OR/15/058 Introduction
| Stone, P. 2015. The geological exploration of the sub-Antarctic island of South Georgia: a review and bibliography, 1871–2015. British Geological Survey Internal Report, OR/15/058. |
The island of South Georgia (centred at about 54°20ʹS, 36°40ʹW) is an isolated terrestrial outpost in the midst of the South Atlantic Ocean. Measuring about 170 km NW–SE, and with a maximum NE–SW width of 30 km, it is the largest emergent feature of the mostly submarine North Scotia Ridge, and a part of the arcuate chain of banks and islands that link Tierra del Fuego, in southernmost South America, with Graham Land, in the north of the Antarctic Peninsula, enclosing the Scotia Sea in a huge, eastward-closing physiographic loop (Figure 1). Together, these features form the Scotia Arc, a widely used but still, sensu stricto, an unofficial geographical descriptor (Hattersley-Smith 1980). The names celebrate the Scotia, the ship that carried the Scottish National Antarctic Expedition, 1902–1904, and from which bathymetric survey work first confirmed the existence of the arcuate submarine ridge (Bruce 1905). Many early workers referred to the arcuate assemblage of features as the Southern Antilles or South Antilles Arc, after the perceived similarity to the island chain enclosing the Caribbean Sea, but the appellation ‘Scotia’ has been in general use following Herdman (1932).
This review (an admittedly subjective summary) documents the growth of geological knowledge of South Georgia and its regional status in the South Atlantic; it is concerned only with research into the island’s bedrock geology and its wider implications. Many of the geologists whose work is cited also made important contributions in other disciplines such as physiography, geomorphology and glaciology, and these areas of research were of course also studied independently by specialists in those fields. There is some overlap, particularly in the earlier publications, but on the whole the results of research into the broader aspects of landscape evolution are neither dealt with in this review nor included in the bibliography.
The first known landing on South Georgia (a British Overseas Territory but also claimed by Argentina under the name Isla de San Pedro) was made in January 1775, from HMS Resolution, during Capt. James Cook’s second voyage of exploration. Geological investigations followed only very slowly and intermittently, and in 1900 South Georgia was still largely terra incognita in geological terms. The uncertainty allowed various accommodations within opposing models as the issue of continental drift was argued through the first half of the 20th century, following the championing of the concept by Wegener from 1912 onwards. Of particular interest is the way in which increasing knowledge played into the arguments for or against the theory of continental drift and, in local terms, for either a dynamic or in situ origin for the components of the Scotia Arc (Stone 2015). As a frame of reference for the assessment of those historical contributions, and their modern successors, it is appropriate to consider first the current consensus on South Georgia’s geology, which has been summarized in geological maps compiled by Macdonald and others (1987) and Curtis and Riley (2011) and is illustrated in Figure 2. For a comprehensive modern assessment of the Scotia Arc, as its origin and evolution are now commonly (although not unanimously) interpreted, see Dalziel and others (2013).
Much of South Georgia is underlain by a thick succession of Lower Cretaceous volcaniclastic sandstone and mudstone (Cumberland Bay Formation) originally deposited by deep-marine turbidity currents. The strata were then asymmetrically folded and thrust towards the north-east to structurally overlie a more intensely deformed and cleaved succession of quartzose turbiditic sandstones (Sandebugten Formation), which may nevertheless be of a similar Cretaceous age. A major shear zone separates the Cumberland Bay and Sandebugten formations and broadly similar (but still more deformed) sedimentary rocks (Cooper Bay Formation) from a very different lithological assemblage that forms the south-east end of the island: gneiss and metasedimentary rocks intruded by gabbro and probably Triassic granite, with mafic dykes locally forming up to 80% of the outcrop (Drygalski Fjord Complex with minor associated metasedimentary formations), Upper Jurassic lavas and volcaniclastic rocks (Larsen Harbour Complex) and Lower Cretaceous tuff and volcaniclastic breccias (Annenkov Island and Ducloz Head formations). In total, the geology of South Georgia illustrates an active continental margin split by the opening of an island arc and backarc basin system, with the subsequent closure of the basin and deformation of its sedimentary fill.