New insight into the structure of the Nuussuaq Basin, central West Greenland

Marine and Petroleum Geology ELSEVIER

Marme and PetroleumGeology 16 (1999) 197-224

New insight into the structure of the Nuussuaq Basin, central West Greenland J.A. Chalmers”, *, T.C.R. Pulvertaft”,

C. Marcussen”, A.K. Pedersenb

Abstract Interpretation of seismic and magnetxc data, forward modellmg of gravity profiles and a reappraisal of all available data on faults onshore provides the first revision m 30 years of our understandmg of the structure of the Nuussuaq Basin, central West Greenland In the western part of the area Mesoro~ sediments at least 6 km and possibly up to 10 km thick occur in an early rift basin dommated by N-S faults. Recently discovered al m surface seeps and in shallow boreholes occurs almost exclusively in the early rift basin. In the east, sediments are thinner, and faults trend both N-S and WNW-SSE, the latter parallel to shear zones in the adjoining basement area. The eastern area may be part of a Late Cretaceous thermal subsidence basin. Renewed faulting mvolvmg both reactivation of older faults and generation of new faults took place in latest Cretaceous-early Paleocene time, and was followed by extensive erosion and phases of incision and infilling of valley systems. Renewed subsidence occurred immediately prior to the eruption of extensive middle Paleocene and Eocene continental flood basalts. The final phase of faulting took place in connection with sea-floor spreading in BatFin Bay and the Labrador Sea during the Eocene. Movement of North America relative to Greenland was transferred from the Labrador Sea to Baffin Bay along a strike-slip fault system m continental crust, the Ungava transform fracture zone. A splay of this system gave rise to a prominent SW-NE fault in the western part of the basin. c’ 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction The most extensive outcrops of Mesozoic-Palaeogene rocks in the entire Labrador Sea-Davis Strait&Baffin Bay region occur in the Disko-Nuussuaq~Svartenhuk area of central West Greenland. The exposed rocks are fluviodeltaic and marine sediments ofAlbian to early Paleocene age, that are overlain by extensive middle Paleocene and Eocene hyaloclastic breccias and continental flood hasalts. The Mesozoic-Paleocene depositional area is referred to as the Nuussuaq Basin. This basin belongs to a complex of sedimentary basins which were established in the Early Cretaceous, if not earlier, and extend from Melville Bay in the north (Whlttaker, Hamann & Pulv&aft, 1997) to the shelf and adjacent deeper waters areas off FrederikshHb Isbhnk (ca. 62’N) to the south (Chalmers et al., 1993; Chalmers, Dahl-Jensen, Bate & Whittaker, 1995). From the point of view of the 011industry, the Nuussuaq Basin was seen for many years as a model for what *Correspo”dmg

author.

e-mad:,acrageus.dk 0264.8172,99/S. seefront PI,: SO264-S,72(98)000,,-4

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might occur in basins offshore central and southern West Greenland, the primary targets for petroleum exploration in Greenland in the 1970s (Ehman, Sodero & Wise, 1976). This has also been the motwe for much of the work carried out in the basin by the Geological Surveys since the l97Os, firstly by the Geological Survey of Greenland (GGU), and since the amalgamation of this survey with the Geological Survey of Denmark in 1995, by the Geological Survey of Denmark and Greenland (GEUS). When, however, in 1992 bitumen was discovered in vugs in basalts near the base of the lava pile, attention began to focus on the petroleum potential of the Nuussuaq Basin itself (Christxmsen, Dam & Pedersen, 1994). Since the original discovery, oil and bitumen have been found m surface outcrops over a wide area in western Nuussuaq and also on the north side of Disko and on the southeast corner of Svartenhuk Halve (‘halw’=penmsula) (Fig. I). Furthermore, oil bled freely from the cores of two of the five slim core wells drilled in western Nuussuaq in 1993-95 (Fig. I): Marraat-I, operator GGU (Chnstiansen et al., 1994), and GANE# I, operator gronArctic Energy Inc., the Calgary-based company that held a concession in western

matter Qm1999Elsewer Saence Ltd All rights reserved.

Nuussuaq from 1994 to 1998 (Christiansen, Bate, Dam, Marcussen & Pulvertaft, 1996). Encouraged by these results, grmArctic drdled a conventional well to 2996 m

in western Nuussuaq in 1996 (GRO#3; al., 1997; see Fig. I). An important step in the investigation

Christiansen of the Nuussuaq

et

Basin was GGU’s acquisition in 1994 of a 13 km reflection seismic line cm the south shore of Nuussuaq. This revealed that the base of the sedimentary basin here is at least 5 km below sea-level (Christiansen, Marcussen & Chalmers, 1995), which was much deeper than hitherto imagined. Obviously one had no idea what the deep structure of the basin was like, nor did one have sufficient information to point to where hydrocarbons could have been generated and where future exploration should be focused. It was natural therefore that some lines in Disko Bugt and the fiords around Nuussuaq should be included in the 1995 sasmic programme sponsored jointly by the Danish State and Greenland Home Rule. In addition to seismic data, gravity data were collected during the cruise and these data have been added to the gravity database held by the Damsh National Survey and Cadastre (formerly the Geodetic Institute). Gravity data have proved to be of great importance m the interpretation of the deep wucture in the basin, because in much of the area it has not been possible to see below the first sea-bed multiple on the seismic lines. Further supplementary informatmn was obtamed from single-channel seismic surveys carried out by GGU in the 1970s (Denham, 1974; Brett &Zarudski, 1979), together with the magnetic data acquired during these surveys and the new aeromagnetic anomaly map arising from the aeromagnetic survey flown over the area m 1997 (Thorning, in press). The present article presents the results of an interpretation of all available geophysical data, integrated with a compilation of all relevant onshore data. In spite of the progress that has been made, the open spacing of the seismic lines and the almost complete lack of information below the first seabed multlple make it impossible to present a definitive structural model at this stage. The results are nevertheless a step forward in the understanding of the basin and provide some constraints which tectonic models for the region will have to respect.

2. Summary

of stratigraphy

of rocks in outcrop

The main outcrops of Cretaceous sediments are in eastern Disko and on Nuussuaq west of the IkorfatSaqqaqdalen fault system (Fig. 1). These sediments, which are of Late Albian to early Campanian age, were deposited in a fluvial- and wave-dominated delta environment (Pedersen & Pulvertaft, 1992). The delta fanned out to the west and northwest from a point east of Disko island, reachmg into deeper water in the position of present-day northwest Nuussuaq and Svartenhuk Halvs. Fluvial sandstones with minor mudstone and coal characterize the south and east of the outcrop area (Johannessen & Nielsen, 1982; Koppelhus & Pedersen, 1993). To the northwest these gwe way to stacked, typical deltaic, coarsening-upwards successions, each starting with interdistributary bay mudstones and endmg with

coal (Olsen, 1993). The non-marine and marginal marme sediments of this delta system have been referred to as the Atane Formation (Pedersen & Pulvertaft, 1992). There is a general tendency for outcrops of the Atane Formation to become younger towards northwest, although dip directions are mostly to NE. This requires the presence of faults with downthrow to the west; these are discussed in a later section. Sediments equivalent m age to the upper part of the Atane Formation were penetrated below 960m in the GRO # 3 well (Fig. 2) and are well exposed on the southeast side of the Itilli valley. These sediments, which are provisionally referred to as the Itilli succession. consist of mudstones alternating with up to 50m thick turbldlte channel sandstones that were deposited on a aubmarme slope (Dam & Ssnderholm, 1994). The Itilli succession is at least 2.5 km thick. The oldest datable sediments in this succession are Coma&n, but in the lowest 1500 m of the succession, no identifiable palynomorphs have been recorded due to the effects of heating (H. Nnhr-Hanson, pus. comm. 199X). In northern Nuussuaq, west of the Ikorfat fault, the Cretaceous is represented by dark marine mudstones with thin streaks of turbidite sandstone (Dam, 1996; NshrHanson & Dam. 1997). These sediments are of Campaman and Maastrichtian age (Birkelund, 1965; NnhrHansen, 1996) and hence are the age equivalent of the upper part of the Itilh successmn (Fig. 2). Cretaceous sediments are also exposed on the north side of Nuussuaq east of the lkorfat fault (Midtgaard, 1996a). Here a ca. ROOm thick succession forms part of a N- to NE-dippmg ramp linking the lkorfat and Kuuk faults (the Qaarsut ramp, Fig. I). This successmn consists mainly of sandstones, heteroliths and mudstones of Ruvial, estuanne, deltaic and lacustrine origin. At the base of the succession, coarse breccias and conglomerates are banked up against basement highs, which may have been fault-controlled. About l50m higher up there 1s a unit consisting mainly of inner shelf marine mudstones with thin storm-deposited sandstones lenses and layers (Midtgaard, lYY6b). Brackwater dinoflagellates found in this unit indicate that the sediments are of Late Albian age (H. Nshr-Hansen, pas. comm. 1996) and hence provide the earhest evidence of a marine transgression in the exposed parts of the Nuussuaq Basin. A 4-Y angular unconformity separates this unit from the overlymg Ruvial channel-fill sandstone. The succession is overlain unconformably either by mudstones yielding Maastrichtxm ammonites (Birkelund. 1965) or, where these are absent. by almost horizontal Paleocene volcanic rocks. Palaeocurrent data from the succcssmn, combined with the westwards thickening of the lowermost units, led Midtgaard (1996a) to suggest that the whole succession was deposited in an actwe half-graben wth its boundmg fault to the west. Orientation of wave-ripple crestlines m

