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Global and Planetary Chang

Palaeosurfaces in central West Greenland as reference foridentification of tectonic movements and estimation of erosion

Johan M. Bonow a,b,⁎, Karna Lidmar-Bergström b, Peter Japsen a

a Department of Physical Geography and Quaternary Geology, Stockholm University, SE-10691 Stockholm, Swedenb Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10 DK-1350 Copenhagen, Denmark

Received 14 April 2004; accepted 20 December 2005

Abstract

Landform analysis of basement rocks has been undertaken with the aid of digital elevation data, aerial photographs and fieldobservations in central West Greenland (69°15′N–66°00′N). Palaeosurfaces have been identified, dated relatively to each other,used to quantify uplift and fault movements and also used to estimate differential erosion. Two types of palaeosurfaces weremapped across the Precambrian basement: a surface at low elevation with distinct hills (hilly relief), and two planation surfacesformed across different types of basement rocks. The hilly relief surface emerges as an inclined surface from Cretaceous coverrocks in Disko Bugt and is interpreted as a stripped late Mesozoic etch surface. This surface is cut off towards the south by a lessinclined planation surface, which is younger and thus of Cenozoic age. It is similar to the post-Eocene (Miocene?) planationsurfaces identified on Disko and Nuussuaq in other studies. The planation surface splits in two southwards towards high areasaround Nordre Isortoq and Sukkertoppen Ice Cap. The upper planation surface forms near-summit areas of tectonic blocks dippingin different directions and with different tilts. The uplift centres define the crests of two mega blocks, separated by the ‘SisimiutLine’ which coincides with the Precambrian Ikertôq thrust zone. A partially developed lower planation surface indicates a firstuplift of maximum 500 m followed by a second uplift of maximum 1000 m. We infer that these uplift events occurred during thelate Neogene based on correlation with similar surfaces on Nuussuaq and the timing of exhumational events estimated from apatitefission track analyses of samples from a deep borehole on Nuussuaq (reported elsewhere). The difference between a reconstructionof the upper planation surface across the entire area and the present topography was used as an estimate of erosion of basement rocksince the formation of the upper planation surface. The erosion is unevenly distributed and varies from almost none on the well-preserved planation surfaces to 800–1300 m along valleys, and even more in the fjords. Erosion is less within areas of gneiss ingranulite facies, than in areas of gneiss in amphibolite facies.© 2006 Published by Elsevier B.V.

Keywords: palaeosurface; West Greenland; uplift; tectonic movements; differential erosion; Neogene; planation surface; etch surface; hilly relief;Landform analysis

⁎ Corresponding author. Present address: Geological Survey ofDenmark and Greenland (GEUS), Øster Voldgade 10 DK-1350Copenhagen, Denmark. Tel.: +45 38142251; fax: +45 38142050.

E-mail address: [email protected] (J.M. Bonow).

0921-8181/$ - see front matter © 2006 Published by Elsevier B.V.doi:10.1016/j.gloplacha.2005.12.011

1. Introduction

The relationships between uplift, erosion andsedimentation along the elevated passive continentalmargins of the North Atlantic and which the drivingforces are, tectonic or climatic, have become a key

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issue for geomorphologists, geophysicists and geolo-gists (Japsen and Chalmers, 2000; Doré et al., 2002).Important questions are when the areas have risen, ifthe uplift was simultaneous across the region, and howmuch different areas were uplifted. Other questions arefrom where the offshore sediments emanate and howmuch is eroded where? Central West Greenland waschosen for a pilot study, in which the landforms areused as a data set for conclusions on uplift anderosion.

West Greenland is part of a passive margin east of theDavis Strait, where N–S faults connect extinct NW–SEoriented Palaeogene sea-floor spreading axes in theLabrador Sea and Baffin Bay (Chalmers and Pulvertaft,2001). Separate uplift events of the Nuussuaq Basin inthe Palaeogene (Dam et al., 1998; Dam, 2002) as well asin the late Neogene (Mathiesen, 1998; Chalmers, 2000;Japsen et al., 2005) have been documented.

The relationships between denudation surfaces inbasement (denuded rocks from old orogenies) andcover rocks (undeformed sedimentary strata or volcanicrocks directly on basement) (Lidmar-Bergström andNäslund, 2005) have turned out to be a most importantresearch field to reveal the denudational and diastroph-ic history of an area (Lidmar-Bergström, 1988, Hall,1991; Lidmar-Bergström, 1993, 1995; Demoulin,1995; Peulvast et al., 1996; Lidmar-Bergström, 1996,1999; Hall and Bishop, 2002; Japsen et al., 2002a;Hall, 2003; Bonow, 2004). In central West Greenlandthe Precambrian basement is exposed over large areas(Fig. 1), but Cretaceous strata are preserved in theNuussuaq Basin, which extends from the Melville Bayin the north to approximately 62°N in the south(Chalmers et al., 1999). Palaeogene volcanics rest bothon Cretaceous strata and directly on basement, bothoffshore and onshore (Chalmers et al., 1999; Henriksenet al., 2000).

The aim of the present study is to use theseconditions in identifying and mapping denudationsurfaces on central West Greenland, in determiningtheir relative ages, and to use the mapped surfaces asreference for identification of tectonic movements andestimations of amounts of uplift and erosion.

Fig. 1. Regional map of basement and cover rocks in the NuussuaqBasin, central West Greenland, containing Cretaceous and Paleocenesediments and Palaeogene volcanic rocks presently exposed on Disko,Nuussuaq and Svartenhuk. Basement is in certain areas covered byCretaceous sedimentary rocks while in other areas it is covered byPalaeogene basalt. The black lines show faults. CBFS — CretaceousBoundary Fault System. MB — Melville Bay. SIC — SukkertoppenIce Cap. Geology after Chalmers et al. (1999), Chalmers and Pulvertaft(2001) and Pulvertaft and Larsen (2002).

