OR/14/040 Introduction

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Stone, P. 2014. Recent contributions on Falkland Islands bedrock geology, with an inventory of representative lithostratigraphical specimens held by the British Geological Survey. British Geological Survey Internal Report, OR/14/040.

The Falkland Islands lie in the South Atlantic Ocean, about 650 km east of the Strait of Magellan, at the western end of the Falkland Plateau, a rectilinear, relatively shallow water (<2500 m approximately) bathymetric feature that extends eastwards from the South American continental shelf (Figure 1a). Mesozoic sedimentary basins to the north (North Falklands Basin) and the south-east (Falkland Plateau Basin) of the archipelago are currently the focus of offshore hydrocarbons exploration within the designated Falkland Islands Exploration Area. The two principal islands are known as West Falkland and East Falkland (Figure 1b). Seen there, and on the surrounding smaller islands, is a clastic sedimentary sequence ranging in age from Siluro–Devonian to Permian that is cut by numerous dolerite dykes of Mesozoic age; the dykes are particularly numerous in West Falkland (Figure 1c).

Figure 1 Location map and outline geology of the Falkland Islands after Richards and others (2013)[1]:
a.  Regional setting at the western end of the Falkland Plateau, South Atlantic Ocean, with bathymetry represented by the grey scale from darkest = shallowest water (<1500 m approximately) to palest = deepest water (>2500 m approximately).
b.  Outrop areas for the West Falkland (unshaded) and Lafonia (pale grey) groups (WF = West Falkland, EF = East Falkland, L = Lafonia).
c.  Linear aeromagnetic anomalies interpreted as dolerite dykes, showing the locations of analysed and dated dyke samples. Note that the aeromagnetic survey covered only the two main islands and extended neither to the western promontories of West Falkland nor to the smaller offshore islands.

The first recorded geological observations were made by Charles Darwin during two visits to the archipelago by HMS Beagle, in 1833 and 1834. Darwin described a folded and cleaved succession of quartzite, sandstone and mudstone, from which he recovered Devonian fossils (Darwin 1846[2]; Morris and Sharpe 1846[3]). In a footnote to his 1846 paper, Darwin recorded the dolerite dyke swarm that had been discovered and reported to him by Bartholomew Sulivan in 1845 (Stone and Rushton 2013[4]). Little more was added until two Swedish expeditions visited the Falkland Islands: the Swedish South Polar Expedition (1901–1903) and the Swedish Magellanic Expedition (1907–1909). Reporting after these expeditions, Andersson (1907)[5] and Halle (1911)[6] recorded the presence on West Falkland of a Precambrian basement complex exposed at Cape Meredith (Figure 1b), improved the Palaeozoic stratigraphy and extended it into the Permian, and described a glacigenic tillite unit that Halle correlated with the Upper Carboniferous to Lower Permian Dywka Tillite of South Africa. This association of Falklands and South African geology had been noted much earlier on palaeontological grounds, and with evident surprise, by Sharpe and Salter (1856)[7] and was reiterated by Clarke (1913)[8] in his seminal work on the Devonian fauna of the Falkland Islands.

Although Halle (1911)[6] had included an outline geological map of the Falkland in his report, the first attempt at comprehensive geological mapping was by Baker (1924)[9] who acted as ‘Government Geologist’ from 1920 to 1922 but was principally concerned with mineral prospecting. In that respect he was unsuccessful, but his work laid the foundations for the modern interpretation of Falklands geology, and he notably utilised Wegener’s recently formulated ideas of continental drift to explain the geological similarities with South Africa. Wegener had championed the theory of continental drift from 1912, but it was not until 1924 that an English translation of his work was published, based on the third German edition of his book. Baker’s geological map was at a relatively small scale and was only available as a fold-out figure within an obscure report. Nevertheless, for fifty years it remained the definitive representation of Falkland Islands geology until the publication by the British Antarctic Survey of a photogeological interpretation by Mary Greenway (1972)[10], who utilised much of Baker’s (1924)[9] lithostratigraphy. In the interim, and with no additional fieldwork, Baker’s map and cross-section had been reproduced with minor amendments in a Spanish-language review by Borrello (1963)[11] published in Argentina. The division of Baker’s specimen collection between The Natural History Museum, London, and Imperial College, London, was documented by Stone and Rushton (2006)[12].