Pig. 2. Generahsedstratigraphy of the Cretaceous-Palaeogenesedimentson Nuussuaq, West Greenland Note that the haghts of the stratrgraphlc columns relateto the chronostratrgraphx dwisions on the left, not to the thlckncsses of the aedmxntary succcss~ons.Fig. prepared by G Dam.

the shallow marine mtervals indicate that the coastline had a general E-W orientation. Sediments that have been approximately dated as Late Albian to Early Cenomanian (Ehman et al., 1976; Croxton, 1978) outcrop on the southwest corner of Upernivik 0 (‘~‘=~sland). About half of the exposed section here consists of up to 30 m thick multi-storey layers of coarsegained sandstones that were deposited by braided rivers (Midtgaard, 1996a). The braded river deposits are interbedded with sandstones, heteroliths and mudstones deposited in tidal estuarine and coastal plain environments. Tidal currents were directed between NNW and NE, mean N. Sediments either belonging to or at least similar to the Atane Formation must underlie the basalts in northwest Disko, since inclusions of sandstones with chert pebbles very similar to some of the Atane Formation sandstones exposed in east Disko occur in both lavas and volcanic necks in this area. Furthermore, contaminated dykes and laws containing iron, sulphides and numerous inclusions

of dark, fissile mudstone occur throughout western Dlsko (Pedersen, 1977a,b,c, 1985; Ulff-Miller, 1977), bearmg wtness of the existence of underlying sediments with extensive dark, organic-rich mudstone. At some time during the Maastrichtian, the area became tectomcally unstable. On Nuussuaq there IS evidence that at least three phases of uplift during the Maastrichtian and early Paleocene were each followed by incision of valleys in the underlying sediments (Fig. 2). Conglomerates, turbiditic and fluvial sands and estuarine-marine mudstones of late Maastrichtmn to middle Paleocene age filled the valleys (Dam & Sanderholm, 1994, 1998; Dam, Larsen & Sernderholm, 1998). In places there is an angular unconformity between tilted A&me Formation sediments and the overlymg sediments of Maastrichtian or early Paleocene age. Palaeogene volcanism began in a subaqueous environment, and the earliest lavas formed pillow lava and hyaloclastite mounds on the sea floor. As the volcanic edifices grew, they emerged above sea-level so that lava began

to be erupted subaerially. However, at the shorehne the lava entered the sea and formed eastwards-prograding Gilbert-type delta structures with cross-bedded hyaloclastite sets up to 700111 thick (Pedersen, Larsen & Dueholm, 1993). The growing volcanic pile dammed up lakes to the east in which organic-rich lacustrine mudstones were deposited (Pedersen, 1989; Pedersen, Larsen, Pedersen & Hjortkjaer, 1998). In southeast Nuussuaq these lacustrine mudstones rest unconformably on tilted Auvm-deltaic sedmxnts of the Atane Formation; the surface of unconformity shows local relief up to 250m (Pulvertaft, 1989). In due course all remaining embayments and lakes were eliminated, and eruptlon became entirely sub-aerial. Plateau laws spread farther east, finally overlapping onto Precambrian basement. Petrologically, the early basalts are high-temperature picrites; these are referred to the Vaigat Formation which IS generally less than 2 km thick. Wlthin the Valgat Formation there are contaminated lava units that in the field, in colour aerial photographs and also geochemically are distinctwe and easy to identify. Mappmg of these has provided accurate information on the position and postbasalt displacement of many faults in the area The overlying basalts are referred to the Maligst Formation (Hald & Pedersen, 1975). This consists mainly of feldsparphyrlc tholaites and is at least 2km thick. Six ““ArpAr age determinations of samples from the Vaigat and MaligPt formations ylelded good plateau ages between 60.4kO.5 and 59.4kO.5 Ma (Storey, Duncan, Pedersen, Larsen & Larsen. 1998). These ages are within magnetochron 26r (Cande & Kent, 1995) but the lowermost laws of the Vaigat Formation show normal magnetization and must therefore have been erupted durmg chron 27n (Rilsager & Abrahamsen, 1998). Conspicuous dolerite sheets intruded into the boundary fault and surrounding rocks in Saqqaqdalen (the Tartunaq intrusions) belong to a group of intrusions that have been dated at 54.8kO.4 Ma, which IS within magnetochron 24r. At the northwest extremity of Nuussuaq peninsula there are approximately 2 km of lower Eocene basalts. A comendlte tuff within these basalts has yielded an age of 52.550.2 Ma correspondmg to the transition between magnetochrons 24n and 23r (Cande & Kent, 1995). There is evidence of even younger volcanism in the area in the form of dykes and necks as young as 27.4kO.6 Ma (Storey et al., 1998). Over most of the area, the plateau lava dip less than 5 in different dlrectlons. However, m northwest Disko and northwest Nuussuaq and on Ubekendt Ejland much steeper dips occur, mainly to the northwest and west. On Ubekendt Ejland the basalts show arcuate strike and ubiquitous dips up to 44 towards W. Simple trigonometry indicates that the thickness of the laws on Ubekendt Ejland is Y km (Larsen, 1977). This however IS probably a deception. There are faults on the island with downthrow to theeast, but due to themonotonous nature

of the Vaigat Formation here, it has not been possible to measure the amount of displacement on these. This problem also occurs on Svartenhuk Halva. 3. Faults and related structure in the exposed area The orientation and displacement of faults that can be demonstrated in outcrops have a strong influence on how faults and other structures seen on the very widely spaced sewnic lines and interpreted on gravity profiles are extrapolated in plan view. For this reason the faults and other structures in the exposed area are described before the interpretation of the seismic and gravity data are discussed. 3. I. The boundarq J&/r

system

The present-day eastern boundary of the Nuussuaq Basin is marked by a system of faults with an overall NNW-SSE trend that runs from Svartenhuk Halva in the north through western Upernivikland central Nuussuaq mto Disko Bugt (Figs. I and 4). In detail the fault system can be seen to be made up of fault segments oriented in dxectmns between NW -SE and NNE-SSW that are linked by transfer faults trending between W-E and WNW--ENE. Basinward dips between 47’ and 73 have been recorded on the fault planes (Rosenkranta & Pulvertaft, 1969; Pulvertaft, 1979, 1989). Downthrow on the boundary faults can exceed 2 km, as for example in southwest Upernivik @and at lkorfat on the north side of Nuussuaq (see later). The fault blocks adjacent to the boundary faults have been rotated up to 20’ towards the faults; this rotation has however not affected the Maastrichtian and Paleocene sediments that overlie the tilted blocks on Nuussuaq, providing a constraint on the timing of the tilting. On the north side of Nuussuaq, the Kuuk and IkorfatSaqqaqdalen segments of the boundary fault system are linked by the Qaarsut ramp. The gneiss surface and overlying Upper Alblan sediments in this ramp dip 9-16’ towards N and NE (Pulvertaft, 1979; H. Cry. field notes 1939). Three subsidiary N- to NNE-trending faults, each with a downthrow of 35@400 m to the west. dissect the ramp, and there are several small WNW-ESE faults wth downthrow to the north (Mldtgaard, lYY6a), The situation resembles on a larger scale the well-exposed softlinked ramps in Canyonlands Nattonal Park, Utah, illustrated by Trudgill & Cartwright (1994). The tnning of movement on the boundary fault system has been discussed by Rosenkrantz & Pulvertaft (1969) and Pulvertaft (1979, lY89), in particular the question as to whether the Cretaceous sediments are syn-rift relative to movements on the boundary faults. Indications of syn-rift sedimentation are seen on Upernivik 0. On the northwest corner of the island there is a

