Geology of the Falkland Islands
The Malvinas Islands are located on a projection of the Patagonian continental shelf. According to British geologists, about 400 million years ago, the islands broke away from what is now South Africa, rotating 180º on their way, drifting westward until they collided with present-day South America.
However, there is no evidence of continental suture to support this statement (neither on the platform nor in the Patagonia region), the marine subsoil of the area does not coincide with a tectonic structure that evidences compression and collision, the data paleomagnetics are not coincident with the supposed paleo-location proposed; In addition to the fact that there is a clear sedimentary correlation with the Devonian and Carboniferous sediments of the Sierra de Ventania in Buenos Aires, and with those of the North Patagonian Massif; evidencing that these areas were close to each other. The latter led several South American geologists to propose that the islands practically remained in their current location, initially forming part of Patagonia in present-day South America.
Studies of the seabed surrounding the islands have indicated the possibility of oil. Intensive exploration of such a resource began in 1996, although some previous seismic surveys had occurred in the region.
Descriptions of the geology of the Falkland Islands have appeared in various publications and are detailed below.
Formation of the Falkland Islands (according to British geologists)
It all began more than 400 million years ago with the appearance of intrusive dykes in the crust of the supercontinent Gondwana, from which a large number of smaller fragments were formed, including the Malvinas Islands. At first the fragment corresponding to the islands separated from the southeastern part of Africa and over a section of what would be Antarctica to undergo a turn of about 180 °. Gondwana's interior was speckled with crystalline rocks over 1 billion years old, present today in the Cabo Belgrano Complex, and its margins were covered by mud and sand. Sand and mud filled the edge of the continent as it sank and finally hardened, and for this reason it is possible to find sequences of these rocks as far apart as South Africa, West Antarctica, and Brazil. In the Malvinas Islands they are known as the Gran Malvina Group.
About 150 million years ago they began to suffer a displacement derived from the movement of tectonic plates that pushed the islands against South America.
200 million years ago, the changes in Gondwana were beginning to mark their end: tectonic forces tore it apart and sheets of liquid basalt were forced to fill the cracks that cut the sedimentary layers. These solidified lava flows can be seen today in the form of dikes that cut through the oldest sedimentary strata, which are found mainly in the southern part of Soledad Island and in South Africa.
After the Ice Age ended, tectonic forces deformed the islands, folding them, pressing them together, giving rise to the formation of a mountain range or range, a fragment of which is now part of Alturas Rivadavia on Soledad Island. The weight of this mountain range served for the earth's crust to sink, giving rise to a basin in front of it. Layers of sand and mud filled the basin as it sank and then petrified to form the rocks of the Lafonia Malvinas Group. Similar rocks fill the Karoo basin throughout southern Africa.
About 290 million years ago the Earth experienced an ice age and glaciers advanced from the polar region eroding and transporting rocks that were deposited in moraine expanses, or sank into the sea where the glacier ended up floating on a layer of ice. Once petrified, the sediments formed the rocks that now make up the Fitzroy Tilite Formation in the Falklands. Identical rocks are found in southern Africa.
Formation of the Malvinas Islands (according to South American geologists)
The Malvinas Islands do not come from Africa, their origin being more related to Patagonia and the Craton of the Río de La Plata. It is likely that you have always had a position similar to the current one.
The igneous-metamorphic basement of the Islands is similar in age to those exposed in Patagonia. The sedimentary cover of the Island presents a clear correlation with the Devonian deposits of the Sierra de la Ventania (Buenos Aires); and with the Cabo Blanco deposits present in the Patagonia of Santa Cruz; sharing both the Malvinocratic fauna. This was even observed and described by Charles Darwin on his trip to South America.
The glacial carboniferous deposits of Lafonia (present in the islands) have the same age and deposit origin as the Tillitas of Sauce Grande (present in Buenos Aires); both being correlated. The latter have clasts with the same Archeocyatids present in Malvinas, Patagonia, Antarctica, and Ventania; their correlation being even more evident.
A sampling of zircons comparing the Malvinas Islands with Patagonia, shows that both have similar ages, being divided into three groups: Grenvillian, Pampean, and Ordovician. South Africa, for its part, has zircons with Archean (not present in the Falklands) and Grenvillian ages; the Pampas and Ordovician zircons being absent.
