Geology of the Falkland Islands
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The geology of the Falkland Islands originates from events more than 400 million years ago, before the Falkland Islands existed, with the appearance of intrusive dikes in the crust of the supercontinent Gondwana. The resulting breakup of Gondwana led to the formation of a large number of minor crustal fragments, including the Falkland Islands. In the beginning, the islands' fragment separated from the southeastern part of Africa on a section that would be Antarctica and later underwent a rotation of almost 180°. The interior of Gondwana was sprinkled with crystalline rocks more than a billion years old, today found in the Complex of Cape Belgrano. Sediments of sand and mud filled and eventually covered the developing continental rifts rift. Later the sediments covering the rifts hardened into rock, forming layers of rock known as rock sequences. Due to Gondwana's breakup these rock sequences exist in places as far apart as South Africa, western Antarctica, and Brazil. In the Falkland Islands these are known as the West Falkland Group.
Two hundred million years ago, the changes in Gondwana were starting to reach their end: tectonic forces tore Gondwana apart and sheets of liquid basalt intruded into the cracks that formed between the sedimentary layers. The result was solidified sheets which can now be seen in the form of dikes that cut the oldest sedimentary layers, those which lay principally in the southern part of East Falkland and in South Africa.
Tectonic forces formed the region. A mountainous chain formed, part of which now forms Wickham Heights (known in Spanish as Altos de Wickham) on East Falkland Island (Solitude Island). A basin developed and was filled with terriginous sediments. These layers of sand and mud filled the basin as it sank and soon they hardened to produce the rocks of the Lafonia Group of the Falklands. Similar rocks fill the Karoo basin in southern Africa. About 290 million years ago an ice age engulfed the area as glaciers advanced from the polar region eroding and transporting rocks that were deposited as extensive moraines and glacial till, or they sank in the sea where the glacier ended up floating in an ice layer. Once the glacial sediments were lithified they formed the rocks that now constitute the tillite Formation of Tilita de Fitzroy in the Falklands. Identical rocks are found in southern Africa.
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[edit] Geological structure of the Falkland Islands
The oldest rocks in the Falklands are gneiss and granite in the Cabo Belgrano Complex, which radiometric studies place at around 1100 million years old. These types of rocks are visible in cliffs at the south end of West Falkland. The complex corresponds to the crystalline rocks that comprised of the interior of the Gondwana supercontinent. This type of rock has, in addition, a great geological similarity to present rocks in South Africa and those that are in Queen Maud Land in Antarctica. On top of gneiss and the granite lie other layers of quartzite, sandstone, and shales or mudstone in West Falkland. These rocks were deposited in shallow waters by currents that transported submarine sediments in a delta environment producing cross-bedding and ripple marks. In the case of the Falklands these stratifications run northward, and are very similar to formations in South Africa which run southward. Comparison of the two serves as evidence that the arenaceous block which contains the islands has been rotated. Rocks in the central part of West Falkland contain fossils of marine organisms that lived in shallow water.
[edit] Formation of basalt dikes
The plain of Lafonia is constituted by arenaceous sediments of the Lafonia Group. Depressions in the sediments formed where they were cut vertically by basalt dikes. The presence of a dike is indicated by the erosion of less resistant rocks, this can be observed in Boungainville Island (Lively Island). In West Falkland there are several dikes that cut the rocks of the Great Malvina group, but these dikes, unlike the previous ones, are chemically more unstable and have been eroded and the only indication of their existence is the aligned linear depressions. In the margins of these depressions there is evidence of contact baking or hornfels formation adjacent to the once molten basalt dike.
[edit] Folding of West Falkland
Most of the layers of West Falkland (also known by its Spanish name, Gran Malvina) and its satellite islands are slightly inclined from the horizontal. This inclination shows different types from rocks in different places. The quartzites of Port Stephens and Stanley are more resistant than the arenaceous sediments of the formation at Fox Bay. The Hornby Mountains, near Narrow of San Carlos have experienced tectonic forces of uplift and folding by which the quartzite beds of Argentine Port are inclined to the vertical.
[edit] Folding of East Falkland Island
Where East Falkland (Soledad) island is surrounded by rocks from the Great Falkland Group the rock layers are severely deformed. The layers are tightly folded with steep to vertical dips, some are even recumbent or horizontal overturned folds. Where the rocks are very soft, for example near the Fox Bay Formation, they are very susceptible to erosion. The hardest quartzites are more resistant to erosion and have created a very irregular landscape with steeply inclined rock layers found along the length of the mountain chain on Soledad island from the city of Stanley (Puerto Argentino) westward to Wickham Heights (Altos de Wickham).