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2. Study area

2.1. Location and major physiographic features of WestGreenland

The study area is located along the west coast ofGreenland, between Sukkertoppen Ice Cap in the southand Disko Bugt in the north (Figs. 1 and 2). The area islimited by the Greenland ice sheet in the east and byDavis Strait in the west. The distance from the coast tothe ice sheet is up to 150 km, whereas the N–S extent ofthe study area is up to 350 km. The area is thus one of

Fig. 2. Geology of the study area (simplified). Geology after Henriksen et al.

the largest continuous ice-free areas in Greenland.Greenland is approximately an asymmetric plateauwith high rims, higher on the eastern side than on thewestern side (Weidick, 2000). A major drainage routehas a water divide situated along the eastern rim ofGreenland and a watershed draining the major parts ofsouthern and central Greenland. The depositionalpattern of fluvial Cretaceous sediments in the NuussuaqBasin shows that this major drainage system wasalready established in the Cretaceous (Funder, 1989;Pedersen and Pulvertaft, 1992; Dam et al., 1998). Atpresent it is a major drainage outlet into Disko Bugt

(2000). Position for topographical profiles in Figs. 4 and 6 is indicated.


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(bay), occupied by the Jakobshavn Isbræ immediatelynorth of the study area (Fig. 1).

2.2. Glacial record

Greenland was completely, or almost completely,covered by ice during parts of the Pliocene and most ofthe Quaternary, and glacial deposits are widespread onice-free land areas and on the adjacent shelf (e.g.,Funder, 1989). Climatic cooling started in the LateMiocene with full glacial conditions of Greenland at7 Ma (Larsen et al., 1994). The glaciation with thelargest recorded aerial extent occurred near the end ofthe Pliocene and is evidenced on the shelf (Funder,1989). During the Eemian the ice sheet was significantlyreduced, followed by large variations of the ice marginduring the latest glacial cycle. The latest deglaciationbegan 14,000–10,000 years ago, with various oscilla-tions. A minimum position was reached approximately5000 years ago, when the ice margin was up to 50 kminland of its present position in southwest Greenland(Huybrechts, 2002) and at least 15 km in WestGreenland (Weidick et al., 1990).

2.3. Tectonic development, volcanism and Neogeneuplift

Sedimentary rift basins started to form outside WestGreenland probably during the Palaeozoic and devel-oped during the Mesozoic and Cenozoic with maximumtotal sediment thicknesses of around 10 km (Chalmers etal., 1999; Chalmers and Pulvertaft, 2001). A continentalmargin developed in the Paleocene when seafloorspreading took place in Baffin Bay and the LabradorSea, areas separated by a major strike-slip fault systemin Davis Strait (Chalmers and Pulvertaft, 2001). Theimpact of the Iceland Plume is recorded in Cretaceous–Paleocene strata on Nuussuaq. Here extensive erosiontogether with development and reactivation of faultstook place in latest Cretaceous times followed by upliftand incision of valley systems in the Cretaceous rocksduring the early Paleocene (Dam et al., 1998; Dam,2002). The valleys were subsequently drowned andfilled with marine Paleocene sediments prior to eruptionof thick sub-aqueous basalt, which extruded over largeparts of the Nuussuaq Basin later in the Paleocene. Thevolcanism became later generally sub-aerial and thebasalt onlapped also the basement areas in the east(Pedersen et al., 2002). Volcanism continued until theEocene in the eastern areas of the Nuussuaq Basin(Storey et al., 1998). Offshore West Greenland,Cenozoic sediments reach thicknesses of more than

4 km (Rolle, 1985; Piasecki et al., 1992; Nøhr-Hansen,2003; Piasecki, 2003). A mid-Eocene unconformityprobably marks the end of major fault movements andcan be correlated to slowing rates of sea-floor spreadingin the Labrador Sea (Chalmers, 2000; Dalhoff et al.,2003).

Uplift of the Nuussuaq Basin in the Neogene hasbeen concluded from the fission track record acquiredfrom the 3 km deep, hydrocarbon exploration wellGRO#3 (Mathiesen, 1998; Japsen et al., 2005). Thefission track record shows three distinct episodes ofcooling, that began between 40 and 30, 11 and 10 aswell as 7 and 2 Ma (Japsen et al., 2005). The two latterepisodes surely included exhumation of rocks andwere interpreted as uplift events. They are inagreement with the observations offshore by Chalmers(2000), who found that a>3 km thick, post-mid-Eocene, seaward dipping section is truncated by anerosional unconformity close to sea bed west ofNuussuaq, and concluded it had formed substantiallyafter the mid-Eocene and probably during theNeogene. These studies suggest uplift and erosionoffshore as well as on the landmass on Nuussuaqduring the Neogene.

A coherent erosion surface formed across largedistances has been identified on Nuussuaq (Bonow,2004). This surface is close to the summits, dips indifferent directions, and changes of tilt occur abruptlyacross major faults. The surface is post-Eocene and itsformation has been independent of rock types as it cutsacross both resistant Precambrian basement and moreeasily eroded Eocene and Paleocene volcanic rocks. Itsfinal shape must have been obtained before the 11 to10 Ma uplift event occurred (see above) and it is thussuggested to be of middle Miocene age (Bonow et al.,in press). The formation of a lower planation surfaceand valley generation is interpreted to be a result of thisuplift and the youngest valley generation is correlatedwith the 7–2 Ma event (Bonow et al., in press).

2.4. Basement and cover rocks

The study area is part of a Precambrian shield (Figs. 1and 2) and is crossed by the Ikertôq thrust zone (vanGool et al., 2002), which roughly coincides with atopographic feature, in this paper called the SisimiutLine. The shield was probably denuded and covered bysedimentary strata in the Ordovician. “Loose blocks” ofPhanerozoic sandstone are present in the study area(Peel and Secher, 1979; Fig. 2) and an Ordovicianoutlier is located at Fossilik just south of the study area(Stouge and Peel, 1979). Ordovician strata occur at sea


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bed in the western part of the Davis Strait (Chalmers andPulvertaft, 2001).

In Disko Bugt, at least 2 km of Cretaceoussediments rest directly on basement. The Cretaceoussediments are bounded in the east by the CretaceousBoundary Fault System (CBFS) and in the west by aN–S trending basement ridge. This ridge is exposed inDisko Bugt on several small islands and skerries andalso at sea bed (Chalmers et al., 1999). On thenorthward continuation of the ridge below Disko,basalt of Paleocene age (Storey et al., 1998) restsdirectly on the basement.

