The geology of the Falkland Islands

This report is complementary to the 1:250 000 scale geological map of the Falkland Islands compiled in 1998. The \nreport and map are products of the Falkland Islands Geological Mapping Project (1996-1998). \nGeological observation and research in the Islands date from 1764. The Islands were visited during two pioneering \nscientific cruises in the 19th century. Subsequently, many scientists visited en route to the Antarctic or Patagonia. \nGeological affinities to other parts of the southern continents, especially South Africa, were noted early in the 20th \ncentury. There have been two previous attempts to create a geological map of the Islands, both motivated primarily \nby the search for economic mineral deposits onshore. In the last few decades much effort has been directed to \nunderstanding the Falklands’ place in Gondwana, the processes by which the Islands have moved to their present \nposition by continental drift and the concomitant development of offshore sedimentary basins. Considerable progress \nin describing the superficial deposits was made in the 1970’s, and during the last ten years. \nThe stratigraphic subdivisions of the geological sequence shown on the previous geological maps have been \nsubstantiated and defined more rigorously than before. In addition, several new stratigraphic units have been \nrecognised. Each unit is described with an introductory summary of composition and distribution, followed by \ncomments on nomenclature and stratigraphic relationships, associated landforms, distinguishing characters, and the \ncriteria used to locate and survey the stratigraphic base. Detailed descriptions of composition, sedimentary structures \nand fossil content then lead to brief comments on the environment of deposition, age and correlation. \nThe bedrock geological formations (‘solid geology’) can be divided into four age groups. The Proterozoic granites \nand amphibolite facies gneisses of the Cape Meredith Complex (about 1150 to 1000 million years old) are overlain \nin turn by sedimentary sequences of the ?Silurian to Devonian West Falkland Group and the Carboniferous to \nPermian Lafonia Group. Jurassic igneous rocks are widespread but only locally abundant. \nThe West Falkland Group is dominated by sandstones, with some siltstones and mudstones. The oldest of four \nformations, the Port Stephens Formation, is divided into seven members, representing marine to fluvial \nenvironments. The basal member on East Falkland is probably the oldest part of the sedimentary sequence and might \nbe latest Ordovician in age, but is more probably Silurian. The overlying Albemarle Member is notable for abundant \ntrace fossils, mainly Skolithos, but also contains a new ichnospecies of Heimdallia. The succeeding marine Fox Bay \nFormation contains the Early to Middle Devonian Malvinokaffric invertebrate fossil fauna. One proximal facies \nmember is recognised in the west. The Port Philomel Formation represents deltaic facies. It is notable for abundant \nfossil plant debris, most conspicuously lycophyte stems. The Late Devonian Port Stanley Formation, which includes \nthe Stanley Quartzite, marks a return to marine conditions. The sandstones in the West Falkland Group are mostly \nquartz arenites and subarkoses, consistent with derivation from an area of stable continent crust. The West Falkland \nGroup can be correlated with parts of the Cape Supergroup of South Africa.