THE GEOLOGY OF THE FALKLAND ISLANDS D T Aldiss and E J Edwards British Geological Survey Technical Report THE GEOLOGY OF THE FALKLAND ISLANDS NOTES FOR DIGITAL VERSION This British Geological Survey Technical Report WC/99/10 is available in a digital version and in a paper version. The contents of this digital version of the report are identical to those of the paper version, except that Figures 1.2 and 4.11 are presented here both in colour and in monochrome. The monochrome version is held on the page following the colour version. Links have been provided between the Contents Pages and the body of the report. Links exist for Chapter headings, second-order section headings, Figures, Plates and Tables. To activate these links, double-click on the relevant line in the Contents Pages. If the software command ‘Go to (page number)’ is used to move through the document, note that although page numbers appear only on the text pages, the software will count all the pages consecutively, treating the Cover Page as page 1, and the Contents Pages as pages 5 to 9, inclusive. Paper copies of this report are available from the Department of Mineral Resources, Ross Road, Stanley, Falkland Islands, telephone (0) 500 27322 or fax (0) 500 27321, e-mail > [email protected], or from BGS Sales, British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK telephone (0) 44 115 936 3241 or fax (0) 44 115 936 3488, e-mail > [email protected] BRITISH GEOLOGICAL SURVEY Overseas Geology Series TECHNICAL REPORT WC/99/10 THE GEOLOGY OF THE FALKLAND ISLANDS D T Aldiss and E J Edwards This report is a product of the Falkland Islands Geological Mapping Project, funded by the Falkland Islands Government. The views expressed are not necessarily those of the Falkland Islands Government. Bibliographic Reference: Aldiss, D T and Edwards, E J. 1999. The Geology of the Falkland Islands. British Geological Survey Technical Report WC/99/10. Cover photograph: Medium-scale anticline in Devonian quartzites of the Port Stanley Formation with stone runs in the Wickham Heights, East Falkland. Mount Usborne (705 m) is in the distance. Photograph by Don Aldiss, BGS (MN28254). © NERC All rights reserved. British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK Department of Mineral Resources, Falkland Islands Government, Stanley, Falkland Islands ©NERC and Falkland Islands Government 1999. All rights reserved. The Geology of the Falkland Islands THE GEOLOGY OF THE FALKLAND ISLANDS D T Aldiss and E J Edwards 1999 SUMMARY This report is complementary to the 1:250 000 scale geological map of the Falkland Islands compiled in 1998. The report and map are products of the Falkland Islands Geological Mapping Project (1996-1998). Geological observation and research in the Islands date from 1764. The Islands were visited during two pioneering scientific cruises in the 19th century. Subsequently, many scientists visited en route to the Antarctic or Patagonia. Geological affinities to other parts of the southern continents, especially South Africa, were noted early in the 20th century. There have been two previous attempts to create a geological map of the Islands, both motivated primarily by the search for economic mineral deposits onshore. In the last few decades much effort has been directed to understanding the Falklands’ place in Gondwana, the processes by which the Islands have moved to their present position by continental drift and the concomitant development of offshore sedimentary basins. Considerable progress in describing the superficial deposits was made in the 1970’s, and during the last ten years. The stratigraphic subdivisions of the geological sequence shown on the previous geological maps have been substantiated and defined more rigorously than before. In addition, several new stratigraphic units have been recognised. Each unit is described with an introductory summary of composition and distribution, followed by comments on nomenclature and stratigraphic relationships, associated landforms, distinguishing characters, and the criteria used to locate and survey the stratigraphic base. Detailed descriptions of composition, sedimentary structures and fossil content then lead to brief comments on the environment of deposition, age and correlation. The bedrock geological formations (‘solid geology’) can be divided into four age groups. The Proterozoic granites and amphibolite facies gneisses of the Cape Meredith Complex (about 1150 to 1000 million years old) are overlain in turn by sedimentary sequences of the ?Silurian to Devonian West Falkland Group and the Carboniferous to Permian Lafonia Group. Jurassic igneous rocks are widespread but only locally abundant. The West Falkland Group is dominated by sandstones, with some siltstones and mudstones. The oldest