3. Landform analysis

3.1. Background

The relation between re-exposed relief, new relief,and cover rocks of different age within areas ofbasement rocks in southern Fennoscandia has turnedout to be a key for analysis of relief development on ashield (Lidmar-Bergström, 1988, 1995, 1996). Particu-larly the effect of Mesozoic deep weathering for shapinga hilly relief with up to 200 m high hills could beacknowledged by studying the regional distribution andlocal position of remnants of both thick kaoliniticsaprolites and Mesozoic cover rocks (Lidmar-Berg-ström, 1982, 1989). The striking difference between thehilly landscape and the totally flat sub-Cambrianpeneplain was useful in this analysis as also thedifference in tilt of the identified surfaces. Togetherwith a literature review on the surrounding sedimentarystrata it was possible to define periods during thePhanerozoic when deep weathering (etching) andstripping on the one hand and stripping and pedimenta-tion on the other dominated basement denudation innorthern and central Europe (Lidmar-Bergström, 1982table 11 p. 172).

It is a common assumption, though not written inany scientific paper, that bedrock hills within formerglaciated areas are caused and totally formed by glacialerosion. On the contrary many studies on reliefdevelopment in Fennoscandia had given support forthe opposite conclusions, viz. that preglacial weather-ing and fluvial erosion are main causes for the origin ofthe present landforms also outside the exhumedsurfaces (Björnsson, 1937; Nordenskjöld, 1944; Matts-son, 1962; Lidmar-Bergström, 1997; Olvmo et al.,2005). Glacial erosion can cause substantial reshaping,particularly within hilly relief by abrasion on top ofhills and plucking on lee sides (Olvmo et al., 1999;Migón and Lidmar-Bergström, 2001), while the glacial

reshaping of the flat sub-Cambrian peneplain isnegligible (Johansson et al., 2001). Another importantcondition, when interpreting glaciated areas, is theprotecting effect of ice sheets and ice caps during mostof the glaciations due to cold based conditions (e.g.,Sugden, 1978; Kleman, 1994). These conditions havebeen at hand in many high areas during the wholeperiod with Late Cenozoic glaciations (Kleman andHättestrand, 1999), while glacial erosion has been mosteffective in low areas (Sugden, 1978; Glasser, 1995;Näslund, 1997).

3.2. Definitions

The following terminology is used in the presentstudy: Denudation surface is used as a general termfor a landscape feature of wide extent formed acrossthe bedrock. If it is characterised by low relief, it iscalled a planation surface and if it is characterised bydistinct bedrock hills we call it a hilly relief. Exhumedor re-exposed surfaces are surfaces that have beenexposed by erosion of protective cover rocks. Suchsurfaces are common over basement rocks (Twidale,1985, 1999). Erosion surface is here used for surfacesconstrained by a common base level for the fluvialsystem.

The described surfaces are palaeosurfaces, defined aslandforms not in accordance with present climatic ortectonic conditions. They are partly destructed orreshaped, but still recognizable. Their former widerextent may be reconstructed (cf. Widdowson, 1997).

Envelope surface is used to denote an imaginedsurface across summits. Even if imaginary it can beuseful in general discussions. In Scandinavia theenvelope surface is often close to either the sub-Palaeozoic peneplain or any sub-Mesozoic surface.The envelope surface does not depict any particularpalaeosurface.

3.3. Identification of palaeosurfaces in the study area

No studies of the large scale geomorphology hadbeen done before in the area. A general topographicaloverview was obtained by a preliminary study based ona field trip and digital elevation data (Japsen et al.,2002b), and documentation of landforms in the southernpart of the study area in 2003.

The relief is depicted in a digital elevation model(DEM) in a 250×250 m resolution square grid,constructed by National Survey and Cadastre (Kort ogMatrikelstyrelsen), Copenhagen (Ekholm, 1996; Bam-ber et al., 2001). The data set includes the


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Sukkertoppen Ice Cap and minor ice caps. Bycomparison with topographical maps and aerialphotographs, the topographical effect of ice wasevaluated. In general the ice is thought not to affectthe overall interpretation of surfaces, due to theirlimited extent and thickness. However the Sukkertop-pen Ice Cap is of substantial extent and thickness,which was taken into consideration during interpreta-tion and reconstruction of surfaces.

An elevation/slope map was constructed according toBonow et al. (2003) (Fig. 3). Slopes were calculated infour groups, >25°, 25–12°, 12–6.5° and <6.5°. Areaswith an inclination <6.5°, the common inclination forpediments (Dohrenwend, 1994), were defined as flat.We think that at high elevations these areas rather welldepict planation surfaces graded to former common baselevels.

The coastal area is characterised by a narrow coastalplain in front of a great escarpment with two high areas,one in the south between Kangerlussuaq and Itilleqfjords (maximum altitude 1848 m a.s.l.) and one aroundNordre Isortoq fjord in the central study area (max1597 m a.s.l.) (Fig. 3). Both high areas can be regardedas alpine landscapes (Sugden, 1974) with horns, arêtesand local glaciers, but some flat summits are stillpresent. Sukkertoppen Ice Cap in the south generallyterminates at about 1400 m a.s.l., but several outletglaciers reach sea-level. Further inland a low-reliefhighland at 1400 to 1000 m a.s.l., the AngujaatorfiupNunaa (Fig. 2), lies between the Sukkertoppen Ice Capand the ice sheet in the east.

North of Nassuttooq the summit plateaus becomeprogressively lower and flat summits are less frequent,and the landscape gradually changes to an undulatingterrain with hills of irregular size. Towards the north thecoastal escarpment fades away.

The hilly relief immediately south of Disko Bugtextends from below Cretaceous and Palaeogene coverrocks. The occurrence of kaolinitic saprolites in thebasement on Nuussuaq (Pulvertaft, 1979) and centralDisko below Cretaceous strata (Chalmers et al., 1999),on southern Disko below Palaeogene strata (Bonow,2005), and offshore in a well SWof the study area (e.g.,Bate, 1997), indicates that deep weathering andsubsequent stripping were the major processes information of this relief, and similar to conditions inFennoscandia.

