Peter R. Dawes
The Thule map sheet, printed in 1991, displays Precambrian rocks overlain by Quaternary deposits and expanses of ice. Two bedrock provinces are preserved: the Archaean-Palaeoproterozoic crystalline shield and the intracratonic, unmetamorphosed, Mesoproterozoic-?Neoproterozoic Thule Basin, both of which extend across Baffin Bay into Canada.
These explanatory notes systematically describe the map units. The notes are introduced by information on the physical environment and geoscientific research, with one section highlighting data obtained in the 15 years since the map sheet was printed. More than 200 papers are cited.
Four metallic commodities are shown on the map: magnetite in shield lithologies, iron sulphides and copper in Thule Basin strata, and ilmenite - derived from Neoproterozoic basic intrusions - in active and uplifted beaches. Other raw materials are reviewed, including agate and soapstone shown on the map.
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Abstract Abstract Dawes, P.R. 2006: Explanatory notes to the Geological map of Greenland, 1:500 000, Thule, Sheet 5. Geological Survey of Denmark and Greenland Map Series 2, 97 pp. + map
These explanatory notes cover part of North-West Greenland defined by latitudes 75°15'N and 78°N and longitudes 57°W and 73°W, a region with appreciable ice cover. The bedrock is dominated by two Precambrian provinces that extend across Baffin Bay into Canada: the highgrade Archaean-Palaeoproterozoic shield overlain by the intracratonic Mesoproterozoic-?Neoproterozoic Thule Basin. Map units are systematically described and introductory sections cover the physical environment, logistics, data sources and geoscientific research.
The crystalline shield
embraces seven complexes: three of Archaean age, two of Archaean-?Palaeoproterozoic age and two of Palaeoproterozoic age. Isotopic ages of c.
2900 Ma indicate that Neoarchaean orthogneisses are present in part of the region while the existence of Mesoarchaean crust is indicated by c.
3200 Ma detrital zircons. The high-grade orthogneisses and paragneisses of the Thule mixed-gneiss complex
were intruded by two plutonic suites, the Kap York meta-igneous complex
2700 Ma and the Smithson Bjerge magmatic association
that includes a major anorthosite body. Subsequent deformation, metamorphism and migmatisation led to the formation of gneisses recognised within the Melville Bugt orthogneiss complex
. Palaeoproterozoic sedimentation and volcanism represented by the Prudhoe Land supracrustal complex
took place after c
. 2250 Ma but had ceased by c.
1985 Ma when the Prudhoe Land granulite complex
was emplaced. Rocks within the Lauge Koch Kyst supracrustal complex
may correlate with the Palaeoproterozoic Karrat Group of West Greenland. Polyphase deformation with isoclinal folding, and regional metamorphism up to granulite-facies grade, affected the region c.
1900 Ma ago, with cooling until c
. 1650 Ma.
Extensional faulting, intracratonic basin formation and periods of basaltic magmatism occurred during the last 1000 million years of Proterozoic time. After regional dyking at c
. 1630 Ma ( Melville Bugt dyke swarm
) followed by mature peneplanation, the Thule Basin
developed as an interior fracture and sag depocentre across the area that is now northernmost Baffin Bay. Defined by the unmetmorphosed Thule Supergroup at least 6 km thick, the basin records fluvial to shallow-marine sedimentation and tholeiitic volcanism at least 1270 million years old. The basin is dissected by the Thule half-graben system
dominated by WNW-ESE-trending faults thought to have developed during the final tectono-magmatic period dated at c.
700650 Ma. Conspicuous products of this are a major sill complex ( Steensby Land sill complex
) and a regional dyke swarm that parallels the half-grabens ( Thule dyke swarm
). Fault reactivation is probably related to the late Phanerozoic tectonic evolution of Baffin Bay.
In addition to the four metallic commodities included on the map - magnetite, copper, iron suphides, ilmenite - there is potential for gold and other mineralisations. The Neoarchaean magnetite province, traceable for over 400 km through the map region, is spatially the largest in Greenland and it is a correlative of the Mary River iron deposits of Baffin Island, Canada. Several raw materials have potential for local handicraft industries, including soapstone and agate.
The region hosts a multi-event glacial and marine Middle-Late Quaternary stratigraphy dating back to at least the Saalian/Illinoian (pre-130 ka B.P.). The entire region was probably overriden by the Inland Ice during the Weichselian/Wisconsinian glacial maximum and deglaciated in the early Holocene, 11 000 to 9000 years ago. Author's address:
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
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