Southeastern Atlantic and southwestern Indian Ocean: reconstruction of sedimentary and tectonic development since the Cretaceous
Sedimentation patterns in the Transkei Basin:
Objectives
The questions to be solved for this area, where a number of oceanic currents lead to heat and salt exchange and hence to the keep-up of the global conveyor belt, are:
- when did the exchange of water masses between the Atlantic and the Indian Ocean commence?
- when do we find first indications for aggressive, erosive water masses similar to present day North Atlantic Deep Water (NADW) and Antarctic Bottomwater (AABW)?
- which effect did the opening of the gateway south of Africa have on production, transport and deposition of sediments?
- which indications for modifications of AABW, NADW, Antarctic Circumpolar Current and/or the Agulhas Current in time and space can we identify?
About 4200 km of high resolution seismic reflection data (Fig. 1) are being analysed to learn about the sedimentary environment, deposition and re-depositional processes. Figure 2 shows the Agulhas Drift. Material re-deposition due to oceanic currents led to the creation of this sediment drift.
Results
The Transkei Basin deposits document the sediment transport around South Africa and, hence, reveal details of palaeocurrent activity of this region for the past 36 my. Thermohaline driven water masses like the North Atlantic Deep Water (NADW) and the Antarctic Bottom Water (AABW), which are part of the global conveyor belt and the main motor for the heat transfer worldwide, have to pass the southern tip of Africa. A large amount of their sedimentary freight i deposited in the submarine Transkei Basin. By the investigation of high resolution seismic reflection data from central Transkei Basin sediments, we reconstructed depocentres and interface outlines for five different time slices since Cretaceous times. Since at least Late Eocene times, we observe an increasing activity of proto-AABW and later proto-NADW in the Transkei Basin. The current's settings were recurrently modified by various large scale global events, like the opening of the Drake Passage Gateway and the Tasman Gateway (∼34 Ma), respectively, or the closure of the Panama Isthmus (∼3 Ma). The investigations reveal a strong influence of global tectonic and climatic events on NADW and (proto-) AABW and their influence on Transkei Basin deposition.
Figures:
Figure 2: Seismic reflection profile AWI-20050011 showing acoustic basement and seismostratigraphic units 1-5. Blow-up above the seismic reflection profile shows subbottom profiler data from the uppermost 30 m of the drift crest. E: Late Eocene, M: Middle Miocene, O: Eocene/Oligocene boundary, P: Early Pliocene. The arrow indicates the intersection with seismic reflection profile AWI-20050014.
Figure 1: Bathymetric plot of the measurement site: offshore reflection and refraction seismics (AWI), onshore refraction seismics (GFZ Potsdam); black lines are reflection profiles, red dots are OBS positions, blue dots are REFTEK positions.
Publications
Schlueter, P., Uenzelmann-Neben, G. (2007), Seismostratigraphic analysis of the Transkei Basin: A history of deep sea current controlled sedimentation. Mar. Geol., 240, 99-111, doi: 10.1016/j.margeo.2007.02.15
Schlueter, P., Uenzelmann-Neben, G. (2008), Indications for bottom current activity since Eocene times: The climate and ocean gateway archive of the Transkei Basin, South Africa. Glo. Plat. Cha. Letts., 60, 416-428, doi: 10.1016/j.glolacha.2007.07.002.
Schlueter, P., Uenzelmann-Neben, G. (2008), Concpicious seismic reflections in Upper Cretaceous sediments as evidence for black shales off South Africa. Marine and Petroleum Geology, 25, 989-999, doi: 10.1016/j.marpetgeo.2007.10.003
Uenzelmann-Neben, G., Schlueter, P., Weigelt, E. (2007), Cenozoic oceanic criculation within the South African gateway: indications from seismic stratigraphy, S. Afr. J. Geol., 110, 275-294, doi: 10.2113/gssajg.110.2/3.000.
Acknowledgements:
This project is funded by the Bundesministerium fuer Bildung, Forschung und Technologie under contract No 03G0182A 'AISTEK-I'.