Onshore Geology
Introduction
In 1998, the British Geological Survey created the first modern geological map of the Falkland Islands, updating an outdated map from the early 1900s. The majority of the islands are composed of Silurian to Devonian sedimentary rocks, while the southern part of East Falkland consists mainly of Carboniferous to Permian rocks. The oldest rocks on the islands are Pre-Cambrian gneisses and granite while the youngest are Jurassic igneous dykes.
The Geological Record
The Falkland Islands were located between Africa and Antarctica when they were part of the supercontinent Gondwana over 400 million years ago. The Falklands had ancient crystalline rocks and were surrounded by shallow seas. The Fitzroy Tillite Formation was deposited during the Carboniferous glaciation, and the rocks were later deformed and over-thrust into a mountain range. The South Atlantic opened during the early Cretaceous period, and the Falklands drifted to their current position offshore South America.
Pre-Cambrian
The oldest rocks in the Falkland Islands, which are about 1100 million years old, are gneiss and granite of the Cape Meredith Complex. They are located in the southern extremity of West Falkland and are a fragment of the ancient crystalline core of Gondwana. These rocks have geological similarities with parts of Natal in South Africa and Dronning Maud Land in Antarctica.
Siluro-Devonian
The West Falkland Group consists of quartzites, sandstones, and mudstones that overlay Pre-Cambrian gneiss and granite making them Siluro-Devonian. The quartzite forms much of the high ground in the Falklands and is similar to Table Mountain's rocks in Cape Town. These rocks were deposited in a shallow, coastal marine setting, with depositional currents running towards the north. The Fox Bay Formation sandstones in the middle of the West Falkland Group contain similar fossils to those found in Brazil, South Africa, and Antarctica, suggesting that the Falklands were once part of the Gondwana continent.
Permo-Carboniferous
The Permo-Carboniferous glaciation resulted in the deposit of the Fitzroy Tillite across Gondwana. The remainder of the Permian section was deposited under mostly post-glacial conditions. The Lafonia Group's Black Rock Member has up to 40% carbon and could still be a viable hydrocarbon source rock offshore. The Brenton Loch Formation contains a large portion of volcanic detritus, while younger sediments of the Lafonia Group show less volcanic detritus.
Mesozoic intrusions
During the early stages of Gondwana fragmentation, dolerite dykes were emplaced across the Islands. A paper published by Stone et al. in 2008 shows that three sets of dykes can be identified, ranging in age from early Jurassic to mid Cretaceous. Magnetic data from these dykes record a magnetic field running in the opposite direction to that derived from dykes of the same generation in South Africa. This has been used as evidence to support a rotation hypothesis for the Islands.
Structure and metamorphism
During the Permian and Mesozoic, the Falkland Islands experienced five phases of deformation (D1-D5). D1 deformation created an east-west belt of folding and sub-parallel faulting. D2 deformation produced large north-south and east-north-east to west-north-west trending folds. D3 deformation generated strike-slip faults, and D4 resulted in brittle deformation and a south-south-west-verging thrust sheet. D5 produced west-north-west through east-north-east-trending extensional faults, reactivating major thrusts.
Fossils
Charles Darwin collected fossils from the Falkland Islands during his Beagle voyage, with the earliest signs of life found in the sandstone of the Siluro-Devonian Port Stephens Formation. Fossils in the Falkland Islands are most abundant in the Devonian Fox Bay Formation, including trilobites, crinoids, and brachiopods. Glaciers that spread across Gondwana about 300 million years ago carried rock debris to the Falkland Islands, depositing tillite with assorted pebbles and boulders in a sandy-clay matrix. The Permian Lafonia Group, a lake system, was home to fish and soft-bodied, crawling animals. Fish trails and delicate leaf structures from Glossopteris-type trees are still preserved from this time period.
Stone Runs
The Falkland Islands have a diverse landscape, with the East featuring craggy uplands overlooking grassy plains and valleys, while the West has broad hills and open valleys. The Falklands' landscape was shaped by the polar climate of the last Ice Age, which created the unique stone runs. These are vast accumulations of boulders and stripes on hillsides, resulting from cycles of freezing and thawing breaking up and sorting the debris. The stone runs are composed of locally derived quartzite blocks, with the largest ones up to four kilometers long and several hundred meters wide. They are most widespread on East Falkland and are a remarkable feature of the Falklands' landscape.