of four formations, the Port Stephens Formation, is divided into seven members, representing marine to fluvial environments. The basal member on East Falkland is probably the oldest part of the sedimentary sequence and might be latest Ordovician in age, but is more probably Silurian. The overlying Albemarle Member is notable for abundant trace fossils, mainly Skolithos, but also contains a new ichnospecies of Heimdallia. The succeeding marine Fox Bay Formation contains the Early to Middle Devonian Malvinokaffric invertebrate fossil fauna. One proximal facies member is recognised in the west. The Port Philomel Formation represents deltaic facies. It is notable for abundant fossil plant debris, most conspicuously lycophyte stems. The Late Devonian Port Stanley Formation, which includes the Stanley Quartzite, marks a return to marine conditions. The sandstones in the West Falkland Group are mostly quartz arenites and subarkoses, consistent with derivation from an area of stable continent crust. The West Falkland Group can be correlated with parts of the Cape Supergroup of South Africa. The Lafonian Group is lithologically more varied and is divided into five formations. The fine-grained marine sediments of the Late Carboniferous Bluff Cove Formation are confined to East Falkland. They are followed by the thick tillites and thin mudstones of the Permo-Carboniferous Fitzroy Tillite Formation. One distinctive bedded interval is defined as a member. The Port Sussex Formation, with three members, includes carbonaceous mudstones with up to 40 per cent total organic carbon. The Brenton Loch Formation is also divided into three members. It is composed of sandstones with siltstones and mudstones, which represent combinations of basinal and turbiditic deposition in a large lake. The basinal sediments include varvites and some striking examples of the trace fossils Umfolozia and Undichnia. The Bay of Harbours Formation is generally similar lithologically, but was deposited in deltaic environments. It includes a sandy proximal facies member and has common plant fossils, including Glossopteris. Most of the sandstones in the Lafonia Group contain detritus derived from a distant contemporary volcanic terrane and there are also sporadic thin tuffs. The lowest part of the Lafonia Group can be correlated with i The Geology of the Falkland Islands the Dwyka Group of South Africa, and the remainder with the parts of the Ecca Group and the lowermost Beaufort Group. Better understanding of the distribution and structure of the sedimentary sequences has allowed their thickness to be estimated more reliably. In general, each stratigraphic unit is thinner on West Falkland than on East Falkland. It is likely that West Falkland lay near one margin of the Permo-Carboniferous depositional basin, and that no part of the Lafonia Group was ever deposited in the southernmost part of West Falkland. Mid-Carboniferous sedimentary dykes occur locally in the south-west of West Falkland. Jurassic igneous dykes are widespread, especially in West Falkland. They are mostly composed of various types of dolerite, but more evolved rocks, including felsite, also occur. Those to the west of Falkland Sound can be divided by their field characteristics into six dyke swarms. Most of the dykes date from the Early Jurassic (about 190 million years ago) but Middle or Late Jurassic dykes could also occur. The latter are likely to be of similar age to Jurassic rhyolitic volcanic rocks which are inferred to occur offshore, close to the Islands. These could be accompanied by basalts, which might be the source of the agates known as ‘Falklands pebbles’. An east-west to NW-SE trending fold and thrust belt in the northern part of the Islands formed during the Permo- Triassic Gondwanide regional deformation. That was followed by at least four episodes of more localised deformation. A series of large-amplitude drape folds trending between north-south and ENE-WSW formed during the second phase as a result of passive deformation of the sedimentary cover over deep-seated faults. Each fold pair represents up-to-the-west fault movements in the underlying basement. During the third deformation, dextral strike- slip faulting with minor folding occurred in Falkland Sound and the adjacent parts of both main islands. About five to ten kilometres of dextral displacement occurred on the Falkland Sound Fault Zone. The fourth phase of deformation is represented onshore by a thrust sheet in the far north of the Islands. This thrusting is possibly younger than the Early Jurassic dolerite dykes. During the final phase of deformation the eastern part of East Falkland was uplifted by about three kilometres relative to the rest of the Islands. This uplift caused the formation of the Goose Green