small outcrop of mid-Cretaceous sedmxnts that include conglomerate with gneiss boulders up to 2m m sire (Henderson & Pulvertaft, 1987). In southwest Upernivik 0 there are conglomerates wth cobbles and boulders up to 2Ocm in SIX in the Upper Albian~Cenomanian sediments adjacent to the E-W segment of the fault system and in outcrops near the south coast, suggestmg the proximity of steep slopes here during sedimentation. On the other hand, palaeocurrents in the tidal facies of the succession here flowed in directions between NNW and NE (Midtgaard, lYY6a), which does not suggest that there was a syn-sedimentary fault scarp in the position of the present-day faults. On the north side of Nuussuaq, channel sandstones and dark silty mudstones of the Late Albian fan-delta plain are cut off abruptly to the east by the Kuuk fault (Pulvertaft, 1979). There is no change whatsoever in the character of the sediments and no coarsening approaching the fault, nor does interpretation of palaeogeography during deposition of the Upper Albian sediments east of lkorfat (Midtgaard, lYY6a) suggest the existence of a fault scarp or coastline in the position of the Kuuk fault at this time. It is therefore concluded that the Upper Albian sediments originally extended east of their present-day outcrop in this area and that movement on the fault and associated tilting of the sediments was largely post-late Albian. Tilting must however have been preMaastrichtian, because Maastrichtian mudstones lie unconformably on the tilted Upper Albian sediments east of Ikorfat. The relations between Paleocene basalts and the Kuuk fdt are also seen in outcrop south of Kuuk. Here the basalts, which are not tilted, are in sub-aqueous facie& and it can be seen that the eastwards-flowing hyaloclastites have been dammed up against the Kuuk fault scarp. Since there is virtually no bevelling of the fault scarp by erosion, only a short time can have elapsed between formation of the fault scarp and eruption of the hyaloclastites. This suggests a second, post-tilting, phase of movement on the Kuuk fault. At lkorfat the Ikorfat-Saqqaqdalen fault IS a combinatlon of an extcnsmnal fault and a monocline. Displacement here took place in at least three phases. The net post-Vzaigat Formation downthrow IS 510 m; this is accurately known from the displacement of the Tunoqqu Member, one of the contaminated marker units m the formation (Pedersen, Larsen, Pedersen & Dueholm, 1996). Maastrlchtian mudstones are displaced at least 600111 and perhaps as much as 725111. The oldest sediments exposed at sea-level west of the fault are of Campaman age, so the entire pre-Campanian has been downfaulted out of sight, implying a total downthrow of more than 2 km on the fault. Displacement during the first, pre-Maastrichtian, phase(s) of faulting must therefore have been in excess of 1275 m. In Saqqaqdalen the boundary fault plane was intruded

by prominent dolerite sheets belonging to the Tartunaq sute that has been dated at 54.X&0.4 Ma (Storey et al., 1998). The dolerltes in the fault plane are not deformed, so movement on the Ikorfat-Saqqaqdalen fault must have died out completely by 55 Ma. 3.2. Faults within the busin: pre-basaltfaults Field and precision photogrammetric mapping of contaminated lava marker horizons in the Vaigat Formation has provided accurate control of the position and displacement of most faults that have been active since eruption of this formatmn. Faulting that has affected only the Cretaceous~Paleocene sediments is less well documented, partly because of lack of marker horizons in the Atane Formation, and partly because these sediments are exposed mainly in ravines, while the intervening slopes are covered by scree and solifluction deposits. Two pre-basalt faults have been observed on the south side of Nuussuaq west of Ataata Kuua; these trend 124 and 161’ respectively. Both faults downthrow upper Maastrichtlan-lower Paleocene valley till sediments about 300111 to the northeast, and both are truncated by the base of the overlying middle Paleocene valley system (C. Dam&M. Sonderholm, pas. comm., 1997). Another post-Maastrichtian, pre-middle Paleocene fault trending about 124’ has been mapped at Ataata Kuua (Pulvertaft & Chalmers, 1990); this has a downthrow to the southwest of about IOOm. Other pre-basalt faults must occur in the basin, but due to poor outcrop, their position IS conjectured. One such fault is believed to cross the southwest shore of Nuussuaq at Kingittoq, where a fault is requred to account for the fact that Santonian~Campanian sediments occur northwest of Kingittoq while east of Kingittoq the sediments are Cenomanian in age and dip NE. This fault may extend towards north-northwest to join the boundary fault system on the south side ofAaf&rsuaq (Figs. I, 4 and 16). Although the displacement of the Cretaceous strata was down to the west, the overlying subaerial laws define a flexure here with a relative downsagging to the east of about 400 m (Pedersen & Dueholm, 1992). indicating a post-middle Paleocene reversal of dlsplacement along the Kingittoq fault. Similar age relationships and dip directions require the existence of other prebasalt faults with downthrow to the west both between Ataata Kuua and Nuuk Killeq (Pulvertaft, 1987) and in the Aaffarsuaq valley. Faulting immediately prmr to eruption of the Vaigat Formation hyaloclastite breccias is mdlcated by abrupt steps III the sub-hyaloclastm. surface. These are seen at three localities: 3.5 km east of the mouth of Kuugannguaq on the north coast of Disko, Nuuk Kdleq on the south coast of Nuussuaq, and on the south side of Aaffarsuaq. At all three localities the surface on which the hyaloclastite breccias accumulated rises abruptly east-

wards by 400-500m. These steps are regarded as expressmns of fault scarps formed during the rapid subsidence that preceded the extrusion of the breccias. They are not due to younger faultmg because they have no influence whatsoever on the lowest subarxd flows above the breccias. 3.3. Pm&husult

faulting

wrhln

the husin

A fault along the valley Agatdalen trends about 125” and has a downthrow of 13s300m to the northeast, as indicated by the displacement of the distinctive Tunoqqu Member and beds below that carry early Danian corals (Flons, 1972). Several N-S faults displace the basalts in northwest Disko and between Marraat and Qunnilik on Nuussuaq. The most unportant of these is the Kuugannguaq-Qunnilik fault. This can be traced from west of Qeqertarsuaq m south Disko to north of Qunnilik on Nuussuaq where it bifurcates. Displacement is down to the west, greatest in the north where the Tunoqqu Member is downthrown 700 m and least in central Dlsko where post-basalt throw is less than 100 m. Another significant N-S fault, the Gassn fault, with downthrow to the west occurs east of Marraat and is interpreted to run just west of the GRO#3 well. The post-Vaigat Formation downthrow of this fault together wth a lesser N-S fault to the west is of the order of 900 m. Most of the N-S faults in northwest Disko have a modest downthrow to the west, but a few downthrow to the east. gwing rise to small horsts and grabens. In the area around Marraat there are several faults, most of which trend about 150’; this IS oblique to the approximately N-S strike of the laws here which have an easterly dip of up to 25’ (Henderson, 1975). The cumulative downthrow across the 3 km wide fault zone is about 80011~ to the southwest. The Marraat fault zone appears to continue across the large delta at the mouth of AafYxsuaq to Nlaqornaarsuk on the south coast of Nuussuaq, where there are numerous prominent fractures on strike with the Marraat faults. However, the rocks here are mainly hyaloclastite breccias, which makes it dificult to assess the displacement on the fractures. 3.4.

T/w III//;

fault zone

The ltilli fault done is a conspnmus structural feature trending 37 from the southeast corner of Hareaen to the north cwnst of Nuussuaq. The general displacement on the fault zone is a downthrow to the northwest. On Harenen the basalt stratigraphy requires a downthrow of more than a kilometre. In the central part of the Itilli valley the lower part of the Upper Cretaceous-Paleocene ltilh succession abuts to the northwest against sub-aerial basalts of the Vaigat and Maliggt formatmns; the entire hyaloclastic breccia unit and the upper part of the ltilli

succession have been downfaulted out of sight, while on the southeast side of the valley the ltilli succession has been arched up into an eastwards-plunging anticline, the Ukalersalik anticline (Fig. 3). The net vertical displacement here must be more than 3 km, most of which is uplift in the axial zone of the Ukalersahk anticline rather than downthrow on the northwest side of the fault. The fault zone is complex, and there are pop-up lenses in places, for example at Tupersuartaa where an outcrop of W-dipping mudstones containing Santonian ammonites (Birkelund, 1965) is fault-bounded and Ranked both to the east and to the west by younger rocks. Northwest of where the ltilli fault zone emerges on the southwest shore of Nuussuaq there are several extensional faults trendmg between 120’ and 150 Both these extensional features and the compressional Ukalersahk anticlme are considered to be evidence in support of the suggestion that the ltilh fault zone is a left-lateral splay from the northern extension of the Ungava Fracture Zone (Chalmers et al., 1993). At the termination of a strike-slip fault zone, axes of folds in compressive areas and the strike of normal faults in dilatatmnal areas are both at a high angle to the zone (Fig. 3; Sanderson & Marchini, 1984). As already mentioned, there appears to be a relationship between relatively steep dips in northwest Nuussuaq and the ltilli fault zone. On the southeast side of the fault, sediments in the proximity of the fault dip up to 30’ to east, not only m the Ukalersalik antichne but also near the north coast of Nuussuaq. Northwest of the fault zone the basalts strike fairly consistently between 24’ and 52 and dip 13-22 to NW. except close to the fault where

both horizontal and erratic steep dips occur, the latter m connection with the extensional faults that strike at a wide angle to the main fault. The tilting of the basalts northwest of the fault zone is a young feature, since the comendite tuff dated at 52.5 Ma (Storey et al., 1998) is interbedded with basalt laws showing the same general NW dip as the remainder of the basalts northwest of the fault. Hence the Itilli fault zone is also regarded as a relatively young feature 3.5. Faults and shear zones in the Precambrian urea

