HOLE 1087D

Position: 31°27.9299'S, 15°18.6541'E

Start hole: 0705 hr, 6 October 1997

End hole: 0700 hr, 7 October 1997

Time on hole: 23.92 hr

Seafloor (drill pipe measurement from rig floor, mbrf): 1383.5

Total depth (drill pipe measurement from rig floor, mbrf): 1584.8

Distance between rig floor and sea level (m): 11.7

Water depth (drill pipe measurement from sea level, m): 1371.8

Penetration (mbsf): 201.3

Coring totals:

    Type: APC

    Number: 13

    Cored: 119.20 m

    Recovered: 121.04 m (101.54%)

    Type: XCB

    Number: 1

    Cored: 9.60 m

    Recovered: 9.88 m (102.92%)

Lithology:

    Unit I: foraminifer-nannofossil ooze, foraminifer-rich nannofossil ooze, foraminifer-bearing nannofossil ooze, and nannofossil ooze

 

Principal results: Site 1087 is located in the southernmost area of the Cape Basin in 1371 m deep water. The primary objective for drilling at this site is to explore the Neogene history of the Benguela Current in the Southern Cape Basin and to detect possible Agulhas Current influences. We expect to obtain information about the supply of warm water from the Indian Ocean, through the Agulhas Retroflection, and from the Subtropical Convergence Zone, which are nearby. Both warm-water and cold-water eddies can be shed from the retroflection and the front, but the position of the Subtropical Convergence Zone and the transport by the Benguela Current will be crucial in determining which type of eddy is more likely to reach the site. The site also is located close to the continent and should detect upwelling signals and signals from continental climates, as well as sea-level changes.

 

Four holes were cored with the advanced hydraulic piston corer/extended core barrel (APC/XCB) at Site 1087 to a maximum depth of 492 meters below seafloor (mbsf), which recovered a relatively continuous section down to 430 mbsf spanning the last 9 m.y. Hole 1087A was cored with the APC to 255.2 mbsf, and Hole 1087B was cored to 72.5 mbsf. Hole 1087C was cored with the APC to 248.60 mbsf and was extended with the XCB to 491.9 mbsf. The hole was logged with the seismostratigraphic suite. Logging was aborted after the logging tool got stuck and operations had to be terminated at Hole 1087C. Hole 1087D was drilled from 0 to 72.5 mbsf and cored to 201.3 mbsf in continuation of Hole 1087B. This hole ended Leg 175 operations with a total record-setting recovery of 8003.23 m.

 

Sedimentation rates range from 20 to 70 m/m.y. The bottom 70 m contain a middle Miocene to lower Oligocene/upper Eocene package interrupted by at least two major discontinuities. The sediments form two lithostratigraphic units that are composed of nannofossil ooze with varying abundances of clay and foraminifers. The sediments strongly resemble the lithologies observed at Sites 1086 and 1085. The sequence most likely contains various unconformities. The uppermost lithostratigraphic unit (0–425 mbsf) consists of nannofossil ooze with varying amounts of foraminifers. Sandy nannofossil foraminifer ooze is present in 50- to 100-cm-thick beds in the upper 45 m. These beds have generally sharp bases, grade upward into more clay-rich, olive foraminifer nannofossil ooze, and are interpreted as either turbidites or winnowed layers. The underlying unit (425–492 mbsf) comprises 2- to 100-cm-thick horizons of foraminifer-bearing and foraminifer-rich nannofossil ooze. A large unconformity is identified by an erosional contact at 450 mbsf and by biostratigraphic evidence. Below the erosional contact, fine laminations are present, which are microfolded with sharp upper and lower contacts.

 

The detrital component is dominated by clay and trace abundances of silt-sized, subangular mono- and polycrystalline quartz grains. Pyrite is present as silt-sized aggregates of euhedral crystals or framboids.

 

A preliminary biostratigraphy was developed using calcareous nannofossils and planktonic foraminifers. The biogenic component of both lithostratigraphic units consists of abundant to very abundant nannofossils. Foraminifers are abundant to few. Siliceous spicules, dinoflagellate cysts, and radiolarian tests are present in trace amounts in upper Pliocene and Pleistocene sediments only.

 

Radiolarian species indicate generally low productivity under subtropical warm-water conditions. The radiolarian assemblages that are characterized by common Cycladophora davisiana suggest upwelling conditions for samples from 60 to 70 mbsf. The occurrence of an Antarctic species at 112 mbsf indicates an influence of cooler water masses.

 

As at Site 1085, the upper Pliocene sediment at Site 1087 contains an interval with a mixed/Thalassiothrix antarctica–rich assemblage composed of Southern Ocean and warm-oceanic species, with an approximate age of 1.9–2.8 Ma. Dinoflagellate cysts are common below 213 mbsf and between 262 and 338 mbsf.

 

APC cores experienced a significant coring-induced remagnetization with a radial-inward direction. Only magnetic inclinations showed distinct polarity biases after alternating-field (AF) demagnetization at 20 mT, which allowed an interpretation of the magnetic polarity sequence from the Brunhes (C1n) Chron to the Gilbert (C2A) Chron (~ 4 Ma).

 

Magnetic susceptibility and gamma-ray attenuation porosity evaluator (GRAPE) wet bulk density data were measured at 5- and 10-cm intervals. The correlation of features present in the physical properties measurements at adjacent holes were used to demonstrate completeness of the stratigraphic sequence between 0 and 214 meters composite depth (mcd).

 

Sediments are carbonate rich and organic carbon poor. Interstitial water chemistry is controlled dominantly by the high carbonate and low organic carbon concentrations in the sediment, which results in modest variations in the chemical gradients of many dissolved species. Alkalinity rises to a broad maximum of ~30 mM between 60 and 126 mbsf and subsequently decreases downhole. Sulfate is not completely consumed until 60 mbsf. Carbonate and phosphate precipitation reactions throughout the sequence also are inferred from the profiles of dissolved calcium, magnesium, and phosphate. Among the three Cape Basin sites, 1087 is most similar to Site 1085 in its geochemical profiles.

 

Logging in Hole 1087C encountered serious problems when the tool got stuck while entering the pipe after the first run. Good-quality logs were recorded with the seismostratigraphy tool string. In the upper 300 m of the logged interval, downhole measurements show very homogeneous patterns. The lower part of the logged interval is characterized by more variations in physical records related to the carbonate vs. detrital content.

 

Site 1087 is the southernmost site of a north–south transect along the west African coast, from the Congo to South Africa. Sediments at this site were deposited continuously over the last 9 m.y. and will permit the reconstruction of the advection of water masses from the Indian Ocean and the Subantarctic Region. It also will provide important new data about early carbonate diagenesis and the processes involved in sediment redeposition on the continental slope.