The Labrador Sea is a small oceanic basin that developed when the North American and Greenland plates separated. An initial period of stretching in Early Cretaceous time formed sedimentary basins now preserved under the continental shelves and around the margins of the oceanic crust. The basins subsided thermally during Late Cretaceous time and a second episode of tectonism took place during latest Cretaceous and early Paleocene time, before the onset of sea-floor spreading in mid-Paleocene time. Around the northern Labrador Sea, Davis Strait and in southern Baffin Bay, voluminous picrites and basalts were erupted at and shortly after the commencement of sea-floor spreading. Volcanism occurred again in early Eocene time at the same time as sea-floor spreading commenced in the northern North Atlantic. Farther southeast, along the Labrador and southern West Greenland margins, oceanic crust is separated from continental crust by highly stretched but non-magmatic transition zones which developed before sea-floor spreading. A complex transform zone, which developed during sea-floor spreading in late Paleocene and early Eocene time, separates continental and oceanic crust along the Baffin Island margin. The Greenland and Labrador ocean-continent transitions are asymmetric across the only available conjugate cross-sections. However, a cross-section through the Labrador margin farther north resembles the Greenland cross-section in the conjugate pair more than it does the Labrador cross-section of this pair. Consideration of the geological history of the area suggests that the non-magmatic transition zones may have formed by slow extension of a few millimetres per year through a period of 53 Ma during Cretaceous and early Paleocene time.

NOTE: This monograph was published in a former series of GEUS Bulletin. Please use the original series name when citing this monograph, for example: Henriksen, N., Higgins, A., Kalsbeek, F., & Pulvertaft, T. C. R. (2000). Greenland from Archaean to Quaternary. Descriptive text to the Geological map of Greenland, 1:2 500 000. Geology of Greenland Survey Bulletin, 185, 2-93. https://doi.org/10.34194/ggub.v185.5197 _______________ The geological development of Greenland spans a period of nearly 4 Ga, from the earliest Archaean to the Quaternary. Greenland is the largest island in the world with a total area of 2 166 000 km2, but only c. 410 000 km2 are exposed bedrock, the remaining part being covered by an inland ice cap reaching over 3 km in thickness. The adjacent offshore areas underlain by continental crust have an area of c. 825 000 km2. Greenland is dominated by crystalline rocks of the Precambrian shield, which formed during a succession of Archaean and early Proterozoic orogenic events and which stabilised as a part of the Laurentian shield about 1600 Ma ago. The shield area can be divided into three distinct types of basement provinces: (1) Archaean rocks (3100-2600 Ma old, with local older units) almost unaffected by Proterozoic or later orogenic activity; (2) Archaean terraines reworked during the early Proterozoic around 1850 Ma ago; and (3) terraines mainly composed of juvenile early Proterozoic rocks (2000-1750 Ma old). Subsequent geological developments mainly took place along the margins of the shield. During the later Proterozoic and throughout the Phanerozoic major sedimentary basins formed, notably in North and North-East Greenland, and in places accumulated sedimentary successions which reached 10-15 km in thickness. Palaeozoic orogenic activity affected parts of these successions in the Ellesmerian fold belt of North Greenland and the East Greenland Caledonides; the latter also incorporates reworked Precambrian crystalline basement complexes. Late Palaeozoic and Mesozoic sedimentary basins developed along the continent-ocean margins in North, East and West Greenland and are now preserved both onshore and offshore. Their development was closely related to continental break-up with formation of rift basins. Initial rifting in East Greenland in latest Devonian to earliest Carboniferous time and succeeding phases culminated with the opening of the North Atlantic in the late Paleocene. Sea-floor spreading was accompanied by extrusion of Tertiary plateau basalts in both central West and central and southern East Greenland. During the Quaternary Greenland was almost completely covered by ice sheets, and the present Inland Ice is a relic of the Pleistocene ice ages. Vast amounts of glacially eroded detritus were deposited on the continental shelves offshore Greenland. Mineral exploitation in Greenland has so far mainly been limited to one cryolite mine, two lead-zinc deposits and one coal deposit. Current prospecting activities in Greenland are concentrated on the gold, diamond and lead-zinc potential. The hydrocarbon potential is confined to the major Phanerozoic sedimentary basins, notably the large basins offshore East and West Greenland. While proven reserves of oil or gas have yet to be found, geophysical data combined with extrapolations from onshore studies have revealed a considerable potential for offshore oil and gas. The description of the map has been prepared with the needs of the professional geologist in mind; it requires a knowledge of geological principles but not previous knowledge of Greenland geology. Throughout the text reference is made to the key numbers in the map legend indicated in square brackets [ ] representing geological units (see Legend explanation, p. 79), while a Place names register (p. 83) and an Index (p. 87) include place names, geological topics, stratigraphic terms and units found in the legend. The extensive reference list is intended as a key to the most relevant information sources.

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The development of the continental margin of West Greenland is closely related to the processes that led to the opening of the Labrador Sea. The opening of the Labrador Sea began in the Early Paleocene (anomaly 27N), and not in the Late Cretaceous as previously supposed. Modelling of magnetic data and new interpretation of seismic data indicate that a large area previously regarded as underlain by oceanic crust is in fact underlain by block-faulted continental crust overlain by syn- and post-rift sedimentary sequences. The ocean–continent transition is now placed 100–150 km southwest of the foot of the continental slope instead of at the foot of this slope. Rifting in the Labrador Sea area began, however, in the Early Cretaceous. The earliest sediments are the syn-rift lower and upper members of the Bjarni Formation on the Labrador shelf and their likely equivalents, the pre- to syn-rift Kitsissut and Appat sequences on the Greenland margin. The age of these units is Barremian (or older) to Albian. The units are overlain by widespread mudstone-dominated units, the Markland Formation of the Labrador shelf and the Kangeq Sequence on the Greenland margin. The former is Cenomanian–Danian in age. By analogy the base of the Kangeq Sequence is probably Cenomanian (or Turonian), while the top is known from well ties to be at the Cretaceous–Tertiary boundary. Rifting was subdued during deposition of these mudstone units. Rifting was renewed in the Early Paleocene, and mudstones, siltstones and very fine sandstones were deposited. With the initiation of sea-floor spreading there was considerable igneous activity at the ocean–continent transition, as well as in the onshore area where picrites followed by plagioclase-porphyritic basalts were erupted. After the end of the Paleocene there was little rifting in the region, but compressional structures were formed locally as a response to transpression related to strike-slip movements that transferred plate motion from the Labrador Sea to Baffin Bay. A marked Early Oligocene unconformity separates the syn-drift Paleocene–Eocene succession from the post-drift middle Oligocene–Quaternary sediments. Sediments deposited since the Paleocene are dominated by sands. The main hydrocarbon play types offshore West Greenland are related to tilted fault blocks. Source rocks are anticipated near the base of the Kangeq Sequence, which is also the seal, and reservoirs are sandstones in the Appat and Kitsissut sequences. These two sequences were not reached by any of the exploration wells drilled in the 1970s.

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