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Offshore evidence for Neogene uplift in central West Greenland
Global and Planetary Change 24 Ž2000. 311–318 www.elsevier.comrlocatergloplacha
Offshore evidence for Neogene uplift in central West Greenland James A. Chalmers ) Geological SurÕey of Denmark and Greenland (GEUS), ThoraÕej 8, DK-2400 Copenhagen NV, Denmark Received 8 November 1999
Abstract Multi-channel seismic lines off southern and central West Greenland show a ) 3-km-thick sedimentary section of mid-Eocene and younger age that dips seaward and is truncated either at the seabed or by an erosional unconformity a short distance below the seabed. This pattern indicates that there has been uplift and erosion of the section and probably of the nearby landmass. The timing of the uplift is not well constrained by borehole data, but certainly took place after the early Eocene, probably during the Neogene and possibly as late as the onset of glaciation in West Greenland in the early Pliocene. The uplift took place substantially later than the cessation of magmatism in the early Eocene and the abrupt slowing or cessation of sea-floor spreading in the Labrador Sea between Chrons 20 and 13 Žmiddle–late Eocene.. This means that, whatever the cause of the uplift, it is unlikely to be directly related to processes either of magmatic emplacement or sea-floor spreading. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Neogene; uplift; West Greenland
1. Introduction There is a substantial evidence that Neogene uplift affected Scandinavia Že.g. Rohrman et al., 1995; papers in Solheim et al., 1996. and the British Isles ŽJapsen, 1997 and references therein. on the European margin of the northern North Atlantic. Cloetingh and Kooi Ž1992., Cloetingh et al. Ž1990., Dore´ and Jensen Ž1996. and Japsen Ž1997. have shown that the uplift and exhumation of the surrounding land areas was accompanied by accelerated subsidence in the central parts of the North Sea. Limited evidence is available that uplift, possibly much of it Neogene, also affected East Greenland on the other side of the northern North Atlantic ŽLarsen, 1990; Christiansen et al., 1992; Hansen, 1996.. )
Tel.: q45-3814-2508; fax: q45-3814-2050. E-mail address: [email protected] ŽJ.A. Chalmers..
Riis Ž1996. showed that uplift offshore Norway could be recognised on seismic reflection sections. Mesozoic, Palaeogene and Neogene sediments under the Norwegian continental shelf have been tilted and eroded, and Pleistocene sediments deposited onto the resulting unconformity. Extrapolation of the tilted surfaces shows that they can be correlated with the peneplaned surface that is now uplifted to form the Scandinavian summit surface.
2. Offshore West Greenland Cenozoic sediments, in places more than 4 km thick, are present offshore West Greenland ŽRolle, 1985; Chalmers et al., 1993, 1995; Whittaker, 1995, 1996; Whittaker et al., 1997.. Between 688N and 728N, Cenozoic sediments rest on Lower Palaegene lavas, the top of which dips westwards from outcrop
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to at least 4 km depth ŽWhittaker, 1995, 1996.. Cloetingh et al. Ž1990. and Cloetingh and Kooi Ž1992. used data from the five commercial boreholes that were drilled off southern West Greenland in 1976 and 1977 ŽRolle, 1985. ŽFig. 1. to calculate accelerated rates of subsidence during the Neogene. However, Nøhr-Hansen Ž1998. has substantially revised the biostratigraphy of four of the five wells, which means that Cloetingh et al.’s Ž1990. and Cloetingh and Kooi’s Ž1992. subsidence rate calculations need to be revised. Additional uncertainty is caused because, for both geological and technical reasons, the uppermost 700 to 1500 m of section in the wells is poorly dated or undated, and the youngest reliable ages in the northern part of the southern West Greenland Basin are Eocene. In 1990 and 1992, the Geological Survey of Greenland ŽGGU, now part of the Geological Survey of Denmark and Greenland, GEUS. acquired regional, multi-channel seismic data off southern and central West Greenland ŽChalmers et al., 1993, 1995.. Part of one line that passes through the Ikermiut-1 well is shown in Fig. 2. Folded and faulted sediments in the deepest part of the section on Fig. 2 are truncated upwards by an unconformity, here called the mid-Eocene unconformity, that has been dated to be of late early to early middle Eocene age by Nøhr-Hansen Ž1998.. The deformation of these sediments is thought to have taken place as a result of transpression from strike-slip movements of the Ungava fault zone ŽChalmers et al., 1995.. Chalmers et al. Ž1993. proposed that the areas of sea-floor spreading in the Labrador Sea and Baffin Bay were connected through continental crust in the Davis Strait area by the Ungava transform fault zone ŽKerr, 1967; Klose et al., 1982.. The mid-Eocene unconformity clearly marks the end of the deformation, and therefore presumably the end of major movement on the Ungava faults. The unconformity is coeval with the
Fig. 1. Southern and central West Greenland showing the structure of the mid-Eocene unconformity that is discussed in the text. The wells are: H-1: Hellefisk-1; I-1: Ikermiut-1; K-1: Kangamiut-1; ˆ N-1: Nukik-1; N-2: Nukik-2 ŽRolle, 1985.; and G3: GRO a3 ŽKristensen and Dam, 1997..