The presence of basic Jurassic dikes on the islands evidences a tensional tectonics, which contradicts the expected compression in a collision zone. The absence of sutures, both on the South American continental edge and in Patagonia; does not support the collision stance. The mapping of the marine subsoil does not coincide with a transcurrency-compression zone, but rather with a stretch zone, typical of the opening of the Atlantic Ocean. The latter precisely helped to generate the hydrocarbon basins of the North Malvinas.
Finally, the igneous-metamorphic basement of the Greevilian age of the Islands (Cape Meredith Complex), continues underground to the east; being present on the Ewing Bank (located under the sea), on San Pedro Island (South Georgia), and on the Las Águilas Bank (near the African coasts); evidencing that the entire complex behaved as a single continental block. It is inadmissible to postulate the allochthony of the Islands, since they are related to different sectors of the Antilles, in addition to Patagonia and Buenos Aires; and if that were the case, their displacement would not be related to South Africa.
Geological structure of the Falkland Islands
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The oldest rocks in the Malvinas are the gneiss and the granite present in the Cabo Belgrano Complex, with an age of 1.1 billion years (according to radiometric dating studies). These types of rocks are visible on the cliffs and the southern tip of the Gran Malvina Island. The complex corresponds to the crystalline rocks that were part of the interior of the supercontinent Gondwana. This type of rock also has a great geological similarity to the rocks present in Natal (South Africa) and those found in Maud's Land, in Antarctica.
Other layers of quartzite, sandstone and non-fissile shale lie on top of the gneiss and granitic layers in the Gran Malvina. The hardest quartzite is present in Monte María (located on Gran Malvina Island). These rocks were deposited in shallow waters by currents that carried sediments in submarine dunes, this process gave rise to the formation of cross-stratifications. From the study of these stratifications that in the case of the Malvinas Islands run in a northerly direction and comparing them with those present in South Africa, which run to the south, it serves as evidence about the rotation experienced by the block that contained the islands.
Sandstones in the central part of the Gran Malvina Group contain fossils of animals that lived in shallow waters about 400 million years ago. Snails, flower petals, and even trilobites have been found in some places. Exactly the same assemblage of fossils is found in rocks in southern Brazil, South Africa, and the Ellsworth Mountains in Antarctica.
As the ice sheet on Gondwana spread, it carried rocks of all sizes, forming tillites, a type of rock that is made up of a group of small grains of different sizes. They are located on the north coast of the Gran Malvina. Similar rocks are found in South Africa.
At the end of the ice age, rocks were compressed and folded by the movement of tectonic plates. In the Falklands the rocks were pushed from north to south, while in the Cape Fold Belt in South Africa the opposite occurred, corroborating once again the theory of rotation of the Falklands block.
The islands also have the presence of igneous rock dikes that filled the cracks produced by the fragmentation of Gondwana. When these dams solidified, they left traces of the magnetic field that were locked up in the minerals being formed. This serves to determine that the magnetic field registered in these rocks (according to the orientation of the minerals) is opposite to that registered in the dykes of the same generation in South Africa. For this reason, the igneous dykes are the strongest evidence of the rotation of the Malvinas.
One of the most distinctive features of the Falkland landscape are the rock slides near Mount Challenger and are the result of the last ice age between 14,000 and 25,000 years ago. The action of the intense cold broke the fragile layer of quartzite that was crushed and scattered several times during the freezing and thawing cycles, distributing them along strips.
Formation of basalt dikes
The undulating Lafonia plain is made up of sandstones from the Lafonia Group during the Carboniferous-Permian. Low rims of sandstones formed where they were cut vertically by basalt dikes. The presence of a dike is evidenced thanks to the erosion process by which some lighter-colored rocks stand out on the surface, this can be seen on Boungaiville Island. In Gran Malvina there are several dikes that cut the crust of the Gran Malvina Group, but these dikes, unlike the previous ones, are chemically more unstable, so they have been eroded and all that remains of their existence are aligned holes. At the small edges of these holes you can see where the sandstone is that was burned and hardened by the hot basalt.
Folds on Gran Malvina Island
In most of the Gran Malvina Island, the strata of the Gran Malvina Group are slightly inclined with respect to the horizontal. This tilt shows different types of rocks in different places. The quartzites of Puerto Esteban and Puerto Argentino are more resistant than the sandstones of the Bahía Fox Formation. The Hornby mountain range, near the Strait of San Carlos, has experienced uplifting and folding forces by which the quartzite beds of Puerto Argentino are found. inclined to the vertical. As the western part grew towards the east, the layer of rocks covered the junction.