Flat areas occur both in low positions as plains(covered by Quaternary sediments) or as glaciallywidened valley floors, and in high positions as largeplateaus and shallow valleys. The summit plateaus withshallow valleys could, by our definition, be palaeosur-

faces and of interest for mapping. They are partlydissected by deeply incised valleys.

3.4. Mapping of palaeosurfaces

Based on the information given above a detailedanalysis has been performed according to the methodsdeveloped for mapping of palaeosurfaces in Scandinaviaby Lidmar-Bergström (1988, 1995, 1996) and Bonow etal. (2003). The landforms were analysed in two steps.First, three palaeosurfaces were mapped (see below), anupper and a lower planation surface, and a surface withdistinct hills at low elevations (Fig. 4). The mappingresulted also in a description of the envelope surface andidentification of fault blocks. In a second step the formerextent of the upper planation surface was reconstructedand used for estimation of erosion since its formation.

3.4.1. Hilly relief areasBedrock hills limited by fractures were mapped by

interpretation of aerial photographs at the scale1:150,000 in a Zeiss–Jena interpretoscope. All together415 hills were mapped in the study area. Theapproximate border between the hilly relief areas andthe planation surface is marked as the hilly reliefboundary (HRB).

South of Nassuttooq hills occurs on a coastal plainbelow a high escarpment. The hills here are moresparsely distributed than to the north. Between Sisimiutand Itilleq the escarpment is mainly obliterated byglacial erosion (see below) but the hills are still presentat the western end of protective ridges in lee-sidepositions for eroding ice. The appearance of hills doesnot obviously change from one area to the other.

3.4.2. Planation surfacesA combination of a contour map (100 m contour

interval) and a grid of topographical profiles 20 km apart(Fig. 5), was used to map the sub-horizontal and/orinclined surfaces. The relative relief was denoted byusing a 20 km wide corridor divided into 250 m longsections. The result of the mapping was checked againstthe bedrock map for control of bedrock influence onsurface appearance.

The major upper surface is best preserved in thesoutheast part of the study area, which was used as a keyarea for its identification. The lower surface hasdeveloped as a valley generation along a rectangularfracture system and is only slightly incised in the uppersurface. The two surfaces are inclined and merge moreor less northwards and eastwards. They split into twosurfaces in the central and south-western parts of the

Fig. 3. Elevation and slope map from a digital elevation model (DEM) with 250 m grid. Areas with slope angle less than 6.5° have been colouredaccording to height above present-day sea level. Areas with slopes >6.5° have been coloured in grey scale. Elevation data obtained from Kort andMatrikelstyrelsen, Denmark.


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Fig. 5. All profiles used for correlation of palaeosurface remnants. Crossing profiles were used in combination with a contour map (100 m interval) toget a three-dimensional picture, which made it possible to map both inclined and sub-horizontal surfaces as well as faults.


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study area, where the land surface has a higherelevation. Four profiles are selected to illustrate themapping (Fig. 6).

Fig. 4. Palaeosurface map of the study area with an upper and a lower planatisurface with a few residual massifs is a major feature, and is preserved mainlyonly slightly incised in the upper surface and mainly forms a valley generationlow angle (profile e). The surfaces are offset by fault scarps.

The N–S profile [a] is drawn along areas with wellpreserved planation surfaces making up a plateaulandscape at about 500 m in the north. At the crossing

on surface and a hilly relief surface (etch surface). The upper planationin areas underlain by gneisses in granulite facies. The lower surface is(profile d). The hilly relief surface is cut of by the planation surface at

Fig. 6. Topographical profiles (shaded area) with minimum and maximum profiles covering a 20 km wide corridor centred along the topographicprofile. The palaeosurfaces dip in different directions and are occasionally offset by faults. The lower surface (dashed line) is only identified inthe higher areas. In the north the planation surface cuts off the tilted hilly relief surface at low angle [d], cf. Fig. 4, profile e. See Figs. 2 and 4for location.


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with the Sisimiut Line there is a clear break in thesummit surface. South of the Sisimiut Line the surfacerises and splits into two. Coincidence of the summitprofile and the topographical profile denotes areaswhere the upper planation surface is well preserved.This is the case in areas with Archaean gneisses ofgranulite facies, while the surface is eroded in areas withArchaean gneisses of amphibolite facies and of gabbro-anorthosite (Figs. 2 and 6).

Profile [b] illustrates the situation close to the coastwith two high areas. The upper surface starts at about250 m a.s.l. in the north, where it cuts off the hillyrelief surface with a low angle. This condition is alsoclear from east–west profiles (Fig. 4, profile e). Inprofile [b] the upper surface rises slowly towards thesouth and splits into two surfaces. They are offsetalong structural lines that delimit the Nordre Isortoqhigh area where the two surfaces are clearly apart. Alow-lying area is delimited by the Sisimiut Line to thenorth and by the structurally controlled Itilleq fjord tothe south. Further south is a high area where the two

surfaces are clearly separated, but their exact positionwas difficult to ascertain and therefore their position inthe profile is marked with question marks. The summitplateaus here show no glacial reshaping (Sugden,1974) and are therefore important for the validity of thereconstruction.

The east–west profiles cross the low plateaus in theeast and high areas along the coast. Close to theSukkertoppen Ice Cap, the upper surface is wellpreserved also in the gneisses in amphibolite facies(profile [d]). In the high area, west of the Sisimiut Line,the two surfaces are clearly separated, but along theeastern part of profile [c] only one surface was identified(cf. crossing of profile [a]). Here the profile crosses thegneiss in amphibolite facies and most of the planationsurface is obliterated.

