Onshore Geology - Introduction

The onshore geology map

Until 1998, the only geological map of the Islands was a reconnaissance map constructed in the early parts of the last century, which had been subsequently modified from aerial photography interpretations without any ground-truthing. Between 1996 and 1998 the British Geological Survey re-surveyed the Islands and produced the first modern geological map.

The bulk of the Islands are composed of Silurian to Devonian sedimentary rocks, while the southern part of East Falkland (also known as Lafonia) is predominantly Carboniferous to Permian in age.

The oldest rocks on the Island are Pre-Cambrian gneisses and granite found at Cape Meredith in the very southern part of West Falkland, and the youngest occur as several suites of igneous dykes of Jurassic age that cut across the islands with various orientations.

A detailed solid geology map of the Falkland Islands, on two sheets at a scale of 1:250 000, is available from the Department of Mineral Resources, and from the British Geological Survey.

The geological history of the region

About 400 million years ago the continents of the Southern Hemisphere all lay adjacent to each other, forming the giant supercontinent of Gondwana. The Falkland Islands were tucked in between the future south-east coast of Africa and part of what was to become Antarctica.

The interior of Gondwana was underpinned by ancient crystalline rocks more than 1000 million years old, now represented in the Falklands by the Cape Meredith Complex, but its margins were covered by shallow seas.

Sand and mud deposited on the slowly subsiding continental edge are now found as far apart as South Africa, West Antarctica and Brazil. In the Falklands they are known as the West Falkland Group.

While the continental fragment that was to become the Falklands was still locked into Gondwana, it lay at a fairly high southern latitude. Around 290 million years ago the Carboniferous glaciation resulted in the deposition of the Fitzroy Tillite Formation in the Falklands; identical rocks are extensive in what is now South Africa.

Starting about 280 million years ago, the rocks were deformed and over-thrust into a mountain range, a fragment of which now forms the Wickham Heights in East Falkland.

A low-lying basin formed ahead of the mountain belt during the Permian, into which sand and mud deposits accumulated to form the Lafonia Group. Similar rocks fill the Karoo basin across southern Africa.

Early fragmentation of Gondwana in the Jurassic facilitated fracturing that was exploited by dyke intrusion across the islands.

During the early Cretaceous the South Atlantic opened, and the Falklands drifted to their present position offshore South America. 

The two models for the break-up of Gondwana and the original orientation of the Falklands are summarised in the page on plate tectonic framework.