basement

The development of rift basins is often influenced by older structures m the underlying basement (Patton, Moustafa, Nelson & Abdine, 1994; Rmg, 1994), and to some extent this appears to be the case in the Nuussuaq Basin. In the basement east of Disko Bugt and in the southeast part of Nuussuaq peninsula there are both ductile shear zones and brittle faults with strike between 110” and 150’ (Garde, 1994). Transfer segments along the IkorfatSaqqaqdalen fault and of several of the faults described within the basin also strike in this direction, with a preference for directions around 125”. Therefore it is reasonable to interpret faults in this direction when interpreting the geophysical data and extrapolatmg faults between seismic lines offshore and the onshore area. East of Uummannaq and south of Disko Bugt there are several faults striking approximately 20’. These are faults wth left-lateral displacements up to I .2 km, sometimes accompanied by a downthrow to the west of up to 100 m. Faults m the same direction occur in the Nuussuaq Basin at Kuuk and between Kuuk and Ikorfat, and rarely also on Disko, so this direction is also taken into consideration when interpreting the geophysical data. 3.6. The Disko gneiss rrdge Outcrops of Precambrian gneisses and metasediments along the shores of fjords and in valleys m an 18km wide zone extending north from Qeqertarsuaq are the expression of a N-S-trending gneiss ridge m west central Disko. The surface of the ridge rises to as much as 700 m a.s.1. in central Disko. The ridge extends at least as far north as the Kuugannguaq valley, where gneiss was struck at a level of 161 m a.s.1. in borehole FP93-3-l drilled by Falconbridge in their search for hard minerals (K. Olshefsky, pew comm.). The gneiss ridge 1s onlapped and overlain by plateau basalts. The southern part of the Kuugannguaq-Qunnilik fault probably represents reactivation of a fault along the western margin of the ridge, but there has been no significant displacement of basal& along the eastern margin of the ridge, which must therefore be a pre-basalt structure. Its relationship to the Cretaceous sediments

will be discussed later when the seismic and gravity data are described. 4. Shallow structure

from seismic and magnetic data

4.1. Seismic data (Fig. 4) Seismic data from four surveys have been used in the interpretation presented in this article: A 13 km long digital seisnuc line, GGU/NU94-01, acquired onshore along the south coast of Nuussuaq m 1994 using explosives as source. Processing was to stack stage only, 71 I km multichannel seismic data along 8 lines offshore in Dlsko Bugt, Vaigat and north of Nuussuaq acquired by GGU (GEUS) in 1995. The lines m Disko Bugt were acquired using sleeve airguns firing at 25 m intervals as the source and a 240.channel, 3-km long digital recaver array. The data were processed to produce 2 x ho-fold nugrations. Because of the high density of icebergs in Vaigat and in Uummannaq Fjord and consequent danger to towed equipment, the length of seismic streamer that could be deployed on the lines in these areas was limited to 1200m (Christiansen et al., 1996) using 96.channels and a shotpoint interval of IX.75 m. These data were processed to yield 3 x 32.fold migrations (12.5 m trace spacing). This streamer length is insufficient to attenuate the sea-bed multiples using differentxd move-out, so on hnes @X/95-06, -08, -18 and -19 only reflections that originate from two-way tunes (TWT) less than those of the first sea-bed multiple can be interpreted. Single-channel analogue seismic data and magnetic data acquired in 1970 by M.S. Brundal (Denham, 1974) and in 1978 by M.V. Dana (Brett & Zarudski, 1979). The Brandaldata were acquired using an airgun source and the Dana data were acquired with a sparker source. Navigation on Brandalwas only by dead reckoning and radar fixes of coastal features so there is an uncertainty of up to a few km in the location of the Brandal profiles. The maximum depth of reflections that can be seen in the single-channel data 1s about 500 In. 4.2. Magnelrc data Total field magnetic data were recorded using proton precession magnetometers during both the Brandal (Denham, 1974) and the Dana (Brett & Zarudski, 1979) cruises. 4.3. Interpretation

methods

The migrated multichannel seismic data, supplemented where necessary by interpretation of the magnetics

205

“d m

Table I

sediments pass laterally into ridges at the sea-bed that have complex small-scale rnternal structure but are not associated with a magnetic anomaly. These ridges are interpreted as moraines. Examples can be seen on Fig. 5 and Fig. 19. 4.42.

profiles, were used to distinguish areas of basalt, sediment and basement outcrop from one another. Further interpretation, especially within the areas of sediment, consisted m picking prominent unconformities, faults, dykes and sills, and sufficient sedimentary retlections to give a good image of the structure. On lmes GGU/9502, -03, -04 and -05 in Disk” Bugt, where a 3 km streamer was deployed, many real reflections below the sea-bed multiple can be seen, but no unambiguous top basement reflector could be dlstinguished. On the bnes acqured using a 1200 m streamer (lines GGU/YS-06, -08, -18 and IY), only very few real reflections were visible below the first sea-bed multiple. This procedure was supplemented by an examination of the single-channel sasmic data, on which boundaries between areas wth outcrop of sedunents and areas of crystalline rocks (basalt and basement) at sea-bed could be distmguished. Supplementary examination of the single-channel seismic data also revealed several faults which could be correlated with faults identified on the multichannel data. With the onshore geological maps as further control, a simple sea-bed outcrop map was then prepared (Fig. 4). The envelope of the deepest visible reflections, depthconverted using the veloatles shown m Table 1, was also picked to be used as the start for gravity modelling.

The geophysical interpretation has been controlled by the geology known from the outcrop onshore. Onshore line GGU/NUY4-01 was acquired in an area where outcrops give control of the’shallow part of the section. However, the remainder of the sasmic data were acquired offshore, so it has been necessary to extrapolate control from the known onshore geology to the seismic lines, since no offshore well or sea-bed sample data exist apart from a few dredge samples of basalt SSW of Disk” (Park, Clarke, Johnson&Keen, lY71). 4.4.1. Qunfernar~ Quaternary sediments are interpreted in two facies. In many places there are small ponds of flat-lying sediments up to about 130 ms TWT (cd. 130111) thick immediately below the sea-bed. Some of these ponds of flat-lying

A4esozoic (andpanibly

older) sedimenrs

Reflection patterns from the pre-Quaternary sediments have been grouped into two seismic facies. Facie I is characterized by many continuous reflections that are generally parallel or sub-parallel to one another and to the base of the facies interval when this can be seen above the first sea-bed multiple. Where the upper surface of the facles I intervals reaches sea-bed or the base of the Quaternary, the reflectors are truncated by erosion. Sediments outcropping onshore that could give rise to seismic facie I are coarsening-upwards deltaic sequences seen m the Atane Formatlon on the southwest side of Nuussuaq (Olsen, 1993), and the Itilli succession in northwest Nuussuaq which consists ofmudstones, thin turbidite sandstone beds, thick channelized amalgamated turbidity sandstone bodies, and chaotic beds (Dam & Ssnderholm, 1994). Examples of f&es I can be seen between S.P.s 5300 and 5650 on line GGU/Y5-05 (Fig. 5). Fac~es 2 is characterized by reflections that are fewer, weaker and of more limited lateral extent than those of facies I. Sediments outcropping onshore that could give rise to this reflection pattern on seismic lines are the sandstone-dominated fluwal sediments of the Atane Formation in eastern Disk” (Pedersen & Pulvertaft, 1992). An example of facies 2 can be seen between S.P.s 4150 and 4500 on line GGU/95-05 (Fig. 5). Because of the weak reflections, it is difficult or impossible to ascertain the thickness of f&es 2 sediments or the depth to basement from seismic data alone. However interpretation of gravity data can provide a solution as shown in Fig. 5. On the smgle-channel seismic data, reflections from within facies I sediments are in some places visible down to a few hundred m below sea-bed. 4.4.3. Sill:, und dykes In places, strong reflections cut across more flat-lying reflections from the sediments (see for example Fig. 19, seismic line GGU/Y5-06, S.P.s 3000-3450). Where the cross-cutting reflections reach the sea-bed they often form ridges (e.g., seismic line GGU/95-06, S.P.s 200-700, 1500-1950 and 225&2YOO, Fig. 19). mdlcating that they come from material more resistant to erosion than the sedmxnts. These and similar features can also be identified on the single-channel lines, along which magnetic data are also available, and it can be seen that the features are associated with magnetic anomalies, commonly negatwe, indicating that the resistant material is reversely magnetized (Fig. 6). We mterpret these features as sills

^

a

-

obsewed

.-----

‘sedimentmodel’

‘basement

m

Basement,p = 2.8 Mg/ml

0

Sediment,

D

Sediment

0

Water,p

or basement

Shot

and dykes similar to the Tartunaq mtrusions exposed in and west of Saqqaqdalen (Nuussuaq) and on Grmme Ejland (Fig. 1) in southern Dlsko Bugt.

4.4.4.

