Lithological variations of sedimentary succession within a meteorite impact crater: Jwaneng S Structure, Botswana

The Jwaneng South Structure is a meteorite impact crater located in the Kalahari region of Botswana. The structure has the shape of a bowl 1.3 km in diameter and a maximum depth of 275 m in the centre. It was discovered by an airship-mounted full tensor gravity gradiometer and penetrated by nine vertical diamond drill-holes. The crater is underlain by the Gaborone Granite (2785 Ma) and basalts of the Karoo Supergroup (182 Ma). The covering aeolian sediments of the Kalahari Group (Late Cretaceous-Recent) completely obscure the structure.A succession of the following lithofacies overlying authigenic in situ brecciated granite was intersected in the boreholes (from base to top): (i) allogenic heterolithic/oligomictic “fallback” and resedimented breccia (ii) sedimentary breccia and conglomerate with sand matrix; (iii) six intervals of carbonate sediments, with traces of evaporites and mudstone interbeds, which are interlayered with (iv) five intervals of sandstone and sedimentary breccia composed of granules, pebbles and cobbles, mostly of granite, embedded in a matrix of well-sorted medium-grained sand; (v) bioturbated, mostly massive sandstone rich in mud matrix (wacke), with locally preserved interbeds of mudstone and cross-bedded sandstone, and abundant root traces; (v) silcrete and calcrete that occur at the top of the succession.This lithological association suggests that deposition within the Jwaneng South meteorite impact crater took place in a playa lake surrounded by steeply-dipping talus piedmont fans. The depositional cycles were controlled by pronounced climatic oscillations. Wet periods are recorded by lithofacies (iii), which reflects intense supply of sand eroded from the Kalahari dune field surrounding the crater and coarse detritus derived from its rim and steep talus below. Dry intervals of high evaporation and fall of the lake level are reflected by lithofacies (iv). During the youngest wet period (v) the lake filled up with alluvia sands interbedded with muds, abundantly vegetated and homogenised by bioturbation. The silcrete and calcrete layer at the top of the succession is the product of pedogenic processes that affected the Kalahari Desert environment. An asymmetry of lateral distribution of the lithofacies (ii) – (iv) and the presence of sedimentary breccia redeposited into the marginal E (and NE) parts of the crater suggest an asymmetry of the crater depression and its coarse clastic rim, which may imply an oblique trajectory of the impactor approaching from the SW

Multiple sulphur and iron isotope composition of detrital pyrite in Archaean sedimentary rocks : a new tool for provenance analysis

Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 286 (2009): 436-445, doi:10.1016/j.epsl.2009.07.008.Multiple S (δ34S and δ33S) and Fe (δ56Fe) isotope analyses of rounded pyrite grains from 3.1 to 2.6 Ga conglomerates of southern Africa indicate their detrital origin, which supports anoxic surface conditions in the Archaean. Rounded pyrites from Meso- to Neoarchaean gold and uranium-bearing strata of South Africa are derived from both crustal and sedimentary sources, the latter being characterised by non-mass dependent fractionation of S isotopes (Δ33S as negative as -1.35‰) and large range of Fe isotope values (δ56Fe between -1.1 and 1.2‰). Most sediment-sourced pyrite grains are likely derived from sulphide nodules in marine organic matter-rich shales, sedimentary exhalites and volcanogenic massive sulphide deposits. Some sedimentary pyrite grains may have been derived from in situ sulphidised Fe-oxides, prior to their incorporation into the conglomerates, as indicated by unusually high positive δ56Fe values. Sedimentary sulphides without significant non-mass dependent fractionation of S isotopes were also present in the source of some conglomerates. The abundance in these rocks of detrital pyrite unstable in the oxygenated atmosphere may suggest factors other than high pO2 as the cause for the absence of significant non-mass dependent fractionation processes in the 3.2 – 2.7 Ga atmosphere. Rounded pyrites from the ca. 2.6 Ga conglomerates of the Belingwe greenstone belt in Zimbabwe have strongly fractionated δ34S, Δ33S and δ56Fe values, the source of which can be traced back to black shale-hosted massive sulphides in the underlying strata. The study demonstrates the utility of combined multiple S and Fe isotope analysis for provenance reconstruction of Archaean sedimentary successions.AH acknowledges support by NAI International Collaboration Grant and NRF grant FA2005040400027. AB participation was supported by NSF grant EAR-937 05-45484, NAI award No. NNA04CC09A, and NSERC 938 Discovery grant. Rouxel's contribution was supported by NSF OCE-0622982

Darwin as a geologist in Africa – dispelling the myths and unravelling a confused knot