The two planation surfaces are best preserved in twoareas: in Angujaatorfiup Nunaa, east of SukkertoppenIce Cap and in the Nordre Isortoq area (Fig. 4). Thesurfaces and incised valleys as identified in the profilesare illustrated in the eastern part of profile [d] in Fig. 4


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(cf. profile [d] in Fig. 6). The surfaces in AngujaatorfiupNunaa seem to continue beneath Sukkertoppen Ice Cap.The higher surface is a major erosion surface (lightgreen in Fig. 4), covering vast areas and has few residualmassifs. Most massifs appear to consist of the same typeof bedrock as underlies of the surrounding surface. Thelower surface (dark green in Fig. 4) is mainly a valleygeneration, developed along the edges of the primarysurface. In some cases the exact position of the surfaces

Fig. 7. 3D-map of the southern part of the study area. The planation surfaceswhile in the northern areas the planation surfaces are mainly obliterated. The hsurface remnants. Photograph a: the upper planation surface makes up theglacially reshaped, but closely following the fracture pattern, which guided tremnants of the lower planation surface (cf. Fig. 4). A deeply incised and glacb: a shallow valley within the upper planation surface. Photographs: J.M. B

was difficult to ascertain, but the correlations fromprofile to profile with the crosscutting analyses (Fig. 5)make us think that we have given a fairly good picture ofthe position for the different palaeosurfaces.

In summary the mapping gave the following result:an upper planation surface has been formed acrossdifferent types of bedrock. The surface is best preservedin areas with Archean gneisses in granulite facies and,east of Sukkertoppen Ice Cap also in other types of

in the Angujaatorfiup Nunaa area are well preserved in the southeast,igh area in the southwest shows mainly alpine relief, but with planationskyline. To the left is a valley incised in the upper planation surface,he original fluvial incision. Both left and right of this valley are minorially reshaped valley (Sarfartoq) is seen at the background. Photographonow, 2003.


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bedrock. A 3D picture combined with landscape viewsillustrates its dominance in the skyline and shows theshallow valleys of the lower planation surface, as well asthe deeply incised valleys (Fig. 7).

3.5. Landform development

3.5.1. Relative timing of surface developmentThe hilly relief in the northern part of the study area

extends from a Cretaceous cover offshore and isinterpreted as an exhumed surface (Bonow, 2005). Theplanation surface (only one here) cuts off the moreinclined surface with hilly relief and is thereforeinterpreted to be younger and of Cenozoic age. In theeastern part of the study area the upper planation surfaceis at about 500 m a.s.l. and the surface continuesnorthwards and can be followed on Nuussuaq acrossboth Precambrian basement and Palaeogene basalt(Bonow et al., in press). This observation supports aCenozoic age of the planation surface.

The relationship between the hilly relief and theplanation surfaces at high elevations becomes unclearsouth of Nassuttooq, where a coastal escarpmentdelimits the hilly relief to the east (see Section 3.4.1).A coastal plain with residual hills with the character of astrandflat has developed. The origin of strandflats isproblematic (Guilcher et al., 1994). Studies of itsrelation to offshore cover rocks are needed to ascertainif it is partly an exhumed feature as parts of theNorwegian strandflat is (Holtedahl, 1998), or if it is atotally new feature. This problem will not be treated inthis paper.

3.5.2. Tectonic blocks and the envelope surfaceThe result of the palaeosurface mapping in West

Greenland is different from that obtained for theSouthern Scandes, where four well developed surface/valley generations were identified between 1000 and2000 m a.s.l. (Bonow et al., 2003). The valleys in thepresent study area cut each other more or lessperpendicular and the valley systems show a strongstructural control. The situation is similar to what isobserved across flat relief in the Precambrian rock ofsouthern Sweden, where rock blocks are clearlydefined (Lidmar-Bergström, 1991; Tirén and Beckhol-men, 1992; Lidmar-Bergström, 1993, 1994). Themapping of the palaeosurfaces showed clear jumpsor change of inclination across some major structurallycontrolled valleys, indicating faults or breaks aftersurface formation. One of the most prominent jumps isacross the Sisimiut line (Fig. 6), where also thepalaeosurfaces change inclination. This indicates that

the palaeosurfaces here have been offset by areactivated fault. Thus a major result of the palaeosur-face mapping is identification of fault scarps andbreaks separating tectonic blocks.

To further outline major tectonic blocks, twosummit envelope maps were constructed from theDEM by extracting the maximum height value fromrectangular calculation in windows with 4 and 10 kmside respectively. The resulting maps show differentdegree of surface generalisation (Fig. 8a, b). The mostgeneralised map (Fig. 8a) clearly denotes two megablocks, one around Nordre Isortoq and one aroundSukkertoppen Ice Cap, which are separated by theSisimiut Line. The more detailed map (Fig. 8b)indicates that the mega blocks are subdivided intominor blocks.

The fault lines separating individual blocks were thenused in a reconstruction of the former shape of the upperplanation surface over the whole study area in the formof a contour map (Fig. 8c). A grid, 20 by 20 km, wascreated with calculated height values for the position ofthe upper planation surface as interpreted in the profiles.The fault lines were inferred to depict the contours of theplanation surfaces of the different tectonic blocks ascorrectly as possible. However it was noted that the20 km grid was too sparse in the two high areas and itdid not satisfactorily show obvious tectonic blocks (Fig.8a, b). A denser net of height values (10 km spacing)was therefore created in two east–west profiles coveringthese areas. The total number of height values for thegrid was 157.

The combination of the envelope maps (Fig. 8a, b)and the reconstructed upper planation surface (Fig. 8c)is the base for interpretation of individual tectonicblocks in Fig. 8d. The southern mega block consists ofthree minor blocks. A major E–W fault scarp alongItilleq crosscuts the western part of this block with adown-throw to the north (Figs. 6b and 8d). The twosouthern blocks reach over 1800 m a.s.l. in the west. Thesurface of the larger block slopes down to about 600 ma.s.l. in the northeast. The northern mega block reaches1600 m in the southwest, in an area crosscut by tectoniclines, some with fault scarps limiting individual blockswith a NE to ENE tilt. The summit surfaces of the blocksslope down to about 600 m in the northeast.