[edit] The effects of glaciation
The effects of the process of ice age erosion that occurred between 25,000 to 15,000 years ago can be seen in the surface of the islands. The tops of the hills have been exposed to most of the effects of freezing and thawing. It also can be noted that, due to the region in which the islands are located, strong wind currents carry sand grains up to heights of a metre above ground level. This metre high sand-blasting effect causes rocks and pillar-shaped rock structures to show the most erosion in their lower levels (i.e. one metre and below). This unique pattern of erosion is very evident in the higher elevations of West Falkland where quartzites of the Port Stephens Formation are exposed to the surface. During the last glaciation with its cycles of freezing culminating 15,000 years ago, glaciers formed in some elevated zones modifying the landscape on the eastern slopes of the mountains. This was the leeward side protected from the westerly winds where snow accumulated year after year. One reason for glacier accumulation on the eastern slopes could be that the climate could have been very dry. Another reason could be that the prevailing wind prevented glaciers from forming in places other than eastern slopes. On East Falkland, basins called glacial cirques can be seen on Mt. Alberdi. On West Falkland glacial cirques are on Mt. Independencia and on the Hornby mountains.
Rock flows or moraines are another characteristic left by glaciation. All of the boulders in the flows are remnants of quartzites that were repeatedly ground down as a consequence of repeated cycles of freezing and thawing and are found principally in the Port Argentino Formation and to a lesser degree in the Port Stephens Formation. Excavations show that the colour of the upper parts of the rocks is different from the lower part due to the process of erosion by rainwater that has whitened the stones leaving them a light grey colour. Below ground level, where the rocks have been protected from erosion, they acquire an orange colour due to iron oxide.
[edit] Economic geology
[edit] Petroleum exploration and reserves
The petroleum survey area of the Falklands Islands is in the sea to the north of the islands, covers 400,000 square kilometers (km²), and contains several sedimentary river basins from the Meozoic period. After conducting various seismic studies and three-dimensional exploration, six test wells were drilled. Five of the six wells produced samples of petroleum. Nevertheless, none produced indications of commercial amounts. According to British Geological Survey studies led by the geologist Phil Richards, the petroleum occurs at 2,700 meters below sea level and a maximum generation would be found beginning at 3,000 meters below sea level. The main petroleum source rocks have still not been penetrated because they are located at a depth deeper than 3,000 meters. It is considered probable that more than 60 billion petroleum barrels (10 km³) have been generated in the North Falkland Basin. These data are based on the studies of pyrolysis obtained of wells and assuming the existence of a mature rock interval of 400 m thickness and covering an area 40 km by 40 km.
The brown lacustrine sediments are similar to the lacustrine source rocks of the Upper Permian to the south from the Junggar River basin which are among the richest source rocks in the world. According to calculations of the Potential Production index (obtained when multiplying the organic content of the rock by its thickness and potential production of hydrocarbons) they suggest the rocks of the North Falkland River basin are in the second place after the Junggar River basin in terms of petroleum potential.
Although the wells indicate a viable source rock, they also indicate that many of the target reservoirs are composed of volcaniclastic rocks with low porosity due to secondary mineralisation (Richardson & Underhill, 2002) and are thus unlikely to store hydrocarbons in large quantities. This is not an anomalous view based solely on published data from one well, but based on information from multiple sources. Volcaniclastic-rich deposits are probably present in many parts of the basin, which is to be expected, given the major volcanic sediment source that lay to the west at the time of sedimentary deposition.
[edit] Structure of the North Falkland River Basin
The filling of the north part of North Falkland River basin is divided in eight parts. Toward the end of the uplift sedimentation occurred within several deltaic systems in a great system of lakes. Towards the end of the post-uplift phase, from the end of the early Albian or Cenomanian to the beginning of the late Paleocene, the North Falkland River basin was characterized by the initial establishment of marginal coastal conditions and finally totally marine conditions, as a marine connection with the river basin was formed. In the case of the Falkland River basin, the marine conditions occurred first (in the early Jurassic) in the Falkland River basin and later in the San Jorge River basin to the northwest, which suggests the development of marine conditions originated from the south and spread towards the north. The process of uplift in the Paleocene was followed by a process of subsidence in addition to the marine-deltaic deposition that took place during the rest of the Cenozoic period.