Two myths persist concerning the role played by Charles Darwin as a geologist in Africa during his epic voyage around the world (1831–1836). The first myth is that Darwin was a completely self-taught geologist, with no formal training. The second myth is that it was Darwin who finally solved the problem of the granite–schist contact at the famous Sea Point coastal exposures in Cape Town, after deliberately setting out to prove his predecessors wrong. These myths are challenged by the now ample evidence that Darwin had excellent help in his geological education from the likes of Robert Jameson, John Henslow and Adam Sedgwick. The story of Darwin and his predecessors at the Sea Point granite contact has become confused, and even conflated, with previous descriptions by Basil Hall (1813) and Clark Abel (1818). Here, the historical record is unravelled and set straight, and it is shown from the evidence of his notebooks that Darwin was quite unaware of the outcrops in Cape Town. His erudite account of the contact was a result of the 8 years spent in writing and correspondence after his return to England and not because of his brilliant insights on the outcrop, as the myth would have it. While there has been little to indicate Darwin’s landfalls in Africa, a new plaque now explains the geology of the Sea Point Contact, and includes a drawing of Darwin’s ship, the Beagle, and quotes from his work

The first stratigraphic column in South Africa, from Hondius (1652), and its modern correlatives

In 1647 the Dutch ship Haarlem, en route from Batavia to the Netherlands Republic, was wrecked in Table Bay. The survivors were encamped over the next year before they were rescued in a fort they constructed called Sandenburgh. Their successful sojourn in the Cape led directly to the establishment of the Dutch colony there in 1652. They survived by living on hunted cormorants and penguins, bartered cattle and sheep, and by drinking fresh water obtained from a well which they sank to a depth of 20 m. The sequence of sediments encountered in the well was recorded by Jodocus Hondius III, grandson of the famous mapmaker, in a book published in 1652, based on accounts given to him by the sailors from the Haarlem. A comparison of the stratigraphy recorded in the well (five sedimentary units) with the Pleistocene and Holocene stratigraphy known from modern studies of these coastal sediments, shows a very good correspondence in terms of lithologies and thicknesses, and attests to the veracity of the sources that provided Hondius with his information. This singular case of a detailed stratigraphic column is interesting in the light it throws on the rudimentary understanding of rock types, stratigraphy and hydrology by Dutch sailors in the mid-17th century, at the beginnings of South African colonial history, more than a decade before the study of stratigraphy was initiated by the work of Steno. The measurements recorded in the description of the well are some of the earliest quantitative data recorded in the history of South African science

A few hours in the Seychelles with Alex du Toit in 1938

Alexander Logie du Toit (1878–1948) was South Africa’s most famous geologist during his lifetime, having authored five books which brought him world renown. In December 1937 to January 1938, accompanied by his wife Evelyn, he visited India in order to attend the Jubilee Indian Science Congress in Calcutta and to do field work in coal and diamond mines. On the return journey to Africa by ship, they stopped for a few hours in Port Victoria on Mahé Island in the Seychelles archipelago. They also passed by Silhouette Island. Du Toit recorded his activities in a diary, and his geological observations in a notebook, where he also drew a sketch of Mahé, and recorded steep structures on the east coast of Silhouette. Although he had not visited the Seychelles before, his deep understanding of the problems of Seychelles geology resulted from his comprehensive research on Indian Ocean geology for his 1937 book Our Wandering Continents. He made remarkably accurate observations on the geomorphology and structure, some of which were only confirmed decades later when the Seychelles were mapped in the 1960s to 1990s. His bold and prescient ideas on the breakup of the Gondwana continent, and on the formation of the Indian Ocean, have been amply confirmed by modern studies, especially by those of Lewis D. Ashwal and his collaborators. Significance: South African geologist Alexander Logie du Toit’s impressions of the Seychelles in 1938 are recorded for the first time, based on entries in his diaries. His observations of structures on Mahé and Silhouette Islands were prescient. His deep understanding of Seychelles geology was the result of his research for his 1937 book Our Wandering Continents. His bold conjecture that the Mascerene Ridge, made of continental crust, was the nucleus of Mauritius, was finally proved in 2017

Darwin as a geologist in Africa - dispelling the myths and unravelling a confused knot

Two myths persist concerning the role played by Charles Darwin as a geologist in Africa during his epic voyage around the world (1831 - 1836). The first myth is that Darwin was a completely self-taught geologist, with no formal training. The second myth is that it was Darwin who finally solved the problem of the granite - schist contact at the famous Sea Point coastal exposures in Cape Town, after deliberately setting out to prove his predecessors wrong. These myths are challenged by the now ample evidence that Darwin had excellent help in his geological education from the likes of Robert Jameson, John Henslow and Adam Sedgwick. The story of Darwin and his predecessors at the Sea Point granite contact has become confused, and even conflated, with previous descriptions by Basil Hall (1813) and Clark Abel (1818). Here, the historical record is unravelled and set straight, and it is shown from the evidence of his notebooks that Darwin was quite unaware of the outcrops in Cape Town. His erudite account of the contact was a result of the 8 years spent in writing and correspondence after his return to England and not because of his brilliant insights on the outcrop, as the myth would have it. While there has been little to indicate Darwin's landfalls in Africa, a new plaque now explains the geology of the Sea Point Contact, and includes a drawing of Darwin's ship, the Beagle, and quotes from his work