3.5.3. Conclusions on uplift and block tiltingThe identified palaeosurfaces are used to set up a

relative chronology of events after the formation of theupper planation surface. The lower surface is developedon the expense of the upper, which is thus left more orless dissected. The present position of the two planation

Fig. 8. Summit envelope surface maps for identification of tectonic blocks, constructed from a rectangle with a) 10 km side b) 4 km side. Note that theice surface of Sukkertoppen Ice Cap is part of the data base, but that the ice thickness is mainly not very great. c) Reconstruction of the upper planationsurface over the whole study area. Crosses mark the grid net. d) Interpretation of tectonic blocks. The north and south mega blocks are separated bythe Sisimiut Line (S.L.) (thick line), approximately along an early Proterozoic structure (cf. Fig. 2).


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surfaces, with a lower surface incised in an uppersurface and on blocks differently tilted, suggests a firstuplift and tilting after formation of the upper planationsurface. A planation surface, which is inclined morethan 0.4% will not survive (Rudberg, 1970; Spönemann,1979). All erosion governed by gravity aims athorizontality, and thus a planation surface cannot formas an inclined surface across regions of large extent withseveral fluvial systems. It must have formed at a uniformlevel and the sea is the most probable base level. Toachieve such a wide surface as the extensive andregionally developed upper planation surface, stabletectonic conditions must have prevailed for a very longtime. The rearrangement of this surface to a highposition is therefore indicative of a period of activetectonic movements.

This first uplift has had a magnitude of less than500 m, which is the approximate height differencebetween the two planation surfaces in the areas ofmaximum uplift (Fig. 6). A second major event raisedthe two surfaces an additional 1000 m in the two upliftcentres and less along their flanks. The Sisimiut Lineseems to have been reactivated during these uplift eventsand the mapped fault scarps to have formed as aconsequence (Fig. 8). The uplift also initiated theerosion processes that led to the removal of Cretaceousor Palaeogene cover rocks and re-exposure of an etchedsurface with hilly relief in the north.

The upper and lower planation surfaces can becorrelated with similar surfaces on Nuussuaq and Diskodescribed in Section 2.3. If our study area experiencedthe same uplift events the upper planation surface is ofmiddle Miocene age, the lower planation surfacedeveloped after uplift starting at 11–10 Ma and thedeep valley incision is connected to a major uplift eventstarting at 7–2 Ma.

3.5.4. Dissection of palaeosurfacesThe western part of Angujaatorfiup Nunaa is a key

area for analysing the successive dissection of theplanation surfaces by valley incision. The tributaryrunning north to the Sarfartoq valley has a fluviallyformed canyon (Figs. 4[d], 6, 7)), between the twoglacially shaped valleys, Tasesiaq and Sarfartoq. Theupper valley is formed by glacial reshaping of a shallowfluvial palaeovalley. The step from the upper to thelower valley is caused by fluvial incision. The presentSarfartoq valley is glacially widened and lowered but itsoriginal incision and the formation of a step we thinkwas triggered by the preglacial uplift. The presentposition of the step has moved backwards in interactionbetween glacial lowering of the main valley and fluvial

backward erosion. In this way the two upper planationsurfaces have been partly destroyed north of Angujaa-torfiup Nunaa (Fig. 7).

The palaeosurfaces in the two high areas close tothe coast have, besides being dissected by valleys,partly been obliterated by erosion from cirque glaciers,resulting in a characteristic alpine landscape (Fig. 7).

3.6. The upper planation surface − a reference surfacefor estimation of erosion

3.6.1. MethodThe map (Fig. 8c) with the reconstructed shape of the

upper planation surface (see Section 3.5.2) was used asreference for estimation of erosion since its formation. Amap of erosion was obtained (Fig. 9) by subtracting thepresent topography from the reconstructed upperplanation surface. As no information about depth infjords has been used, the map only reflects erosionabove present sea level. The position of the upperplanation surface along the west coast is not known andtherefore there is no estimation of erosion here. Theestimated amount of erosion is dependent on the correctidentification of the upper planation surface ondifferently tilted tectonic blocks. The area marked witha question mark in Fig. 8c, has bended contours and thiseither marks a downfaulted block or that the identifica-tion of the palaeosurface is incorrect.

3.6.2. ResultThe map of erosion (Fig. 9) shows clear differences

in amount of erosion of the upper planation surface. Ithas in certain areas experienced almost no erosion sinceformation, while in other areas it has been totallyobliterated. Most erosion, 800–1300 m, since itsformation has occurred in the area of highest uplift.The summit surface of the low lying block to the southof the Sisimiut Line has suffered deeper erosion (up to400 m) than other inland areas. Areas underlain byamphibolite gneiss are much more eroded than areasunderlain by granulite gneiss, e.g. the area south ofSisimiut Line. The exception to this is the high coastalareas eroded by cirque and valley glaciers, where bothrock types are eroded, although minor flat remnants ofthe planation surface do occur (Fig. 4). On the otherhand areas underlain by amphibolite gneiss are wellpreserved close to the present border of the Sukkertop-pen Ice Cap indicating its protective role.

We think that the validity of the erosion map isfairly good (with exception of the area with questionmark: Fig. 8c). This is a first attempt to estimatedifferentiated erosion over a large area in West

Fig. 9. Absolute amount of erosion since uplift, calculated as the difference between the reconstructed upper planation surface (Fig. 8c) and thepresent relief.


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Greenland, and the method can be refined by still betterlandform analysis.

3.6.2.1. Relative effect of glacial erosion. The relativeeffect of glacial erosion versus fluvial erosion in thestudy area has not been evaluated in detail. The areasof preserved planation surfaces at high elevation haveonly experienced limited erosion (green to yellowareas in Fig. 9), while erosion has been morepronounced in other parts of the landscape. Theidentification of two distinct erosion generations, afully developed upper planation surface and a lowergeneration, mainly represented by wide valleys,possible to correlate with the aid of an inclined planeand distinct from the deeply incised valleys, givesreasons to regard erosion from the upper to the lowerplanation surface as mainly fluvial. Where thepalaeosurfaces are obliterated between valleys, erosionmust have been mainly glacial, while along the incisedvalleys erosion has been both fluvial and glacial. Evenif in detail the preserved planation surfaces arecharacterised by glacial scouring (Sugden, 1974; Fig.7a), our quantification shows a great variation in theerosional effect. Shallow valleys in the planated reliefshow minor glacial modification (Fig. 7b). Heavyerosion has affected the upper planation surface at theeastern part of the low-lying tectonic block south ofSisimiut Line (Figs. 4, 6, 9). The block is underlain byamphibolite gneiss and the erosion is interpreted asmainly glacial. An argument for this is the preservationof the preglacial surface on this type of gneiss belowthe Sukkertoppen Ice Cap (see above). Thus the glacialerosion here could be caused by a combination ofmechanically easily erodible bedrock (cf. Olvmo andJohansson, 2002) and by the position between twohigh areas with merging ice flows. Glacial erosion hasalso been severe in the western part of this block,which can be explained by confluent ice in combina-tion with easily erodible amphibolite gneiss. Severeglacial erosion has also affected the main valleys thathosted outlet glaciers.

