I had the honour to present Shell's current view on the formation and early evolution of the Orange Basin at the EAGE Sub-Saharan African Energy forum in Maputo/Mozambique in Mid-April this year. The talk was a summary of many years of work by a large group of specialists and explorers. Abstract below.

Abstract:

Evolution of the Orange and Cape Basins in the context of the South Atlantic rifting

Christian Heine & regional/local exploration teams and supporting specialists, Shell plc

The SW African margin, composed of the Orange Basin and Cape Basins in Namibian and South African waters, is, both, a type location for magma-rich passive margins as well as showing characteristics of a narrow, weakly magmatic rifting. Recent exploration efforts have produced a wealth of data which illuminate the dynamic processes of continental lithosphere extension and magmatism forming the basement and early basin infill in those two basins. These observations have a profound bearing on our understanding of the evolution of passive margins, a refinement of the kinematics of the South Atlantic rift as well as the response of the earliest source to sink systems.

Relatively widespread continental extension across the Proto-Argentine and SW African continental shelf localised in the present-day proximal Orange Basin and on the Argentine Shelf in late Jurassic-Berriasian times. Towards late Berriasian/Valaginian, emplacement of large volumes of magmatism resulted in a termination of continental extension and a switch to magmatic accretion in the Orange Basin. Regionally mapped magmatic mega-sequences, abandoned eruption centers, and flow sequences demonstrate an initial north-westerly displacement of the South American plate relative to Africa. Evolution of the Cape Basin rift segment was more complex, with lower volume magmatism centered on the Argentine shelf and migrating eastwards, resulting in an asymmetric margin that is relatively relatively wide on the South American side and a narrow, weakly magmatic African conjugate.

As plate separation continues, changes in rift kinematics towards a more westerly separation direction results in a westward jump of the spreading location and the abandonment of the earlier magmatic centers in the Proto-Orange Basin. The result of this change was a “reset” of the South Atlantic rift axis, with a symmetric conjugate pair of SDR mega-sequences documenting the final breakup phase, and a stranded, Afar-like Proto-Orange basin on the African margin. Seismic reflector geometries indicate the presence of possible continental crustal blocks in the distal part of the Orange Basin, analogous and possibly conjugate to the outer high in the Colorado basin in Argentina. Magmatic mega-sequences indicate an initial southward propagation of extension and an aborted rift on the Argentine margin, which, in the context of our regional tectonostratigraphic framework provides evidence that rifting and breakup in the Cape Basin segment occurred relatively late, contradicting the idea of a “northward unzipping” of the South Atlantic.

Petrographic observations of sediments directly above SDRs in Shell’s Cullinan-1X well provide insights into a potential working source to sink system during late Hauterivian/early Barremian times. Together with flexural decompaction modelling of transects across the Orange Basin which show a gentle ramp geometry of the margin this challenges the view of an early outer marginal high

EAGE Conference proceedings on EarthDoc