[edit] History of petroleum exploration
Geological surveys of the Falklands began toward the end of the 1970s, when two petroleum services companies undertook seismic surveys of the Falklands and the surrounding seafloor. Although the data appeared to indicate the area was a viable site for exploratory drilling, the islands' government was not prepared to grant licences for drilling. With the growth of oil extraction in the North Sea, most crude extraction in British waters was confined to this area. Some limited surveying did continue, but this was halted entirely following the invasion of the Falkland Islands by Argentina in 1982 and the subsequent conflict.
In 1992 the Falkland Islands government contracted the British Geological Survey to resume geological survey work in the Falklands. After an initial investigation showed the presence of several Mesozoic river basins in the waters surrounding the islands, the first round of exploratory licences was granted covering the most promising of these basins, an elongatedly-shaped fractured river basin in relatively shallow waters north of the islands. Other candidate basins, located to the south and east of the islands, present a greater technological challenge, as they were located in considerably deeper waters.
During the granting of licenses in 1996, seven companies agreed upon a drilling campaign . Six wells were drilled which were planned for the first five-year period of the licences.
Along with geological and geophysical data obtained during the exploration campaign, environmental data were also gathered. On the other hand, new investigations in this local were carried out during the drilling campaign and were the object of study in recent years . Investigations of oil reserves in the Falklands area have continued, but no large-scale extraction has yet commenced.
[edit] Petroleum System in the North Falkland River Basin
A system of source rock has been found in the North Falkland Basin capable of generating more than 102 kg of hydrocarbons per ton of rock. Although a great part of the vertical thickness of the source rock is immature, it is capable of generating hydrocarbons below 2,000 m . The rock generating the largest quantity of hydrocarbons is located at a depth of nearly 3,000 m. The calculations of the volume of rock that lies just inside the edges of petroleum ranges from 36x109 m3 up to 400x109 m3, depending on the interpretation of the seismic data. In general, it is believed that the basin could have generated up to 60 billion barrels of petroleum.
During exploration, sandstones, also known as arenaceous rock (approximately 100 m thick), were discovered on top of the principal interval of source rock, with porosities reaching 30%. Up to now, very few sandstones with good reserve properties have been found in the succession of rifts below the interval of principal source rock, but very few wells have penetrated into this zone.
The absence of high pressure in the basin suggests that whatever quantity of petroleum was produced could have migrated laterally and thus could be trapped in rift reservoirs developed below and to the side of the source rock and could function -- given its low level of porosity -- as a seal for the deeper rock and would only be cut on the borders where it is intruded upon by faults.
[edit] References
- "Controls on the structural architecture and sedimentary character of syn-rift sequences, North Falkland Basin, South Atlantic." by Richardson, N.J. and Underhill, J.R. 2002. Marine & Petroleum Geology, 19, 417-443
- "Post-drilling analysis of the North Falkland Basin– part 1: tectono-stratigraphic framework", by Phil Richards and B. V. Hillier. Journal of Petroleum Geology, vol. 23(3), July 2000, pp. 253-272. PDF file Accessed 27 March 2006.
- "Post-drilling analysis of the North Falkland Basin– part 2: petroleum system and future prospects" by Phil Richards and B. V. Hillier. Journal of Petroleum Geology, vol. 23(3), July 2000, pp. 273-292.
- "Overview of the petroleum geology, oil exploration and associated environmental protection around the Falkland Islands", by Phil Richards. Published in Wiley InterScience. May 2000.
- "Drilling results from the North Falkland Basin", by Phil Richards. Offshore, Abril 2000, pp. 35-38.
- "Falkland Islands Government: Offshore Oil Exploration", publication of the Falkland Islands Department of Mineral Research. November 2000.
[edit] Bibliography
- The Falkland Islands: a key element in Gondwana palaeography. J. E. A. Marshall. Tectonics, vol. 13, p. 499-514. (1994)
- Geology of the Falkland Islands. R. Clark, E. J. Edwards, S. Luxton, T. Shipp y P. Wilson. Geology Today, vol. 11, p. 217-223. (1995)
- Stone runs in the Falkland Islands. M. Rosenbaum. Geology Today, vol. 12, p. 151-154. (1996)
- The Geology of the Falkland Islands. D. T. Aldiss y E. J. Edwards. British Geological Survey Technical Report, WC/99/10. (1999)
- Reconstruction and break-out model for the Falkland Islands within Gondwana. B. C. Storey y otros. Journal of African Earth Sciences, vol. 29, p. 153-163. (1999)
[edit] Sources
- The Falkland Islands, reading the rocks –a geological travelogue (2000), British Geological Survey
- The Falkland Islands, stone runs –rocks in the landscape (2001), British Geological Survey