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Fig. 2. Seismic line running N–S through the Ikermiut-1 well showing the mid-Eocene and ‘base Quaternary’ unconformities that are discussed in the text. See Fig. 1 for location.
substantial slowing of the rate of sea-floor spreading in the Labrador Sea from 10 to 3 mmryear, which took place at the end of Chron 21 ŽSrivastava and Keen, 1995., which is in the early middle Eocene ŽBerggren et al., 1995.. Sea-floor spreading finally ceased in the Labrador Sea in Chron 13 Žlate middle Eocene.. The section on Fig. 2 above mid-Eocene unconformity, about 1300 ms thick Žapproximately 1.5 km., is truncated upwards towards the north by an erosional surface, which is, in places, a distinct angular unconformity. The upper unconformity can be traced round the southern West Greenland basin on seismic lines and tied to the various wells ŽRolle, 1985.. It ties Hellefisk-1 at 603.5 m below rotary table Žmrtb., Kangamiut-1 at 998 mbrt, Nukik-1 at ˆ 825 mbrt and Nukik-2 at 790 mbrt. The tie to Ikermiut-1 is shallower than cuttings or electric-log data were first recorded during drilling of the well. The sonic logs from the wells show a much more variable character above the unconformity than below it ŽRolle, 1985., possibly indicating that the
lithology above the unconformity is much more heterogeneous than that below it. This observation may indicate that the sediments above the unconformity are glacigenic. While there is no proof of this assertion, in what follows the unconformity is called the ‘base Quaternary’ unconformity. Fig. 3 shows a seismic line that crosses the line shown in Fig. 2 at the location of Ikermiut-1. Both the mid-Eocene and ‘base Quaternary’ unconformities are marked. The ‘base Quaternary’ unconformity forms the base of a channel between Shot Points ŽSPs. 2800 and 3200, and down-lapping, prograding patterns can be seen above it along the shelf break southwest of SP 4450. Elsewhere, it is only a short distance below the seabed and may crop out at about SP 2300. All sediments below the ‘base Quaternary’ unconformity dip steadily westwards at about 1.58 and are truncated either by the ‘base Quaternary’ unconformity or by the seabed. A similar structural pattern can be seen elsewhere offshore West Greenland. Fig. 4 shows a seismic line west of the island of Disko ŽFig. 1.. On Disko and
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Fig. 3. Seismic line running SW–NE through the Ikermiut-1 well and tying the line shown in Fig. 2 at that well. A prograding shelf-margin wedge can be seen above the ‘base Quaternary’ unconformity at the shelf margin between shot points ŽSPs. 5500 and 4500. This unconformity or the sea-bed truncates all underlying horizons, including the mid-Eocene unconformity, which dip south-westwards at about 1.58. See Fig. 1 for location.
Fig. 4. Seismic line running E–W to the west of Disko and Nuussuaq ŽFig. 1.. The ‘base Quaternary’ unconformity or the sea-bed truncates all underlying horizons, including the mid-Eocene unconformity and top of the Palaeogene basalts, which dip south-westwards at about 1.58. The Palaeogene basalts are exposed onshore ŽClarke and Pedersen, 1976. and were encountered by the Hellefisk-1 well ŽFig. 1. ŽRolle, 1985.. Depths to their top have been mapped by Whittaker Ž1995, 1996..