Folds on Soledad Island
Where Soledad Island is surrounded by rocks of the Gran Malvina Group, the strata are highly deformed. The strata are highly folded with vertical inclinations and there is even the presence of recumbent folds. When the rocks are soft, for example near the Bahía Fox Formation, they are very susceptible to erosion. The harder quartzites are more resistant to erosion and have created a more irregular landscape with steeply-sloping rock layers along the Isla Soledad mountain range from Puerto Argentino west to Alturas Rivadavia.
Ice Age Effects
Today you can see on the islands the residues left by the erosion process of the ice age that occurred between 25,000 and 15,000 years ago. The hilltops have suffered most of the freezing and thawing process. It can also be seen that due to the region in which the islands are located, there are large wind currents that transport the grains of sand to heights close to a meter above ground level, for this reason the rocks and pillar-shaped structures present the greatest wear in the lower zones, especially visible in the higher parts of Gran Malvina Island where the quartzites of the Puerto Esteban Formation are exposed on the surface.
The last ice age with its freezing cycles culminated about 15,000 years ago, glaciers formed in some high areas modifying the landscape creating on the eastern slopes of the mountains. This was the windward side protected from the westerly winds where the snow accumulated year after year. The weather could have been quite dry but perhaps it was the wind that prevented glaciers from forming elsewhere. On Soledad Island you can see basins called glacial cirques on Cerro Alberdi, and on Gran Malvina Island they are on Mount Independencia and in the Hornby Mountains.
Rockslides are another feature left behind by the Ice Age. All the rocks that are part of the thrusts are remains of quartzite that were ground repeatedly as a result of repeated cycles of freezing and thawing, they are found mainly in the Puerto Argentino Formation and to a lesser degree in the Puerto Esteban Formation. The excavations carried out show that the color in the upper part of the rocks is different from the lower part due to the erosion process of rainwater that has bleached the stones, leaving them a pale grayish color. Below, where the rocks have been protected from erosion, they take on an orange color due to iron oxide.
Seismology
On November 25, 2013, an earthquake measuring between 6.9 and 7 degrees of magnitude on the Richter scale occurred in the South Atlantic area located 314 kilometers from Puerto Argentino/Stanley, capital of the Malvinas Islands and 877 kilometers from the city of Ushuaia, capital of the entire province of Tierra del Fuego, reported the United States Geological Survey (USGS, for its acronym in English). The epicenter of the earthquake was registered at a depth of 10 kilometers in the high seas, without registering victims.
On December 11, 2014, there was an earthquake measuring 7 on the Richter scale 314 kilometers southeast of the island capital in the South Atlantic, causing no casualties or damage. The epicenter was recorded at a depth of 10 kilometers and 877 kilometers from Ushuaia, at 3:27 in the morning (UTC-3). Later there was a 5° aftershock also at a depth of 10 kilometers.
Oil in the Falkland Islands
The Malvinas Islands exploration area is located in the sea north of them and covers an area of 400,000 km² which contains several Mesozoic sedimentary basins. After having carried out different seismic studies and three-dimensional inspections, six exploration wells were dug, five of which presented oil samples. However, none presented indications of commercial quantities.
According to studies carried out by the British Geological Survey led by geologist Phil Richards, it was determined that oil generation can occur from 2,700 meters below sea level, and a maximum generation would occur from 3000m. The main rocks prone to contain oil have not yet been penetrated because they are located at a depth greater than 3 kilometers below sea level.
It has been concluded that more than 60 billion barrels of oil are likely to have been generated in the North Falkland Basin. >). These data are based on pyrolysis studies obtained from the wells and assuming the existence of an interval of mature rock with a thickness close to 400 m and covering an area of 40 km by 40 km. However, even with more conservative figures for source rock thickness and surface area, richness, and kerosene generating potential, it is estimated that significant quantities may have been expelled.
Example: In a zone of mature rock 200 m thick, over an area of 35 km by 12 km, it may have produced more than 11.5 billion barrels of oil, even with the production of 8 kg of hydrocarbon per ton.
The brownish lake rocks are similar to the Upper Permian lake source rocks south of the Junggar Basin of northwest China, which are the richest and thickest source rocks in the world.
According to calculations of the potential production index (obtained by multiplying the organic content of the rock by its thickness and potential hydrocarbon production) suggest that the rocks of the North Malvina Basin are in second place after the source rocks of the Junggar Basin, in terms of its potential for oil.
