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Offshore geology

 
Continental margin off East,North and West Greenland, Offshore geology
Interpretation of the offshore geology around Greenland is based mainly on seismic surveys, supplemented by aeromagnetic and gravimetric data and, in the case of southern West Greenland, by data from five exploration wells drilled in 1976–77. Offshore South-East Greenland six holes have recently been drilled (three are shown on the map) in connection with the Ocean Drilling Project.
The map is designed to show two general aspects of offshore geology: (1) the extent of continental crust [a], oceanic crust [c–g] and of the intervening, poorly understood, transition zone [b], and (2) the distribution of sedimentary basins and major faults. Where extensive volcanic units are known to occur in areas underlain by continental crust, their distribution is also shown on the map.

The continental margin off East and North Greenland
The continental margin off East Greenland between 60o and 76oN can in general terms be described as a volcanic rifted margin. The position of the continent–ocean transition was drawn on the basis of aeromagnetic data supplemented by characteristic features in the NAD reflection seismic data. The absolute seawards (eastern) limit of continental crust cannot overlap areas where linear magnetic anomalies characteristic of oceanic crust can be identified.
The zone off East Greenland shown on the map as underlain by transitional crust [b] was drawn in a rather arbitrary manner, at least with regards to its width. This zone is thought to consist of continental crust with increasing numbers of dykes and other intrusions as oceanic crust is approached.

The north-east margin of continental Greenland, north of 78oN, has a very different character. It is shown as a former intracontinental transform plate boundary. According to current interpretations of the history of the opening of the Greenland – Norwegian Sea and Arctic Ocean a substantial dextral lateral displacement of Svalbard relative to North-East Greenland took place in the time interval corresponding to magnetochrons 24R–13 (earliest Eocene – earliest Oligocene), prior to the opening of the Fram Strait between Greenland and Svalbard.

The continental margin off West Greenland
The continental margin off West Greenland is complex and the interpretation is a subject of discussions. The transition zone between typical oceanic and normal continental crust is certainly much wider off southern West Greenland than it is thought to be anywhere off East and South-East Greenland. The seawards limit of normal continental crust off southern West Greenland lies well to the south-west of the continental slope, at water depths of more than 1500 m. This interpretation of the distribution of crustal types is supported by the structural pattern seen in the seismic lines. Both the normal continental crust and the zone of transitional crust show large tilted fault blocks overlain by syn- and post-rift sediments. The oldest sediments are most likely of Early Cretaceous age.
The crust under Davis Stræde (Davis Strait) is estimated to be 22 km thick, which is intermediate between the thickness of normal oceanic and continental crust.
The crust is therefore interpreted by the Survey as being formed of thinned continental crust, in accordance with the interpretation of the distribution of crustal types farther south. The nature of the crust underlying Baffin Bugt is not obvious. There are no distinct sea-floor spreading magnetic anomalies in this region.

Offshore sedimentary basins
Reflection seismic surveys have shown that large sedimentary basins occur offshore East Greenland between latitudes 67o–72oN and 75o–77oN, and offshore West Greenland between 63o–68oN and 73o–77oN. In the intervening areas off both East and West Greenland there are extensive Lower Tertiary basalts below which there are expected to be thick sedimentary successions, but these cannot be resolved in the seismic data.

North-East Greenland shelf (72°–80°N)
The existence of thick sedimentary successions on the North-East Greenland shelf was first suggested on the basis of interpretation of aeromagnetic data. Judging from the known geology of the Barents Sea, the Norwegian shelf and onshore North-East Greenland, the age of these sediments is likely to be Devonian to Recent, with unconformities in the middle Permian and in the Cretaceous.
On the shelf between latitudes 72o15' and 75o30'N extensive volcanic rocks of presumed Early Tertiary age have been interpreted from the aeromagnetic and seismic data. In the near-shore area these are exposed at the seabed; eastwards they become increasingly deeply buried under younger sediments. It is considered almost certain that the pre-Tertiary sediments interpreted to the north and south of this area continue beneath the volcanic rocks.

