Geology of the Falkland Islands

The geology of the Falkland Islands has its origins in events of more than 400 million years bp, before the Falkland Islands existed as separate entities, with the appearance of intruded 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 fragment containing the islands separated from the southeastern part of Africa on a section that would become Antarctica and later undergo a rotation of almost 180°. The interior of Gondwana was sprinkled with crystalline rocks more than a billion years old, today found in the Cape Meredith (Spanish, "Cabo de Belgrano") complex. Sediments of sand and mud filled and eventually covered the developing continental rifts. 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 can be identified in places as far apart as South Africa, western Antarctica, and Brazil. In the Falkland Islands these sequences are known as the West Falkland Group (Spanish "el grupo de Gran Malvina").

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 resulting solidified sheets can now be seen in the form of dikes that cut the oldest sedimentary layers, those which lie principally in the southern part of East Falkland (Spanish "La Isla de Soledad") and in South Africa.

Tectonic forces continued to form the region: a mountainous chain formed, part of which now creates Wickham Heights on East Falkland Island. A basin developed and was filled with land-based, or terrigenous, sediments. These layers of sand and mud filled the basin as it sank and as they hardened they produced the rocks of the sedimentary Lafonia Group of the Falklands, which has rocks similar to those in southern Africa's Karoo basin.

About 290 million years ago, in the Carboniferous period, 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 Fitzroy Tillite Formation in the Falklands. Identical rocks are found in southern Africa.

Geological structure of the Falkland Islands

The oldest rocks in the Falklands are gneiss and granite in the Cape Meredith 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 Cape Meredith complex corresponds to the crystalline rocks that made up the interior of the Gondwana supercontinent. This type of rock also has a great geological similarity to rocks currently found in South Africa and in Queen Maud Land in Antarctica. On top of the gneiss and granite lie layers of quartzite, sandstone, and shales or mudstone in West Falkland. Cross-bedding and ripple marks identify the zone where these rocks were deposited as the shallow waters of a delta environment where currents transported submarine sediments. 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 block of sandstone sediments 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.

Formation of basalt dikes

The plain of Lafonia is made of the arenaceous, or sandy, sediments of the Lafonia Group. Depressions in the sediments formed where they were cut by vertical basalt dikes. The dike is revealed by the erosion of the less resistant rock matrix; this can be observed on Lively Island (Spanish "Isla de Bougainville"). In West Falkland there are several dikes that cut the rocks of the Great Malvina group, but these dykes, 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 dyke.

Folding of West Falkland

Most of the layers of West Falkland 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 Falkland Sound have experienced tectonic forces of uplift and folding by which the quartzite beds of Argentine Port are inclined to the vertical.

Folding of East Falkland Island

Where East Falkland 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. The very soft rocks near the Fox Bay Formation are highly susceptible to erosion. The hardest quartzites are more resistant 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 westward to Wickham Heights.

The effects of glaciation

The effects of the ice age erosion that occurred in the Pleistocene glaciation 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 the strong wind currents characteristic of the region carry sand grains up to heights of a metre above ground level; the resulting sand-blasting causes rocks and pillar-shaped rock structures to show the most erosion in their lowest exposed levels, at one metre and below. This unique pattern of erosion is evident in the higher elevations of West Falkland where quartzites of the Port Stephens Formation are exposed at the surface. During the last glaciation, snow that accumulated year after year formed glaciers in some elevated leeward zones. These glaciers modified the landscape on the eastern mountain slopes that were protected from the westerly winds. The reason for glacier accumulation on the eastern slopes could be that the climate could have been very dry, or that the prevailing wind prevented glaciers from forming in places other than eastern slopes.

Another effect of glaciation can be seen on East Falkland in the basins called glacial cirques that were created on Mount Usborne (Spanish "Alberdi"). On West Falkland glacial cirques are on Mount Adam (Spanish "Independicia") 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. They are found principally in the Port Stanley Beds 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.

Economic geology

Petroleum exploration and reserves

The petroleum survey area of the Falklands Islands is in the sea to the north of the islands, covers 400,000km², and contains several sedimentary basins from the Meozoic period. After conducting seismic reflection studies and three-dimensional exploration, six test wells were drilled. Five of the six wells produced samples of petroleum. However none produced indications of commercial amounts.

According to British Geological Survey studies led by the geologist Phil Richards, the petroleum occurs at 2,700m below sea level and a maximum generation would be found beginning at 3,000m below sea level. The main petroleum source rocks have still not been penetrated because they are located at a depth deeper than 3,000m. It is considered probable that more than 60 billion petroleum barrels (10 km³) have been generated, that is, produced by natural forces, in deposits in the North Falkland Basin. These figures are based on the studies of pyrolysis obtained from wells and assume the existence of a mature rock interval of 400m thickness and covering an area 40km by 40km.

However, even with more conservative figures for the thickness and surface area of the source rock, the rich potential generator of petroleum, it is calculated that significant quantities can be extracted. For example, in an area of mature rock 35 km by 12 km and 200 m thick more than 11.5 thousand million barrels of petroleum could exist, including the production of 8 kg of hydrocarbon per ton.

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 for oil 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 second only to the Junggar River basin in petroleum potential.

