A. K. HIGGINS*, A. G. LESLIE & M. P. SMITH

Transcription

1 Geol. Mag. 138 (2), 2001, pp Printed in the United Kingdom 2001 Cambridge University Press 143 Neoproterozoic Lower Palaeozoic stratigraphical relationships in the marginal thin-skinned thrust belt of the East Greenland Caledonides: comparisons with the foreland in Scotland A. K. HIGGINS*, A. G. LESLIE & M. P. SMITH *Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen NV, Denmark Department of Geology, School of Geosciences, The Queen s University of Belfast, Belfast BT7 1NN, UK School of Earth Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK (Received 27 March 2000; accepted 23 November 2000) Abstract Throughout the 1300 km long East Greenland Caledonides, the western exposed marginal thrusts overlie foreland rocks of latest Neoproterozoic Early Palaeozoic age, mainly exposed in tectonic windows. In the western, km wide, marginal thrust belt, the thrust planes outlining the windows appear to follow long flats developed in Lower Palaeozoic carbonates. East of the marginal thrust belt, thrust inclinations steepen, and by implication the remaining part of the Caledonian orogen extending eastwards to the present Atlantic Ocean coast is allochthonous and thick-skinned. The contrast between the restricted Neoproterozoic Lower Palaeozoic foreland succession and the very thick and almost continuous sedimentation of the allochthonous Neoproterozoic Eleonore Bay Supergroup Tillite Group Cambro-Ordovician sequence of the fjord zone of East Greenland confirms the presence of distinct N S trending facies belts on the northwestern passive margin of Iapetus. Comparisons with the Caledonides of Northwest Scotland, which may originally have lain as little as 500 km south of the East Greenland Caledonides, provide further clues to the understanding of Neoproterozoic Early Palaeozoic basin geometry on this sector of the developing Iapetus margin. The areas of the Laurentian margin represented in the foreland windows of East Greenland were inboard of Neoproterozoic rifting but, with respect to the Torridonian basins of Northwest Scotland, the Eleonore Bay Supergroup succession must have been laid down further outboard. Similarly the Lower Palaeozoic developments of the foreland of Northwest Scotland are thicker than the equivalent foreland sequences of East Greenland, but much thinner than the allochthonous East Greenland Cambro-Ordovician succession. 1. Introduction The East Greenland Caledonian orogenic belt extends for more than 1300 km along the coastal region of East Greenland, between latitudes 70 and N, in a zone now up to 300 km wide (Fig. 1). South of 70 N, the Caledonides are concealed beneath Palaeogene plateau basalts. The western frontal region of the fold belt is completely exposed only in Kronprins Christian Land ( N); elsewhere a very thin restricted Neoproterozoic Lower Palaeozoic succession is preserved below the thrusts within a series of tectonic windows which all occur in the western km wide marginal thrust belt of the orogen. The eastern part of the orogen is some km wide onshore and is dominated by Archaean Palaeoproterozoic orthogneiss complexes brought up from depth along steeply inclined thrusts; as such it can be characterized as a thick-skinned thrust belt (Fig. 1). In southern areas the crystalline gneiss complexes are overlain by thick Mesoproterozoic Lower Palaeozoic sedimentary successions. The crystalline gneiss complexes within the * Author for correspondence: akh@geus.dk Caledonian orogen have yielded Archaean and Proterozoic isotopic ages (Steiger et al. 1979; Rex & Gledhill, 1981; Kalsbeek, Nutman & Taylor, 1993), and retain much of their basement character despite often intense Caledonian reworking. Isotopic mineral ages are, however, generally Caledonian and witness to regional amphibolite facies metamorphism during this period (e.g. Rex & Higgins, 1985; Dallmeyer & Strachan, 1994; Dallmeyer, Strachan & Henriksen, 1994). The most conspicuous and best known of the Proterozoic sedimentary successions is the Neoproterozoic Eleonore Bay Supergroup, which in the fjord region of East Greenland (72 74 N) passes upwards into the Tillite Group and Lower Palaeozoic sediments. This succession is up to 18.5 km thick in total, and ranges from amphibolite facies grade metamorphism at the base to non-metamorphic in the upper part; Caledonian deformation of this succession is generally characterized by N S trending major folds. The western marginal zone of the fold belt is dominated throughout its length by transport to the westnorthwest on major Caledonian thrust systems accommodating cumulative displacements which amount to several hundred kilometres (Leslie & Higgins, 1998, 1999; Higgins & Leslie, 2000). Late