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the Nuussuaq peninsula are exposed fluvio-deltaic and marine sediments of Albian to early Paleocene age, that are overlain by extensive middle–late Paleocene hyaloclastic breccias and continental flood basalts ŽClarke and Pedersen, 1976; Henderson et al., 1981.. On Fig. 4, the top of the flood basalts can be seen to be exposed at the seabed northeast of SP 5600, from where the top of the basalts dips southwest at about 1.58 under a cover of Cenozoic sediment. The mid-Eocene unconformity is a short distance above the top of the basalts. Sediments above the ‘base Quaternary’ unconformity are less than 150 m thick and are present only southwest of SP 3500. Between SPs 3500 and 5600 Cenozoic sediments crop out directly at the seabed. Comparison of Figs. 3 and 4 with, e.g. Riis’ Ž1996. Figs. 4 through 8 shows that the uplift in West Greenland is similar in character to that in Scandinavia. Unfortunately, the dating of the West Greenland uplift is unclear, other than that, it was substantially later than the early Eocene, was proba-
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bly Neogene and may have been as late as the onset of glaciation in West Greenland in the early Pliocene ŽKorstgard ˚ and Nielsen, 1989.. In any case, this means that uplift was substantially later than the slowing of sea-floor spreading rates in the Labrador Sea in the middle Eocene and probably later than the cessation of sea-floor spreading in the late Eocene. Uplift was also substantially later than the cessation of the main phases of extrusive volcanism in the Palaeogene ŽStorey et al., 1998.. Whatever the cause of the uplift, it is difficult to envisage how processes related either to sea-floor spreading or to magmatism could have caused it.
3. Profile of top basalt horizon Fig. 5 shows a cartoon cross-section through the off- and on-shore sedimentaryrbasalt basin of central West Greenland. On the western part of the profile, which is based on the seismic line shown in
Fig. 5. Cartoon cross-section from west of Disko and Nuussuaq Žbased on the seismic line shown in Fig. 4. and along the southern coast of the Nuussuaq Peninsula ŽFig. 1.. The onshore profile is based on the profile published by Pedersen et al. Ž1993., the GANE a1 ŽChristiansen et al., 1996. and GRO a3 ŽKristensen and Dam, 1997. wells and a seismic line along the fjord between Nuussuaq and Disko ŽChalmers et al., 1999.. G3 is the GRO a3 well, G1: the GANE a1 well, It: the Itilli fault; G: the Gassø fault;K–Q: the Kuugannguaq–Qunnilik fault; and m-E and ‘b-Q’ the mid-Eocene and ‘base Quaternary’ unconformities discussed in the text. An extrapolation of the general dip of the top of the basalts offshore intersects the Itilli fault at 400 m above sea level Žmasl., is displaced to 1400 masl by the Itilli fault, to 2300 masl by the Gassø fault and to 2600 masl by the Kuugannguaq–Qunnilik fault. See text for discussion.
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Fig. 4, Eocene and younger sediments lie on Palaeogene flood basalts of unknown thickness. The eastern, onshore part of the profile runs along the south coast of Nuussuaq ŽFig. 1., where Paleocene basalts and hyaloclastites lie on Lower Paleocene and Cretaceous sediments. There are three large faults in western Nuussuaq, all of which probably moved in post-basalt times with downthrow to the west. The throw across the Itilli fault is at least 1 km, that across the Gassø fault is about 900 m, and that across the Kugannguaq–Qunniliq fault about 300 m ŽChalmers et al., 1999.. An extrapolation of the general dip of the top of the Palaeogene basalts offshore to its intersection with the Itilli Fault is shown in Fig. 5. The intersection is at a height of about 400 m above sea level. This would indicate that the top of the basalts was at least 1400 m above sea level between the Itilli and Gassø faults, and, if it is assumed that the top basalt surface was flat east of the Itilli fault, at 2300 m between the Gassø and Kugannguaq–Qunniliq faults and at 2600 m east of the Kugannguaq–Qunniliq fault. A fourth fault, about 10 km east of the Kugannguaq–Qunniliq fault, appears to have displaced the sediments but not the basalts. Well GRO a3 well was drilled into the fault-block between the Gassø and Kugannguaq–Qunniliq faults and Mathiesen Ž1998. has used fission-track and maturity data to calculate that between 2 and 3 km of overburden, probably basalt, has been eroded from the GRO a3 site. The highest mountain peaks east of the Kugannguaq–Qunniliq fault consist of Palaeogene basalt and are approximately 2 km high, suggesting that about 1 km of basalt formerly existed above them. The figures quoted are uncertain because various assumptions have been made when calculating them. The extrapolation of the top of the offshore basalt surface is clearly uncertain, and the intersection with the Itilli fault could be either higher or lower than shown. It has also been assumed that the top of the basalts is flat east of the Itilli fault. It could have been assumed that the westerly dip observed offshore continued east of the Itilli fault, in which case the calculated heights are too low. However, the subaerial basalt flows of the Maligat ˆ Formation observed onshore are preserved nearly flat ŽPedersen et al., 1993., so, lacking other information, it is not
unreasonable to assume that the top of the basalt pile was also flat.