4. Discussion

4.1. Formation and preservation of palaeosurfaces

4.1.1. A former sub-Ordovician peneplain?Tiny remnants of Lower Palaeozoic rocks (Secher

and Larsen, 1980; Pedersen and Peel, 1985; Larsen andRex, 1992; Larsen et al., 1999) give reasons to believethat central West Greenland had long-lasting Palaeo-zoic covers like Northern Greenland (e.g., Henriksen et

al., 2000) and the Baltic (Lidmar-Bergström, 1995) andCanadian shields (Ambrose, 1964). In these areasremnants of Palaeozoic covers are present across lowrelief in basement rocks. Ordovician rock fragments inintrusive rocks at Fossilik (south of Sukkertoppen IceCap) show the presence of an Ordovician cover duringthe Jurassic intrusive event in that area (L.M. Larsen,pers. comm. 2002). In the northern part of the studyarea any Palaeozoic cover was removed before thedeposition of the Cretaceous or Palaeogene coverrocks. It is, however, likely that the highest massifs inat least the southern part of the study area (Fig. 4)could be close to a sub-Ordovician planation surfacefrom which the present relief has developed (cf.primary peneplain in Scandinavia, Lidmar-Bergström,1996).

4.1.2. A stripped Late Mesozoic etch surfaceThe northern part of the study area is the

continuation of the basement ridge exposed at southernDisko (Fig. 1). At Disko the basement exhibitsstructurally controlled hills and weathered forms withassociated saprolite remnants close to and even below aPalaeocene basalt cover (Bonow, 2005). The north-eastern part of the study area is close to the erosionalboundary of Cretaceous cover rocks. A former extent inover the study area of these sediments is likely (Fig. 1).The erosional boundary of the basalt is fairly close inthe west (Fig. 1), and the basalt may thus have coveredparts of the study area as well.

The landform analysis led to the identification of ahilly relief along the coast, particularly well developedin the northern part of the study area. It was observedthat the distribution of hills is highly dependent on hostrock and density of joints and fractures and it is knownthat deep weathering exploits structural weaknesses inbedrock and enhances landform patterns controlled byfractures and joints (Thomas, 1966; Kroonenberg andMelitz, 1983; Thomas, 1994). Thus there are reasons tobelieve that the present landforms in the northern part ofthe study area, although glacially altered, have theirorigin in a Mesozoic and/or early Palaeocene deeplyweathered surface. The hilly relief has been preserveddue to protective cover rocks, that have been removedfairly recently, caused by a combination of fluvialprocesses triggered by Neogene uplift and of glacialerosion. The origin and age of the hilly coastal plainsouth of Nassuttooq is however unclear.

4.1.3. The Cenozoic planation surfacesThe analysis of landforms in the study area resulted

in identification of two planation surfaces at high


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altitude. Very little relief rises above the upper planationsurface but the existing residual massifs (Fig. 4) mayreach the level of a lost sub-Ordovician peneplain. Thelower surface is mainly a valley generation, which onlyin restricted areas has grown to wider plains. Theplanation surfaces are under destruction. They musthave formed by a fluvial system to former common baselevels and subsequently been uplifted (cf. Section 3.5.3).

The largely intact planation surfaces in the study areashow that the fluvial system was not efficient enough todissect the surfaces after uplift in preglacial times, andtherefore that uplift must have occurred late. The highlysituated planation surfaces are better preserved wherethe gneiss is in granulite facies. The surfaces are alsowell preserved in more easily eroded rock types near theSukkertoppen Ice Cap. It seems that ice-sheets duringthe Late Cenozoic have contributed in preserving theplanation surfaces.

4.2. Fluvial valley incision and glacial erosion

It has often been taken for granted that glacial erosionwas the main cause for valley incision in formerlyglaciated areas. However, studies of U-shaped valleys inrecently uplifted areas in New Zealand (Kirkbride andMatthews, 1997), Washington, USA (Montgomery,2002) and Scandinavia (Bonow et al., 2003) indicatethat major river systems were incised and graded prior toonset of glaciations.

The central parts of southern Norway have experi-enced long periods with cold based ice sheets duringthe Cenozoic glaciations and much of the preglaciallandscape is preserved (Sollid and Sørbel, 1994). HereBonow et al. (2003) chose an area where the preglacialfluvial landscape development was analysed. The majorvalley, Gudbrandsdalen, is wide but clearly glaciallyreshaped, however it has a tributary valley (Lordalen)of the same wideness but with almost no glacialreshaping. Further, resistant rocks have at two locationsalong the main valley preserved its old valley bottom.These valley benches were correlated with valleybenches further upstream, beyond the present waterdivide. This reconstructed valley floor fitted theconnection to the wide fluvial Lordalen. Thus, theincision of Gudbrandsdalen in the uplifted palaeosur-face was shown to originally have been fluvial.Lordalen was used as reference in evaluating the glacialeffect in other parts of the river system. It wasconcluded that the glacial erosion of the main valleyGudbrandsdalen was up to 150 m (valley fill notincluded) but over 300 m (valley fill not included) in aneighbouring major tributary.