Following the appearance of Greenway’s map, a number of local projects were undertaken by visiting Argentine geologists, but this involvement came to an abrupt end in 1982 with the Argentine military invasion and subsequent recovery of the islands by British armed forces. The Argentine contributions have been summarised by Aldiss and Edwards (1999)[13] whose work represents the next major advance in the understanding of Falkland Islands geology. As part of the post-conflict regeneration programme, the Falkland Islands Government commissioned a field-based geological survey which was carried out between 1996 and 1998 by Don Aldiss and Emma Edwards, supported by the British Geological Survey. The products were a published bedrock geology map in two sheets at a scale of 1:250 000 (Aldiss and Edwards 1998[14]) and a comprehensive geological report (Aldiss and Edwards 1999)[13]. Key features of Falkland Islands geology, as interpreted and described by Aldiss and Edwards 1998[14], 1999[13]) and the researchers cited by them, are shown in Figure 2; the lithostratigraphy is summarised in Table 1. At the southern extremity of West Falkland, a small outcrop of Proterozoic crystalline basement — Cape Meredith Complex' — comprises an assemblage of gneiss, granite and amphibolite, from which a number of radiometric dates have given an age of approximately 1100–1000 Ma. The Proterozoic rocks are unconformably overlain by a Silurian–Devonian succession of fluvial to neritic, clastic strata predominantly comprising quartzite, sandstone and mudstone. This succession is known as the West Falkland Group; it crops out over most of West Falkland and in the north of East Falkland. The south of East Falkland and its smaller peripheral islands are underlain by the tillite, mudstone and sandstone of the Carboniferous to Permian Lafonia Group. Pre-Devonian, mafic dykes cut the Cape Meredith Complex with widespread, polyphase swarms of mafic, Mesozoic dykes cutting the basement and all of the overlying sedimentary succession. At the time of the Aldiss and Edwards survey the Mesozoic dykes were known principally from West Falkland and were regarded as exclusively Jurassic. They are now known to be both Jurassic and Cretaceous in age, with the Cretaceous dyke swarm spanning equally across East and West Falkland (Stone and others 2008[15]; Richards and others 2013[1]). The new dyke data will be discussed in more detail later in this report.

Figure 2 Geology and locality maps for the Falkland Islands:
a.  Outline onshore geology after Aldiss and Edwards (1998) [14].
b.  Additional detail with the position of the principal localities mentioned in the text.
Table 1 Lithostratigraphy of the West Falkland Group (Silurian to Devonian) and the Lafonia Group (Carboniferous to Permian). The maximum likely thickness of each formation is shown by the superscript figures after the formation name. After Aldiss and Edwards (1998[14], 1999[13]), Trewin and others (2002)[16], Hunter and Lomas (2003)[17]
Lithostratigraphy Principal lithologies Depositional environments
Lafonia Group
Bay of Harbours Fm.3000 m Sandstone, mudstone Delta top and channels
Brenton Loch Fm.3000 m Sandstone, laminated mudstone Deltaic and prodeltaic basin floor
Port Sussex Fm.400 m Mudstone, sandstone, diamictite Glaciomarine to marine or lacustrine
Fitzroy Tillite Fm.850 m Massive diamictite Glacial and glaciomarine
Bluff Cove Fm.250 m Sandstone, mudstone Shallow marine or proglacial deltaic
Low-angle unconformity
West Falkland Group
Port Stanley Fm.1100 m Quartz-sandstone, mudstone Shallow marine and shoreface
Port Philomel Fm.350 m Sandstone, mudstone Deltaic to shallow marine
Fox Bay Fm.1500 m Micaceous sandstone, mudstone Marine inner shelf and shoreface
Port Stephens Fm.2500 m Quartzo-feldspathic sandstone Fluvial to intertidal and shoreface
Major, angular unconformity
Cape Meredith Complex Proterozoic granite and gneiss, c. 1000 Ma

Despite the proximity of the archipelago to South America, the geological association of the Falklands with the Cape Province of South Africa has long been recognized: the general similarities were appreciated by Halle (1911)[6] and Baker (1924)[9], the latter accounting for the similarities in terms of the then-radical theory of continental drift. However, the first proposal to move the islands into a geologically more explicable position came from Du Toit (1927) in whose reconstruction of Palaeozoic palaeogeography they were positioned much farther north, forming a structural link between the coeval and correlative Ventania and Cape Fold Belts of Argentina and South Africa respectively. Du Toit’s solution did not find general favour and an alternative reconstruction was proposed by Adie (1952)[18] wherein the Falkland Islands were rotated through 180o into an original position adjacent to the east coast of South Africa. Palaeomagnetic results from the Jurassic dykes (Taylor and Shaw 1989)[19] appeared to support that proposition, and the Falkland Islands are now commonly regarded as forming part of a microplate created and rotated during the break-up of Gondwana (Mitchell and others 1986[20]; Marshall 1994[21]; Storey and others 1999[22]). Further evidence for rotation between intrusion of the Jurassic and Cretaceous dykes has been presented by Stone and others (2009)[23] and will be discussed later in this report.

What follows hereinafter is a summary of the principal contributions to a geological understanding of the Falkland Islands that have arisen since completion of the Aldiss and Edwards (1998[14], 1999[13]) survey. To complement the report a representative suite of rock specimens illustrating the Falkland Islands lithostratigraphy has been lodged in the collection of the British Geological Survey at the UK’s National Geoscience Data Centre, Keyworth, Nottingham. Full details of the specimens are listed in Appendix 1 – Representative lithostratigraphical reference specimens held by the British Geological Survey in the National Geoscience Date Centre, Keyworth, Nottingham. The lithostratigraphy used (Table 1) follows that established by Aldiss and Edwards (1998[14], 1999[13]).

