View source for OR/15/058 The beginnings of the modern synthesis: 1950-1965

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The resolution of South Georgia’s stratigraphy and geological structure was initiated in the  1950s by the work of A F Trendall during the first two of the three South Georgia Survey Expeditions: 1951–52 and 1953–54 (Carse 1959<ref name="Carse 1959">CARSE, V D. 1959. The Survey of South Georgia, 1951–57. ''Geographical Journal'', '''125''', 20–37.    </ref>; Trendall 1953<ref name="Trendall 1953">TRENDALL, A F. 1953. The Geology of South Georgia: I. ''Falkland Islands Dependencies Survey Scientific Reports'', No. 7, 26 pp, 2 plates, fold-out map.      </ref>, 1959<ref name="Trendall 1959">TRENDALL, A F. 1959. The Geology of South Georgia: II. ''Falkland Islands Dependencies Survey Scientific Reports'', No.19, 47 pp, 5 plates, fold-out map.</ref>). His participation in the first expedition was terminated prematurely when he fell into a crevasse high on the Ross Glacier and broke his leg; the second expedition was beset by logistical problems (Trendall 2011)<ref name="Trendall 2011">TRENDALL, A F. 2011. ''Putting South Georgia on the map. Duncan Carse’s South Georgia Surveys of 1951-56''. Published by the author, Albany, Western Australia. 216 pp.</ref>. Despite these difficulties, Trendall achieved an impressively comprehensive overview. Initially, he thought it possible that the Cumberland Bay and Sandebugten ‘series’ had an unconformable relationship, but subsequently revised that opinion in favour of them representing facies (and/or provenance) variations within a single, thick succession of turbidite strata; he accordingly modified his terminology, describing Cumberland Bay type and Sandebugten type strata. The early Cretaceous age of his Cumberland Bay type rocks was confirmed by new fossil discoveries on Annenkov Island. Trendall also provided much new information on the intrusive sequence of the igneous rocks in the south-east of the island (defining a ‘South-eastern Igneous Complex’), where he also confirmed the interbedding of basaltic pillow lavas and ‘Cumberland Bay type’ turbidite strata. His explanation of the structural problems invoked overfolding, refolding and thrusting but, nevertheless, the precise nature of the relationship between Cumberland Bay and Sandebugten type rocks remained uncertain.

Some progress towards resolving that ‘relationship problem’ was made by the first Falkland Islands Dependencies Survey (FIDS) geologists to work on South Georgia. N Aitkenhead and P H H Nelson were en route to the FIDS base at Hope Bay on the Antarctic Peninsula, but were able to make a short visit to the Cumberland Bay area during the December 1959 to January  1960 interval. Their observations were summarized in an unpublished report (Aitkenhead and Nelson 1962)<ref name="Aitkenhead and Nelson 1962">AITKENHEAD, N, and NELSON, P H H. 1962. The geology of the area between Cumberland West Bay and Cape George, South Georgia. ''British Antarctic Survey Preliminary Geological Report'', '''15''', 13pp. Unpublished.</ref> now held by the British Antarctic Survey (the FIDS was renamed the British Antarctic Survey&nbsp;—&nbsp;BAS&nbsp;—&nbsp;in 1962). From the north-west coast of the Barff Peninsula they described a discordant contact between tightly folded and steeply inclined Sandebugten type strata and an overlying succession of strata that was more similar to the Cumberland Bay type and which showed a markedly different, more open structural style. However, the interpretation remained equivocal, with the preferred option an unconformity subsequently modified by thrusting.