2 144 A. K. HIGGINS, A. G. LESLIE & M. P. SMITH 82 N 40 W Peary Land Wandel Sea Station Nord POST-CALEDONIAN Palaeogene basalts Palaeogene intrusions Kronprins Christian Land thin-skinned thrust belt (parautochthonous foreland) 78 N 35 W 74 N Hamberg Gletscher foreland Målebjerg window Charcot Land window Gåseland window 70 N INLAND ICE Eleonore Sø window Lambert Land C ALEDONIAN SOLE THRUST Centrumsø Jameson Land Scoresby 25 W Kronprins Christian Land Nørreland window Dronning Louise Land MARGINAL THRUST BELT SSZ THICK SKINNED THRUST BELT Sund Traill Ø Danmarkshavn Bessel Fjord Wollaston Forland 100 km Wandel Sea Basin: Carboniferous Palaeogene sediments East Greenland basins: Carboniferous Cretaceous sediments LATE TO POST-CALEDONIAN Devonian continental sediments CALEDONIAN OROGENIC BELT Caledonian granites Neoproterozoic Ordovician sediments (East Greenland) Neoproterozoic Silurian sediments (eastern North Greenland) Palaeo Mesoproterozoic sediments and basalts (eastern North Greenland) Crystalline complexes and sediments (Archaean Mesoproterozoic) CALEDONIAN FORELAND Neoproterozoic Silurian sediments (North Greenland) Palaeo Mesoproterozoic sediments and basalts (eastern North Greenland) Mainly crystalline rocks parautochthonous windows Thrust Fault/shear zone Tectonic zone boundaries Greenland CALEDONIDES Figure 1. For legend see facing page.

3 Foreland stratigraphy of East Greenland Caledonides 145 Caledonian crustal scale extension associated with orogenic collapse in the eastern part of the Caledonides has been invoked as one of the principal controls on Devonian Carboniferous basin development (e.g. Larsen & Bengaard, 1991; Hartz & Andresen, 1995; Andresen, Hartz & Vold, 1998; Hartz et al. 2000); post-caledonian Upper Palaeozoic and Mesozoic successions are widely exposed along the outer coastal regions of East Greenland. This paper reviews foreland stratigraphical developments which are exposed in window-like settings below major thrusts in the western marginal thrust belt of the East Greenland Caledonides. New observations from the southern part of the East Greenland Caledonides made during the expeditions by the Geological Survey of Denmark and Greenland have enabled an improved understanding of the coherence of the foreland stratigraphy, and are included here together with comparisons with similar successions from the Caledonian foreland of Northwest Scotland. Such comparisons provide broad insights into Neoproterozoic basin geometry on the developing Laurentian margin of Iapetus. 2. Stratigraphy of the foreland and parautochthonous foreland 2.a. Kronprins Christian Land ( N) The western marginal zone of the East Greenland Caledonides is completely exposed only in Kronprins Christian Land in the northernmost part of the fold belt (Figs 1, 2), and thus the Neoproterozoic Early Palaeozoic stratigraphy of the Caledonian foreland is well known only from this region. Foreland Proterozoic successions are well exposed west of Danmark Fjord and there comprise thick Palaeo- to Mesoproterozoic sequences overlain by Neoproterozoic Lower Palaeozoic shelf sediments. The former comprise the Independence Fjord Group and Zig Zag Dal Basalt Formation (Collinson, 1980, 1983; Jepsen, Kalsbeek & Suthren, 1980; Kalsbeek & Jepsen, 1984; Sønderholm & Jepsen, 1991), while the shelf sediments are represented by the Hagen Fjord Group comprising the Jyske Ås, Campanuladal, Kap Bernhard and Fyns Sø formations (Fig. 3; see also Clemmensen & Jepsen, 1992). The Independence Fjord Group is cut by intense swarms of doleritic dykes and sills known as the Midsommersø dolerites (Kalsbeek & Jepsen, 1983); these are equated with the Zig Zag Dal Basalt Formation. There is a hiatus between the Riphean