Basults and basement

Basalts and Precambrian basement can seldom be dw tinguished from one another on seismic data alone, because often no reflections are received from within these rock types and their surfaces are similarly rough at outcrop. This would be a problem where the two rock types arejuxtaposed, as for example south of Disko, were it not for the fact that they are readily distingushable on magnetic data. Precambrian basement in the area consists mainly of granodioritic and tonalitic gneisses which normally con-

pint

p = 2.55

model’

Mglmj

= 1.0 Mg/mj

no.

tain little remanent magnetization, and magnetic anomalies from them are generally positive and due to different susceptiblhties (Thorning, 1986). In contrast, the basalts normally have reversed remanent magnetization, the greater part having been erupted during Chron 26r (Storey et al., 1998; Rilsager & Abrahamsen, 1998), and give rise to negative magnetic anomalies. A seismic and magnetic line across the contact between basalt and basement is shown III Fig. 7. As described in a previous sectton, the basalts in the DiskeNuussuaq area occur in two facies: subaerial plateau lavas and subaqueous hyaloclastite breccias, which give rise to very different magnetic signatures (Fig. 19). Because there are only few seismic lines in the areas where the hyaloclastite breccias occur, the two types of basalt are not distinguished on the map Fig. 4.

Fig. 6 Part ofmgle-channel hne BRA/72-39 showng the ridgesat seabed that are reversely magnetized and are therefore interpreted as sdls that are more resistant to erosion than theEurroundingsediments

4.5. Regional description 4.5.1. Offshore east, south and west ofDisk South of Disko, basement at the sea-bed forms the southern extension of the Disko gneiss ridge (Fig. 4). Basement is exposed on various skerries and small islands withm this area. Basalt is interpreted to be exposed at the sea-bed to the west of the area of exposed basement and the basalt is covered by younger sediments near the western limit of the map. Sediment is interpreted to be present under most of Disko Bugt to the east and southeast of Disko. Facies I is present only locally and is nowhere thicker than a few hundred metes. Between and under the segments of facies I, facie 2 is commonly present (Fig. 5). Because of the complexity, no attempt has been made to distinguish the occurrences of facies I and facies 2 on the map in Fig. 4. Gravity interpretation (see later and Chalmers (1998)) shows that the sedimentary basin is bounded to the east by Faults, whose outcrop pattern is probably more complex than shown on Fig. 4. The pattern drawn here 1s mfluenced partly by interpretation of aeromagnetic data (Thornmg, in press) and partly by the trend of shear zones onshore (Garde, 1994). No faults are interpreted at the southern limit of the sediments, which may indicate that erosion has removed formerly more extensive sediments. There are many faults of no great throw visible within the sediments on the multichannel lines. The data quality

Rg.7 Magnetlcdataalongllne DANA/78-35 alignedwltbseismicdata along part ofhne GGU,95-03. The two hnes are approximately parallel and nowhere more than 3 km apart. A feather edge al sedimentcan be seen at the eastern end of GG",95-03. ,.,est of which metamoroh,c basement sexposed at the sea-bed.The mcreasem magnets ana~aly from FIX No. 3110 to Fix No 3072 an DANA,%35 IS due to the basement being shallower at S.P. 4000 an GGU,95-03 than at S.P 3000 The > 1000 nTneeat,vemagnet~ anomalywestolRx No.3070 on DANA178-35 ~smter&ted to bedue to reve;selymagnet~red basalt exposed at the sea-bed west of S P. 4300 on GGU95-03 A weak reAect~onfrom,ustbelow500mstwa-wayt~mebetweenS.P.s4500and 4750 onGGU'95.03 maycome from the baseofthe baealts

on the Brandal and Dana lines is generally too poor to show these small faults unambiguously. There 1s thus little indication on the seismic data of the tectonic trend in this area. One exception is shown in Fig. 8, where indications of energy returned from a fault crossed by both line GGU/95-03 and line GGU/95-04 can be seen. That this is the same fault is shown by the traces intersecting at about 1900 ms. This fault strikes 130’ and 1s labelled fault E on Fig. 4. Its extension to the southeast may be visible on line GGU/95-2. Farther southwest, the sediment-basement contact visible on two of the Brandul lines is steep and may be a fault. If so this second fault, fault D on Fig. 4, has a similar strike to fault E. Dykes, sills and other intrusions are interpreted to be present under much of Disko Bugt east of Diako. The features are complex in plan and, because of the relatively wide spacing between seismic lines and the uncertain navigation on the Brando/ Itnes, they cannot be traced from line to line with any certainty. However, areas where they can be identified are extensive and are shown in Fig. 4. Comparison of Fig. 4 with Denham’s (1974) map plate suggests that Denham (1974) had difficulty in dis-

Ow GGU/95-03 skm I

E N GGU/95-04

S fault E

,

/

tinguishing areas of basement outcrop from areas where there are many sills at sea-bed outcrop. Much of the area in Disko Bugt shown as sedimentary basin with sills in Fig. 4 is shown as ‘crystalline rocks at outcrop’ on Denham’s (1974) map. 4.5.2. Inner Vaigat Facie 2 is present at sea-bed in much of inner Vaigat, except at the southeastern end where a small graben contams over 1500111 of facies 1 sediments (Fig. 5 and Fig. 19). Line GGU/95-05 ties wth line GGU/95-06 within this graben. Calculation of the true dips of the reflections at the intersection of lines GGU/95-05 and GGU/95-06 shows that those in the upper unit dip 17’ towards 224 and those in the lower unit dip 20’ towards 70”. An attempt was made to use the Brandal seismic and magnetic data to map the outcrop pattern of faults, sills and sediments in this area. Unfortunately, it was found that there were mist& of up to about 3 km, both between the Brandal lines and the GGU/95 lines and between different Brandal hnes. The amount of mistie also appears to vary along an individual Brandal line. It was thus not possible to use these data to produce a defimtive map,

but they do gwe information about the geology. For instance, the Brandal lines indicate the presence of another small graben about 5 km northeast of the graben traversed by GGU/95-05 and GGU/95-06, and separated from It by exposed basement and faults. They also indicate that the fault on GGU/95-05 at S.P. 5200 is the same fault as that on GGU/95-06 at S.P. 1170, which therefore runs NNW-SSE. The reflections within the upper unit dip into this fault. No fault that could be an extension of the Saqqaqdalen fault into Vaigat is visible on the seismic hnes, so this fault is shown in Fig. 4 as being terminated to the south by a fault that strikes about 120 along southeastern Vaigat. This fault is indicated on Brandal seismic lines and is also modelled on gravity profiles.

Outer Vaigat Northwest of the hult at S.P. 1170 on line GGU/9506 (Fig. 19), a very different character is visible on line GGU/95-06. In fact, between S.P.s 1170 and 1800, there are very few reflections at all and it was at first thought that basement might be exposed at the sea-bed here. However, gravity interpretation suggests that there is 4.5.3.

210

J.A

C'txdnirrr~i

al

I Monne

ondPrrrohm

about a kilometre of sediment in this area, which is therefore of facies 2. Northwest of about S.P. 2560, facie I reflections are visible between strong, cross-cutting reflections from sills. The facies I reflections, although interrupted by minor faults, form a synclinal structure from S.P. 2790 to S.P. 5260, where there is a large fault (fault P) that throws down to the northwest. Northwest of fault P there IS a second synclinal structure containing facie I reflections, somewhat more asymmetric than the structure between S.P.s 2790 and 5260. The Kuugannguaq-Qunnilik (K-Q) fault is known onshore Disko and Nuussuaq. It crosses seismic lme GGU/95-06 at S.P 6100, west of which the facies I reflections are much more flat-lying, but irregular, probably because of large numbers of small faults. The structures described above indicate that the facies I sediments are contained within a serxs of major faultblocks separated by faults that throw down to the west Drag on the hanging walls of some of these faults gwes

Geology

16 j,WU,

,Y,-224

rise to the synclinal patterns and there are also faults antithetic to the main ones. Because of the sea-bed multiple, the base of the facies I sediments is not visible on lme GGU/95-06, but, because of the generally easterly dip of the sediments, a stratigraphlc thickness of up to 2 km can be seen in places. The facies I reflections on @X/95-06 between S.P s 2560 and 6350 are truncated upwards either at the sea-bed or by an unconformity onto which Quaternary sediments have been deposited. The strongly reflectmg sea-bed may be due to the presence of a layer of methane clathrates. Between at least S.P.s 4200 and 5900, a reflection is visible parallel to and about 150 ms below the sea-bed that cuts across the facies I reflections. Such bottom-following reflections are commonly interpreted as coming from the base of clathrate layers. Methane clathrate would be stable under Vaigat if the water temperature is around 0 ‘C. Multichannel seismic hne GGU/NU94-01 (Fig. 9), acquired onshore along the southwest coast of Nuussuaq