In the present study area the planation surfaces havepartly been dissected by fluvial and glacial erosion.Kangerlussuaq, Nordre Isortoq and Itilleq fjords all cutwater gaps across areas with preserved planationsurfaces above 1000 m a.s.l. The step from the lowererosion surface to the valley bottoms is interpreted asoriginally caused by fluvial incision (cf. Section 3.5.4).The valleys have been deepened and widened by glacialerosion to different degrees depending on location (Fig.7). Interaction of fluvial and glacial erosion alongvalleys during repeated glaciations and interglacialshave contributed in the backward retreat of the step.Fluvial and glacial interactions have been a commonphenomenon in reshaping preglacial landscapes informerly glaciated areas (Rudberg, 1993; Kleman andStroeven, 1997).

4.3. Palaeosurfaces as geomorphological markers

4.3.1. Palaeosurfaces for identification of tectonicmovements

In this study we have been able to confirm alandscape development by fluvial incision in a valley-in-valley system as was concluded for southern Norway(Bonow et al., 2003). From our studies we have gotinput for the formulation of a landscape developmentmodel, which integrate different parts of older models.First, we adapt the Baulig (1935) idea of a valley-in-valley system often forced by land uplift. Second, weadopt the idea of formation of valley bottoms with lowinclination, which we call pediment (King, 1967; 1976;Ahnert, 1998: rock-floored terraces p.183–184, pedi-ments and piedmonttreppen p.223–227). It was noted inthe study from southern Norway (Bonow et al., 2003)that slopes are maintained between surface and thereforewe adopt a model with slope retreat (with its exact formdepending on bedrock) and not slope decline asproposed by Davis (1899). The Davis model does notallow the formation of stepped surfaces (Twidale, 1976)and is therefore not applicable. Ideas similar to our wayof thinking we find in Ahnert (1982), Demoulin (1995)and Huguet (1996). The formation of large flat surfaceswe think largely have occurred in semi-humid and semi-arid climates during the late Palaeogene and theNeogene. Regarding the dissection of the flat surfaceswe also think that uplift has been more important thanclimate change for triggering enhanced fluvial incision(cf. Ahnert, 1970), and that climate change in certaincases may be caused by uplift.

Other researchers favour a dynamic approach and tryto establish “variations in rates of denudation acrosslandscapes, rather than attempting to diagnose the


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amount and timing of tectonic uplift on the basis ofcorrelating erosion-surface remnants” (Brown et al.,2000). The importance of former base levels for thefluvial system that once shaped now uplifted landscapeswas, however, acknowledged by Schoenbohm et al.(2004), Sugden and Denton (2004), and Clark et al.(2005), and the former base level positions were used asevidence for late uplift.

Sea level has been the most probable general baselevel for surface formation in the study area. Conse-quently, identification and mapping of planation sur-faces turned out to be important tools for conclusions onboth block tilting and amounts of uplift.

4.3.2. Palaeosurfaces as reference for fluvial andglacial erosion

Sugden and Denton (2004) use the existence oferosion surfaces with upstanding inselbergs and valleybenches backed by rectilinear slopes as strong evidencefor preservation of former fluvial landscapes inAntarctica and for only limited glacial erosion. Thereis always a risk for circularity in the reasoning of what isglacial and what is pre-glacial, but there are today muchindependent evidences for the selectivity of glacialerosion (Sugden, 1978) from both paleoglaciologists(e.g., Kleman and Hättestrand, 1999) and from studiesof preserved palaeosurfaces of different shape andinclination (e.g., Lidmar-Bergström, 1988), that non-glacial explanations cannot be ignored. The twoplanation surfaces in central West Greenland, the fullydeveloped and the partly developed, are evidences ofpreglacial fluvially formed palaeosurfaces. As we haveshown, the surfaces can be used as markers forestimation of erosion and qualitative judgements ofthe glacial impact.

5. Conclusions

A major planation surface cuts across different typesof basement rocks in central West Greenland south ofDisko Bugt. In the northern parts of the study area theplanation surface cuts off an inclined surface with hillyrelief, exhumed from Upper Cretaceous cover rocks,which determines the age of the planation surface to theCenozoic. South of Nassuttooq, the hilly relief surface isdelimited by a coastal escarpment in the east, and hereits age is unclear. The planation surface was broken andtilted in different directions during two uplift events,concentrated around two centra, Nordre Isortoq andSukkertoppen Ice Cap. A partially developed lowerplanation surface indicates a first uplift of maximum500 m followed by a second uplift of maximum 1000 m.

Correlation with similar erosion surfaces on Nuussuaqsuggests that uplift in the study area occurred in the lateNeogene, as the onset of two exhumation events at 11–10 and 7–2 Ma, have been interpreted from analysis offission track data from Nuussuaq.

Differential erosion has affected the area sinceformation of the upper planation surface, which is bestpreserved in areas with Archean gneisses in granulitefacies. The highest amount of erosion (800–1300 m) hasoccurred in the areas of maximum uplift, while highlysituated plateaus are partly not at all affected. Fluvialprocesses are considered responsible for most of theerosion to the level of the lower planation surface.Fluvial processes, triggered by uplift, were alsoresponsible for the initial incision of deep valleys. Theexhumation of the etch surface in the north was causedby a combination of fluvial processes due to uplift andthe late Cenozoic glaciations. Glacial erosion has had agreat effect on low lying areas and especially withinareas underlain by amphibolite gneisses.

The mapping of planation surfaces in central WestGreenland, by landform analysis in combination withelevation data, provided a direct measure of the lateralvariation of uplift, recognition of block movements, andestimation of amounts of erosion since the formation ofthe upper planation surface.

Acknowledgements

We acknowledge the support of the Danish NaturalScience Research Council, GEUS, the Swedish Re-search Council, the Carlsberg Foundation, ArktiskStation, Stiftelsen Margit Althins stipendiefond(KVA), John Söderbergs stiftelse and Svenska Sällska-pet för Antropologi och Geografi (Andreéfonden). Wewant to thank James A. Chalmers, Clas Hättestrand,Jens-Ove Näslund, Asger Ken Pedersen and ChrisPulvertaft for fruitful discussions. Challenging andprovoking comments by Paul Bishop, Mauro Coltortiand one anonymous referee forced us to clarify ourviews, and thus the paper was significantly enhanced.The digital elevation model is with courtesy from KMS,Copenhagen. The paper is published with permission ofthe Geological Survey of Denmark and Greenland.

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