Acoustic stratigraphy and current-generated bedforms in deep ocean basins off southeastern Africa

doi:10.1016/0025-3227(79)90083-5

Marine Geology

Volume 33, Issues 3–4, November 1979, Pages 239-260

https://doi.org/10.1016/0025-3227(79)90083-5 Get rights and content

Abstract

A bathymetric map of the deep-sea floor off southeastern Africa shows the Agulhas Plateau to be separated from the continental margin of southeastern Africa by a narrow (50 km) elongate depression, the Agulhas Passage, which acts as a deep-water connection between the Agulhas and Transkei basins.

Three regionally developed sediment layers occur in the deep (> 4500 m) Transkei Basin/Agulhas Passage area. With the aid of a simple ocean crust sinking/carbonate compensation level (CCL) model these layers are related to the sedimentation history of the area. The model suggests that acoustic basement (Horizon X) represents Lower Cretaceous limestones draped over oceanic basement, and that this is overlain by an acoustically transparent sequence of pelagic/terrigenous material (Horizon A) that was deposited during a lengthy period (95 m.y.) beneath the carbonate compensation level. Post-Late Miocene sedimentation (Horizons B and C) has probably taken place above the CCL. Local sedimentation has always been influenced by strong sea-floor currents, but since middle Palaeogene times these currents have operated on a regional scale and have generated numerous large ridge and billow-like bed forms.

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Cited by (20)

