Review of Greenland Avtivities 2001 - Geus

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56° 54° 52° 72° 72° nT 71°30′ 71° 70°30′ 50 km 71°30′ 71° 0.335 0.223 0.163 0.122 0.092 0.068 0.051 0.037 0.027 0.019 0.014 0.010 0.006 0.004 0.001 – 0.001 – 0.003 – 0.005 – 0.007 – 0.009 – 0.011 – 0.014 – 0.016 – 0.021 – 0.027 – 0.035 – 0.049 – 0.077 – 0.126 – 0.210 – 0.402 56° 54° 52° Fig. 3. Calculated vertical gradient of the total magnetic field intensity with shaded relief for the area in central West Greenland covered by project Aeromag 2001. The location of the aeromagnetic survey in 2001 is circumscribed by a grey line and the new data have been merged with data from project Aeromag 1997. orogeny at around 1.85 Ga, and intruded by a 1.65 Ga old swarm of dolerite dykes. The Palaeoproterozoic metasediments (the Karrat Group) host the now exhausted Black Angel Pb-Zn ore bodies (13.5 mill. tonnes ore at 4.0% Pb and 12.3% Zn) which were mined from 1973–1990. Similar marblehosted lead-zinc occurrences at other localities indicate an additional potential for this type of mineralisation. The clastic facies of the Karrat Group include extensive sulphide facies iron formation and vein-type base and precious metals mineralisation, and offer a potential for shale-hosted massive sulphides and turbidite-hosted goldbearing veins and shear zones (Thomassen 1992). The Karrat Group crops out within the Rinkian Belt of central West Greenland, a region that is characterised by gneiss domes and fold nappes with sheath-like geometry and flat-lying axial surfaces that affect both Archaean basement gneisses and the Palaeoproterozoic Karrat Group. Grocott & Pulvertaft (1990) identified seven phases in the tectonic evolution of the region, of which three involve extension and four compression. The southern and south-western part of the surveyed region preserves extensive areas of Palaeogene volcanic rocks, related to the plate break-up between Greenland and Baffin Island. These volcanic rocks represent, together with similar volcanic rocks on the Canadian side of Davis Strait and on the east coast of Greenland, the results of mantle plume activity along the track of the present Iceland mantle plume (Storey et al. 1998). In the surveyed area volcanic activity started in mid-Paleocene time with deposition of the Vaigat Formation. The earliest volcanics were extruded below sea level as hyaloclastite breccias, while most later volcanism was subaerial. The Vaigat Formation is dominated by picritic and other olivine-rich tholeiitic basalts; it is between 0 and 3 km thick on Svartenhuk Halvø and more than 5 km thick on Ubekendt Ejland (Hald & Pedersen 1975). In the northern part of the basalt- 70
covered area, the Vaigat Formation was followed by deposition of about 50 m of fluvio-lacustrine sediments and hyaloclastite breccias. The succeeding volcanic rocks of the Svartenhuk Formation are characterised by plagioclase-porphyritic and aphyric basalts, and have a minimum thickness of 2.8 km. Determinations of the magnetic palaeodirection and palaeointensity from samples of the Vaigat Formation at Nuussuutaa on Nuussuaq have been presented by Riisager & Abrahamsen (1999) and Riisager et al. (1999). The lower part of the Vaigat Formation (the Anaanaa Member and part of the Naujánguit Member) has normal magnetisation (C27N), whereas the upper part of the Vaigat Formation is reversely magnetised. Palaeomagnetic data for the Maligât Formation on Nuussuaq, a stratigraphic equivalent to the Svartenhuk Formation, yielded reverse magnetisation directions; a detailed chronostratigraphy has not yet been established. The Palaeogene volcanics are characterised by units of strongly reduced lavas with native iron, indicating a potential for Norilsk-type nickelcopper-PGM in the plateau basalt areas. Magnetic anomaly maps and geological implications The geophysical data illustrated in Figs 2 and 3 are considerably simplified representations of the original data sets, but major anomalies can be discerned. Onshore, six sub-parallel linear anomalies striking NNW–SSE cross the entire survey area. Some of these coincide with the locations of mapped Mesoproterozoic mafic dykes of the Melville Bugt dyke swarm (Nielsen 1987). The dolerite dykes and thin layers of hornblende schist and amphibolite at many inland locations (Henderson & Pulvertaft 1987) are seen very clearly in the magnetic response. Elongated anomalies