Some geological work was also carried out during the 1964-65 austral summer by members of the Combined Services Expedition. Although this was primarily a mountaineering and topographical surveying venture, J D C Peacock had responsibility for geological observations and the collection of specimens. Geological results were not independently published but were summarised in an expedition report by Burley (1966)<ref name="Burley 1966">BURLEY, M K. 1966. Combined Services Expedition to South Georgia 1964–5. ''Explorers Journal'', '''44''', 106-118. </ref>. Large-scale folding was widely noted, intrusive igneous sills were located between Moraine Fjord and St Andrews Bay, whilst small-scale, intense folding was recorded around the north side of Royal Bay where the main surveying effort was focussed (Fagan 1966)<ref name="Fagan 1966">FAGAN, P F. 1966. Surveys in South Georgia. ''Geographical Journal'', '''132''', 61–64.</ref>. The British Antarctic Survey holds the expedition’s unpublished archive (ref. AD6/2M/1964/L) and specimen collection (ref. ES3/GYX26).

''Regional correlations''

Trendall did not speculate on the regional associations of the South Georgia sedimentary succession, although the stratigraphical revision made between his 1953 and 1959 reports was most probably influenced by the reappraisal of Pirie’s (1905) putative graptolite from the South Orkney Islands, a fossil that had underpinned, via correlations southward, the proposed Lower Palaeozoic age for the Sandebugten division.

The ‘graptolites’ from the South Orkney Islands had been verified by no lesser palaeontological authorities than Gertrude Elles and Ben Peach, the latter also identifying phyllocarid remains (Pirie 1905,<ref name="Pirie 1905">PIRIE, J H H. 1905. On the Graptolite-bearing Rocks of the South Orkneys. ''Proceedings of the Royal Society of Edinburgh'', '''25''', 463–470.</ref> 1913<ref name="Pirie 1913">PIRIE, J H H. 1913. Geology of the South Orkneys. Galley proof for part of the unpublished ''Scottish National Antarctic Expedition Scientific Report'', '''8''', 11 pp. Copy held by the British Antarctic Survey, Cambridge.</ref>); Pirie’s 1905<ref name="Pirie 1905"></ref> paper was presented to the Royal Society of Edinburgh by John Horne, another eminent Scottish geologist well-versed in Lower Palaeozoic geology. With such an ''imprimatur'', the fossils were not unmasked (as most probably indeterminate plant material) for another 50 years, and not before another erroneous ‘graptolite’ discovery by Argentine geologists (Cordini 1955, 273–277),<ref name="Cordini 1955">CORDINI, I R. 1955. Contribución al conocimiento del sector Antártico Argentine. ''Publicaciónes del Instituto Antártico Argentino'', '''1'''. 277 pp.</ref> as reported by Thomson (1977)<ref name="Thomson 1977">THOMSON, M R A. 1977. An annotated bibliography of the paleontology of Lesser Antarctica and  the  Scotia  Ridge.  ''New Zealand Journal of Geology and Geophysics'', '''20''', 865–904.</ref>. When Pirie’s specimens (National Museum of Scotland specimen numbers 1954.2.28 and 29) were re-examined by the Birmingham University palaeontologist (and graptolite specialist) Dr Isles Strachan, he reported, as quoted by Adie (1957b, p.22): “The fragmentary specimens from the South Orkney Islands are extremely poorly preserved. No thecae can be seen on the supposed graptolite stipes and the markings on the other organic fragments can be explained in several ways. There is no positive evidence for an Ordovician-Silurian age for the shales although, of course, the specimens can be interpreted to agree with such an age. They could, however, equally well be identified as plant fragments and assigned to the Carboniferous.” It took some time for this reassessment to become established with, for example, King and Downard (1964)<ref name="King and Downard 1964">KING, L C, and DOWNARD, T W. 1964. Importance of Antarctica in the hypothesis of continental drift. ''In:'' Adie, R J. (ed.) ''Antarctic Geology''. North Holland Publishing Co., Amsterdam. 727–735.</ref> still citing the graptolitic evidence for a Lower Palaeozoic age, but eventually the supposedly graptolitic rocks yielded Triassic  conodonts and radiolaria (Dalziel 1979; Dalziel and others 1981).