4. Total amount of Neogene relief Whittaker Ž1995, 1996. shows the top basalt surface at a maximum of more than 6 km below sea level west of Disko. There is some doubt about the identity of this surface at its deepest levels, and an estimate of a maximum of 4 km below sea level is probably more realistic. Thus, it is possible that, prior to onshore erosion, there has been a total relief on this surface of around 6.5 km. However, it is unlikely that the top basalt surface was ever flat. Onlap patterns in the overlying sediments Žsee, e.g. Fig. 4, SPs 2600 to 3400. suggest that the top basalt surface had a depositional relief of around 1.5 km. It is also thought that the Itilli, Gassø and Kuugannguaq–Qunnilik faults moved during the early Eocene ŽChalmers et al., 1999., during sea-floor spreading in the Labrador Sea ŽChalmers et al., 1993.. The total throw of at least 2.2 km across these three faults should, therefore, also be subtracted from the total relief. When this is done, there is a residual, approximately 2.8 km of relief of the top basalt surface, which may have been produced by Neogene uplift. The sediments and basalts of the Nuussuaq Basin, central West Greenland, were then exposed by glacial erosion.
5. Conclusions West Greenland appears to have undergone 2.5–3 km of uplift in Neogene times similar to the uplift that has been documented for Scandinavia. The uplift in West Greenland probably took place substantially later than the cessation both of major Palaeogene magmatism in the area and of sea-floor spreading in the Labrador Sea. If it is assumed that similar uplift is due to similar causes, explanations of Neogene uplift in both Scandinavia and West Greenland must take into account the absence of both magmatism and sea-floor spreading in the latter area.
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Acknowledgements I thank Gregers Dam and referee Sierd Cloetingh for comments. The paper is published with the permission of the Geological Survey of Denmark and Greenland.
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Impact of Glaciations on Basin Evolution; Data and Models from the Norwegian Margin and Adjacent Areas. Global Planet. Change 12, 415–436. Hansen, K., 1996. Thermotectonic evolution of a rifted continental margin; fission track evidence from the Kangerlussuaq area, SE Greenland. Terra Nova 8, 458–469. Henderson, G., Schiener, E.J., Risum, J.B., Croxton, C.A., Andersen, B.B., 1981. The West Greenland Basin. In: Kerr, J.W., Ferguson, A.J. ŽEds.., Geology of the North Atlantic Borderlands. Mem. Can. Soc. Pet. Geol. 7, 399–428. Japsen, P., 1997. Regional Neogene exhumation of Britain and the western North Sea. J. Geol. Soc. 154, 239–247. Kerr, J.W., 1967. A submerged continental remnant beneath the Labrador Sea. Earth Planet. Sci. Lett. 2, 283–289. Klose, G.W., Malterre, E., McMillan, N.J., Zinkan, C.G., 1982. Petroleum exploration offshore Southern Baffin Island, Northern Labrador Sea, Canada. In: Embry, A.F., Balkwill, H.R. ŽEds.., Arctic Geology and Geophysics. Mem. Can. Soc. Pet. Geol. Vol. 8, 233–244. Korstgard, ˚ J.A., Nielsen, O.B., 1989. Provenance of dropstones in Baffin Bay and Labrador Sea, Leg 105. In: Srivastava, S.P., Arthur, M.A. ŽEds.., Proceedings of the Ocean Drilling Program, Scientific Results Vol. 105, 65–69. Kristensen, L., Dam, G., 1997. Lithological and petrophysical evaluation of the GRO a3 well, Nuussuaq, West Greenland. Danmarks og Grønlands Geologiske Undersøgelse Rapport 1997r156, 30 pp. Larsen, H.-C. et al., 1990. The East Greenland Shelf. In: Grantz, A., Johnson, L., Sweeney, J.F. ŽEds.., The Arctic Ocean Region. The Geology of North America-L. Geological Society of America, Boulder, pp. 185–210. Mathiesen, A., 1998. Modelling of uplift history from maturity and fission track data, Nuussuaq, West Greenland. Danmarks og Grønlands Geologiske Undersøgelse Rapport 1998r87, 90 pp. Nøhr-Hansen, H., 1998. Dinoflagellate cyst stratigraphy of the Upper Cretaceous to Paleogene strata from the Hellefisk-1, Ikermiut-1, Kangamiut-1 and Nukik-1 wells, offshore central ˆ West Greenland. Danmarks og Grønlands Geologiske Undersøgelse Rapport 1998r54, 58 pp. Pedersen, A.K., Larsen, L.M., Dueholm, K.S., 1993. Geological section along the south coast of Nuussuaq, central West Greenland. 1:20 000 coloured geological sheet. Grønlands Geologiske Undersøgelse, Copenhagen. Riis, F., 1996. Quantification of Cenozoic vertical movements of Scandinavia by correlation of morphological surfaces with offshore data. In: Solheim, A., Riis, F., Elverhoi, A., Faleide, J.I., Jensen, L.N., Cloetingh, S. ŽEds.., Impact of Glaciations on Basin Evolution; Data and Models from the Norwegian Margin and Adjacent Areas. Global Planet. Change 12, 331– 357. Rohrman, M., van der Beek, P., Andriessen, P., Cloetingh, S., 1995. Meso-Cenozoic morphotectonic evolution of southern Norway: Neogene domal uplift inferred from apatite fission track thermochronology. Tectonics 14, 700–714. Rolle, F., 1985. Late Cretaceous–Tertiary sediments offshore central West Greenland lithostratigraphy, sedimentary evolution and petroleum potential. Can. J. Earth Sci. 22, 1001–1019.
