View source for OR/15/058 Specialist research and regional models: 1973-1982

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In parallel with the closing contributions to the BAS geological mapping programme, specialist studies sought to integrate accumulated data into a regional interpretation and development model. Key to this phase of the BAS programme were the structural interpretations of P W G Tanner and the sedimentological work of D I M Macdonald. As previously noted, Tanner visited many localities around South Georgia during the 1973–74 austral summer. He returned for the 1975–76 summer, working mostly in the coastal region between Fortuna Bay and Cumberland East Bay, whilst at the same time Macdonald worked in the extreme north-west mainland of South Georgia in the Elsehul to North Cape area, and along the south-west coast of the island between Queen Maud Bay and Cape Darnley. Both researchers made landings on the Hauge  Reef and Pickersgill Islands in collaboration with Storey, and jointly interpreted their geology in terms of an island arc assemblage (Tanner and others 1981)<ref name="Tanner 1981">TANNER, P W G, STOREY, B C, and MACDONALD, D I M. 1981. Geology of an Upper Jurrasic–Lower Cretaceous island-arc assemblage in Hauge Reef, the Pickersgill Islands and adjoining areas of South Georgia. ''British Antarctic Survey Bulletin'', '''53''', 77–117.</ref>. A sedimentary succession of tuff and mudstone overlain by volcaniclastic breccia was correlated with the Annenkov Island Formation, and overlay pillowed basaltic lavas correlated with the Larsen Harbour Formation. The geochemistry of the assemblage confirmed its tholeiitic association (Storey and Tanner 1982)<ref name="Storey 1982">STOREY, B C, and TANNER, P W G. 1982. Geochemistry of a Mesozoic island-arc assemblage and ocean-floor basalts from South Georgia. ''British Antarctic Survey Bulletin'', '''56''', 7–24.</ref>. The sedimentary unit was cut and hornfelsed by a wide range of mafic sills and gabbroic to dioritic intrusions, also largely tholeiitic. Radiometric dates (Rb-Sr and K-Ar) reported by Tanner and Rex (1979)<ref name="Tanner 1979">TANNER, P W G, and REX, D C. 1979. Timing of events in an Early Cretaceous island arc–marginal basin system on South Georgia. ''Geological Magazine'', '''116''', 167–179.</ref> were approximately in the 80–100&nbsp;Ma range.

As part of his sedimentological studies Macdonald investigated the nature of the fossil wood fragments that had been widely reported from the Cumberland Bay Formation (Jefferson and Macdonald 1981)<ref name="Jefferson 1981">JEFFERSON, T H and MACDONALD, D I M. 1981. Fossil wood from South Georgia. ''British Antarctic Survey Bulletin'', '''54''', 57–64.    </ref>. The data gathered informed both the assessment of sediment diagenesis and, by virtue of the fragments’ orientations, the assessment of regional palaeocurrent patterns. Macdonald (1982)<ref name="Macdonald 1982">MACDONALD, D I M. 1982. Palaeontology and ichnology of the Cumberland Bay Formation, South Georgia. British Antarctic Survey Bulletin, 57, 1–14.</ref> also reviewed the trace fossil assemblages from the Cumberland Bay Formation. Again, these had been widely reported by previous workers but not systematically assessed in terms of depositional facies. Macdonald regarded the ichnofaunal assemblages as indicative of deposition in relatively deep water, with low faunal diversity (and indications of a high physical stress) pointing to a partially anoxic environment.

All of the palaeontological evidence for the age of the South Georgia sedimentary succession  was reviewed by Thomson and others (1982)<ref name="Thomson 1982">THOMSON, M R A, TANNER, P W G and REX, D C. 1982. Fossil and Radiometric Evidence for Ages of Deposition and Metamorphism of Sedimentary Sequences on South Georgia. In: Craddock, C. (ed.) Antarctic Geoscience. The University of Wisconsin Press, Madison. 177–184.</ref> and integrated with K-Ar radiometric dates that reflected uplift and cooling following regional metamorphism. Apart from the faunal assemblage found in the Annenkov Island Formation evidence was very sparse, but sufficient material was available to indicate a uniform Aptian (Early Cretaceous) depositional age, ca&nbsp;113–126&nbsp;Ma (Gradstein and others 2012)<ref name="Gradstein 2012">GRADSTEIN, F M, OGG, J G, SCHMITZ, M D and OGG, G M. (eds). 2012. ''The Geologic Time Scale''. Elsevier, Oxford, Amsterdam and Waltham MA.</ref>. One previous K-Ar age of ca&nbsp;70&nbsp;Ma had been given for the Cumberland Bay Formation by Grikurov and others (1967)<ref name="Grikurov 1967">GRIKUROV, G E, KRYLOV, A Ya and SILIN, Yu I. 1967. Absolute age of some rocks from the Scotia Arc and Alexander I Land (western Antarctica). Originally published in Russian. English translation: ''Proceedings of the Academy of Sciences of the USSR., Geological Sciences Section'', '''172''', 19–22.</ref>. The more extensive data reported by Thomson and others (1982)<ref name="Thomson 1982"></ref> gave an age range of 51–135&nbsp;Ma for the Cumberland Bay Formation and 64–84 for the Sandebugten Formation. The older ages were thought to be influenced by the presence of detrital white mica. The best estimate for the minimum age of metamorphism was thought to be 82–91&nbsp;Ma, with post-orogenic uplift continuing to at least 50–60&nbsp;Ma. Thomson and others compared their results from South Georgia with the comparable evidence  from  the Yahgan Formation of southern Patagonia and supported its previously proposed correlation with the Cumberland Bay Formation.

