Dolomite horizons occur as discrete centimeter-thick layers within Cenozoic organic-rich hemipelagites from the Angola Basin and Walvis Ridge and Basin (Fig. F1) (Pufahl et al., 1998; Wefer et al., 1998). In the Angola Basin, authigenic layers were intersected at Sites 1078 and 1080 within moderately bioturbated Holocene to Pleistocene calcareous clays containing high proportions of silt and organic carbon of mixed marine-terrestrial origin (total organic carbon [TOC] values range from 1.11 to 5.35 wt%) (Table T1). The clastic fraction is dominated by silt-sized grains of smectite, kaolinite, and/or illite, muscovite, quartz, albite, and microcline. Framboidal and subhedral pyrite are common accessory phases. Sedimentation within the Angola Basin records moderate levels of productivity within a coastal environment free of strong fluvial and upwelling influences. Deposition is dominated by the offshore transport and hemipelagic rain out of silts and clays derived from vigorous coastal erosion (Berger et al., 1998). Sedimentation rates are consequently extremely high with maximum values of 60 cm/k.y.
Site 1078 is located in 427 m of water between the high-productivity regions of the Congo River to the north and the Namibia coast to the south. Several dolomite layers were recovered from this site at depths of 83, 112, and 131 meters below seafloor (mbsf). Three intervals of laminated sediments consisting of intercalated diatom mats and thinly bedded Bouma division DE turbidites were also intersected at Site 1078. The authigenic layer at 131 mbsf occurs within the upper portion of a laminated interval. Dolomite horizons at 83 and 112 mbsf occur in nannofossil-rich clay. Site 1080 is located in 2766 m of water north of the Walvis Ridge. At Site 1080, much of the late Quaternary record is missing and dolomite horizons were intersected at shallower depths of 38 and 51 mbsf in Pleistocene nannofossil- and foraminifer-rich clay. Sediments from Site 1080 represent a disturbed and incomplete hemipelagic succession recording successive episodes of winnowing associated with stratigraphic condensation. The presence of intervals of laminated sediment and benthic foraminifers tolerant of low-oxygen conditions indicates that bottom waters in the Angola Basin have had persistent low-oxygen concentrations.
Sites 1081 and 1082 are located in 760 and 1279 m of water, respectively, within the Walvis Basin (Table T1). Sediments at each site span the late Miocene to Holocene and consist of organic-rich (TOC values range from 0.03 to 16.08 wt%) pyrite-bearing calcareous clays with varying abundances of foraminifers, diatoms, and nannofossils. As in the Angola Basin, the clastic fraction consists of silt-sized grains of smectite, kaolinite, and/or illite, muscovite, quartz, albite, and microcline. Although the sites are seaward of the upwelling center, they contain a strong upwelling signal that has been transported outward from its coastal zone by eddies and filaments of the Benguela Current. Sediment accumulation rates at the Walvis sites are typically between 5 and 10 cm/k.y. but can be as high as 20 cm/k.y.
Fifty-three dolomite horizons were found within sediments from Sites 1081 and 1082. Horizons were identified mainly on the basis of downhole logging but were verified through samples in cores. Holes were logged four times. The first tool string (siesmostratigraphy) included the spectral gamma-ray (NGT), sonic, electrical induction, and temperature (TLT) sondes. This combination is useful for describing lithology, sedimentary fabric, and degree of lithification. The second tool string (lithoporosity) included the NGT, neutron porosity, gamma density, and TLT sondes. The third tool string (Formation MicroScanner [FMS], two passes) included the NGT, inclinometer, and FMS sondes. The FMS tool string produces high-resolution electrical resistivity images of the borehole wall, which can be used to study the bedding structures and diagenetic features. The fourth tool string (geological high-sensitivity magnetic tool [GHMT]) included the NGT, magnetic susceptibility, and vertical component magnetometer. This string provides a magnetic stratigraphy of the sediments. Distinct spikes in resistivity, density, and sound velocity downhole are interpreted as discrete dolomite horizons. Because of their high resistivity, these layers are readily identifiable by FMS (Wefer et al., 1998). Of the 53 dolomite horizons identified, only 16 were recovered during drilling.
Site 1084 is located within the Northern Cape Basin, north of Lüderitz, in 1992 m of water where filaments of cold nutrient-rich water from the coastal upwelling area mix with lower productivity oceanic water, producing a zone of intermediate productivity. Sediments range in age from early Pliocene to Pleistocene and consist of bioturbated clays containing varying abundances of diatoms, nannofossils, foraminifers, and radiolarians (Table T1). Sediments here contain the highest organic carbon contents of any site studied during Leg 175 (TOC values are as high as 18.01 wt%). Diagenetic activity, as reflected in the ubiquitous presence of carbon dioxide and methane and in the rapid reduction of sulfate within pore waters, is intense; alkalinity concentrations reached a maximum value of 172 mM, which is the second highest ever measured in sediments recovered by the Deep Sea Drilling Project (DSDP) or the Ocean Drilling Program (ODP) (Murray et al., 1998). Sedimentation rates vary between 10 and 27 cm/k.y.
Dolomite horizons from Walvis Ridge and Basin and Cape Basin sites are 10 to 60 cm thick and are restricted to intervals of sediment below 100 mbsf. Two types of dolomite horizons were recovered: lithified and semilithified (Fig. F2A, F2B). Lithified layers commonly possess a hard center that becomes increasingly less cemented above and below. Semilithified horizons are friable and disaggregate easily and were only intersected at Site 1081; they have high intercrystalline porosities that range from 20% to 25%. Porosities of lithified layers are much lower and are typically between 5% and 7%. A continuum exists between these two end-members that reflects the relative degree of authigenesis. There appears to be no obvious relationship between host sediment composition and horizon type. The lateral extent of horizons is unknown, but by analogy with similar authigenic carbonate layers that punctuate compositionally similar Cenozoic strata in the circum-Pacific region, we assume that layers pinch and swell over a distance of a few hundred meters (Garrison et al., 1984, and references therein; Pisciotto and Mahoney, 1981).
No dolomite was found at Site 1083, located only 25 km to the northwest of Site 1082 but at greater water depth (2200 m). Carbonate contents at Site 1083 are relatively high (carbonate carbon values range from 19.8 to 82.2 wt%), and organic carbon values are lower than the other sites (TOC values range from 1.02 to 5.52 wt%).