Late Cretaceous onset of current controlled sedimentation in the African–Southern Ocean gateway
2018, Marine Geology
Citation Excerpt :
We therefore suggest that the erosive circulation in the study area commenced ~ 91 Ma during the Turonian. Both the occurrence of a ~ 25 Myr long hiatus at the Agulhas Plateau (Tucholke and Carpenter, 1977; Tucholke and Embley, 1984; Uenzelmann-Neben, 2002) and a strongly condensed sedimentary column in the Agulhas Passage (Dingle and Camden-Smith, 1979) was attributed to strong current activity. The particularly long lasting hiatuses at the topographic highs in the study area may be a result of strong intensification of the flow due to the occurring steep flanks or narrow passages at the structures (Stow et al., 2002).
During the breakup of Gondwana the Mozambique Ridge, a Large Igneous Province emplaced between 140 and 125 Ma, was located in the evolving African–Southern Ocean gateway. Therefore, it represents an archive of the evolving exchange of water masses between the South Atlantic and Indian Ocean via the development of surface, intermediate, and bottom circulation. Two Cretaceous seismic units (S1 and S2a) were deposited on top of magmatic basement separated by a hiatus. Unit S1 mostly shows seismic reflections parallel to the top of the basement and no indications of current activity. The occurrence of several sediment drifts within seismic unit S2a indicates the onset of current controlled sedimentation. Based on our observations we propose deposition under partly euxinic conditions in the area of the Mozambique Ridge until ~ 100 Ma. The onset of a strong shallow circulation affecting deposition at the Mozambique Ridge is inferred by the Late Cretaceous ~ 25 Myr hiatus reported by drilling results and documented in the seismic records, whereas black shales deposited in the nearby deep Transkei Basin indicate a restricted deep circulation at least until ~ 85–80 Ma. We propose that the observed hiatus might be a consequence of a late Early Cretaceous uplift of the Mozambique Ridge and the progressive opening of the Agulhas Passage allowing inflow of surface (Upper Pacific Water, Upper North Atlantic Water) and intermediate water (Intermediate Southern Ocean Water) into the study area. The intense circulation that caused the hiatus seems to have weakened in Campanian times, which is documented by the occurrence of sediment drifts in seismic unit S2a. We suggest that the onset of current controlled sedimentation was caused by palaeogeographic modifications in the Atlantic Ocean along with relocation of circulation pathways. Our results illustrate the crucial role of the African–Southern Ocean gateway in the commencing water mass exchange between the Atlantic and Indian Ocean and highlight the complex interactions that eventually lead to the initiation of a proto–Antarctic Circumpolar Current in Turonian times.
Anomalous seafloor mounds in the northern Natal Valley, southwest Indian Ocean: Implications for the East African Rift System
2014, Tectonophysics
Citation Excerpt :
This paper aims to describe these features from a morphological and shallow seismic perspective and to use their occurrence as a means to better understand the geological and/or oceanographic evolution of this basin. As in all other basins, early work in the Natal Valley relied heavily on high frequency seismic echo-character to describe the bathymetry and shallow sub-bottom characteristics of the seafloor (cf. Dingle and Camden-Smith, 1979; Dingle et al., 1978; Kolla et al., 1980). The primary focus of this was to establish the acoustic stratigraphy and magnetic character of the Natal Valley.
The Natal Valley (southwest Indian Ocean) has a complicated and protracted opening history, as has the surrounding southwest Indian Ocean. Recently collected multibeam swath bathymetry and 3.5 kHz seismic data from the Natal Valley reveal anomalous seafloor mounds in the northern Natal Valley. The significance, of these domes, as recorders of the geological history of the Natal Valley and SE African Margin has been overlooked with little attempt made to identify their origin, evolution or tectonic significance. This paper aims to describe these features from a morphological perspective and to use their occurrence as a means to better understand the geological and oceanographic evolution of this basin. The seafloor mounds are distinct in both shallow seismic and morphological character from the surrounding seafloor of the Natal Valley. Between 25 km and 31 km long, and 16 km and 18 km wide, these features rise some 400 m above the sedimentary deposits that have filled in the Natal Valley. Such macro-scale features have not previously been described from the Natal Valley or from other passive margins globally. They are not the result of bottom water circulation, salt tectonics; rather, igneous activity is favoured as the origin for these anomalous seafloor features. We propose a hypothesis that the anomalous seafloor mounds observed in the Natal Valley are related to igneous activity associated with the EARS. The complicated opening history and antecedent geology, coupled with the southward propagation of the East African Rift System creates a unique setting where continental rift associated features have been developed in a marine setting.
Sedimentary deposits on the southern South African continental margin: Slumping versus non-deposition or erosion by oceanic currents?
2009, Marine Geology
Seismic profiles extending from the southern South African shelf into the deep sea reveal a strong erosional activity, which affects large parts of the continental margin. Quaternary to Oligocene units and, in places, the complete sedimentary column appear to have been removed. Mass movements were previously considered as the origin of this erosion. However, structures indicating slumping can be identified in only a few places. The erosional activity is confined to specific water depths, which correlate well with the activity levels of water masses observed here. We thus suggest that the Agulhas Current, Antarctic Intermediate Water, North Atlantic Deep Water, and Antarctic Bottom Water have intensively shaped the sedimentary units over a long period. This indicates a stable circulation comparable to that at present day since the Neogene.
Conspicuous seismic reflections in Upper Cretaceous sediments as evidence for black shales off South Africa
2008, Marine and Petroleum Geology
The Upper Cretaceous is commonly associated with greenhouse climate, Oceanic Anoxic Events (OAE), and the ongoing break up of Gondwana, which resulted in strong variations of the ocean's currents flow paths and climate change. Little is known about these changing conditions, in particular south of South Africa. A high-resolution seismic reflection data set from the submarine Transkei Basin off South Africa revealed various depositional stages for this region for the past ∼90 Ma. In these seismic sections, a recurrently appearing high-amplitude horizon within weak to homogeneous Upper Cretaceous reflections was observed. Due to the inaccessibility of any drill hole data from the Transkei Basin and adjacent regions, the origin of this reflector is difficult to specify. It is roughly dated at ∼80–∼85 Ma, which falls within the last big OAE in Upper Cretaceous (OAE 3). A seismostratigraphic analysis led to an exclusion of several features as the origin for this reflector, such as gas hydrates, opal bearing horizons or biogenic oil. Other possible features, such as lithologic variations, intrusions or black shales, are discussed in more detail. According to its appearance and reflection characteristics it could possibly be the first report of black shales in this region.
Indications for bottom current activity since Eocene times: The climate and ocean gateway archive of the Transkei Basin, South Africa
2008, Global and Planetary Change
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 is 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.
Seismostratigraphic analysis of the Transkei Basin: A history of deep sea current controlled sedimentation
2007, Marine Geology
The Earth's climate is controlled by various factors, with large scale ocean currents playing a significant role. In particular, the global thermohaline circulation of water masses like the Antarctic Bottom Water (AABW), or the North Atlantic Deep Water (NADW), is a global motor for maintaining the exchange of water masses. The AABW and NADW have met and interacted off South Africa since Oligocene times. Here, the narrow deep Agulhas Passage gateway, located between South Africa and the submarine Agulhas Plateau, constrains bottom water exchange between the southeast Atlantic and the southwest Indian Ocean. A seismostratigraphic analysis of sedimentary structures in the Transkei Basin, which opens up at the eastern end of the Agulhas Passage, was carried out, to reconstruct the palaeocurrents off South Africa. The analysis of newly collected high resolution seismic reflection data showed the effect of large scale current deposition. There are at least 5 major sedimentary phases to observe, some of which seem to be influenced by NADW and AABW. The first stage represents ongoing deep sea sedimentation from middle Cretaceous to middle Tertiary times. Later stages are separated by discordances, which represent the onset of AABW and NADW, among others, triggered by the opening of the Drake Passage gateway (∼ 35 Ma) and the closure of the Isthmus of Panama (∼ 3 Ma). We found two large drift bodies located one above the other. Corresponding to their shape and position, the older drift is inferred to have been deposited by currents flowing in a north–southerly direction, whereas the younger drift lies perpendicular to it and seems to be built up by west–east flowing currents.

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Research paperAcoustic stratigraphy and current-generated bedforms in deep ocean basins off southeastern Africa

Abstract

Mar. Geol.

Earth Planet. Sci. Lett.

Mar. Geol.

Deep-Sea Res.

Deep-Sea Res.

Mar. Geol.

Earth Planet. Sci. Lett.

Agulhas Plateau off southern Africa: a geophysical study

Geol. Soc. Am. Bull.

Seismic refraction observations in the Transkei Basin and adjacent areas

Mar. Geophys. Res.

Sedimentary processes in the North Atlantic

The anatomy of a large submarine slump on a sheared continental margin (SE Africa)

J. Geol. Soc. London

Continental breakup and the development of post-Paleozoic sedimentary basins around southern Africa

Geol. Soc. Am. Bull.

Research paper
Acoustic stratigraphy and current-generated bedforms in deep ocean basins off southeastern Africa