The North Sea basin is located in northern Europe and lies between the United Kingdom, and Norway just north of The Netherlands and can be divided into many sub-basins. The Southern North Sea basin is the largest gas producing basin in the UK continental shelf, with production coming from the lower Permian sandstones which are sealed by the upper Zechstein salt. The evolution of the North Sea basin occurred through multiple stages throughout the geologic timeline. First the creation of the Sub-Cambrian peneplain, followed by the Caledonian Orogeny in the late Silurian and early Devonian. Rift phases occurred in the late Paleozoic and early Mesozoic which allowed the opening of the northeastern Atlantic. Differential uplift occurred in the late Paleogene and Neogene. The geology of the Southern North Sea basin has a complex history of basinal subsidence that had occurred in the Paleozoic, Mesozoic, and Cenozoic. Uplift events occurred which were then followed by crustal extension which allowed rocks to become folded and faulted late in the Paleozoic. Tectonic movements allowed for halokinesis to occur with more uplift in the Mesozoic followed by a major phase of inversion occurred in the Cenozoic affecting many basins in northwestern Europe. The overall saucer-shaped geometry of the southern North Sea Basin indicates that the major faults have not been actively controlling sediment distribution.
Geological history
Paleozoic era Two major orogenic events occurred in this era, the Caledonian Orogeny and the Variscan Orogeny, allowing a complex geologic history to begin. During the late Silurian and early Devonian the Caledonian Orogeny occurred with episodes of uplift and erosion leaving unconformities. The Caledonian event occurred due to the collision of three land masses – Laurentia, Baltica, and Avalonia – which would eventually lead to the creation of Pangea. This collision allowed for a mountain belt to form NW–SE in the northern portion of the current basin, and in the south extending SW–NE. Following the Caledonian Orogeny approximately 380 Ma the Variscan Orogeny started and ended near the Permian. During this time period the orogeny caused Carboniferous rocks to become folded and faulted. The last collision occurred in the late Carboniferous where two supercontinents collided leading to the Varsican mountain range, Laurasia and Gondwanaland. Late Permian deposition of evaporites created the Zechstein supergroup which act as a salt cap for the fine grained sediment.
Mesozoic era During this era the end of extensional tectonics had been well constrained in the southern North Sea basin; the extension occurred from the late Carboniferous to the Triassic. There had been some reactivation of Varsican basement faults due to the subsidence of the Sole Pit Basin and allowing basin tilts creating a peripheral graben system around the basin. Due to the reactivation of the basement faults it led to the beginning of halokinesis in the basin. The halokinesis allowed major uplift during the Mesozoic because of the presence of salt and the reactivation of basement faults; the thrusting permitted the sediment to thrust over the diapirs and float on top of the Zechstein salt. Due to the Kimmerian phase uplift in the northern portion of the North Sea, it allowed subsidence and deposition to fill the basin, creating sandstone. Due to differential loading along the faults, salt diapirs developed and played a huge role in the southern North Sea basin and all salt tectonic structures. Reverse faulting associated with late Carboniferous basin inversion is recorded by a wide range of Carboniferous stratigraphy subcropping the Permian sediments. The subcrop pattern indicates a strong influence of NW–SE tectonic trends during this inversion. This inversion event was followed by deposition of upper Carboniferous red beds, which pass up into sands of the Permian Rotliegend Group; these are overlain by evaporites of the Zechstein Supergroup. A major phase of basin inversion during or at the end of the Late Cretaceous affected many basins in northwestern Europe, including the Sole Pit Basin and the Cleveland Basin, and has been attributed to strike-slip reactivation of basement faults.
Cenozoic era During the end of the Mesozoic and into the Cenozoic era the Alpine orogeny occurred which led to reactivation of faults and structures. In the beginning of the Tertiary, inversion involving basin tilt and reactivation of basement faults transpired. The center part of the southern North Sea basin comprises the Silver Pit and Sole Pit trough and the Cleaver Bank High, which are all distinguished by a series of salt swells and walls which occurred in the Tertiary. A reversal of basin tilt during the Tertiary uplifted the thick sedimentary wedge in the Sole Pit Trough to form the Sole Pit High. Since the orogeny reactivated the Mesozoic rifts it permitted the Zechstein salts to act as a buffer or detachment layer separating two structural regimes, which can lead to traps for natural resources.
Tectonic phases
Caledonian phase During the Paleozoic there were three major landmasses that collided, Laurentia, Baltica, and Avalonia closing the Iapetus ocean. The event created a mountain chain trending North to South in the northern portion and an East to West trend in the South. The reason being that there is a North to South trend in the North is because Laurentia coming from the West and Baltica coming from the east meeting at the center to create a compressional regime. Through time eventually Avalonia coming from the south closing the Iapetus ocean, collided with the two landmasses to create a T-junction giving an East to West trend in the southern portion. This event is the first major event that would lead to the creation of Pangea. The tectonic event comprised the entire Ordovician and into the early Devonian, the Caledonian rocks are the basement of the current North Sea.
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