One Billion years ago, along the edge of a long forgotten sea, coarse grains of sand were deposited in thick layers. The waves washed them, the wind blew them around and they slowly lost any angular edges and became smooth and round. Over time they were buried and became rock which was then intruded by a volcanic batholith, slowly cooling to form a beautiful rich pink granite. This small section of the Earth’s crust is still around today and is exposed along the cliffs of Cape Meredith, the most Southerly point of mainland West Falklands. These Pre-Cambrian exposures are the oldest rocks in Southern South America and form the Cape Meredith Complex. In fact the geology of the Falkland Islands is unique in this region and for many decades Geologist couldn’t explain how the Falkland Islands could fit in the theory of plate tectonics and was used by some scientists to try and disprove Alfred Wegener 1915 theory of continental drift.
There is a gap in the rock record of nearly 600 million years. All we can conclude is that during this time, what was to become the Falkland Islands, was a positive feature on the Earth’s surface and exposed to the prehistoric storms of many millennia. Then about 420 million years ago (we don’t know for sure), the sea started to lap our shores and a beach was formed. There was life living in the area of the shore, as we find huge trace fossils( Heimdallia ‘meredithensis) of an animal that is long extinct and never described by science. These rocks form part of the Post Stephens formation, a sequence of river and shore sediment, mainly coarse grained sandstones and siltstones, dated from the Silurian into the Devonian period. There is a high concentration of feldspar within the sandstone, which has been weathered to form Kaolinite Clay. As a result the rock is very friable and amazing rock formations, such as the Indian Village and the cauliflower rocks have formed.
About 380 Million years ago, the shallow sea deepened, to about 200m. Amazing Devonian armour plated fish would have swum in the depths, and orthocone nautiloids would have followed the currents near the surface. The seas were rich in life, bivalves, brachiopods, crinoids, gastropods and trilobites. During this period the Falkland Islands were located in the hurricane belt, of the tropics, where storms would pick animals up from the coastal shallows and carried rapidly out into deep water and then buried in the soft sediment, only to be preserved as fossils in the Fox Bay formation. Towards the end of the Devonian period the Falklands formed part of a delta, covered in a forest of Lycopsid fern like trees, creating the Port Philomel formation before being covered in an intricate interlacing river system, rich in quartz sandstones and forming the Port Stanley Formation.
320 million years ago, a huge mountain range stretching from Brazil, through South Africa and on into Antarctica was created. As the Cape Fold belt was being formed the Bluff Cove member was being deposited, where you can see frozen into the rock evidence of sedimentary escape structures and the rock was caught in the folding and tectonic activity.
The climate suddenly changed and the Falkland Islands were plunged into an icy coffin as the Carboniferous polar climate advanced north, threating to turn the whole Earth into a snowball. Increasing CO2 resulted in the ice retreating and huge quantities of sediment being dumped in a sea, forming the Fitzroy Tillites. The sea was anoxic and as the sediment rich rock slowed, it was replaced with organic rich mudstones. These mudstones, were heated and would have been a source of hydrocarbons, long since burnt off, but there are still sections of the black rock slates with enough organic matter in them to be used a s poor type of coal.
With the large amounts of water being released, the area to the south of East Falklands was flooded and formed a large lake. During the winter the lake froze, and then was ice free in the summer months creating a varve deposit, creating part of the Permian Brenton Loch Formation. Other sections of this formation contain mud and silt stones, with delicately preserved traces of fish as thief fins and tail skimmed over the ancient soft surface.
As the lake filled in the Bay of harbours formation was created. These fine grained sandstones often contain fossils of Glossopteris, a wide spread tree, that was found in many environments across the ancient continent of Gondwana. The Falklands were warm covered in tress growing along the banks of meandering rivers. It sounds idyllic, however, during the Permian all the major continents were joined, forming a huge area of land, most of which was covered in a harsh desert; it was during this period that we see the largest mass extinction event with over 80% of life of the planet disappearing. Few animal fossils have been found in these rocks, only a bivalve, but early animals would have been roaming these forests, so one day we might find the remains of the first prehistoric native animal.
190 Million years ago, the Falklands were exposed to extensional tectonic forces. The joints created in the rock were intruded by doloritic dykes, that can be easily seen on West Falklands, but not so well exposed on East Falklands. This igneous material, rich in Iron, was vital in helping geologist piece together the story as to why the Falklands geology is not the same as the geology of South America. When the igneous material cools, the iron is petrified, pointing in the direction of the magnetic pole. When samples were taken of these rocks the orientation of the magnetic pole was 1800 out of where it should have been. The only way we could make it work was to rotate the Islands 1800. Not only that, the deformation and folding found in the rock of East Falklands is more intense than on West Falklands, suggesting that East & West were separated by as much as 200 miles. As the South American, Antarctic and African plates began to separate, the Falkland Islands became a micro continent. Being pulled and rotated independently of their larger neighbours, until about 100 million years ago we became stuck to the South of the South American continent. By this time 1200 of rotation has occurred and East and West had become joined. The final 600 of rotation occurred as Antarctica and South America twisted apart.
We then see another gap in our rock records. The next evidence is a small deposit of clay and woody material, buried under the beach on West Point Island. The wood dates back 30 million years ago; when for a short period of time the Falklands were covered in southern beech trees and the landscape would have been similar to Tierra del Fuego today. At this time, major tectonic forces were at work, The Andes Mountains were being forced to rise and the Antarctic Peninsular and the Antarctic continent began its journey to the Southern Pole. As they broke away from South America, the Drakes Passage opened up. Antarctica became geothermally isolated and the continents ice caps grew. The southern ocean was formed and the cold polar sea currents began to swirl around the Falkland Islands, decreasing the temperature to a tundra climate and resulting in the loss of the trees and the emergence of our grass Tierra del Fuego lands.
The vegetation of the Falklands has not changed very much for many millennia. The next usable record are the peat deposits, the oldest of which is about 40,000 years old, buried under Pitaluga Bay. The majority of peat in the Stanley area is 13,000 years old and marks the end of the last ice age. The spores and vegetation show that what we see in the way of plants today, we would have seen here 13,000 and 40,000 years ago. We have been a grassland for a very long time…trees do not belong here.
The final features to comment on are Stone runs, described by Charles Darwin as rivers of stone. They were created over 100,000 years ago and some dares put them at 1.3 million years old. These strange features show a number of different fabrics, and how exactly they were formed hasn’t been fully answered. The most popular theory is that in colder periods freeze thaw occurred in the mountains. As the ground froze, it cracked open allowing the loose material to fill the cracks. Over time the ground opened and closed, allowing little rocks to trickle down to the permafrost below. The weight of the large rocks forced the small rocks to move and they became rounding, acting like barring’s carrying the large, angular blocks down the hillside and along the valley floor. We still have unanswered questions, why do some stone runs appear to move over the hills? Why do stone runs, other than the one on Keppel Island, never reach the sea?
The fact that we still don’t know all the answers and we can still go into the field and find new things, makes working in Falkland geology very interesting.
By Emma Brook BSc MSc FGS |