Nevertheless, despite the palaeontological revisions, there was a continuing view that, whilst not Lower Palaeozoic, the Sandebugten type strata might still be older then the demonstrably Mesozoic Cumberland Bay type. Trendall (1959, p.42)<ref name="Trendall 1959">TRENDALL, A F. 1959. The Geology of South Georgia: II. ''Falkland Islands Dependencies Survey Scientific Reports'', No.19, 47 pp, 5 plates, fold-out map.</ref> thought it “probable that the Sandebugten-type is also of Mesozoic age, though there is still the possibility that it is Palaeozoic.” Adie (1964, p.122)<ref name="Adie 1964">ADIE, R J. 1964. Geological History. In: Priestly, R E, Adie, R J and Robin, G de Q. (eds) ''Antarctic Research''. Butterworth, London. 118–162.</ref> agreed, citing the unpublished work of Aitkenhead and Nelson. He re-emphasised the lithofacies similarities between a resurrected Sandebugten Series and the Trinity Peninsula Series of Graham Land and the Greywacke-Shale Series of the South Orkney Islands (source of Pirie’s ‘graptolite’ and now known to be Triassic), both of which divisions he considered to be probably Carboniferous.

Correlations westward from South Georgia, towards Tierra del Fuego, had also been contentious. At first it was thought unlikely that there was any geological association between the two areas, a view given prominence by Professor J Gregory’s (1929)<ref name="Gregory's 1929">GREGORY, J W. 1929. The Geological History of the Atlantic Ocean. ''Quarterly Journal of the Geological Society, London'', '''85''', Proceedings lxviii–cxxii.      </ref> Presidential Address to the Geological Society of London. Therein, South Georgia, and the other islands of the Scotia Arc  were regarded as vestiges of a foundered continent that had once occupied the South Atlantic region. Gregory would have based his opinion largely on the work of his protégé Ferguson, and justified his regional interpretation with the observation that “[t]he sedimentary rocks also indicate for South Georgia a different history from that of the Andes” (Gregory 1929, p.cxi)<ref name="Gregory 1929">GREGORY, J W. 1929. The Geological History of the Atlantic Ocean. ''Quarterly Journal of the Geological Society, London'', '''85''', Proceedings lxviii–cxxii.</ref>. This opinion was immediately challenged by Holtedahl (1929, p.105–106)<ref name="Holtedahl 1929">HOLTEDAHL, O. 1929. On the Geology and Physiography of some Antarctic and Sub-Antarctic Islands. ''Scientific Results of the Norwegian Antarctic Expedition'' ''1927–1928'', '''3'''. I Kommisjon Hos Jacob Dybwad, Oslo. 172 pp + 52 plates.</ref> writing after the 1927–28 Norwegian Antarctic Expedition: “To me it seems that the sedimentary rocks of the extreme south of South America … remind one very strongly of the South Georgia series.” Subsequent work confirmed this view. After extensive work in Tierra del Fuego, (Kranck 1934)<ref name="Kranck 1934">KRANCK, E H. 1934. The South Antillean Ridge. ''Bulletin de la Commission Géologique de Finlande'', '''104''', 99–103.    </ref> noted the geological similarities with South Georgia and made specific lithostratigraphical correlations with the South Georgia divisions proposed by Ferguson (1915)<ref name="Ferguson 1915">FERGUSON, D. 1915. Geological observations in South Georgia. Transactions of the Royal Society of Edinburgh, 50, 797-814, 10 plates.</ref>. In Tierra del Fuego, Krank established the Yahgan Formation, which later work on Navarino Island, southern Chile, confirmed as having a close correspondence to the Cumberland Bay Formation of South Georgia (Katz and Waters 1966)<ref name="Katz and Waters 1966">KATZ, H R, and WATERS, W A. 1966. Geological investigation of the Yahgan Formation (Upper Mesozoic) and associated igneous rocks of Navarino Island, Southern Chile. ''New Zealand Journal of Geology and Geophysics'', '''9''', 323–359.</ref>. These aspects of South Georgia geology played into the wider, early 20th century debate over the validity of continental drift, and its role in the formation of the Scotia Arc.

==References==
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[[category: OR/15/058 The geological exploration of the sub-Antarctic island of South Georgia: a review and bibliography, 1871-2015 | 06]]