Liverpool Land Basin, East Greenland (70°–72°N
A very thick succession of sediments can be recognised in the seismic data offshore Liverpool Land. The sediments are particularly thick within the part of the area underlain by continental crust, where the base of the sediments cannot be identified on the existing data. The upper part of the sedimentary succession is a virtually complete Tertiary succession up to 6 km thick; this formed a large prograding wedge that spread out across both continental and oceanic crust from the mouth of present-day Scoresby Sund. In the part of the area underlain by continental crust the Tertiary succession lies with angular unconformity on block-faulted and tilted sediments of pre-Tertiary (Late Palaeozoic – Mesozoic) age, while where the Tertiary sediments have prograded into the area underlain by oceanic crust, the subsurface consists of subaerial lavas seen as seaward-dipping reflectors in the seismic data.

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E-W cross-section across the transition from continental crust to oceanic crust and the overlying Liverpool Land Basin at c.71°N, East Greenland. Vertical exaggeration x 3.8.
Blosseville Kyst Basin, East Greenland (67°–70°N)
More than 4 km of post-middle Eocene sediments occupy an elongate, coast-parallel sedimentary basin off the Blosseville Kyst. The sediments lie entirely on a subsurface of Lower Tertiary basalts. In the area underlain by continental crust there are almost certainly Mesozoic and Paleocene sediments beneath the basalts, as there are onshore and farther to the south. However, it is not possible to interpret the geology underlying the basalts on the basis of existing seismic data.

Southern West Greenland (60°–68°N)
The earliest sediments offshore southern West Greenland are pre- and syn-rift sequences up to 3 km or more in thickness, the Kitsissut and Appat sequences; by analogy with the better known Labrador Shelf, these are believed to be Early Cretaceous (Barremian–Albian) in age.

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Cross-setion of offshore southern West Greenland at c. 64o30´N, west of Nuuk/Godthåb, based on interpreted and depth-converted seismic data. Vertical exaggeration x 5. Lup

These successions are overlain by a widespread Upper Cretaceous mudstone sequence, the Kangeq sequence, the upper part of which was penetrat in the Ikermiut–1 well (c. 670N). A major hiatus spanning the interval Campanian – early Paleocene probably reflects the same episode(s) of faulting, uplift and erosion as are recorded in the succession in the Nuussuaq Basin. Following these disturbances fan sands intercalated with mudstones were deposited. Deposition of mudstones continued into the early Eocene, but from the middle Eocene sedimentation was dominated by coarser clastic sediments deposited in simple prograding sequences.

Central West Greenland (68°–73°N)
The Lower Tertiary basalts exposed onshore in the Disko – Nuussuaq – Svartenhuk Halvø area continue offshore where they have been mapped from seismic and magnetic data over the entire area between latitudes 68o and 73oN. In the eastern part of this area the basalts are exposed at the seabed and have been sampled by dredging, but to the west they become increasingly buried under a cover of Eocene and younger sediments. Below the basalts underlaying sediments may be as much as 5 km in thickness.

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Cross-section of offshore central West Greenland at c. 69o30'N, west of Disko.Depth from depth-connerted seismic data. Vertical exaggeration x 7.
North-West Greenland (73°–78°N)
North of 73oN the new seismic data acquired as part of the KANUMAS project have confirmed the existence of a very deep graben or half-graben in the west and south-west part of Melville Bugt. This had earlier been outlined from aeromagnetic and gravity data acquired in the late 1960s and early 1970s. The new data have also revealed several other graben and half-graben structures extending to the northern limit of the survey at 76o30'N. In the Melville Bugt graben the thickness of sediments exceeds 13 km.
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Representative cross-section compiled from reflection data from offshore North-West Greenland at c. 75oN, south-west of Melville Bugt / Qimusseriarsuaq. Vertical exaggeration x 2.5. Lup

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Offshore geology