Fyns Sø Formation carbonates and the sandstones of the Kap Holbæk Formation with well-developed palaeokarst surfaces present locally (Smith et al. 1999). The sandstones of the Kap Holbæk Formation contain deep Skolithos burrows which extend vertically for many tens of centimetres (Clemmensen & Jepsen, 1992, fig. 29) and indicate a Tommotian (Early Cambrian) or younger age (Crimes, 1992a). Owing to this significant break in deposition, Smith et al. (in press a) have removed the Kap Holbæk Formation from the Hagen Fjord Group. Another hiatus, covering most of the Cambrian and the lower part of the Lower Ordovician sequence, occurs between the Kap Holbæk Formation and the overlying mid-lower Ordovician Silurian platform carbonates (Fig. 3). Field work carried out by the Survey in Kronprins Christian Land during considerably simplified the earlier interpretations of the frontal thrust systems made by Hurst & McKerrow (1981a, b, 1985). A single thrust sheet (the Vandredalen thrust sheet) is now recognized as the westernmost major nappe structure, and this displaces Neoproterozoic clastic metasediments of the Rivieradal Group (Smith et al. in press a) across the parautochthonous foreland succession (Figs 2, 4; see also Rasmussen & Smith, 1996). The Vandredalen thrust sheet lies within the eastern part of the marginal thrust belt, and the approximately linear trend of the Vandredalen thrust front coincides with the position where the thrust ramps up through the Ordovician Silurian platform carbonate sequence (Fig. 5a) eventually to overlie Silurian turbiditic sandstones. East of the thrust front, the Vandredalen thrust follows a long flat in the Lower Ordovician Wandel Valley Formation. Further eastwards, the Vandredalen thrust ramps abruptly downwards along the NNE SSW trending Brede Spærregletscher Hekla Sund lineament (Figs 2, 4) which coincides approximately with the original rifted margin of the basin in which the Rivieradal Group accumulated. A westward displacement of about 40 km can be demonstrated for the Vandredalen thrust front from matching cut-offs of the Fyns Sø Formation in the hangingwall and footwall, although about half of this arises from displacement on the thrusts of the thinskinned fold-and-thrust belt west of the Vandredalen thrust front (Higgins et al. in press a, b; Fig. 4). The up to 40 km wide thin-skinned fold-and-thrust belt beneath and to the west of the Vandredalen thrust front (Figs 2, 4) deforms a Neoproterozoic to Lower Silurian platform succession that is continuous with the undisturbed foreland sequences west of Danmark Fjord further west. The succession in this belt is disrupted by a series of east-dipping NNE SSW trending thrusts, generally with displacements of up to a few kilometres. Mapping of the area by M. P. Smith, J. A. Rasmussen and J. S. Peel in confirmed the thin-skinned models of earlier work (Fränkl, 1954, Figure 1. Geological map of the East Greenland Caledonides, with the location of the foreland windows discussed in the text. The approximate location of the Caledonian sole thrust and the division into western marginal and thick-skinned thrust belts is also shown. SSZ: Storstrømmen Shear Zone. Modified after Higgins & Leslie (2000).

4 146 A. K. H I G G I N S, A. G. L E S L I E & M. P. S M I T H Lauge Koch Land Formation Fjo nm ark Da Greenland rd 81 N Sø m er Hagen Fjord Gr. Vandredalen thrust Rivieradal sheet Group Zig Zag Dal Basalt Fm. and Independence Fjord Gr. Kap Holbæk Formation Wandel Valley Formation Fyns Sø, Kap Bernhard, Hagen Fjord Group Campanuladal Fms. Turesø Formation Børglum River and Sjælland Fjelde Formations Odins Fjord Formation Samuelsen Høj Formation CALEDONIDES Wandel Sea Basin sequence (post-caledonian) Ro Crystalline basement Vandredalen thrust Amdrup Land Thrust Fault Vand re dale n BS Ing o F jo lf rd Holm Land d S un Di jmphna Su n d al ad er vi Hekla i Elv Ri sø Sæfax um tr Cen EN 80 N LLI Hovgaard Ø SKA NG al jd ve d Sy section line Kap Bernhoft Blåsø g Nio fje halv rd s f j o rd en 25 km 20 W Figure 2. Geological map of Kronprins Christian Land, the northernmost segment of the East Greenland Caledonides. BS: Brede Spærregletscher. Section line indicates the position of the cross-section of Figure 4.