References

  1. 1.0 1.1 RICHARDS, P C, STONE, P, KIMBELL, G S, MCINTOSH, W C, and PHILLIPS, E R. 2013. Mesozoic magmatism in the Falkland Islands (South Atlantic) and their offshore sedimentary basins. Journal of Petroleum Geology, Vol. 36, 61–74.
  2. DARWIN, C R. 1846. On the geology of the Falkland Islands. Quarterly Journal of the Geological Society of London, Vol. 2, 267–274.
  3. MORRIS, J, and SHARPE, D. 1846. Description of eight species of brachiopodous shells from the Palaeozoic rocks of the Falkland Islands. Quarterly Journal of the Geological Society of London, Vol. 2, 274–278.
  4. STONE, P, and RUSHTON, A W A. 2013. Charles Darwin, Bartholomew Sulivan and the geology of the Falkland Islands: unfinished business from an asymmetric partnership. Earth Sciences History, Vol. 32, 156–185.
  5. ANDERSSON, J G. 1907. Contributions to the geology of the Falkland Islands. Wissenschaftliche Ergebnisse der Schwedischen Sudpolar-expedition 1901–1903, Vol. 3, (Lief. 2), 38pp.
  6. 6.0 6.1 6.2 HALLE, T G. 1911. On the geological structure and history of the Falkland Islands. Bulletin of the Geological Institution of the University of Uppsala, Vol. 11, 115–229.
  7. SHARPE, D, and SALTER, J W. 1856. Description of Palaeozoic fossils from South Africa. Transactions of the Geological Society of London, Series 2, Vol. 7, 203–225.
  8. CLARKE, J M. 1913. Fósseis Devonianos do Paraná. Monographia do Serviço Geológico y Mineralógico do Brasil, No. 1, 353pp.
  9. 9.0 9.1 9.2 BAKER, H A. 1924. Final Report on Geological Investigations in the Falkland Islands, 1920–1922. Government Printer, Stanley. 38pp, map, cross-section and 18 figures.
  10. GREENWAY, M E. 1972. The geology of the Falkland Islands. British Antarctic Survey Scientific Reports, No. 76, 42pp.
  11. BORRELLO, A V. 1963. Sobre la geologia de las Islas Malvinas. Ministerio de Educación y Justica, Buenos Aires, Argentina.
  12. STONE, P, and RUSHTON, A W A. 2006. The Baker Collection of Falkland Island fossils at Imperial College, London. The Falkland Islands Journal. Vol. 8 (5), 17–22.
  13. 13.0 13.1 13.2 13.3 13.4 13.5 ALDISS, D T, and EDWARDS, E J. 1999. The Geology of the Falkland Islands. British Geological Survey Technical Report, WC/99/10. 135pp.
  14. 14.0 14.1 14.2 14.3 14.4 14.5 Aldiss, D T, and Edwards, E J. 1998. Geology of the Falkland Islands. Solid Geology 1:250 000. Two sheets, East and West. British Geological Survey for Falkland Islands Government.
  15. STONE, P, RICHARDS, P C, KIMBELL, G S, ESSER, R P, and REEVES, D. 2008. Cretaceous dykes discovered in the Falkland Islands: implications for regional tectonics. Journal of the Geological Society, London, Vol. 165, 1–4.
  16. TREWIN, N H, MACDONALD, D I M, and THOMAS, C G C. 2002. Stratigraphy and sedimentology of the Permian of the Falkland Islands: lithostratigraphic and palaeoenvironmental links with South Africa. Journal of the Geological Society, London, Vol. 159, 5–19.
  17. HUNTER, M A, and LOMAS, S A. 2003. Reconstructing the Siluro-Devonian coastline of Gondwana: insights from the sedimentology of the Port Stephens Formation, Falkland Islands. Journal of the Geological Society, London, Vol. 160, 459–476.
  18. ADIE, R J. 1952. The position of the Falkland Islands in a reconstruction of Gondwanaland. Geological Magazine, Vol. 89, 401–410.
  19. TAYLOR, G K, and SHAW, J. 1989. The Falkland Islands: New palaeomagnetic data and their origin as a displaced terrane from southern Africa. In: Hillhouse, J W. (ed.) Deep structure and past kinematics of accreted terranes. Geophysical Monographs, No. 50, 59–72.
  20. MITCHELL, C, TAYLOR, G K, COX, K G, and SHAW, J. 1986. Are the Falkland Islands a rotated microplate? Nature, Vol. 319, 131–134.
  21. MARSHALL, J E A. 1994. The Falkland Islands: a key element in Gondwana palaeogeography. Tectonics, Vol. 13, 499–514.
  22. STOREY, B C, CURTIS, M L, FERRIS, J K, HUNTER, M A, and LIVERMORE, R A. 1999. Reconstruction and break-out model for the Falkland Islands within Gondwana. Journal of African Earth Sciences, Vol. 29, 153–163.
  23. STONE, P, KIMBELL, G S, and RICHARDS, P C. 2009. Rotation of the Falklands microplate reassessed after recognition of discrete Jurassic and Cretaceous dyke swarms. Petroleum Geoscience, Vol. 15, 279–287.