THE GEOWALL AS A RESEARCH TOOL: 3D VIRTUAL-REALITY ANALYSIS OF LATE QUATERNARY SHELF-EDGE EVOLUTION ALONG THE SOUTHERN AFRICAN MARGIN

The spatial relationship between the lowstand shoreline and the shelf edge has yet to be quantitatively described on a global scale. In many cases, it is assumed that the shelf edge lies at or near the shoreline during a relative sea-level lowstand, allowing direct sediment transport into the deep ocean. However, in some regions the lowstand shoreline may remain midway on the shelf, leaving the deep ocean disconnected from the continents. It is not yet known how common this scenario is.

This study seeks to quantify the spatial relationship between the shoreline and the shelf edge along the southern African margin during the Last Glacial Maximum (LGM). A geophysical model which takes into account the glacio-isostatic and hydro-isostatic effects of glaciation was used to generate a global digital elevation model during the LGM. This model was imported into a Geographic Information System (ArcView), and a curvature algorithm was used to calculate the position of the shelf edge. The stereo projection system, GeoWall, was used to examine the validity of the calculated shelf-edge position in 3D. Preliminary results show that the shelf edge on the southern African margin was not coincident with the LGM shoreline. The shoreline remained on the shelf, even though sea level was around 120 m below the present. This concept challenges traditional views concerning the evolution of passive margins and the modes of sediment transport from terrestrial to marine environments, suggesting that deep marine sedimentation may not be facilitated during a lowstand in this region.