5 Foreland stratigraphy of East Greenland Caledonides 147 STRATIGRAPHY DEPOSITIONAL ENVIRONMENT TECTONIC SETTING Silurian Ordovician Lauge Koch Land Formation Samuelsen Høj Formation Odins Fjord Formation Turesø Formation Børglum River Formation Sjælland Fjelde Formation Wandel Valley Formation thrust loaded flysch basin thermal subsidence block tilting Baltica collision lapetus passive margin Cambrian thermal subsidence Kap Holbæk Formation KH Vendian extensional rifting and block tilting lapetus opening Sturtian Fyns Sø Fm Kap Bernhard Fm Campanuladal Fm Jyske Ås Fm Hagen Fjord Group post-rift thermal subsidence pre-lapetus rift-sag cycle Riphean Rivieradal Group (allochthonous Vandredalen thrust sheet only) RG extensional rifting Zig-Zag Dal Basalt Formation Independence Fjord Group Hekla Sund basalts ZZ IF HS pre-grenville intracratonic extensional events Figure 3. Stratigraphical scheme showing the Proterozoic Lower Palaeozoic stratigraphy and tectonic history of Kronprins Christian Land. Vertical ruling indicates hiatuses. Modified after Smith et al. (1999). 1955; Peel, 1980) and demonstrated that individual thrusts frequently coalesce, while many thrusts die out altogether in a southwards direction. Total displacement in an E W cross-section through this zone along the valley containing Centrumsø has now been estimated at about 18 km on ten separate thrusts (Higgins et al. in press b). Conodonts isolated from the Ordovician carbonates in this region show systematic alteration due to the elevation of temperature beneath tectonic overburden during Caledonian orogenesis. These conodont alteration indices (CAI) show the maximum temperatures adjacent to the Vandredalen thrust front to have been about C, equivalent to an overburden of km (J. A. Rasmussen and M. P. Smith, unpub. data; Fig. 4). The thickness of the Vandredalen thrust sheet alone is insufficient to account for this amount of overburden, and the former presence of additional thrust sheets derived from further east must be invoked. East of the Brede Spærregletscher Hekla Sund lineament, highly deformed belts of Proterozoic quartzites and dykes do indeed make up a separate and higher thrust sheet (riding on the Spærregletscher thrust, SPT on Fig. 4), bordered to the east by another prominent N S trending lineament marking the transition to thick-skinned geometry. The crystalline rocks extending eastwards from this lineament to the outer coast of Kronprins Christian Land represent the roots of deep-seated thrust sheets which characterize the thick-skinned thrust belt, and which may have displacements of as much as 100 km.

6 148 A. K. HIGGINS, A. G. LESLIE & M. P. SMITH W ST VT E S TIMATE D OV ERBURDEN 22 km 18 km Ord. Sil. 50 km? 100 km? SPT E 5 0 km 0 10 km Allochthonous thrust sheets Crystalline basement Independence Fjord Group Rivieradal Group Hagen Fjord Group Foreland and parautochthonous fold and thrust belt Crystalline basement Independence Fjord Group Hagen Fjord Group Ordovician Silurian Figure 4. W E cross-section of the thrust systems in Kronprins Christian Land, showing the Rivieradal Group thrust over the parautochthonous fold-and-thrust belt on the Vandredalen thrust (VT). The estimated overburden deduced from conodont alteration indices (CAI) is also shown. SPT: Spærregletscher thrust. ST: Caledonian sole thrust. 2.b. Lambert Land (79 20 N) The large semi-nunatak of Lambert Land (Fig. 1) is dominated by crystalline basement gneiss complexes together with sandstone successions of the Independence Fjord Group, which have been interleaved in major Caledonian thrust sheets (K. A. Jones & J. C. Escher, pers. comm. 1999). In western Lambert Land, the Mesoproterozoic Independence Fjord Group is overlain by Ordovician carbonates of the Wandel Valley Formation, and the unconformity is disrupted by numerous minor Caledonian thrusts. The unconformable relationship is significant in demonstrating that the overstep recorded at the base of the Wandel Valley Formation in eastern North Greenland (Bryant & Smith, 1990) progressed southwards as well as eastwards (Smith et al. 1999); in Lambert Land the erosion surface beneath the Wandel Valley Formation breached the Hagen Fjord Group to lie within the Independence Fjord Group. Conodonts extracted from the Ordovician carbonates in the western part of Lambert Land show similar CAI values to conodonts from below the front of the Vandredalen thrust sheet in Kronprins Christian Land, again indicative of burial at depths of km below higher Caledonian thrust sheets (J. A. Rasmussen and M. P. Smith, unpub. data). 