The interpretation of passive margin depositional processes using seismic stratigraphy: examples from offshore Namibia

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Abstract Seismic stratigraphic analysis of the post-rift megasequence of the Namibian passive continental margin has enabled depositional features to be interpreted in terms of the likely controls on their formation. The scale of the information derived from this manner of depositional system analysis is fundamental in understanding processes of sedimentation on passive margins; processes which may not be so readily apparent at field or well-log scale studies. Inner shelf channel forms are interpreted as incised valleys, created during a fall in relative sea level. The horizontal nature of the seismic facies infilling these valleys suggests that they filled during progressive periods of rising relative sea level, implying late lowstand or transgressive sediments. A series of low mounded features can also be identified on the shelf, which may be interpreted as palaeodune fields, carbonate build-ups or longshore clastic mounds. The outer shelf and upper slope positions reveal interpreted canyon and channel forms. The infill to these features contrasts with that of the incised valleys by having a more draped from (sometimes with a chaotic unit below); a geometry consistent with submarine origins. The submarine canyons may be termed attached or detached depending on whether they can be shown to link back into fluvial systems on the shelf; attached canyons will be more likely to be able to support the development of deepwater, sand-rich submarine fans. In the slope/base of slope area both regionally extensive and more localized mounds can be interpreted as mass transport deposits, the trigger for which may be attributed to a relative sea-level fall (due to their connection to the valley and canyon systems), though a transgressive origin may also be consistent in some cases. The mounds seen in positions deepest in the basin have a markedly different seismic facies to those at the base of slope and a different depositional system may be invoked, perhaps that dominated by distal turbidity currents or waning bottom currents.