Although the wells indicate a potentially productive source rock, they also indicate that many of the target reservoirs are composed of volcaniclastic rocks with low porosity due to secondary mineralisation (Richardson and 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.

Tectonic-Stratigraphic Structure of the North Falkland River Basin: Post-Exploratory-Well Analysis

The sedimentary material that fills the north part of North Falkland River basin is divided in eight tectonic-stratigraphic geological units. A geologic unit is a volume of rock or ice of identifiable origin and age that is defined by distinctive characteristics. At present the biostratigraphic information collected is not sufficient to establish a stratigraphical sequence based on palynological and paleontological data. Also, the lithostratigraphical units are useful to describe the stratigraphy only of individual wells, but are not useful to make comparisons since there are lateral lithological variations between the units.

The North Falkland basin was where the fluvial-lacustrine (river-lake) deposition took place during the first and last stages of crest formation in the area, which created a permanent lake toward the end of the process of the subsidence or sinking of the crest. At the end of the formation of the crest a process of sedimentation occurred which formed various delta systems creating a large system of lakes.

Toward the end of the tectonic 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 began in the early Jurassic and continued in the San Jorge River basin to the northwest, which suggests that marine conditions spread northwards from the south. The process of uplift in the Paleocene was followed by a process of tectonic subsidence in addition to the marine-deltaic deposition that took place during the rest of the Cenozoic period.

History of petroleum exploration

Geological surveys of the Falklands began in the late 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 the North Sea 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, a fractured river basin of elongated shape 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 are located in considerably deeper waters.

When the Falkland Islands government granted licenses in 1996, seven companies agreed upon an exploratory drilling campaign . Six exploratory wells were drilled as 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.

The petroleum system in the North Falkland River Basin

The petroleum exploration discovered a system of source rock in the North Falkland Basin capable of generating more than 102 kg of hydrocarbons per ton of rock. Although a great portion of the thickness of the source rock is geologically immature, it is capable of generating hydrocarbons below 2,000 metres. 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 bordering fringe of the petroleum deposit ranges from 36x10⁹ m³ up to 400x10⁹ m³, depending on the interpretation of the seismic survey data. In general, it is believed that the basin could have generated up to 60 billion barrels of petroleum.

The petroleum exploration discovered that under the principal interval of source rock is a layer of sandstone approximately 100 metres thick 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.

ee also

*Drainage basin
*Depression (geology)
*Geological fold
*Geologic unit
*Lithostratigraphy
*Maturity (geology)
*Paleontology
*Palynology
*Petroleum geology
*reflection seismology
*Sequence stratigraphy
*Sedimentary basin
*Structural basin
*Stone runs

References

*Richardson, N.J. and Underhill, J.R. 2002. "Controls on the structural architecture and sedimentary character of syn-rift sequences, North Falkland Basin, South Atlantic." "Marine & Petroleum Geology", 19, 417-443
*Richards, Phil and B. V. Hillier. "Post-drilling analysis of the North Falkland Basin– part 1: tectono-stratigraphic framework", "Journal of Petroleum Geology", 23.3, July 2000, pp. 253-272. [http://www.bgs.ac.uk/falklands-oil/download/download_files/Richards_Hillier_Part1.pdf PDF file] Accessed 27 March 2006.
*Richards, Phil and B. V. Hillier. "Post-drilling analysis of the North Falkland Basin– part 2: petroleum system and future prospects" "Journal of Petroleum Geology", "'23("., July 2000, pp. 273-292.
*Richards, Phil, "Overview of the petroleum geology, oil exploration and associated environmental protection around the Falkland Islands", in [http://www.interscience.wiley.com Wiley InterScience] . May 2000.
*Richards, Phil, "Drilling results from the North Falkland Basin", "Offshore", Abril 2000, pp. 35-38.
*"Falkland Islands Government: Offshore Oil Exploration", publication of the Falkland Islands Department of Mineral Research. November 2000.

Bibliography

*Mitchell, C., Taylor, G.K., Cox, K.G. & Shaw J. 1986. "The Falklands- A rotated microplate?" "Nature", 319, pp 131-134.
*Taylor, G.K. & Shaw, J. 1989. "The Falkland Islands: New paleomagnetic data and their origin as a displaced terrane from Southern Africa." In: "Deep structure and past kinematics of accreted terranes", ed. J.W. Hillhouse, (AGU Monograph 50), 59-72.
*Marshall, J. E. A. "The Falkland Islands: a key element in Gondwana palaeography." "Tectonics", 13 (1994) p. 499-514.
*Clark, R., E. J. Edwards, S. Luxton, T. Shipp and P. Wilson. "Geology of the Falkland Islands." "Geology Today", "11"' (1995) 217-223.
*Rosenbaum, M. "Stone runs in the Falkland Islands" "Geology Today", 12, p. 151-154. (1996)
*Aldiss, D. T. and E. J. Edwards. "The Geology of the Falkland Islands" "British Geological Survey Technical Report", WC/99/10. (1999)
*Storey, B. C. et al. "Reconstruction and break-out model for the Falkland Islands within Gondwana." "Journal of African Earth Sciences", 29 (1999) p. 153-163.

ources

*"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

External links

* [http://www.bgs.ac.uk/ British Geological Survey]
* [http://www.bgs.ac.uk/falklands-oil/images/onshore_images/mapandkey.jpggeological map]