(Fig. 4), is roughly parallel to and about 8 km north of line GGU/95-06 between approximately S.P.s 5500 to 6200 (Fig. 19). The data quality is fairly good and reflections can be seen down to about 3.5 s TWT. The apparent dips of6-16’ towards the southeast in the shallow section agree with the structural informatmn at outcrop where Cretaceous sediments are exposed. The stratigraphic thickness of the non-marine and marginal marine Cretaceous sediments exposed on Disko and Nuussuaq is at most about 2~ 3 km, and older sediments are not known from outcrops. The large thickness of sediment shown here is therefore unexpected Interval velocitxs derived from the stacking velocities from line GGU/NU94-01 are shown in Fig. IO. Below about 0.8-0.9s two-way time (TWT), the rate of increase in interval velocity is sigmficantly slower than the rate of increase above 1 s TWT. This change from one trend to the other may take place at the unconfortmty visible on the line between 0.7 s TWT at the west end of the section and I. I s TWT at the east end. corresponding to depths between about I km and about 2 km, rather than at a constant TWT. This could indicate that the unconformity marks a significant change in the successmn, possibly the base of the Atane Formatmn. What lies below the unconformity can only be a matter of speculation. The sediments between about I and about 2.5 s TWT, where there is another pattern of strong reflections, could be Cretaceous, perhaps COTresponding to the Appat and/or Kitsissut sequences offshore (Chalmers et al., 1993). Alternatively, they could be Ordovician hmestones comparable to those exposed in eastern Canada (Bell & Howie, 1990) remnants of which are found in Greenland (Stage & Peel, 1979). Geochemical fingerprints and abundant xenoliths in the Paleocene basalts show that they have definitely passed

through elastic and organic-rich sediments, but there are neither geochetmcal nor petrographic fingerprints from carbonate sediments. It IS therefore possible that the sediments are completely unknown, possibly of Mesozoic age. The apparent unconformity at about I .8 s TWT could be a thin sill cutting across the sedimentary succession. Reflections down to about 2.5s TWT clearly come from sediments plus possibly some thin, cross-cutting sills, but what lies below 2.5 s TWT is not clear on the seismic ewdence alone. It is possible that the event at about 3.3s TWT is a peg-leg multiple. If so, then the band of reflections at about 2.5s TWT may indicate basement at a depth of between 4.5 and 6 km. If the event at about 3.3 s TWT is real, basement could be between 7 and 8 km depth. This deepest unit may not be sedimentary, however. Several of the prograding hyaloclastite flows within the Vaigat Formatmn exposed on Nuussuaq consist of s~hca enriched basalts (Pedersen et al., 1993), as does the volcano at Ilugwoq, I2 km to the north of seistmc line GGU/NU94-01 (L. M. Larsen, pers. comm. 1997). Storage of magma in a magma chamber or sill complex intruded into upper-crustal basement or deeply-buried sedments is the most likely explanation for such contamination and it is possible that the reflectmns from below about 2.5 s TWT could come from such a sill complex. What may be a large fault can be interpreted below 2.5 s TWT on the eastern part of the section. The extrapolatmn of this fault reaches surface at Nuuk Kllleq where, as mentioned earher, the top of Cretaceous sediments rises suddenly about 40011~ in altitude to the east (Pedersen et al., 1993). Approximately I .5 km west of the data coverage on the line, the Kuugannguaq-Qunnilik fault that throws basalts down to the west is observed at the surfxe (Pedersen et al., 1993). It is therefore likely that line GGU/NU94-01 lies on the sane rotated faultblock that is visible on GGU:95-06 between fault P at S.P 5300 and the Kuugannguaq-Qunnihk fault at S.P. 6100 (Fig. 19). The evidence from seismic line GGU/NU94-01 suggests that the easterly-dipping. block-faulted, facies-I sediments visible on seismic line GGU/95-06 between approximately S.P.s 3000 and 6400 belong to the Atane Formation. but that beneath them lie at least 34 km of unknown sediments. The sediments have been faulted and rotated during the major episode of extension between deposition of the Atane Formation sediments and the eruption of the basalts that has been described from the onshore data 4.5.4.

The sftmf between Hurawi

und Nuussuay

Water depths shallow abruptly to the northwest at about S.P. 6350 on seismic line GGU/95-06 (Fig. 19) and there are few, if any, reflections from below the seabed. The magnetic data along the nearby Brandal line

onshore and free air anomahes offshore. The corrections applied to the data imply that all the gravity measurements have been adjusted to the same, sea-level datum and that the effects of rocks above sea-level should have been removed. Consequently, all modelling shows only the effect from rocks that lie below sea-level.

5.2. Modelling

techniques

Forward modelling of the gravity data was carried out along the profiles shown in Fig. I2 using the densities shown in Table 2. Because modelling of the gravity data proved to be difficult and ambiguous, a separate, detailed

T‘iblC

2

description of the techniques used, problems encountered and results obtained has been written (Chalmers, 1998) and only a summary is gwen here. Attempts at identifying a regional field (Nettleton, 1976) by using the observed values (around -60 to -70 mgals) over areas of known basement were ambiguous and inconclusive, so an alternative modelling technique was adopted. A reference model was defined as shown in Fig. I3 and all modelling was done with respect to It. Assuming that the densities of the crust and mantle are known, the only spatial variable in areas where basement is exposed is the depth to the base of the crust, which was adjusted until a satisfactory agreement between modelled and observed fields was obtained. Gravity profiles (Fig. 12) were constructed along the seismic hnes plus extensions, where possible, for several tens of km into areas where basement is exposed, either onshore or at the sea-bed The gravity model profile along seismic line GGU/95-x has been named GGUx (e.g., the grawty profile along seismic line GGU/95-05 1s GGUS). Additional ‘artificial’ profiles (Disko-I to Disko-7) were constructed from gravity observations that lie approximately along a straight line. Where possible, profiles were constructed in such a way that they passed over two areas of basement outcrop, in order to be able to interpolate the ‘reglonal’ (base of the crust) profiles. Depth-converted interpretations of ‘acoustic basement’ from the multichannel seismic lines were used to start modelling total sediment thickness, and in places the gravity model showed this to be depth to actual basement (Fig. 14). Where lack of reflectivity in the deeper sedlmentary section means that actual basement is deeper than ‘acoustic basement’, additional thicknesses of sediment were modelled until a satisfactory agreement between modelled and observed fields was obtained (Fig. 14). In places, it was found necessary to adjust the interpolated ‘base of the crust’ profile in the basin areas. The techmque described above relies on having at least two areas of exposed basement between which the modelled depth to the base of the crust can be interpolated simply and uniformly This condition is met where areas of basement are exposed onshore south and east of Disko Bugt, in eastern Nuussuaq, in central Disko (the Dlsko gneias ridge) and the area where basement crops out at the sea-bed south of Dlsko (Fig. 4). The condition is not

met along Vaigat, in western Nuussuaq and areas north of Nuussuaq and west of the Disko gneiss ridge. Because the base of the sediments along most of lines GGU/9506, -08, -18 and -19 is deeper than the first sea-bed multiple, constraint of the gravity modelling is not available from these lines. Seismic hne GGU/NU94-01 (Fig. 9) on the south coast of Nuussuaq (Chnstiansen et al., 1995) provides some constraint. However the uncertainty concernmg depth to basement on that hne means that there is a corresponding uncertainty m the cahbration of sediment thicknesses and depths to the base of the crust on the gravity profiles along Valgat, in western Nuussuaq and areas north of Nuussuaq and west of the Disko gneiss ridge. Because of this, two alternatwe models, one for each of the depthto-basement mterpretations, have been calculated for the profiles whose results depend on the interpretation on seismic line GGU/NU94-01. Models based on the assumption that basement hes between 4.5 and 6km depth on GGU/NU94-01 are labelled wth suffix ‘a’ (e.g., GGU6a). while those based on the assumption that bascment lies between 7 and 8 km depth on GGU/NU94-01 are labelled with suffix ‘b’ (e.g., GGU6b). Models GGU6a and GGU6b are shown in Fig. 15. The modelling has been extended north and west from seismic line GGU/NU9-401. However, the farther from that line, the less constraint there 1s on the modelling, so models in these areas must be treated circumspectly. AdditIonal construnts on the gravity models are obtained from known outcrop hmits of sediments, basement and basalts. A further constraint 1s the requirement that the modelled profiles should tie at thar intersections.

A map showing depths to basement as interpreted from the ‘a’ gravity profiles is shown in Fig. 16~ and Fig. l6b shows the corresponding depths from the ‘b’ models where they are different from L-in northwestern Disko, western Vaigat and western Nuussuaq. A map of the interpreted depths to the base of the crust is shown m Fig. I7 To the south and east, and in a north south trending ridge on Disko, top basement is either above sea-level onshore or exposed at the sea-bed. These areas are marked. Care has been taken during the drawing of Figs. l6a and l6b to show the faults with realistic heaves. It was assumed that the faults all had dips of60’, and the heaves were drawn accordingly. A 60’ dip 1s consistent with the gravity models and with measured dips of 47-60” on the Saqqaqdalen fault (Pulvertaft, l989), 55’ on the Kuuk fault (Pulvertaft, 1979) and 65’ on the KuugannguaqQunnilik fault (Pedersen et al., 1993). Several of the faults shown on Figs. 16a and 16b are known at outcrop and were discussed earlier. In these cases, the contoured depth to top basement was used to calculate the heave from the

outcrop of block and block. Various in drawing

the fault, first to top basement on the footwall then to top basement on the hanging wall assumptions and conjectures have been made these maps as described below.