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Solheim, A., Riis, F., Elverhøi, A., Faleide, J.I., Jensen, L.N., Cloetingh, S. ŽEds.., Impact of Glaciations on Basin Evolution; Data and Models from the Norwegian Margin and Adjacent Areas. Global Planet. Change 12, 450 pp. Srivastava, S.P., Keen, C.E., 1995. A deep seismic reflection profile across the extinct mid-Labrador Sea spreading center. Tectonics 14, 372–389. Storey, M., Duncan, R.A., Pedersen, A.K., Larsen, L.M., Larsen, H.C., 1998. 40Arr39Ar geochronology of the West Greenland Tertiary volcanic province. Earth Planet. Sci. Lett. 160, 569– 586.
Whittaker, R.C., 1995. A preliminary assessment of the structure, basin development and petroleum offshore central West Greenland. Open File Series Grønlands Geologiske Undersøgelse 95r9, 33 pp. Whittaker, R.C., 1996. A preliminary seismic interpretation of an area with extensive Tertiary basalts offshore central West Greenland. Bull. - Grønl. Geol. Unders. 172, 28–31. Whittaker, R.C., Hamann, N.E., Pulvertaft, T.C.R., 1997. A new frontier province offshore northern West Greenland: structure, basin development and petroleum potential of the Melville Bay area. Bull. Am. Assoc. Pet. Geol. 81, 979–998.
Offshore evidence for Neogene uplift in central West Greenland James A. Chalmers ) Geological SurÕey of Denmark and Greenland (GEUS), ThoraÕej 8, DK-2400 Copenhagen NV, Denmark Received 8 November 1999
Abstract Multi-channel seismic lines off southern and central West Greenland show a ) 3-km-thick sedimentary section of mid-Eocene and younger age that dips seaward and is truncated either at the seabed or by an erosional unconformity a short distance below the seabed. This pattern indicates that there has been uplift and erosion of the section and probably of the nearby landmass. The timing of the uplift is not well constrained by borehole data, but certainly took place after the early Eocene, probably during the Neogene and possibly as late as the onset of glaciation in West Greenland in the early Pliocene. The uplift took place substantially later than the cessation of magmatism in the early Eocene and the abrupt slowing or cessation of sea-floor spreading in the Labrador Sea between Chrons 20 and 13 Žmiddle–late Eocene.. This means that, whatever the cause of the uplift, it is unlikely to be directly related to processes either of magmatic emplacement or sea-floor spreading. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Neogene; uplift; West Greenland
1. Introduction There is a substantial evidence that Neogene uplift affected Scandinavia Že.g. Rohrman et al., 1995; papers in Solheim et al., 1996. and the British Isles ŽJapsen, 1997 and references therein. on the European margin of the northern North Atlantic. Cloetingh and Kooi Ž1992., Cloetingh et al. Ž1990., Dore´ and Jensen Ž1996. and Japsen Ž1997. have shown that the uplift and exhumation of the surrounding land areas was accompanied by accelerated subsidence in the central parts of the North Sea. Limited evidence is available that uplift, possibly much of it Neogene, also affected East Greenland on the other side of the northern North Atlantic ŽLarsen, 1990; Christiansen et al., 1992; Hansen, 1996.. )
Tel.: q45-3814-2508; fax: q45-3814-2050. E-mail address: [email protected] ŽJ.A. Chalmers..