2.c. Nørreland (78 40 N) In the area of Nørreland (Fig. 1), an anticlinal culmination exposes a window of quartzitic rocks, 20 km long and 10 km wide, beneath a thrust sheet of basement orthogneisses and amphibolites (J. M. Hull & J. D. Friderichsen, pers. comm. 1998). The quartzites are lithologically comparable with the Mesoproterozoic Independence Fjord Group and, characteristically for that sequence, are cut by a network of basic intrusions that can be correlated with the Mesoproterozoic Midsommersø dolerites. While major mylonitic developments are not present, a blue-grey platy metacarbonate crops out in the contact zone on the western side of the culmination. Samples of the platy carbonate have yielded Ordovician conodonts which survived the intense Caledonian deformation and have CAI values that once again indicate they have been subjected to temperatures corresponding to an overburden of about km (J. A. Rasmussen and M. P. Smith, unpub. data). The thrust sheet of crystalline basement lithologies immediately overlying the carbonates includes amphibolite facies grade amphibolites and orthogneisses that locally contain eclogitic garnet pyroxene enclaves dated as Caledonian in age (Brueckner, Gilotti & Nutman, 1998). Similar high-grade rock types crop out westwards for about 40 km as far as nunataks at the border of the Inland Ice where they are again seen to be in thrust contact with Independence Fjord Group lithologies, here interpreted as foreland. 2.d. Dronning Louise Land ( N) The mountainous nunatak region of Dronning Louise Land (Fig. 1) is well known from the pioneer work of Peacock (1956, 1958) and more recent systematic mapping by the Survey in (Friderichsen et al. 1990; Holdsworth & Strachan, 1991; Strachan et al. 1994). A N S trending imbricate zone separates a foreland area to the west from allochthonous Proterozoic gneiss complexes and metasedimentary rocks to the east. The imbricate zone marks the western limit of intense Caledonian deformation, but less severe

7 Foreland stratigraphy of East Greenland Caledonides 149 OF RG VT a MT S G b Figure 5. (a) The Vandredal thrust (VT) ramping up though Ordovician platform carbonates on the west side of Vandredalen, Kronprins Christian Land. Strike section looking west shows Neoproterozoic sandstones and conglomerates of the Rivieradal Group (RG) in the hangingwall structurally overlie Silurian carbonates of the Odins Fjord Formation (OF) in the footwall. The cliff is 900 m high. (b) Målebjerg (1873 m high) at right, on the east side of the Målebjerg window (glacier surface at about 350 m altitude). Gneisses (G) at the base of the cliff are overlain by a folded sequence of Skolithos-bearing quartzites of the Slottet Formation (S); a few metres of sheared carbonate (Målebjerg Formation), not visible on photograph, occur immediately below the thrust contact (MT). The dark sequence in the hangingwall comprises Mesoproterozoic metasediments. Caledonian deformation is also recorded in the structurally underlying foreland. The foreland area is composed of crystalline basement orthogneiss overlain by sequences of sedimentary rocks assigned to the Trekant and Zebra series. The older Trekant series and underlying basement gneisses are intruded by dolerite dykes, and are overlain unconformably by the Zebra series. The Trekant series in the foreland is up to 510 m thick, and comprises sandstones, siltstones and conglomerates, a similar range of lithologies to the Independence Fjord Group of eastern North Greenland (Clemmensen & Jepsen, 1992). The dolerite intrusions are considered to be equivalents of the Midsommersø dolerites. The Zebra series is a thin sequence (< 100 m) of quartzites, mudstones and iron oxide-cemented sandstones, with grey-black dolomitic limestones at the top. Deep Skolithos burrows are found in situ in the quartzites, demonstrating that the Zebra series is no older than Cambrian (Crimes, 1992a), and on the basis of this ichnofauna the unit correlates with the Kap Holbæk Formation of Kronprins Christian Land (Friderichsen et al. 1990; Strachan et al. 1994) and the Slottet Formation of the Eleonore Sø and Målebjerg windows (see next section). The imbricate zone is characterized by numerous east-dipping thrust sheets of varying thickness, which

