Geology of the Falkland Islands

Phil Stone (UK)

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.

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 1,000Ma 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.

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.

The loss of the marine animal fauna and its replacement by washed-in plant debris reflects changing conditions along the coast of Gondwana, where the West Falkland Group was deposited, and further change brought about a huge influx of clean, white quartz sand. This now forms the 1,000m or so of quartzite that makes up most of the Port Stanley Formation, the highest division of the West Falkland Group. Apart from scattered plant remains, which can locally be quite concentrated, this formation is entirely unfossiliferous. Large-scale, tabular cross-bedding is a common feature (Fig. 5) suggesting deposition of the beds as extensive, migrating submarine sand sheets.

The hard, white quartzite of the Port Stanley Formation produces the highest ground in the islands (Fig. 6) and also gives rise to the famous Falklands ‘stone runs’ – of which more later.

There is a break in the sedimentary record at the top of the Port Stanley Formation that might represent as much as 100Ma, though there is only a slight angular discordance between the quartzite and the succeeding sandstone and mudstone of the Bluff Cove Formation. This division, and those above it, make up the Lafonia Group and crop out over the southern part of East Falkland, including the eponymous Lafonia (Fig. 2). The Bluff Cove Formation is about 250m thick and the presence of rare plant traces shows that the nearby landmass was vegetated. That was about to change. Towards the end of Carboniferous times, as Gondwana drifted into southern polar latitudes, the earth’s climate cooled and huge ice sheets expanded.

In the Falkland Islands’ rock record, the first signs of the approaching ice are the large, isolated boulders, rafted in on icebergs, which appear in the sandstone at the top of the Bluff Cove Formation. Soon, the whole area was covered by a floating ice sheet from which rained down mud, sand, and erratic pebbles and boulders, the whole assemblage now forming the 850m thick Fitzroy Tillite Formation. The typical lithology shows a mixture of clast types supported in a dark matrix of muddy sand (Fig. 7).

Some of the erratic clasts can be likened to rock types in the underlying Falklands’ succession, but others are entirely exotic. Spectacular examples of the latter are the boulders of Early Cambrian limestone containing fossil trilobites and archaeocyaths (Fig. 8).

In a remarkable confirmation of the Falklands-Africa-Gondwana association, exactly the same types of archaeocyaths have been found in limestone clasts in the Dwyka Tillite, the correlative of the Fitzroy Tillite in South Africa. There is no local source for the Cambrian limestone in South Africa either and the erratics, both there and in the Falklands, were most likely derived from a source in what are now the Transantarctic Mountains! Incidentally, the Fitzroy Tillite takes its name from the farming community of Fitzroy, itself named after the captain of Darwin’s ship, HMS Beagle.

Once the ice sheets had retreated, the Falklands area was once again blanketed by mud and sand, which now make up the 400m of the Port Sussex Formation. However, by this time, tectonic activity was stirring and the upper part of the formation contains layers of volcanic ash. The history of the Falklands was about to get violent. Starting around 280mya, the Falklands’ margin of Gondwana became a zone of compression and the rock sequence was crumpled and faulted (Fig. 9) with a strong cleavage imposed in places.

A thrust belt developed which, in terms of the Falklands’ present-day geography, carried the folded strata southwards. As it did so, a mountain range built up and its weight was sufficient to depress the Earth’s crust in the vicinity, so that, ahead of the thrust front, a foreland basin was formed. In this was deposited the younger, Permian rocks, forming part of the Lafonia Group. The correlation with Africa is instructive. In the Falklands, the fold and thrust belt moved south with the Lafonia foreland basin ahead of it. In Africa, the equivalent structure, the Cape Fold Belt, moved north with the Karoo foreland basin forming ahead of it. This contradiction was not cited by Ray Adie in 1952, but it certainly supports his idea that the Falklands microplate has rotated from its once contiguous relationship with southern Africa.

The depositional environment established in the Lafonia/Karoo foreland basin was initially one of shallow water and the basin was, perhaps, lacustrine for at least part of its history. The older succession ahead of the thrust belt was still undisturbed, so in the foreland basin, the Port Sussex Formation is conformably succeeded by the sandstone and mudstone of the Brenton Loch Formation, up to 3,000m thick. Much of the fine-grained strata are laminated and trace fossils abound – one remarkable type comprising sets of parallel, sinuous lines (Fig. 10) is thought to have been made by fish swimming close to the lake bed and trailing their fins in the mud. Despite that implied presence of fish, the only fossil remains to have been found are a handful of rather bland bivalves.

A few plant fragments are found in the upper part of the Brenton Loch Formation but they become much more abundant in the succeeding Bay of Harbours Formation, also up to about 3,000m thick. There is much fine-grained sandstone in this formation, which probably accumulated in a network of inter-connected delta channels, as a result of which, cross-bedding and rippled surfaces are common. The plant fossils come in two varieties: impressions of leaves and stems indicative of Glossopteris trees and giant horsetails, and fragments of silicified wood (Fig. 11). By this time, late in the Permian, the compressive structural regime had come to an end, so the Brenton Loch and Bay of Harbours formations remain relatively undisturbed across low-lying Lafonia. However, the next phase in the Falklands’ geological history was to prove crucially dramatic – the break-up of Gondwana.

Gondwana’s fate was probably instigated by a vast upwelling of magma, a mantle plume, which impacted on the crust somewhere in the Africa-Falklands-Antarctica region of the supercontinent (Fig. 1). Its presence is recorded in the Falklands by several intersecting swarms of dolerite dykes, all of Early Jurassic age – individual dykes are up to 30m wide (only the largest are shown in Fig. 2). The palaeomagnetic signatures of these dykes provides further evidence for the rotation of the Falklands microplate and contrasts with palaeomagnetic data derived from a later set of Early Cretaceous dolerite dykes. The Cretaceous dykes (Fig. 12) were intruded during the opening of the Atlantic Ocean and so post-date any rotation.

They are an onshore manifestation of the extensional tectonic regime that created the Cretaceous-Tertiary offshore sedimentary basins that surround the Falkland Islands. The offshore basins contain up to 5,000m of strata and are currently the focus of oil exploration.

The Falkland Islands onshore geological record ends in the Permian, though there is one tantalising occurrence of tree trunks and branches, underneath a Pleistocene solifluction deposit, which might prove the treeless Falkland Islands to have enjoyed a Neogene forest cover. This would have been wiped out by Pleistocene glacial conditions, though the islands never experienced any substantial ice cover.

There were a few small cirque glaciers on the highest peaks, but for the most part the conditions were of deep-frozen tundra. Therefore, periglacial processes were active over a long period of time and their interaction with the Port Stanley Formation quartzite has produced extraordinary landscape features – the stone runs. These are vast periglacial blockfields (Fig. 13) and patterned ground developed on such a large scale that its extent can only really be appreciated from the air (Fig. 14).

Though periglacial patterned ground is commonplace world-wide, the scale of the Falklands’ examples is extraordinary and sets them in a class of their own. They probably owe their size to a combination of the Port Stanley quartzite, a tough rock that naturally forms large and irreducible boulders, and a complex development history spread over several ice age stadials. Charles Darwin was rather taken by them too – though puzzled. The best suggestion he could offer was that they had been shaken into place by earthquakes.

Dr Phil Stone is a research associate with the British Geological Survey in Edinburgh, and acts as a geological advisor to the Department of Mineral Resources, Falkland Islands Government. This article is published by permission of the Falkland Islands Government and the Executive Director, British Geological Survey, NERC.

References

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