Riis Ž1996. showed that uplift offshore Norway could be recognised on seismic reflection sections. Mesozoic, Palaeogene and Neogene sediments under the Norwegian continental shelf have been tilted and eroded, and Pleistocene sediments deposited onto the resulting unconformity. Extrapolation of the tilted surfaces shows that they can be correlated with the peneplaned surface that is now uplifted to form the Scandinavian summit surface.
2. Offshore West Greenland Cenozoic sediments, in places more than 4 km thick, are present offshore West Greenland ŽRolle, 1985; Chalmers et al., 1993, 1995; Whittaker, 1995, 1996; Whittaker et al., 1997.. Between 688N and 728N, Cenozoic sediments rest on Lower Palaegene lavas, the top of which dips westwards from outcrop
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to at least 4 km depth ŽWhittaker, 1995, 1996.. Cloetingh et al. Ž1990. and Cloetingh and Kooi Ž1992. used data from the five commercial boreholes that were drilled off southern West Greenland in 1976 and 1977 ŽRolle, 1985. ŽFig. 1. to calculate accelerated rates of subsidence during the Neogene. However, Nøhr-Hansen Ž1998. has substantially revised the biostratigraphy of four of the five wells, which means that Cloetingh et al.’s Ž1990. and Cloetingh and Kooi’s Ž1992. subsidence rate calculations need to be revised. Additional uncertainty is caused because, for both geological and technical reasons, the uppermost 700 to 1500 m of section in the wells is poorly dated or undated, and the youngest reliable ages in the northern part of the southern West Greenland Basin are Eocene. In 1990 and 1992, the Geological Survey of Greenland ŽGGU, now part of the Geological Survey of Denmark and Greenland, GEUS. acquired regional, multi-channel seismic data off southern and central West Greenland ŽChalmers et al., 1993, 1995.. Part of one line that passes through the Ikermiut-1 well is shown in Fig. 2. Folded and faulted sediments in the deepest part of the section on Fig. 2 are truncated upwards by an unconformity, here called the mid-Eocene unconformity, that has been dated to be of late early to early middle Eocene age by Nøhr-Hansen Ž1998.. The deformation of these sediments is thought to have taken place as a result of transpression from strike-slip movements of the Ungava fault zone ŽChalmers et al., 1995.. Chalmers et al. Ž1993. proposed that the areas of sea-floor spreading in the Labrador Sea and Baffin Bay were connected through continental crust in the Davis Strait area by the Ungava transform fault zone ŽKerr, 1967; Klose et al., 1982.. The mid-Eocene unconformity clearly marks the end of the deformation, and therefore presumably the end of major movement on the Ungava faults. The unconformity is coeval with the
Fig. 1. Southern and central West Greenland showing the structure of the mid-Eocene unconformity that is discussed in the text. The wells are: H-1: Hellefisk-1; I-1: Ikermiut-1; K-1: Kangamiut-1; ˆ N-1: Nukik-1; N-2: Nukik-2 ŽRolle, 1985.; and G3: GRO a3 ŽKristensen and Dam, 1997..
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Fig. 2. Seismic line running N–S through the Ikermiut-1 well showing the mid-Eocene and ‘base Quaternary’ unconformities that are discussed in the text. See Fig. 1 for location.