Tanner (1982a)<ref name="Tanner 1982a">TANNER, P W G. 1982a. Geologic Evolution of South Georgia. In: Craddock, C. (ed.) ''Antarctic Geoscience''. The University of Wisconsin Press, Madison. 167–176.</ref> brought together all of the available evidence in a review of the geological evolution of South Georgia, a process that could now be described in terms of a late Jurassic to Early Cretaceous island arc and marginal basin system. The regional importance of the mylonitic shear zone (the Cooper Bay dislocation in Tanner’s account), separating the igneous rocks of the basin floor and its associated continental relics from the turbidite strata deposited in the basin, was also given prominence. The shear zone was one of the geological elements utilised by  Tanner in a correlation of the litho-tectonic divisions of South Georgia with those of southern Patagonia and Tierra del Fuego. From these relationships Tanner inferred a pre-drift position for South Georgia to the south of Burdwood Bank, a more easterly position than had featured in previous reconstruction (e.g. Dalziel and Elliot 1971)<ref name="Dalziel 1971">  DALZIEL, I W D, and ELLIOT, D H. 1971. Evolution of the Scotia Arc. Nature, London, 233, 246–252.    </ref>. Tanner was also able to include the Shag Rocks microcontinental block in his reconstruction with more confidence than had been previously possible, having examined and described rare specimens from those inaccessible rock pinnacles (Tanner 1982b)<ref name="Tanner 1982b">TANNER, P W G. 1982b. Geology of Shag Rocks, part of a continental block on the North Scotia Ridge, and possible regional correlations. ''British Antarctic Survey Bulletin'', '''51''', 125–136.</ref>.

In 1982, the island of South Georgia was invaded and briefly occupied by an Argentine military force. The events surrounding the occupation and the recovery of the island by British forces have been described by Headland (1984)<ref name="Headland 1984">  HEADLAND, R. 1984. ''The Island of South Georgia''. Cambridge University Press, Cambridge. 293 pp.    </ref>. The ensuing need for rehabilitation and the clearance  of mines and unexploded munitions, prevented the resumption of scientific field work for some considerable time. Publication of the results of the earlier research continued however, with three papers summarising the depositional and structural history of the sedimentary successions as the result of opening and closing of a Mesozoic back-arc basin.

Tanner and Macdonald (1982)<ref name="Tanner 1982">TANNER, P W G, and MACDONALD, D I M. 1982. Models for the deposition and simple shear deformation of a turbidite sequence in the South Georgia portion of the southern Andes back-arc basin. ''Journal of the Geological Society, London'', '''139''', 739–754.</ref> and Macdonald and Tanner (1983)<ref name="Macdonald 1983">MACDONALD, D I M. and TANNER, P W G. 1983. Sediment dispersal patterns in part of a deformed Mesozoic back-arc basin on South Georgia, South Atlantic. ''Journal of Sedimentary Petrology'', 53, 83–104.</ref> focussed in particular on the Cumberland Bay Formation, thought to be more than 8&nbsp;km thick. Previous interpretations of the palaeocurrent patterns were confirmed by a much enlarged database. For the Aptian (Early Cretaceous) Cumberland Bay Formation, andesitic, volcaniclastic sediment was derived from the south–east by turbidity currents flowing along the axis of the depositional basin in a linear, tectonically-controlled trough. In contrast, the Sandebugten Formation, probably of similar age, had a lateral derivation from a continental and silicic volcanic provenance on the north side of  the basin. Deformation during closure of the basin resulted in increasingly tight and overturned folds toward the NE, with shortening calculated at up to 55% and an original basin width of about 60&nbsp;km. Both the sedimentary and tectonic models were extended westward to link South Georgia with the contemporary back-arc basin in the southern Andes and Tierra del Fuego.

In the third paper, Storey and Macdonald (1984)<ref name="Storey 1984">STOREY, B C, and MACDONALD, D I M. 1984. Processes of formation and filling of a Mesozoic back-arc basin on the island of South Georgia. In: Kokelaar, B.P. and Howells, M.F. (eds) ''Marginal Basin Geology''. Geological Society of London, Special Publication '''16''', 207–218.</ref> reviewed the earlier stages in the history of basin extension, the relic continental crust of the Drygalski Fjord and the ophiolitic Larsen Harbour complexes, and related the island arc succession of the Annenkov Island Formation to the main depositional basin. The thick units of volcaniclastic breccia in the Annenkov Island succession were regarded as channel-fill, debris flow deposits; laterally equivalent turbidites formed the Cumberland Bay Formation.

==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 | 08]]