5 research outputs found
Cenozoic siliciclastic sediment budget at continent-scale, Africa.
No full textInternational audienceSiliciclastic sediment budget measurements was performed along the margins and onshore basins of Africa for Cenozoic times. Our objective was first to quantify the ratio between onshore and offshore sediment preservation in the case of a relief with mostly no mountain belt and secondly to understand the factors forcing the sediment supply along the passive margins of Africa that can be long to very-long relief deformation (mantle dynamics, ridge push...) or climate changes (with the major aridification of Africa since Middle Miocene). This study is based on basin-scale regional sections (seismic reflection data from industry and academics, wells correlation), calibrated in age and lithology on different types of wells (industry, DSDP/ODP). Most of the effort was on the revaluation of the ages (calibration and uncertainties). The volumes of sediments and uncertainties on depth conversion velocity laws, lithology and ages were measured using software developed by J. Braun (Grenoble University, France). * The sediment preserved onshore (750 000 km3) is one of magnitude less than was is preserved offshore * The main deformations controlling the sediment supply are (1) the growth or the domes of the East African rift and (2) the marginal bulge of the central and equatorial segments of the South Atlantic Ocean (from southern Angola to Guinea). * The aridification of Africa since at least Middle Miocene is very sensitive in the south (fossilization of the relief of the South African Plateau) and in the northwest, with a sharp decrease of the sediment supply. * Some buffer effects are very important, for example for the Nile and the Zambezi, where sediments were first stored in onshore basins, Sudan or Malawi rift, and later drained because of a capture (Nile) or a regional stress change (Zambezi). Keywords: Africa, Cenozoic, Siliciclastic sediment fluxes, Deformation, Climat
VERTICAL MOVEMENTS IN NW AFRICA MARGIN: controls on accomodation and sedimentary partionning
No full textInternational audiencePresent day central Atlantic margins of West Africa are flat margins with no significant reliefs onshore. Nevertheless, recent thermochronological studies shows denudation, related to major vertical movements (Ghorbal et al., Terra Nova, 2008 ; Bertotti et al., Int J Earth Sci, 2012) along some parts of the margins. Using basin-scale regional sections, calibrated in age and lithology on different types of wells (industry, DSDP/ODP), the aim of this study was (1) to analyse the sediments geometry of the whole margin (Morocco to Senegal) from its hinterland to the distal deep-water basin, (2) to constrain and quantify the vertical movements along the margin and (3) to discuss impact of those deformations on margin morphology (accommodation, sedimentary partitioning between the shelf and the distal basin through time. . . ) and their geodynamic significance. 1. The structure of the Triassic rift controls the aggradational geometry of the platforms from Jurassic (carbonate aggradation: Tethys type margins) to Early Cretaceous (mixed terrigenous/carbonate) times. The present day geometry of the margins is inherited from the end of the thermal subsidence period (Cenomanian - Turonian) and the decrease of the accommodation that lead to progradational geometries characteristics of Atlantic types margins, 2. Major uplifts events, probably associated with Early Cretaceous global plate reorganisation (Austrian deformations) are recorded during Valanginian and Hauterivian-Barremien times along the Moroccan margins (from Dakhla to Tarfaya). There is no major "final uplift" (Oligocene - Miocene) that characterizes most of the South Atlantic margins. 3. Some siliciclastic wedges (e.g. Oligocene - Miocene) are not necessary recording uplift of the upstream proximal onshore, but distant deformation events (e.g. Hoggar uplift)
Controls on the provenance of late Eocene to Quaternary Mozambique Channel shales (DSDP 25 Site 242)
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