8 150 A. K. HIGGINS, A. G. LESLIE & M. P. SMITH W ARNOLD ESCHER LAND 0 10 km ELEONORE SØ WINDOW Boyd Bastion fault LOUISE BOYD LAND E ANDREÉ LAND MÅLEBJERG WINDOW Allochthonous thrust sheets Lower thrust units mainly metasediments Foreland windows Palaeoproterozoic gneisses and granites Upper thrust units high grade metasediments and granites Palaeoproterozoic Eleonore Sø complex Neoproterozoic Eleonore Bay Supergroup Cambro-Ordovician Slottet and Målebjerg formations Figure 6. Schematic W E cross-section through the Eleonore Sø and Målebjerg windows, modified from Leslie & Higgins (1998). The distance from the westernmost nunataks to the east margin of the Målebjerg window (> 90 km) provides a minimum figure for displacement of the overlying thrust. include lithologies assigned to all the units recognized in the foreland. Quartzites of the Zebra series in the imbricate zone locally contain ichnofossils assigned to Cruziana sp. (Strachan et al. 1994), which indicate a maximum age of Atdabanian (mid-early Cambrian) (Crimes, 1992b). The uppermost preserved levels of the Zebra series in the imbricate zone are grey dolomitic limestones. The precise age of the dolomitic limestones at the top of the Zebra series in both the foreland and imbricate zone is uncertain, but they must be of Cambro-Ordovician age. 2.e. Eleonore Sø and Målebjerg ( N) The volcano-sedimentary complex of the Eleonore Sø region (Fig. 1) crops out in a series of nunataks extending from latitudes N, a region at least 120 km from north to south and up to 20 km from east to west. The complex was originally described by Katz (1952), who proposed a correlation with the Neoproterozoic Eleonore Bay Supergroup and Tillite Group. Subsequently, following the suggestion of Wenk (1961), the sequence was assigned to the Basal Series of the Eleonore Bay Supergroup (Haller, 1971), but Haller had also speculated that the complex might occupy a window through a Caledonian thrust and represent part of the over-ridden Caledonian foreland. Field work in 1997 and 1998 confirmed that the outcrops occur in an anticlinal window (Fig. 6). It also showed, however, that the sequence informally known as the Slottet quartzite (now Slottet Formation; Smith et al. in press b), which was interpreted by Katz (1952) as a separate thrust unit, in fact rests unconformably on different units of the Eleonore Sø complex. The Eleonore Sø volcano-sedimentary complex comprises arkosic psammites and semipelites overlain in turn by a carbonate sequence several kilometres thick, a further sequence of quartzose psammites and black shales, and a variably developed volcanic sequence in which pillowed lavas are prominent; a rift setting is envisaged (Leslie & Higgins, 1998). A recent SHRIMP U Pb zircon age of c Ma from a quartz porphyry body intruded into the sequence indicates a Palaeoproterozoic age (F. Kalsbeek, pers. comm. 1998), and invites correlation with the closely comparable mix of Palaeoproterozoic supracrustal rock types found 150 km to the south in the Charcot Land window (see Section 2.g). The unconformably overlying Slottet Formation is up to 350 m thick and oversteps different members of the Eleonore Sø volcano-sedimentary complex. A thin basal conglomerate (10 cm to 1.5 m thick) is often present, and the lowest quartzite beds are frequently of granule or very coarse sand grade. Most of the sequence comprises a very pure, and conspicuously white, quartzite in cm thick beds, frequently with well-developed trough cross-bedding. The uppermost part of the sequence includes levels with abundant well-preserved, deep Skolithos tubes, observed in situ at several localities, indicating a maximum age of Tommotian (Early Cambrian) and correlation with the Zebra series of Dronning Louise Land and the Kap Holbæk Formation of Kronprins Christian Land. At several localities, the Slottet Formation is overlain by up to 50 m of pale blue-grey and yellow-orange weathering carbonates of the Målebjerg Formation (Smith et al. in press b). These were observed on both east and west sides of the window, immediately underlying a major Caledonian thrust which transports dominantly metasedimentary rock units west northwestwards (Fig. 6). Although no macrofossils were recovered from the Målebjerg Formation carbonates, the character of the sequence and the presence of abundant organic material in micropalaeontological

