HOLE 1081C

Position: 19°37.2128'S, 11°19.1620'E

Start hole: 0925 hr, 12 September 1997

End hole: 2015 hr, 12 September 1997

Time on hole: 10.83 hr

Seafloor (drill pipe measurement from rig floor, mbrf): 805.2

Total depth (drill pipe measurement from rig floor, mbrf): 960.4

Distance between rig floor and sea level (m): 11.4

Water depth (drill pipe measurement from sea level, m): 793.8

Penetration (mbsf): 155.2

Coring totals:

    Type: APC

    Number: 17

    Cored: 155.2 m

    Recovered: 160.94 m (103.7%)

Lithology:

    Unit I: clay with varying amounts of diatoms, nannofossils, foraminifers, and radiolarians

 

Principal results: Three holes were cored with the advanced hydraulic piston corer/extended core barrel (APC/XCB) at Site 1081 to a maximum depth of 452.7 meters below seafloor (mbsf), which recovered an apparently continuous hemipelagic sedimentary section spanning the Holocene to upper Miocene (0–9 Ma). Hole 1081A was cored with the APC to 134.0 mbsf and extended with the XCB to a depth of 452.7 mbsf. At Hole 1081B, 21 cores were taken with the APC to 187.6 mbsf. Hole 1081C was cored with the APC to 155.2 mbsf. Detailed comparisons between the magnetic susceptibility and the gamma-ray attenuation porosity evaluator (GRAPE) density records generated on the multisensor track (MST) demonstrated complete recovery of the sedimentary sequence down to 215 meters composite depth (mcd).

 

Sediments from Site 1081 consist of two lithostratigraphic units. Unit I is composed of olive-gray to black clays that contain varying amounts of diatoms, nannofossils, foraminifers, and radiolarians. Three subunits are defined based on the various abundances and types of microfossils in the sediments: nannofossil- and foraminifer-rich clay (0–77 mbsf), diatom-rich clay (77–230 mbsf), and nannofossil-rich clay (230–390 mbsf). Lithostratigraphic Unit II (390–452 mbsf) is composed of olive-gray clayey nannofossil ooze. The detrital component of the sediments is clay with rare silt-sized, angular and subangular, mono- and polycrystalline quartz grains, feldspar, and mica. Muscovite and biotite are also present in trace amounts. The biogenic component is represented by varying abundances of foraminifers, nannofossils, and diatoms. Varying amounts of particulate organic matter were observed. Authigenic minerals, such as glauconite, rare framboidal pyrite, and dolomite, were observed. Sedimentation rates are fairly constant within the upper Miocene and lower Pliocene (40 m/m.y.) sequences. Sedimentation rates within the upper Pliocene sequence are the highest recorded at this site (90–150 m/m.y.) but are reduced at the Pliocene/Pleistocene boundary (70 m/m.y.).

 

Fine biostratigraphic resolution was achieved by integrating datums from all microfossil groups. All microfossil groups, except for planktonic foraminifers, show marked coeval fluctuations in abundance, which are reflected in the lithology. Planktonic foraminifers are barren, rare, or replaced by pyrite from 105 mbsf to the bottom of the hole. Calcareous nannofossils are abundant and well preserved within the upper (0–77 mbsf) and lower (147–452 mbsf) parts of the section. Samples between 77 and 147 mbsf are commonly barren or poor in calcareous nannofossils, although some nannofossil-rich sediments are occasionally found within short intervals. The biostratigraphy of the Neogene is poorly constrained because of the scarcity of index species. Diatom abundance shows a substantial increase in upper Pliocene and lower Pleistocene sediments and reaches a maximum in the upper Pliocene interval, whereas overall abundance levels remain low in upper Miocene and lower Pliocene sediments. This pattern resembles that of Deep Sea Drilling Project (DSDP) Site 532. The diatom assemblage consists of a mixture of upwelling-related and oceanic species. Upwelling-related species dominate the diatom assemblage during highest abundance times in the upper Pliocene interval. They are not common in lower Pliocene and Miocene sediments where oceanic species tend to dominate. The diatom content in the sediments possibly reflects a varying nutrient supply that could be related to upwelling of nutrient-rich deeper waters and high biological productivity over the Walvis Ridge, especially in the late Pliocene.

 

After alternating-field (AF) demagnetization at 20 mT, a complete magnetostratigraphy was determined in the uppermost 120 mbsf of the APC section at Site 1081. All chrons from the Brunhes (C1n) to the termination of the Olduvai (C2n) at 1.77 Ma could be identified.

 

Well-developed nannofossil ooze–clay cycles, in which concentrations of calcium carbonate and organic carbon vary between 1 and 53 wt% and between 1.4 and 8.2 wt%, respectively, reflect fluctuations in the elevated marine production associated with the Benguela Current. Higher concentrations of organic carbon from 0 to 200 mbsf record higher productivities during the last 2 m.y. than earlier in the history of this upwelling system. The interstitial water chemical profiles at this deeply drilled site record a relatively shallow (0–80 mbsf) region affected by the diagenetic degradation of organic matter, which results in sulfate consumption as well as increases in alkalinity, ammonium, and phosphate. These changes are accompanied by calcite dissolution and dolomite precipitation, which are recorded by increases in dissolved strontium and decreases in dissolved magnesium and calcium. Additionally, the diatomaceous sequence in lithostratigraphic Subunit IB causes an increase in dissolved silica and a second peak in dissolved phosphate deeper in the sediments.

 

Physical sediment properties were determined both by high-resolution MST core logging and index properties measurements. Magnetic susceptibility and GRAPE signals reveal pronounced cyclicities, which were used for high-quality stratigraphic correlation in conjunction with digital color data.

 

The geophysical downhole logs show well-identified levels of high velocity, resistivity, and density attributed mainly to dolomitic layers. The Formation MicroScanner (FMS) shows dolomitic layers and numerous conductive horizons that might be related to high-porosity assemblages of diatomaceous microfossils. Porosity, density, and natural gamma radiation logs show high-frequency cyclical patterns that will be used as valuable indicators of paleoclimatic history and will provide age estimates for the sedimentary section.

 

Site 1081, in 793-m deep water on the Walvis Ridge, is the shallow-water drill site of the Walvis Ridge/Walvis Basin transect. The other anchors are DSDP Sites 532 and 362 in 1300 m water depth and Ocean Drilling Program (ODP) Sites 1082 and 1083. The DSDP sites are seaward of the upwelling center but contain an upwelling signal that has been transported by the Benguela Current. Site 1081 will give a better record of the upwelling itself. The transect, located above the calcite compensation depth (CCD) in a passive-margin area of high sedimentation rates, will provide high-resolution records of these important processes and add important new dimensions to the records now available. This transect, situated on the only topographic high over which the Benguela Current passes, is central to the reconstruction of the history of the current.