Tsunami hazard assessment of coastal South Africa based on mega-earthquakes of remote subduction zones

After the mega-earthquakes and concomitant devastating tsunamis in Sumatra (2004) and Japan (2011), we launched an investigation into the potential risk of tsunami hazard to the coastal cities of South Africa. This paper presents the analysis of the seismic hazard of seismogenic sources that could potentially generate tsunamis, as well as the analysis of the tsunami hazard to coastal areas of South Africa. The subduction zones of Makran, South Sandwich Island, Sumatra, and the Andaman Islands were identified as possible sources of mega-earthquakes and tsunamis that could affect the African coast. Numerical tsunami simulations were used to investigate the realistic and worst-case scenarios that could be generated by these subduction zones. The simulated tsunami amplitudes and run-up heights calculated for the coastal cities of Cape Town, Durban, and Port Elizabeth are relatively small and therefore pose no real risk to the South African coast. However, only distant tsunamigenic sources were considered and the results should therefore be viewed as preliminary.The Nuclear Structural Engineering (Pty) and the National Research Foundation through the Technology and Human Resources for Industry Programme project (THRIP) TP2011061400009.https://link.springer.com/journal/242019-04-01hj2018Geolog

Hydrocarbon generation and migration from Barremian – Aptian source rocks, Northern Orange Basin, offshore Western South Africa: A 3d numerical modelling study

A 3D numerical modelling workflow was applied to the Barremian—Aptian source rock interval in a shelfal to lower slope area of the northern Orange Basin, offshore western South Africa. The main objective was to investigate the timing of hydrocarbon generation and migration. Hydrocarbon migration has previously been investigated in the south of the basin by relating gas escape features with structural elements as seen on seismic sections, but migration pathways are still poorly understood. The modelling study was based on data from three exploration wells (AO-1, AE-1 and AF-1) together with 42 2D seismic sections totalling 3537 km in length, and a 3D seismic cube covering an area of 750 sq. km. Modelled formation temperatures increase from north to south in the study area and were consistent with downhole temperatures at well locations. However, there is variation between measured and modelled values of vitrinite reflectance (VR), especially in the Turonian and Cenomanian intervals. The measured VR is lower than the modelled VR within the Turonian section in the north of the study area, suggesting that erosion has affected the thermal maturity of the sediments. However, in the Cenomanian interval, the measured VR is higher than the modelled VR. Uplift, increased erosion in the hinterland and sediment transport to the coastal areas resulted in Cenomanian progradation of the Orange Basin fill

Post-Palaeozoic stratigraphy of the eastern Agulhas Bank, South African continental margin

The Agulhas Bank, which forms the continental margin on the southern tip of Africa, consists of a thick Meso-Cainozoic sedimentary sequence (up to 6.2 km) resting on and behind pre-Mesozoic continental acoustic basement. The stratigraphy of this sequence is outlined and its history and facies variations mentioned where they are known. Refraction seismic velocity and bottom sample data indicate a basic three-fold subdivision of the Mesozoic sequence, which can be correlated with the onshore succession in the Algoa Basin. It is separated by a major hiatus from the Cainozoic sediments, which consist of a Palaeogene and Neogene sequence subdivided by another well-defined level of erosion. Various formations within the Cainozoic are defined and named. An outline of the bathymetry of the eastern Agulhas Bank is also given

Mesozoic palaeogeography of the Southern Cape, South Africa

Four palaeogeographical reconstructions are presented for the southern Cape covering the period Late Permian to Late Cretaceous. This time spans the commencement to an advanced stage of breakup of Gondwanaland, during which the area moved from a mid-continental, high latitude, to an ocean-dominated, middle latitude position. These movements can be traced in facies changes and erosional cycles associated with the rift between West Gondwana and Antarctica (proto southwest Indian Ocean) and the later rift between South America and Africa (proto southeast Atlantic Ocean)

Large allochthonous sediment masses and their role in the construction of the continental slope and rise off southwestern Africa

Large Neogene slumps have affected over 260,000 km2 of the outer continental margin and adjacent Cape Basin off southwestern Africa. Individual structures cover areas up to 68,700 km2 and proximally are commonly composed of huge rotated sediment blocks up to 450 m thick and several kilometers across. Seismic shocks, possibly in conjunction with lower-slope undercutting by bottom-current erosion, are suggested as possible trigger mechanisms for these features which are all thought to be post-Pliocene (possibly Pleistocene) in age. Older slumps are also recognized along the margin and four cycles of sedimentation/slumping are identified: early Upper Cretaceous (I); late Upper Cretaceous (II); Palaeogene (III); and Neogene (IV). In the main part of the Orange Basin depocentre (west of Childs Bank) the Cretaceous slump styles are thought to represent Mississippi delta-type down-slope sediment cascades (with reverse faulting and mud diapirism) over 1 km thick which resulted from very rapid dumping of terrigenous material from the Orange River. Cainozoic slumps show a different tectonic style and locus and this is thought to reflect a change in sedimentation patterns which resulted from lower terrigenous input onto the margin, higher biogenic/authigenic sedimentation, and slowed crustal subsidence. A connection possibly exists between low sea level stands and the Cainozoic episodes of slumping

Implications for high latitude gondwanide palaeozoogeographical studies of some new Upper Cretaceous marine ostracod faunas from New Zealand and the Antarctic Peninsula

Sixty species of Ostracoda have been recovered from Cenomanian, Santonian and Maastrichtian strata in New Zealand, and late Campanian sediments on Snow Hill and James Ross islands in the Antarctic Peninsula. The two main New Zealand sites are in latest Maastrichtian strata, but in contrasting thermal regimes - warm, shelfal facies at Waipara, and cool, outer shelf/upper slope at Pukehou. The palaeozoogeographical history of several important taxa across the K/T boundary in Gondwanaland is clarified by the new data: Rostrocytheridea survived at Pukehou to within a few metres of the K/T, while Majungaella was found ~0.5 m from the top of the Maastrichtian at Waipara. The previously-known retrothermal propensities of Majungaella can be traced to the Maastrichtian at Pukehou, where a similar adaptation is observed in Rostrocytheridea, and possibly in Krithe. The first two genera became extinct across Mesozoic/ Tertiary boundary in Australasia, while in the Patagonia-Antarctic Peninsula region, Majungaella survived and colonised much of the Antarctic seaboard, but Rostrocytheridea probably did not survive into the Palaeogene. The extant genus Ameghinocythere is now known from late Campanian of Snow Hill Island, and also occurs in the late Maastrichtian in New Zealand. The earliest record of the widely distributed Gondwanide genus Apateloschizocythere is probably from the Cenomanian at Coverham, New Zealand. Nine new species are described: Ameghinocythere lutheri, A. eagari, Apateloschizocythere? colleni, Limburgina postaurora, Majungaella wilsoni, M. waiparaensis, Parahystricocythere ericea, Rayneria? punctata, Rostrocytheridea pukehouensis and Trachyleberis hornibrooki. The genus Parahystricocythere is new