substantial slowing of the rate of sea-floor spreading in the Labrador Sea from 10 to 3 mmryear, which took place at the end of Chron 21 ŽSrivastava and Keen, 1995., which is in the early middle Eocene ŽBerggren et al., 1995.. Sea-floor spreading finally ceased in the Labrador Sea in Chron 13 Žlate middle Eocene.. The section on Fig. 2 above mid-Eocene unconformity, about 1300 ms thick Žapproximately 1.5 km., is truncated upwards towards the north by an erosional surface, which is, in places, a distinct angular unconformity. The upper unconformity can be traced round the southern West Greenland basin on seismic lines and tied to the various wells ŽRolle, 1985.. It ties Hellefisk-1 at 603.5 m below rotary table Žmrtb., Kangamiut-1 at 998 mbrt, Nukik-1 at ˆ 825 mbrt and Nukik-2 at 790 mbrt. The tie to Ikermiut-1 is shallower than cuttings or electric-log data were first recorded during drilling of the well. The sonic logs from the wells show a much more variable character above the unconformity than below it ŽRolle, 1985., possibly indicating that the
lithology above the unconformity is much more heterogeneous than that below it. This observation may indicate that the sediments above the unconformity are glacigenic. While there is no proof of this assertion, in what follows the unconformity is called the ‘base Quaternary’ unconformity. Fig. 3 shows a seismic line that crosses the line shown in Fig. 2 at the location of Ikermiut-1. Both the mid-Eocene and ‘base Quaternary’ unconformities are marked. The ‘base Quaternary’ unconformity forms the base of a channel between Shot Points ŽSPs. 2800 and 3200, and down-lapping, prograding patterns can be seen above it along the shelf break southwest of SP 4450. Elsewhere, it is only a short distance below the seabed and may crop out at about SP 2300. All sediments below the ‘base Quaternary’ unconformity dip steadily westwards at about 1.58 and are truncated either by the ‘base Quaternary’ unconformity or by the seabed. A similar structural pattern can be seen elsewhere offshore West Greenland. Fig. 4 shows a seismic line west of the island of Disko ŽFig. 1.. On Disko and
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Fig. 3. Seismic line running SW–NE through the Ikermiut-1 well and tying the line shown in Fig. 2 at that well. A prograding shelf-margin wedge can be seen above the ‘base Quaternary’ unconformity at the shelf margin between shot points ŽSPs. 5500 and 4500. This unconformity or the sea-bed truncates all underlying horizons, including the mid-Eocene unconformity, which dip south-westwards at about 1.58. See Fig. 1 for location.
Fig. 4. Seismic line running E–W to the west of Disko and Nuussuaq ŽFig. 1.. The ‘base Quaternary’ unconformity or the sea-bed truncates all underlying horizons, including the mid-Eocene unconformity and top of the Palaeogene basalts, which dip south-westwards at about 1.58. The Palaeogene basalts are exposed onshore ŽClarke and Pedersen, 1976. and were encountered by the Hellefisk-1 well ŽFig. 1. ŽRolle, 1985.. Depths to their top have been mapped by Whittaker Ž1995, 1996..
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the Nuussuaq peninsula are exposed fluvio-deltaic and marine sediments of Albian to early Paleocene age, that are overlain by extensive middle–late Paleocene hyaloclastic breccias and continental flood basalts ŽClarke and Pedersen, 1976; Henderson et al., 1981.. On Fig. 4, the top of the flood basalts can be seen to be exposed at the seabed northeast of SP 5600, from where the top of the basalts dips southwest at about 1.58 under a cover of Cenozoic sediment. The mid-Eocene unconformity is a short distance above the top of the basalts. Sediments above the ‘base Quaternary’ unconformity are less than 150 m thick and are present only southwest of SP 3500. Between SPs 3500 and 5600 Cenozoic sediments crop out directly at the seabed. Comparison of Figs. 3 and 4 with, e.g. Riis’ Ž1996. Figs. 4 through 8 shows that the uplift in West Greenland is similar in character to that in Scandinavia. Unfortunately, the dating of the West Greenland uplift is unclear, other than that, it was substantially later than the early Eocene, was proba-
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bly Neogene and may have been as late as the onset of glaciation in West Greenland in the early Pliocene ŽKorstgard ˚ and Nielsen, 1989.. In any case, this means that uplift was substantially later than the slowing of sea-floor spreading rates in the Labrador Sea in the middle Eocene and probably later than the cessation of sea-floor spreading in the late Eocene. Uplift was also substantially later than the cessation of the main phases of extrusive volcanism in the Palaeogene ŽStorey et al., 1998.. Whatever the cause of the uplift, it is difficult to envisage how processes related either to sea-floor spreading or to magmatism could have caused it.
3. Profile of top basalt horizon Fig. 5 shows a cartoon cross-section through the off- and on-shore sedimentaryrbasalt basin of central West Greenland. On the western part of the profile, which is based on the seismic line shown in
Fig. 5. Cartoon cross-section from west of Disko and Nuussuaq Žbased on the seismic line shown in Fig. 4. and along the southern coast of the Nuussuaq Peninsula ŽFig. 1.. The onshore profile is based on the profile published by Pedersen et al. Ž1993., the GANE a1 ŽChristiansen et al., 1996. and GRO a3 ŽKristensen and Dam, 1997. wells and a seismic line along the fjord between Nuussuaq and Disko ŽChalmers et al., 1999.. G3 is the GRO a3 well, G1: the GANE a1 well, It: the Itilli fault; G: the Gassø fault;K–Q: the Kuugannguaq–Qunnilik fault; and m-E and ‘b-Q’ the mid-Eocene and ‘base Quaternary’ unconformities discussed in the text. An extrapolation of the general dip of the top of the basalts offshore intersects the Itilli fault at 400 m above sea level Žmasl., is displaced to 1400 masl by the Itilli fault, to 2300 masl by the Gassø fault and to 2600 masl by the Kuugannguaq–Qunnilik fault. See text for discussion.