9 Foreland stratigraphy of East Greenland Caledonides 151 concentrates, together with the stratigraphical context, indicates a Phanerozoic, probably Cambro-Ordovician, age. Around 35 km east of the Eleonore Sø window, another window occurs at the foot of Målebjerg (Figs 5b, 6). The prominent unit in this window is an up to 200 m thick quartzite, which Haller (1971, p. 87) had previously noted to be very similar to the Slottet quartzite. Variably sheared examples of Skolithos again occur in the upper part of this quartzite, confirming the correlation with the Slottet Formation and, as in the Eleonore Sø region, the quartz arenite is overlain by Cambro-Ordovician carbonates which are assigned to the Målebjerg Formation (Smith et al. in press b). These comprise alternating buff-weathering dolostones and metamorphosed grey lime mudstones with buff, dolomitic burrow-mottling. The top of the limestone is coincident with a highly attenuated isoclinal syncline, which brings platy mylonitic quartzite overlain by intensely sheared pelitic and semipelitic rocks above the much less deformed sequence on the lower limb. In the Målebjerg area, originally mapped by Haller (1953), the Slottet Formation rests unconformably on sheared and retrogressed gneisses, with pebbly or gritty quartzites at the base, and there is no equivalent of the Eleonore Sø volcano-sedimentary complex. In contrast, a clastic sequence up to 31 m thick locally occurs within basement hollows below the Slottet Formation. It incorporates two beds of diamictite (1.4 m and 7.6 m thick respectively) interpreted as tillites, together with interbedded phyllites, platy quartzites and semi-pelites (Smith & Robertson, 1999a; Smith et al. in press b). A correlation with the Tillite Group of the fjord zone (Hambrey & Spencer, 1987) is proposed. Brasier & Shields (2000) have recently queried the presumed Vendian (Veranger) age for the late Neoproterozoic tillites of Scotland and East Greenland, and favour a Sturtian age on the basis of chemostratigraphical studies. The presence of the Slottet Formation overlain by Målebjerg Formation carbonates in both the Eleonore Sø and Målebjerg areas suggests that the two windows reveal parts of the same underlying foreland area. This implies a westward displacement of at least 90 km for the overlying thrust sheet, which comprises mainly metasedimentary successions (Figs 5b, 6) and, locally, crystalline gneiss complexes. 2.f. Hamberg Gletscher (73 30 N) The scattered westernmost nunataks where inner Hamberg Gletscher merges with the Inland Ice at N (Fig. 1) comprise coarse-grained gabbros and hornfelsed sequences of dark lavas interlayered with calcareous sediments. These rock units are bordered to the east by dark coloured low-grade metasediments, and further east at an apparently higher structural level by high grade psammitic and pelitic metasediments. While some of the gabbros are locally strongly sheared, the structural setting of this sequence is uncertain due to the scattered nature of the nunataks. However, during compilation of the 1: geological map sheet of the region by J. C. Escher (pers. comm. 2000), regional considerations led to the conclusion that the gabbro lava complex and low-grade associated sediments might form part of the intact Caledonian foreland. The speculative thrust marking the boundary of this foreland region is not exposed, but has been drawn through glaciers so as to include a number of other nunataks along the border of the Inland Ice, including an anomalous gneissic nunatak at N (Fig. 1). The presumed foreland segment represented by the Hamberg Gletscher complex is exposed over an area about 30 km from north to south and less than 15 km wide. No representatives of the Slottet Formation quartzites or the Målebjerg Formation carbonates were seen in this region on the reconnaissance helicopter flights. 2.g. Charcot Land ( N) The distinctive rock units which crop out in Charcot Land and adjacent nunataks occur below an arched major thrust (Figs 1, 7a; Henriksen & Higgins, 1976). These rock units include an infracrustal gneiss complex, an unconformably overlying volcanosedimentary sequence known as the Charcot Land supracrustal sequence (Steck, 1971), quartz dioritic and granitic intrusions, and a small outcrop of diamictite interpreted as tillite. This region is traditionally known as the Charcot Land window and the rock units exposed have usually been considered to be part of the Caledonian foreland. The quartz diorite and granite intrusions, which cut the Charcot Land supracrustal sequence and the crystalline basement rocks, have both yielded Palaeoproterozoic isotopic ages (zircon and Rb-Sr whole rock) of c Ma (Steiger & Henriksen, 1972; Hansen, Steiger & Higgins, 1981). These are considered indicative of the time of emplacement, and thus suggest correlation of the