Peripatetic careers of Vsevolod and Eugenie Gorsky, mid-20th century Slovenian-educated geoscientists

In September 1947, South Africa’s most famous geologist, Dr Alexander Logie du Toit, FRS, well known for his support of the concept of Continental Drift, received a visit from a Slovenian-educated Russian émigré couple, Vsevolod and Eugenie Gorsky, who were newly arrived in South Africa. Vsevolod, born in what is now Ukraine, was a mining engineer, geologist and geophysicist with vast experience in the minerals industry, while his wife Eugenie, born in the Russian Caucasus, was an analytical geochemist. Vsevolod had a brief exchange of letters with du Toit, seeking his help in obtaining employment in South Africa’s minerals industry. Included in the first letter to du Toit were detailed curricula vitae of both Vsevolod and Eugenie Gorsky. These detailed CVs allow us to reconstruct the training (at the University of Ljubljana, under the influence of Russian mineralogist V.V. Nikitin) and careers of these two earth science professionals in Slovenia and Macedonia, in the early Twentieth Century, and to follow their peripatetic careers as they left the Kingdom of Yugoslavia before the start of the Second World War, in Cyprus, Egypt, Tanganyika and South Africa. They ultimately ended up in Brazil in the 1960s and 1970s, and probably retired there. Eugenie was constrained to follow her husband wherever his career led him, but she always ended up working in most of the countries and places they found themselves in. As a professional couple who travelled the world, the Gorskys were pioneers in a way of life that is commonplace now in a globalized world

Oldest 'earthquake' in South Africa (Robben Island, 07 April 1620) discredited

The oldest recorded earthquake in South Africa is widely accepted (in several seismic catalogues) to have occurred on 07 April 1620. This earthquake was regarded as having a Modified Mercalli Scale intensity of II-IV, corresponding to a Richter Scale Magnitude of ~4. An examination of the original sources on which the record of this earthquake rests, reveals that it was based on a description of 'two startling thunderclaps like cannon shots while ship was becalmed near Robben Island' by Augustin de Beaulieu, who was the head of a fleet of three ships which put in Table Bay in March-April 1620. A full excerpt of Beaulieu's account reveals that the thunderclaps took place in a short period of calm during an extended period of stormy weather, and that the observations were made on board ship, so that no seismic ground vibration was felt. The Western Cape has a much lower incidence of lightning than the interior of South Africa, and the fact that the thunderclaps were not accompanied by lightning is not unusual. Thus the simplest explanation of the thunderclaps is that they were the result of atmospheric phenomena, and not a result of seismic activity, as interpreted by J.N. Theron in 1974. The events of 07 April 1620 should thus be removed from the catalogues of historical seismicity in South Africa, making the slight shock felt in Cape Town in 1690, with a Modified Mercalli Scale intensity of III, the oldest recorded seismic tremor in the history of South Africa

Oldest ‘earthquake’ in South Africa (Robben Island, 07 April 1620) discredited

The oldest recorded earthquake in South Africa is widely accepted (in several seismic catalogues) to have occurred on 07 April 1620. This earthquake was regarded as having a Modified Mercalli Scale intensity of II-IV, corresponding to a Richter Scale Magnitude of ~4. An examination of the original sources on which the record of this earthquake rests, reveals that it was based on a description of ‘two startling thunderclaps like cannon shots while ship was becalmed near Robben Island’ by Augustin de Beaulieu, who was the head of a fleet of three ships which put in Table Bay in March–April 1620. A full excerpt of Beaulieu’s account reveals that the thunderclaps took place in a short period of calm during an extended period of stormy weather, and that the observations were made on board ship, so that no seismic ground vibration was felt. The Western Cape has a much lower incidence of lightning than the interior of South Africa, and the fact that the thunderclaps were not accompanied by lightning is not unusual. Thus the simplest explanation of the thunderclaps is that they were the result of atmospheric phenomena, and not a result of seismic activity, as interpreted by J.N. Theron in 1974. The events of 07 April 1620 should thus be removed from the catalogues of historical seismicity in South Africa, making the slight shock felt in Cape Town in 1690, with a Modified Mercalli Scale intensity of III, the oldest recorded seismic tremor in the history of South Africa