The geology of the North Sea describes the geological features such as channels, trenches, and ridges today and the geological history, plate tectonics, and geological events that created them.
The basement of the North Sea was formed in an intraplate setting during the Precambrian. Rigid blocks were overlaid with various depositions, sands and salts. These rigid blocks were transformed to a metamorphic base due to tectonic processes such as continental collisions which cause horizontal pressure, friction and distortion in the Caledonian plate cycle as well as the Variscan plate cycle. The blocks were also subjected to metamorphic evolution during the Triassic and Jurassic periods when the rock was heated up by the intrusion of hot molten rock called magma from the Earth's interior.
The Caledonian (Iapetus) plate cycle saw the formation of the Iapetus suture during the Caledonian orogeny. The Iapetus suture was a major weakness creating a volcanic fault in the central North Sea during the later Jurassic period. The Iapetus ocean was replaced with a suture line and mountain range when Laurentia, Baltica and Avalonia continents collided. This collision formed Laurussia.
The Variscan (Rheic) plate cycle resulted in the formation of Pangaea when Gondwana and Laurussia collided. The elimination of the Rheic Ocean caused the formation of a massive mountain range through the border countries of the present day North Sea.
Triassic and Jurassic volcanic rifting and graben fault systems created highs and lows in the North Sea area. This was followed by late Mesozoic and Cenozoic subsidence creating the intracratonic sedimentary basin of the North Sea. This era experienced higher sea levels because of sea floor spreading, cooler lithosphere temperatures. Plate tectonics and continental orogenies combined to create the continents and the North Sea as we know them today. The final events affecting the North Sea coastline features and submarine topography occurred in the Cenozoic era.
Tectonic structure The Mesozoic structures underneath the North Sea can be seen as a failed rift system. After initial crustal extension and the formation of rift basins during the Triassic and Jurassic periods, the extension concentrated on the other side of the British Isles, which would create the northern Atlantic Ocean. The rift basins even saw some inversion during the late Cretaceous and Eocene epochs. From the Oligocene onward, tensions in the European crust caused by the Alpine orogeny to the south cause a new, more modest phase of extension. Some grabens in the area are still active.
The subsurface of the North Sea area is dominated by grabens: the north-west south-east oriented Lower Rhine Graben under the southern North Sea and the Netherlands, the north–south-oriented North Sea Central Graben that begins north of the Dutch coast and ends in the region east of Scotland, and the Viking Graben along the south-east Norwegian coast. The Horn Graben is a smaller graben east of the Central Graben and in front of the Danish coast. Another smaller structure is known as the Terschelling Graben, which borders the Central Graben in the west, just north of the Netherlands.Geology of the North Sea from Petroleum Geology A larger graben is found in the subsurface below the Skagerrak, this north–south structure is called the Bamble-Oslo Graben. Map on page 303 The Viking Graben is separated from the Faeroe Shetland Basin below the Atlantic by the Shetland Platform, the two structures join in the area north-east of the Shetland Islands.
Geologic history
Precambrian The area we now call the North Sea was interspersed amid several separate continental areas (Siberia, Baltica, Gondwana, and Laurentia) as well as the Iapetus Ocean and Tornquist Sea. Plate tectonics and continental collisions (orogenies) brought the features together of the North Sea that we recognise presently.
Paleozoic orogenies
Finnmarkian, Athollian, Caledonian orogenies
The Finnmarkian orogeny affected northern Norway in the early Palaeozoic era. In the late Ordovician period, Avalonia collided with Baltica creating the Tornquist Margin. This corresponds to the suture of the north German-Polish Caledonides. By the end of the Ordovician the Tornquist Sea had been eliminated. The Tornquist-Tesseyre fault system also had its beginnings in the orogeny closing the Tornquist Sea. Baltica subducted below Avalonia.
The Atthollian Orogeny was a collision between the Scottish Highlands and a Midland Valley area which very likely created the Highland Boundary Fault.
During the late Silurian period Baltica then collided with Laurentia which is named the Scandian phase of the Caledonian Orogeny. Baltica was subducted below Laurentia forming the Caledonian Belt. The Caledonian Orogeny caused the closure of the Iapetus Ocean when the continents and terranes of Laurentia, Baltica and Avalonia collided. The combined mass of the three continents formed a "new" continent: Laurussia or Euramerica.See for paleogeographic reconstructions of the collisions for example Stanley (1999), p. 386; Ziegler (1990), pp. 17–19 The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly 490–390 Ma. The basement formations of the North Sea were roughly formed during this Caledonian geosynclinal stage of the Cambrian – Devonian periods. The Caledonian mountains range helped create the 'Old Red Sandstone' continent at the joining of the tectonic plates. The Rheic Ocean began to open to the south.
Acadian Orogeny
The Acadian orogeny occurred in the Lower Devonian period (Paleozoic era), about 500 Ma. The continents of Laurentia and Baltica collided. The Iapetus Ocean disappeared under this continental collision. These plate tectonic forces built up a mountain range extending from what is now termed the Adirondacks north through Nova Scotia, the British Isles, Norway and the eastern margin of Greenland. Greenland and Scandinavia were situated at the amid the equator and mid tropics (20 to 20 degrees north). The area now comprising Greenland-Scandinavia-British Isles began drifting northward in the Late Carboniferous-Early Permian period. A high amount of volatile magmatism caused dikes parallel to the Tornquist Margin which were formative areas of the North Sea – British Isles -Germany – southwest Sweden area.
