During Leg 175, 13 sites were drilled off the western and southwestern coasts of Africa. This area is one of the most productive regions of the world ocean. The combination of high productivity in surface waters and a tectonically quiet setting (passive margin) resulted in the deposition of fossil- and organic-rich sediments accumulating at high rates on the eastern flank of the Congo, Angola, and Cape Basins. Locally, high terrigenous supply from rivers, such as the Congo River, as well as coastal erosion, as occurs off Angola, dilute the biogenic component of the sediment and result in increased sedimentation rates.
The main objective of Leg 175 was to reconstruct the late Neogene history of productivity and associated circulation and upwelling dynamics of the Benguela Current system. Considering the high rates at which sediments accumulate in this region, the shipboard micropaleontologists were called on to provide a high-resolution stratigraphic framework and paleoecological analysis, with particular emphasis on the Pleistocene and Pliocene sections.
The shipboard analysis provided age estimates at an average resolution of 200 k.y. for most of the Pleistocene and upper Pliocene intervals of the recovered section. The biostratigraphic resolution of the lower Pliocene and Miocene intervals was somewhat coarser but still in the range of 500 k.y. This was accomplished despite a very high rate of core recovery and variable preservation of fossil assemblages.
Although all investigated microfossil groups provided valuable information, calcareous nannofossils were critical to the construction of a detailed biostratigraphy. Being abundant in sediments deposited under either high or low productivity conditions, this species group was less affected by environmental constraints than were, for instance, siliceous microfossils. Also, calcareous nannofossils were found to be more resistant than planktonic foraminifers to diagenetic processes, which characterized most of the organic-rich intervals of the recovered sections. Finally, the resolution of the nannofossil-based biostratigraphy was improved by integrating acme intervals of species/taxonomic categories, in addition to the classical first and last occurrence of index species. These isotopically calibrated intervals (see "Explanatory Notes" chapter, this volume) were crucial to the improvement of the stratigraphic resolution of the Pleistocene.
The biostratigraphic utility of planktonic foraminifers was strongly hampered by selective dissolution and pyritization in the most organic-rich sites. However, the carbonate-rich southernmost sites from the Mid-Cape Basin (MAB) and Southern Cape Basin (SCB) (Sites 1085–1087) contained abundant, unaltered planktonic foraminifers that were used to erect a biostratigraphy independent from the calcareous nannofossils and siliceous microfossil groups. The planktonic foraminiferal datum events for the Pliocene and Miocene intervals were particularly useful.
Although mostly used as indicators of environmental changes (upwelling intensity, surface circulation, and freshwater input), siliceous microfossils (diatoms, radiolarians, and silicoflagellates) provided important age control in sediments deposited under high-productivity regimes. This is particularly true for the sites close to the major upwelling centers off Namibia (Site 1081 on the Walvis Ridge; Sites 1082 and 1083 in the Walvis Basin; and Site 1084 off Lüderitz), as well as for the sites occupied in the peri-estuarine environment off the Congo River. Because the calcareous microfossils, especially the planktonic foraminifers, were commonly affected by dissolution in the high-productivity region off the Congo River, the biostratigraphic information provided by siliceous microfossils was invaluable at these shallow-water coastal sites.
The biostratigraphic analysis of the 13 sites indicates an overall continuous hemipelagic sedimentation occurring at a rate of ~10 cm/k.y., or greater. Age estimates and sedimentation rate patterns inferred from the vertical distribution of microfossils was commonly in close agreement with the magnetostratigraphy developed on board, which suggests absence of (or only minor) postdepositional sediment transport. Shore-based refinement of the magnetostratigraphy from analyses of discrete samples, as well as the future development of an oxygen-isotope stratigraphy for every site occupied, will provide an opportunity to improve the calibration of the Neogene microfossil datum events identified in the sedimentary sections from Leg 175.
A synthesis of Leg 175 sites is presented and organized by basin. Comparisons are made between sites drilled from a specific geographic setting. Only 12 of the 13 sites that were drilled are discussed. Site 1080, excluded from this synthesis, was drilled off the Kunene River but only penetrated to 52 meters below seafloor (mbsf). The poor recovery was attributed to the presence of hard dolomite layers.
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Ms
175IR-119
Reproduced online:
31 August 2007