene/Oligocene boundary, dated by calcareous nannofossils and planktic foraminifers, respectively (Barker et al, 1983). Samples from 60 to 90 g were disaggregated Remarks.…”