The six excavated wells encountered reservoir rocks. These reservoirs range from the Upper Jurassic to the Upper Cretaceous.
Tectono-stratigraphic structure of the North Malvina Basin – analysis after the exploration wells
The fill of the northern part of the Malvina Norte Basin is divided into 8 tectono-stratigraphic units. So far, the biostratigraphic information collected is not abundant enough to establish a stratigraphic sequence based on palynological and paleontological data. On the other hand, lithostratigraphic subdivisions are only useful for describing the stratigraphy of individual wells, but are useless for comparison due to lateral lithologic variations that exist between units.
The basin was the site of fluvio-lacustrine deposition during the early and late stages of ridge formation in the area, which led to the formation of a permanent lake towards the end of the ridge subsidence process. At the end of the formation of the ridge, a sedimentation process occurred that came to form several deltaic systems, creating a large system of lakes.
Towards the end of the post-ridge phase, from the end of the Lower Albian or Cenomanian to the beginning of the Upper Paleogene, it was characterized by, first, the establishment of marginal coastal conditions and finally totally marine conditions, as a marine connection with the basin.
In the case of the Malvinas Basin, marine conditions occurred earlier (Lower Jurassic) and later in the San Jorge Basin to the northwest, suggesting that the development of marine conditions originated from the south and spread northward. After a process of uplift in the Paleogene it was followed by a process of thermal subsidence in addition to the marine/deltaic deposition that took place during the rest of the Cenozoic.
History of Oil Exploration
Exploration in the Falkland Islands began in the late 1970s with the acquisition of a regional seismic data set by two oil service companies. At that time, the available data did not allow the start of exploration since the Government of the Malvinas Islands was not prepared to offer excavation licenses, on the other hand, the start of the war on the islands by Argentina in 1982 postponed all exploration. In 1992 the island government contracted the British Geological Survey to begin the exploration process. After an initial investigation that revealed the existence of several Mesozoic basins, the seismic investigation continued. The main exploration interest after the first round of licenses was located in the Malvina Norte Basin area, an elongated fractured basin in relatively shallow water. The basins located to the south and east of the islands present a significant technological challenge since they are located at a greater depth.
During the licensing offer in 1996 seven companies agreed to an excavation campaign. 6 wells were carried out, which were planned for the first 5-year period of the new licences.
Along with the geological and geophysical data extracted during the exploration campaign, environmental data was also collected. On the other hand, new research in this area was carried out during the campaign and has been the subject of study in recent years.
Oil system in the Malvina Norte Basin
A source rock system was found in the Malvina Norte Basin capable of generating more than 102 kg hydrocarbon/t of rock. Although much of the vertical thickness of the source rock is immature, it is capable of generating hydrocarbons below 2,700 m. The rock that generates the largest amount is located at a depth of about 3000 m. Estimates of the volume of rock lying just inside the mature belt of oil range from 36 X 109 m³ to 400 X 109 m³, depending on seismic data.. It is widely believed that up to 60 billion barrels of oil could have been produced in the basin.
During exploration, sandstones (about 100 m thick) have been found above the main source rock interval, with porosities up to 30%. Only very few sandstones with good reserve properties have been found so far in the crack succession below the main source rock interval, but few wells have been penetrated down to that zone.
The lack of great pressure in the basin suggests that any oil produced may have migrated laterally, thus being trapped in rift reservoirs developed below and to the sides of the main source rock. The rock studied is found fundamentally in the upper part of the source rock and could function -given its low level of porosity- as a seal for the lower rock and would only be cut at the edges where it is crossed by faults.
Related bibliography
- The Falkland Islands: a key element in Gondwana palaeography. J. E. A. Marshall. Tectonics, vol. 13, pp. 499-514. (1994)
- Geology of the Falkland Islands. R. Clark, E. J. Edwards, S. Luxton, T. Shipp and P. Wilson. Geology Today, vol. 11, pp. 217-223. (1995)
- Stone runs in the Falkland Islands. M. Rosenbaum. Geology Today, vol. 12, pp. 151-154. (1996)
- The Geology of the Falkland Islands. D. T. Aldiss and E. J. Edwards. British Geological Survey Technical ReportWC/99/10 (1999)
- Reconstruction and break-out model for the Falkland Islands within Gondwana. B. C. Storey and others. Journal of African Earth Sciences, vol. 29, pp. 153-163. (1999)