Variscan Orogeny
The Rheic Ocean or Proto-Tethys Ocean was eliminated during the Hercynian/Variscan orogeny, and the Pangaea supercontinent formed. The continents of Gondwana and Laurussia collided, both were drifting north, however Gondwana was drifting at a faster rate. The mountain range which was created extended along Spain, Brittany, central Germany and into Poland. The closure of the Rheic Ocean in the late Carboniferous formed thrusts in southern England.
The Pangaea supercontinent was added to in the late Permian period when Siberia collided with Baltica and Kazakhstan. Pangaea was surrounded by massive subduction plate tectonics. The North Sea area was central to the continental Pangaea supercontinent. In this period the area now named the North Sea was in the subtropics and was a non-marine arid environment.
Permo–Triassic rifting and thermal subsidence
During the Triassic period, the Viking and Central Graben volcanic systems were formed. Upward thrusting rifts were formed during the Triassic comprising the taphrogenic stage. The Atlantic rift zone is associated with the North Sea rifts zone. The Triassic saw active crustal stretching via the rifting process, lower sea levels and volcanic activity.
Jurassic As the Jurassic (200–145 Ma) started, Pangaea began to break up into two continents, Gondwana and Laurasia. At this time sea levels rose. The burial of algae and bacteria below the mud of the sea floor during this time resulted in the formation of North Sea oil and natural gas, much of it trapped in overlying sandstone by deposits formed as the seas fell to form the swamps and salty lakes and lagoons that were home to dinosaurs.McKirdy et al. (2007) op cit page 146.Gillen, Con (2003) op cit Pages 133–7.
Middle Jurassic domal uplift Volcanism and a rift system developed in the central North Sea area where basaltic lavas were extruded. The mantle warped upwards creating a dome in the middle of the North Sea where the Iapetus Suture intersected the Tornquist-Teisseyre fault system. The dome area was amidst the Viking Graben, Central Graben and Moray Firth Basin. The Scotland rifting and the extrusive centre of the Forties were associated with the uplifted area. The Long Forties and Horda Basin were also volcanic centres. During this time the sea level fell. Major reservoirs were created in the North Sea during this time as clastics and sands were deposited in paralic environments.
Cretaceous period sea floor spreading During the cretaceous the main rift expansion was in an east and west direction. There are two basins the large South Permian Basin, and the smaller Northern Permian basin, which are east–west trending. The basins are separated by the Ringkøbing-Fyn system of highs. The Viking Graben lies between the Shetland Platform and the Fenno-Scandian high. The Central Graben lies amid the Permian basins and their high. The formation of these has been influenced by the zig zag suture line of the Caledonian orogeny with its areas of crustal tension weakness. Sea floor expansion continues, and sea levels rise. The sea level is to higher than present day levels.
In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the Chalk Group, which forms the white cliffs of Dover on the south coast of England and similar cliffs on the French Normandian coast. The group is found in England, northern France, the Low Countries, northern Germany, Denmark and in the subsurface of the southern part of the North Sea. Stagnation of deep sea currents in middle Cretaceous times caused anoxic circumstances in the sea water. In many places around the world, dark anoxic shales were formed during this interval.See Stanley (1999), pp. 481–482 These shales are an important source rock for oil and gas, for example in the subsurface of the North Sea. Across the north central and northern North Sea, the Chalk Group is a major seal unit, overlying a number of blocks of reservoir rocks and preventing their fluid contents from migrating upwards.
The Silverpit crater, a diameter suspected impact crater in the North Sea The age of the feature is constrained between 74 – 45 million years (Late Cretaceous – Eocene). The Sole Pit Basin was created by deep burial of depocentres.
Cenozoic era intracratonic basin formation
Geologically, the Cenozoic is the era when the continents moved into their current positions. The Alpine Orogeny, the spreading of the Mid Atlantic ridge, and the creation of the Atlantic Ocean basin occurred in the Cenozoic era. The Iceland hot spot and North Atlantic rifting helped to exhume the British Isles. In the early Palaeogene period (Caenozoic Era) between 63 and 52 Ma, the North Sea formed, and Britain was uplifted. Some of this uplift was along old lines of weakness from the Caledonian and Variscan Orogenies long before.
The post rift phase followed late Jurassic rift events during the late Mesozoic and Cenozoic thermal subsidence. As the rifting stopped, then regional subsidence occurred creating an intracratonic sedimentary basin. Subsidence occurred due to lithosphere cooling. Some of this uplift was along old lines of weakness from the Caledonian and Variscan Orogenies long before. Intraplate compression was caused when the Atlantic Ocean basin formed. the Mid-Atlantic spreading ridge has been busy separating east from west. Greenland separated from North America and the rifting altered direction during the Paleogene, which caused Northern Europe to separate from Greenland. During the Eocene period, the last land bridge across the Atlantic sank.