with variable strikes between 40–65° and with distinct minimum magnetic total field values are visible in the eastern half of Figs 2 and 3. These structures, many in areas covered by ice, appear not to have been deformed, which implies that they are younger than the Rinkian orogeny. Of the five anomalies of this type, the most prominent is a NE–SW feature with its south-western termination at Upernivik Ø. This anomaly may be an extension of the Itilli fault that crosses the westernmost part of Nuussuaq, and is marked by a sharp transition between positive and negative magnetic total field anomaly values. However, none of these anomalies have so far been correlated with any confidence with known geological structures. The amplitudes of the anomalies are in the order of –150 nT and may have a magnetisation direction opposite to the present geomagnetic field direction. The Palaeogene basalts dominate the magnetic response in the coastal region and the offshore part of the survey. Differences in polarities of the basalts are clearly defined by the low magnetic field values on Svartenhuk Halvø and the positive values west of Nuussuaq. On Ubekendt Ejland both the acid tuff/ignimbrite mapped on the western part of the island and the granite intrusion on the southern part are associated with positive magnetic anomalies. A distinct pattern of elongated, arcuate anomalies is dominated by low magnetic values. A similar anomaly pattern is not visible in the area with high magnetic field values. Comparison with mapped faults and dykes on Svartenhuk Halvø (Larsen & Pulvertaft 2000) shows an excellent correlation between the magnetic lineaments and geologically mapped structures, e.g. dykes and presumed non-exposed dykes associated with faults. However, basalt with normal magnetisation at depth in conjunction with vertical displacement along faults may also contribute to the anomalies. Topographic effects clearly contribute to the anomaly pattern, but are not sufficient to explain the anomalies. The distinct differences in anomaly pattern between areas with positive and negative magnetic anomaly values may be a result of difference in: (1) rheology due to difference in thermal state and pressure at the time of magmatic activity; (2) crustal composition; (3) volcanic source type. Firstly, the time span between the eruption of two different magma types may have been sufficiently long that the thermal state and pressure in the crust may have changed, leading to a difference in eruption style and in the amount of dyke intrusion. Secondly, the initial composition of the crust at the location of the magnetic high may have been unfavourable for the creation of faults and intrusion of dykes. Thirdly, the basalt region with reverse magnetisation may be an amalgamation of lavas from numerous eruptions with limited flow distances whereas the basalt with normal magnetisation may represent episodes with flows of a much larger extent. The magnetic data alone are not conclusive with respect to which of the three interpretations are correct. Establishment of age relationships between the two types of basalts is essential for a reliable interpretation. In particular, if the basalt of the area with high magnetic field values was extruded after the basalt causing low magnetic values, a mechanism must be found that explains why none of the elongated anomalies within the area of low magnetic intensity are truncated by basalts with normal magnetisation. 71
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GEOLOGY OF GREENLAND SURVEY BULLETI
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Geology of Greenland Survey Bulleti
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Contents Numbers of articles corres
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■ ■ ■ ■ ■ ■ ■ ■ ■
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Canada Pituffik O 1 Thule Air Base
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Publications A complete list of geo
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Canada Greenland Greenland Inland I
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northern Nagssugtoqidian orogen (SN
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western to the north-eastern corner
Page 22 and 23: group consists of conjugate sets of
Page 24 and 25: y pure shear. Coincidence of the or
Page 26 and 27: The Precambrian supracrustal rocks