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Fig. 4, Eocene and younger sediments lie on Palaeogene flood basalts of unknown thickness. The eastern, onshore part of the profile runs along the south coast of Nuussuaq ŽFig. 1., where Paleocene basalts and hyaloclastites lie on Lower Paleocene and Cretaceous sediments. There are three large faults in western Nuussuaq, all of which probably moved in post-basalt times with downthrow to the west. The throw across the Itilli fault is at least 1 km, that across the Gassø fault is about 900 m, and that across the Kugannguaq–Qunniliq fault about 300 m ŽChalmers et al., 1999.. An extrapolation of the general dip of the top of the Palaeogene basalts offshore to its intersection with the Itilli Fault is shown in Fig. 5. The intersection is at a height of about 400 m above sea level. This would indicate that the top of the basalts was at least 1400 m above sea level between the Itilli and Gassø faults, and, if it is assumed that the top basalt surface was flat east of the Itilli fault, at 2300 m between the Gassø and Kugannguaq–Qunniliq faults and at 2600 m east of the Kugannguaq–Qunniliq fault. A fourth fault, about 10 km east of the Kugannguaq–Qunniliq fault, appears to have displaced the sediments but not the basalts. Well GRO a3 well was drilled into the fault-block between the Gassø and Kugannguaq–Qunniliq faults and Mathiesen Ž1998. has used fission-track and maturity data to calculate that between 2 and 3 km of overburden, probably basalt, has been eroded from the GRO a3 site. The highest mountain peaks east of the Kugannguaq–Qunniliq fault consist of Palaeogene basalt and are approximately 2 km high, suggesting that about 1 km of basalt formerly existed above them. The figures quoted are uncertain because various assumptions have been made when calculating them. The extrapolation of the top of the offshore basalt surface is clearly uncertain, and the intersection with the Itilli fault could be either higher or lower than shown. It has also been assumed that the top of the basalts is flat east of the Itilli fault. It could have been assumed that the westerly dip observed offshore continued east of the Itilli fault, in which case the calculated heights are too low. However, the subaerial basalt flows of the Maligat ˆ Formation observed onshore are preserved nearly flat ŽPedersen et al., 1993., so, lacking other information, it is not
unreasonable to assume that the top of the basalt pile was also flat.
4. Total amount of Neogene relief Whittaker Ž1995, 1996. shows the top basalt surface at a maximum of more than 6 km below sea level west of Disko. There is some doubt about the identity of this surface at its deepest levels, and an estimate of a maximum of 4 km below sea level is probably more realistic. Thus, it is possible that, prior to onshore erosion, there has been a total relief on this surface of around 6.5 km. However, it is unlikely that the top basalt surface was ever flat. Onlap patterns in the overlying sediments Žsee, e.g. Fig. 4, SPs 2600 to 3400. suggest that the top basalt surface had a depositional relief of around 1.5 km. It is also thought that the Itilli, Gassø and Kuugannguaq–Qunnilik faults moved during the early Eocene ŽChalmers et al., 1999., during sea-floor spreading in the Labrador Sea ŽChalmers et al., 1993.. The total throw of at least 2.2 km across these three faults should, therefore, also be subtracted from the total relief. When this is done, there is a residual, approximately 2.8 km of relief of the top basalt surface, which may have been produced by Neogene uplift. The sediments and basalts of the Nuussuaq Basin, central West Greenland, were then exposed by glacial erosion.
5. Conclusions West Greenland appears to have undergone 2.5–3 km of uplift in Neogene times similar to the uplift that has been documented for Scandinavia. The uplift in West Greenland probably took place substantially later than the cessation both of major Palaeogene magmatism in the area and of sea-floor spreading in the Labrador Sea. If it is assumed that similar uplift is due to similar causes, explanations of Neogene uplift in both Scandinavia and West Greenland must take into account the absence of both magmatism and sea-floor spreading in the latter area.
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Acknowledgements I thank Gregers Dam and referee Sierd Cloetingh for comments. The paper is published with the permission of the Geological Survey of Denmark and Greenland.
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