Lithostratigraphic Summary For Leg 175: Angola-Benguela Upwelling System

Abstract

During Ocean Drilling Program Leg 175, 40 holes were drilled at 13 sites, and 8003 m were recovered with the aim of reconstructing the late Neogene history of the Angola-Benguela upwelling system on the southwest African margin. This system is one of the great upwelling regions of the world and is characterized by organic-rich sediments that provide an excellent record of productivity back to the middle Miocene. Understanding the sedimentology and stratigraphy of these sediments will provide important new information on the paleoceanography of this region, including the complex role the ocean plays in global carbon cycling and climate change. This contribution summarizes the sedimentologic and stratigraphic data compiled on board the JOIDES Resolution between 12 August and 10 October 1997. The 13 drill sites can be divided into four regions based on sediment type and composition: (1) the Lower Congo Basin, (2) the Angola Basin, (3) the Walvis Ridge and Basin, and (4) the Cape Basin. The stratigraphy of each region is distinct and records the competing influences of current regime, shelf topography, and proximity to major river systems (both ancient and modern) and upwelling centers. The Lower Congo Basin is a hemipelagic environment containing finegrained sediments derived from the Congo River. Sedimentation within the Angola Basin is dominated by rain-out of hemipelagic silts and clays derived from coastal erosion and from the Kunene River to the south. Sediments within the Walvis Ridge and Basin consist of carbonate oozes and organic-rich clays, which record a strong upwelling signal from the Benguela Current. Deposition in the Cape Basin is dominated by pelagic settling of biogenic debris at the most southern tip of the Benguela Current upwelling center.