5.3.1. Disko eusf of the Disko gnei.x~ ridge and offshore emf andsouth of Disko

The maps show that depths to basement under eastern Disko and Dlsko Bugt are not large, typically less than 3 km. Depths to basement greater than 3 km are shown only southwest of Awe Prinsens Ejland and (possibly as great as over 7 km) under central Disko. The outcrop pattern of the faults that bound the sedimentary basin to the east is probably more complex than shown on Fig. l6a. The pattern drawn here is influenced partly by interpretation of the Brandal seismic lines. partly by aeromagnetic data (Thorning, in press) and partly by the trend of shear zones onshore (Garde, 1994).

The interpretation shown here implies the existence of a shallow basement ridge that strikes NW-SE under northeastern Disko, bordered on its southwest side by fault C. Fault C is drawn by joining faults interpreted on gravity profiles Dlsko-2 and Disko-3. It strikes at 120’. similar to other known faults and to ductile shear zones exposed onshore (Figs. I and 4). Depth to basement on the hanging wall of this fault is modelled, very uncertainly, as more than 7 km on line Disko-3, based on a single gravity measurement made where the terrain on central Dlsko is alpine and in places capped with ice. All the gravity measurements were made on nunataks within the r-caps, but no attempt was made during terrain corrections to compensate for the unknown thickness of ice (R. Forsberg, pas comm. 1998). Consequently the calculations done to produce the Bouguer anomalies are simplified and there may be considerable uncertamty in the gravity anomalies and therefore in the modelling in central Dlsko.

hni GGU/Y5-&‘and I km corresponds to 40S.P.). The outcrop locatmn of basalt, basement and sediments across D\ko between Distance kms - 120 and 0 was taken from the 1.10” “0” geology map. Bathymetry southeast of “~stance km 60 IS Thown on Rg. 1 Note that between depths of appron,mate,y 4 km and 25 km the vert,ca, scale has been changed and the section of crust between these depths IS omnted from the figure.

5.3.2. Disko wesf of the Disko gnneiss ridge There IS no seismic control of the area west of the Disko gneiss ridge and the outcrop is entirely of basalt (Fig. 4). However, some control over gravity modelling is obtained from the known basalt stratigraphy in the area, together wth evidence from contaminated basalts (e.g., metallic xon and sediment xenoliths) that the erupted magma had passed through organic-rich, terrigenous, siliciclastic sediments. It has also been assumed that the depth to the base of the crust of 26 km calculated at the western end of GGU6 (Fig. IS) is reached as near as possible to the west of the ridge. Gravity models based on these constraints have been made at the western ends of profiles Disko-I, Disko-2, Disko-3, Disko-4, GGU3 and GGU4. The models were made with the sane crustal and sediment densities, 2.8Mg/m’ and 2.55Mg/m’ respectively, as elsewhere in the region. However, it is possible that there are large numbers of Palxogene Intrusions, both in the sediments and in the basement, which would have the effect of raising their average density. Since basalt thickness IS fairly well controlled from outcrop stratigraphy, it is possible that the depths to top basement west of the Dlsko gneiss should be deeper than those shown here, which would have the effect of making them more similar to those modelled on GGU3 and Disko-3 to the south and GGU6a and GGU6b to the north. If so, it is possible that more of

the western limit of the Disko gneiss ridge than shown here consists of faults, and that fault F might continue northwards from Disko-3 with a substantial throw to jam the Kuugannguaq-Qnnnilik fault (K-Q). In this case, the part of fault F that strikes 115-120’ in Kangerluk is an atefact of the uncertainty m gravity modelling. The area where basement may be modelled too shallow because of igneous intrusions is shown on Fig. 16 by stipple. 5.3.3. Norlhern Disko, Vaigat, WES~P~ Nuussuaq and north cf Nuussuaq The Saqqaqdalen fault (Sa) is shown on Figs. 3 and 16a as being terminated to the south by fault G, because no fault that could be an extension of the Saqqaqdalen fault into Vaigat is visible on seismic line GGU/95-06. Fault H resolves the discrepancy in the outcrop stratigraphy at Kingittoq on southern Nuussuaq that was described earlier. The shallow depths to basement between faults H and Sa are difficult to reconcile with the known outcrop stratigraphy in southern Saqqaqdalen. However, the Tartunaq intrusions crop out in this area, so it is possible that there are also many intrusions in the basement, which would imply that the densities used for modelling this area are too low and that basement indicated on Fig. 16~ by stipple should be deeper than shown.

The Kuuk fault is shown as turning to strike 135“ offshore because of the outcropping basement on Appat and Salleq islands and the model on profile Disko-7. The fault pattern shown on Fig. l6a between the Kuuk and lkorfat faults offshore is not the only one that can be drawn from the data available. The Kuugannguaq-Qunnilik fault has been extended northwest from its known outcrop limit to merge with the Itilli fault (It) as the simplest solution of how to contour the area to the southeast of the Itilli fault.

A fault, fault P, is mapped to the east of and roughly parallel to the Kuugannguaq-Qunnilik fault. Fault P is visible on sasnuc line GGU/95-06 at S.P. 5270 (Fig. 19) and on seismic line GCU/95-08 at S.P.2230 (Fig. I I). At Nuuk Killeq on the south coast of Nuussuaq, a change of about 400 m in altitude of the base of the Palaeogene hyaloclastite breccias (Pedersen et al., 1993) may be the eroded scarp of the same fault, and it can possibly be seen on seismic line GGU/NU94-01 (Fig. 9) below 2.5 s TWT on the eastern part of the section. Large depths to

top basement are modelled along the whole of protiles Disko-5b and Disko-5a north of about Km 150, which indicate the presence of a steep slope to the east of profile Disko-5 that is most easily interpreted as fault P. It is possible to interpret a gap of about 20 km in fault P on Fig. 16a (the ‘a’ map) from just north of profile Disko6a, but it IS also possible to draw a continuous fault as shown. It is necessary to interpret excess mass on both ‘a’ and ‘b‘ verbions of profile Disko-5 just north of where it crosses sasm~c line GGU/NU94-01. As discussed in more detail in Chalmers (1998), this excess mass could be ather some form of pluton intruded into the basement or at the basement-sediment interface (Disko-5a and Disko-5b) or a narrow ridge of basement that rxs steeply nmnediately

north of seismic line GGU:NlJ94-01 to less than I km depth then descends again farther north. The ‘basement ridge’ models pose serious problems m drawmg a strutturemap, and theplaneofthefault that theynnplyshould be visible on seismic line GGU/NU94-01 at around 5 to Xkm depth, which is not the case. Evidence for a pluton or sill complex that may be visible on seismic line GGU/NU94-01 (Fig. 9) has been discussed earher. The locatlon of the possible magma chamber is shown on Figs. 16a and 16b. The extent of thallow basement at the northern end of the Disko gneiss ridge is controlled by the borehole Falconbridge FP93-3-l (Figs. I, 4, 16~1and 16b) which intersected basement at a height of 161 m above sea-level below 55 m of probably brackish-water Aptian-Albian

sednnent (Nnhr-Hansen pus. comm., 1998). It is unlikely that such sediment has been preserved without ever having been buried deeper, so it may be an erosional remnant whose presence indicates that the Disko gneiss ridge has been uplifted at some time after deposition of the sediment, probably in the late Maastrlchtian and/or early Paleocene. Seismic line GGU/NU94-01, 33 km north of FP93-3-I, shows basement between 5 and 8 km below sea-level, dependmg on the interpretation chosen. On both gravity profiles Disko-5 and Dlsko-7, the northern limit of the Disko gneiss ridge is modelled as a fault, fault M, north of which is a rotated fault block and another fault, fault N, under the northeast coast of the

island. The strike of the faults is not entirely clear from the data, but must be approximately NW-SE. Alternative patterns for the interaction of fault P wth faults M and N are shown on Figs. l6a and l6b. Either alternatwe could be drawn on either the ‘a’ or ‘b’ models and the choice as to which version is shown on which map is arbitrary. The fault pattern shown in Fig. l6a may be more probable than that shown in Fig. l6b because, unlike at Nuuk Killeq on Nuussuaq, there is no escarpment at the sediment/basalt contact on the north coast of Disko that could have been created by fault P. Fault T is mterpreted only on gravity profile Disko-ba and not on Disko-hb. South of Dlsko-ha, it has been

drawn with the same strike, slightly east of north, as the many small faults that are exposed on northwest Disk& North ofprofile Disk”-ha, fault T has been extended with a NNW strike to join the Itilli fault (It). The change in strike was necessary because fault T is not Interpreted on profile GGU6a. The location of the ltllli fault (It) is controlled by its onshore outcrop on Nuussuaq and Harenen and where it crosses seismic lines GGU/95-06 at S.P. 6100, GGU/9508 at S.P. 3580, GGU/95-I9 at S.P. 1480, grawty profile Disko-6 (Chalmers, 1998) and various Brandal lines, especially where different basalt facies that give rise to different magnetic signatures are separated at outcrop by the fault, and in the crush zone observed in eastern Ubekendt Ejland. Fault V is interpreted west of the ltilh fault only on gravity profle GGUX. Its strike is not known but has been drawn parallel to the Itilli fault.