Page 28 and 29: 68°25′ 52° 68°25′ BIF 0 3 km
Page 30 and 31: Fig. 4. Dolomitic marble in the sou
Page 32 and 33: Bugt (Fig. 1; Henderson 1969). The
Page 34 and 35: Keto, L. 1962: Aerial prospecting b
Page 36 and 37: ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲
Page 38 and 39: increases towards the south. We obs
Page 40 and 41: Kalsbeek, F. & Taylor, P.N. 1999: R
Page 42 and 43: Canada Nuussuaq 0 50 km PROTEROZOIC
Page 44 and 45: deposit. Descriptions of the differ
Page 46 and 47: Pegmatites The gneisses throughout
Page 48 and 49: tacts with quartzo-feldspathic coun
Page 50 and 51: Geological correlation of magnetic
Page 52 and 53: Fig. 2. The hand-held magnetic susc
Page 54 and 55: Susceptibility distribution (%) Sus
Page 56 and 57: have a reducing effect, which hinde
Page 58 and 59: ancy between the susceptibility val
Page 60 and 61: 67° Sisimiut • 53° 52° 51° 50
Page 62 and 63: Table 1. Range of Nb concentrations
Page 64 and 65: chemical composition - may indicate
Page 66 and 67: Concentration/C1 chondrite 100.00 1
Page 68 and 69: Menzies, M.A. & Hawkesworth, C.J. (
Page 70 and 71: 72°30´ Aeromag 2001 50 km Upernav
Page 74 and 75: Conclusions The aeromagnetic survey
Page 76 and 77: 53° 51° Inland Ice Greenland 72°
Page 78 and 79: On the south coast of Nuussuaq, wes
Page 80 and 81: Fig. 5. Blocks and boulders in the
Page 82 and 83: 2000 SW NE 1500 Surface trace 1985
Page 84 and 85: 1979). Along other sections of the
Page 86 and 87: Petroleum geological activities in
Page 88 and 89: Seismic acquisition during summer 2
Page 90 and 91: Other petroleum geological work Alt
Page 92 and 93: A multidisciplinary study of the Pa
Page 94 and 95: Seismic interpretation The grid of
Page 96 and 97: Middle Eocene Lower Eocene Upper Pa
Page 98 and 99: Chalmers, J.A., Dahl, J.T., Bate, K
Page 100 and 101: 95-06 ▲ ▲ 58° 95-12 95-10 56°
Page 102 and 103: 70° 71° 60° 2000 2000 2400 400 1
Page 104 and 105: Storey, M., Duncan, R.A., Pedersen,
Page 106 and 107: 24° 100 km 30° 27° 21 ° Jameson
Page 108 and 109: Oligocene Krabbedalen Fm Eocene Bop
Page 110 and 111: grading. Bivalve shell material is
Page 112 and 113: References Birkenmajer, K. 1972: Re
Page 114 and 115: B A 72° Palaeogene basalts Gl 50 k
Page 116 and 117: a b c d e f g h 20 µm i j k l Fig.
Page 118 and 119: offshore South-East Greenland, onsh
Page 120 and 121: ▲ Albert Heim Bjerge Wordie Gletc
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T D A CW NS placed the lowermost c.
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is abundant and highly diverse, and
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The Heimbjerge Formation comprises
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Late Permian carbonate concretions
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m 25 20 15 B3 L2 B2 Fig. 3. Simplif
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(2001), who pointed out the restric
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Piasecki, S. 1984: Preliminary paly
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66° 68° • • • • • • 7
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absent at Tikeraussaq where a postu
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72° 70° 68° 66° Sample locality
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Table 2. Summary of selected elemen
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Concluding remarks Observations dur
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Lake-catchment interactions with cl
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lakes. As in earlier reports (e.g.
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Discussion Global climate models pr
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Glaciological investigations on ice
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Fig. 2. The new one-mast design of
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A B N 20 km 1993 1995 Fig. 5. Satel
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Krabill, W., Frederick, E., Manizad
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2001/47: Calibration of stream sedi
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Hanghøj, K., Kelemen, P., Bernstei
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Danmarks og Grønlands Geologiske U
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GEOLOGY OF GREENLAND SURVEY BULLETI
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