The geology of the Falkland Islands

Plate tectonics have produced some surprising juxtapositions, as the earth’s continental fragments have drifted and jostled over the eons. Microplates seem to have enjoyed most freedom of movement and none more so than that supporting the Falkland Islands. Though this archipelago is situated in the south-west corner of the South Atlantic Ocean, about 650km east from Tierra del Fuego and the Strait of Magellan, its geology tells of an African heritage. Charles Darwin provided the first evidence for that – although he didn’t appreciate it at the time.

fig-1-gondwana
1. A reconstruction of the Gondwana supercontinent at about 300mya. (© BGS/NERC.)

HMS Beagle visited the Falkland Islands twice, in 1833 and 1834, and during the first visit Darwin discovered fossil shells, mostly brachiopods. His first impression had been unfavourable, but, after that discovery, he noted in his diary: “The whole aspect of the Falkland Islands were however changed to my eyes … for I found a rock abounding with shells; and these of the most interesting age.” Darwin published his account of Falklands’ geology in 1846. The “interesting age” proved to be Devonian and, as more data were acquired, a close and surprising similarity was established with the fauna of equivalent age in South Africa. This similarity was soon extended to other aspects of the Falklands rock succession, while the geology of neighbouring Patagonia proved to be quite different.

These relationships were not readily explicable without recourse to continental drift, so were largely ignored for many years, despite a remarkably prescient interpretation by a South African geologist, Ray Adie, in 1952. Not content with a straightforward African connection, as championed by Alexander Du Toit in his 1937 book, Our Wandering Continents, Adie proposed that what we would now call the Falklands microplate had rifted from the east coast of South Africa and had then been rotated through 180° as it drifted to its present position. His evidence was drawn from the alignment of sedimentological and structural features from the two areas. Half a century later, and though the jury is still out on some of the details, Adie’s proposal is looking to be essentially correct. The close geological correlation between South Africa and the Falkland Islands is now put down to their original proximity in a reconstructed Gondwana supercontinent (Fig. 1).

The Falklands’ rock succession

The Falkland Islands are made up of two main components, East and West Falkland, a dozen or so largish, subsidiary islands, and a myriad of smaller islands, rocks and reefs. These all add up to a total land area of just over 12,000km2, though the islands are spread out over an area about twice that. The oldest rocks seen are the Proterozoic (about 1,000myrs old) granite and gneiss of the Cape Meredith Complex, which has a very small outcrop on the southernmost point of West Falkland (Fig.2). This ‘basement’ complex is there unconformably overlain by a thick succession of marine, near-shore clastic strata, demonstrably Devonian, but perhaps ranging in age from Silurian to Carboniferous, known as the West Falkland Group.

fig-2-geologymap
2. A simplified geological map of the Falkland Islands. The outcrops of the Bluff Cove and Port Sussex formations are thin and, on the map, these units are subsumed into the Fitzroy Tillite Formation. Within the Lafonia Group outcrop, the Brenton Loch Formation mostly occupies ground to the north-east of the Goose Green isthmus. The Bay of Harbours Formation crops out to the south-west of this isthmus. (© Falkland Islands Government.)

The lowest of the West Falkland Group’s four formations, the Port Stephens Formation, comprises about 2,500m of pale brown and grey, quartz-rich but locally ‘arkosic’ sandstone (that is, a usually pinkish or red sandstone consisting primarily of quartz and feldspar). Cross-bedding and rippled surfaces are fairly common and there are plenty of trace fossils, but no body fossils have been found. The Port Stephens Formation forms most of the high ground in the north of East Falkland and the south of West Falkland, but the hardness of the sandstone is quite variable and, on some of the exposed ridges, it has been carved into fantastic monoliths by the sand blasting effect of thousands of years of Falklands’ gales (Fig. 3). In an upward, conformable transition, the Port Stephens Formation is followed by the Fox Bay Formation, a unit comprising about 1,500m of yellowish brown, micaceous sandstone and dark mudstone. These rocks are relatively soft and readily eroded, so form much of the low ground in West Falkland and in the northern part of East Falkland. It was from a coastal exposure in the latter area (Port Louis, Fig. 2) that Darwin collected his fossils. In fact, the Fox Bay Formation is widely fossiliferous – as is the equivalent unit in South Africa, the Bokkeveld Group – with a rich and varied fauna that includes brachiopods (Fig. 4), trilobites, bivalves, snails, crinoids and orthocones. From these can be established an Early Devonian age (Emsian, about 400mya). Higher in the micaceous sandstone succession, the marine fossils are replaced by plant remains and the host strata are assigned to the Port Philomel Formation, which is about 300m thick.


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