6. Development of the structure A map of the structure at outcrop IS shown in Fig. 4, maps of the structure at the top of the basement are shown in Figs. l6a and I6b and maps showing the Nuussuaq basin in its regional setting are shown in Fig. IS. A composite cross-section through the basm is shown in Fig. 20. Within the bum, three trends on the fault system are evident, N-S, WNW-ESE and NW-SE. The N-S trend especially defines the Disk” gneias ridge, and its effect on the basalts shows that faulting in this trend was active on the western margin of the ridge at a late stage in basin development. Segments of the fault system that marks the present-day boundary of sednnentary outcrops to the east also trend c. N-S and they are offset by segments that trend WNW ESE, giving an overall NNW-SSE trend to this fault system. The trend between WNW-ESE and NW-SE is the trend of several shear zones in the Precambrlan basement east of Disk” Bugt, suggestmg that these old shear zones exerted an influence on later faulting. The third trend is NE SW along the ltilh fault in western Nuussuaq. At present the sequence of events that created the Nuussuaq Basin 1s not clear. There appears to be a deep basin under western Disk”, western Nuussuaq and Valgat that extends northwards beyond where it can be delineated by present data. There may also be a deep halfgraben under central Disk” east of the gneiss ridge. A much more extensive and shallower basin extends over eastern Disko and Disk” Bugt. It is possible that the deep basin m the northwest is a rift basm bounded to its east by fault P (Figs. l6a and l6b. and Fig. 20) a hypothesis supported by the base of the crust being shallow in this area (Fig. 17). If this 1s the case, the more extensive, shallower basin east of fault P

that extends over much of eastern Nuussuaq, eastern Disk” and Disko Bugt could represent the thermal subsidence (‘steer’s head’) phase of this rifting episode. If so, the Atane Formation was deposlted into this thermal subsidence basin. These basins were then dissected by a new phase of rifting in the Maastrichtian and early Paleocene which created the faults that form the present eastern hmit of the basin and faulted and rotated the facies I sedunents into the fault blocks visible on line GGU/95-06 in Vaigat (Fig. 19) and on lines GGU/95-08 (Fig I I) and GGU/95-I9 north of Nuussuaq as well as onshore Nuussuaq. Theae faults trend between N-S and WNW-ESE. The rift blocks were eroded before being covered by upper Maastrichtian-lower Paleocene sediments and voluminous rmddle Paleocene basaltic lava These were in turn dissected during the Eocene by faults along the NS trend and a new NE SW (Itilh) trend. This tectonic activity probably subsided during later Palaeogene times and the area was lifted by l-2 km to Its present situation probably during the Neogene The situation of the Nuussuaq Basin in its regional setting prior to the start of sea-floor spreading in the Labrador Sea is shown in Fig. l8a. This map suggests that the Nuussuaq Basin might be a south-southeasterly extension of the MelwIle Bay Basin (WhIttaker et al., 1997). However the arcn between Melville Bay and Nuussuaq 1s obscured below Palaeogene basalts. The Nuussuaq Basin appears not to be directly continuou with the basins off southern West Greenland, although there could be linkage between the faults that border the SISimiut Basin to its east and those that define the western limit of the Disk” gneiss ridge. Off southern West Greenland, the major fzdults shown on Fig. I8a trend N-S (Chalmers et al., 1993, 1995). the bane as one of the trends in the Nuussuaq Basin. The Melville Bay fault, which forms the eastern limit of the sedunents in Melville Bay (Whittaker et al., 1997), trends NNW-SSE. This trend is the same as the orerall trend of the fault system formed by the Saqqaqdal, Ikorfat and Kuuk faults and their extensions which form the present-day eastern limit of sednnents ,n the Nuussuaq Basin. This is not the place to discuss the development of the entire offshore West Greenland basin system. However, one can speculate that the basin system that had formed in Mesozoic and early Paleocene times, prior to the start of sea-floor spreading in the mid-Paleocene (Chalmers & Larsen, 1995), may have been formed by crustal extension in an E-W direction m the southern West Greenland and Nuussuaq basins and WSW-ENE in Melville Bay. This extension resulted in normal faults that strike N-S in the southern West Greenland and Nuussuaq basins and NNW-SSE in Melville Bay. The normal faults appear to be displaced from one another by transfer elements (faults and/or ramps) that strike parallel to

r-

older structural elements. In the Nuussuaq Basin the transfer elements are parallel to NWmSE/WNW-ESEstriking shear zones. Farther south, the transfer elements are parallel to WSW-ENE structural trends within the Nagssugtoqidian erogenic belt (Marker, Mengel, van Goal, & Field Party, 1995). Similar situations are known elsewhere. For example, in the Suer Rift, faults parallel

to the rift (clysmic trend) are offset by transfer faults trending between N-S and NNE-SSW, which is the trend of an older fabric and older dykes in the surrounding basement (Colette, Le Quellec, Letouzey & Moretti, 1988; Patton et al., 1994). The resulting rift margin pattern is very snnilar to that mapped and mterpreted along the present-day eastern margin of the Nuussuaq Basin.

In the Viking Graben of the North Sea, Late Jurassic NS-striking extensional faults are offset by transfer elements along the prevmusly-existing, NE-SW Caledonian trend (Johnson & Dingwall, 1981; Threlfall, 1981). Sea-floor spreading appears to have started in the Labrador Sea in the mid-Paleocene (Chalmers & Laursen, 1995) although there is evidence that there may have been a long period of slow extension between Labrador and Greenland prior to that (Chian et al. 1995; Chalmers, 1997). Undisputed oceanic crust with its magnetic stripes and transform faults can easily be mapped in the Labrador Sea, but not in Baffin Bay. However, Whittaker et al. (1997) have suggested the location of a former NW-SE spreading axis and a NS transform fault in Baffin Bay from gravity data. Chalmers et al. (1993) proposed that the areas of seafloor spreading in the Labrador Sea and B&in Bay were connected through continental crust in the Davis Strait area by the Ungava transform fault zone (Kerr, 1967; Klose, Malterre, McMillan & Zinkan, 1982); this hypothesis is illustrated in Fig. IXb. Fig. l8b shows why the ltilh Fault has a qwte different orlcnt&m from the other faults in the Nuussuaq Basin; it is a splay from the Ungava strike-slip system, and as such was only active after sea-floor spreading started. The other faults are part of the older Mesozoic or early Paleocene extensional system, although they, too, may have been reactneated during the Eocene. No fault with the ltilli SW NE orientation has been identified in the Nuussuaq Basin southeast of the ltilli fault.

7. Petroleum prospectivity As mentioned in the introduction, oil and bitumen have been found in surface outcrops over a wide area in western Nuussuaq, on the north side of Disko and on the southeast corner of Svartenhuk Halvs (Fig. I) and oil bled freely from the cores of two of the five slim core wells drilled in western Nuussuaq in 1993-95 (Fig. I). Analysis of the oils (Bojesen-Koefoed, Christiansen, Nytoft & Pedersen, in press) shows that they come from at least five source rocks that are probably situated near to the seeps. The source rocks are probably of Mesozoic and possibly early Paleocene age and must be situated below the basalts. Other than this, the petroleum system in the basin is not understood. Comparison of the distribution of the seeps (Fig. I) with the structure maps shown in Figs 16~1and l6b shows that the seeps occur in the block-faulted region north of the Disko gneiss ridge where nearby depths to basement are large. The significance of this is not understood, but the observation may be significant, because If hydrocarbons are movmg in this area of the basin, the faultblocks wslble on the sasm~c lines in this area (Figs. I9

and 20, and Fig. I I) could form structural hydrocarbons.

traps for the’

Acknowledgements Funds for acquisition and processing of the 1995 seismic and gravity data were provided by the Government of Greenland, Bureau of Minerals and Petroleum and the Danish State through the Mineral Resources Administratmn for Greenland. R. Forsberg is thanked for access to the KMS gravity data and K Olshefsky of Falconbridge Ltd and E. Andersen of Platinova A/S are thanked for permission to publish the Information from Falconbridge borehole FP93.3-I. Torben Bistrup provided a useful and critical discussion on the gravity interpretatmn and two anonymous referees suggested useful improvements to the text. Draughtsman St&n Sralberg showed considerable patience through a long and complex period. This article is published with permission of the Geological Survey of Denmark and Greenland.

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