Charcot Land supracrustal sequence with the comparable supracrustal rocks in the Eleonore Sø region. The tillite is the youngest unit known within the Charcot Land window and, as at Målebjerg, appears to occupy original depressions in the pre-tillite basement. It is not present at the thrust contact on either west or east margins of the Charcot Land window, nor is there evidence here of the presence of Cambro- Ordovician sediments. The tillite has been correlated with the upper of the tillites of the fjord zone (the Storeelv Formation of the Tillite Group: Moncrieff, 1989), although a Sturtian age for these presumed Vendian (Veranger) tillites has recently been suggested by Brasier & Shields (2000). Along much of the eastern

10 152 A. K. HIGGINS, A. G. LESLIE & M. P. SMITH margin of the window, the rocks immediately beneath the thrust are metacarbonates of the Charcot Land supracrustal sequence. The hangingwall of the thrust is made up of Archaean Palaeoproterozoic crystalline gneisses in the east (Flyverfjord infracrustal complex) and metasedimentary successions (Krummedal supracrustal sequence) in the west (Higgins, 1982). Estimates of the westward displacement of the thrust sheets above the window range from km (Henriksen, 1986) to at least 110 km (Higgins, 1982). 2.h. Gåseland ( N) The most southerly of the Caledonian foreland windows, the Gåseland window, crops out in western Gåseland and adjacent areas in the inner Scoresby Sund region (Figs 1, 7b), and was originally described by Wenk (1961). There are similarities with the Charcot Land and Målebjerg windows in that a crystalline basement complex is unconformably overlain by a diamictite interpreted as tillite, which is preserved in pockets on the gneiss surface at the base of a thin sequence of calcareous sediments. Wenk had assumed that the weakly metamorphosed sediments represented a Basal Series of the Neoproterozoic Eleonore Bay Supergroup. Later Survey work pointed out the similarities between the tillites of the Gåseland area and those of the Tillite Group in the fjord region (Phillips et al. 1973), with the implication that the strongly laminated cream-coloured marbles above the tillites and beneath the thrust in the Gåseland window might be part of a Cambro-Ordovician foreland succession (Phillips et al. 1973; Phillips & Friderichsen, 1981). There is no development of Skolithos-bearing quartzites in either the Charcot Land window or the Gåseland window. The thrust sheet overlying the foreland in the Charcot Land window comprises both basement gneiss complexes (orthogneisses and amphibolites of the Flyverfjord infracrustal complex) and overlying metasediments (Krummedal supracrustal sequence); Phillips et al. (1973) estimated a minimum westward displacement of 35 km for the overlying thrust sheets, but higher thrust sheets which comprise migmatitic metasediments with granite sheets could have been displaced much greater distances. Wenk s (1961) crosssection clearly shows how the almost horizontal attitude of the thrust abruptly steepens and ramps downwards at the eastern margin of the window in Gåseland (Fig. 7b). 3. Stratigraphical overview South of Kronprins Christian Land, windows through Caledonian thrust sheets give a scattered and incomplete impression of stratigraphical developments; while nearly all the successions exposed in the windows show some degree of Caledonian tectonic disturbance, it is argued below (Section 4) that they have close affinities with the autochthonous foreland now hidden beneath the Inland Ice, west of the innermost nunataks. The Proterozoic Lower Palaeozoic successions in the foreland areas over a 1300 km long N S trending stretch are summarized in Figure 8. In all the foreland areas, except Hamberg Gletscher and Charcot Land, the uppermost preserved units are carbonates of certain, or probable, Early Palaeozoic age. Ordovician conodonts have been isolated from samples collected in Kronprins Christian Land, Lambert Land and Nørreland; abundant organic material in concentrates from the Eleonore Sø and Målebjerg carbonates also indicates a post-proterozoic age. In addition, Ordovician conodonts and gastropods have been recorded from erratic limestone boulders on Cecilia Nunatak (72 30 N: Wegmann, 1935; Haller, 1971; J. S. Peel in Higgins, Friderichsen & Thyrsted, 1981). Cambrian sediments above the Skolithos-bearing quartzites are either absent or greatly attenuated and may indicate an extension of the Cambrian hiatus demonstrated in eastern North Greenland, southwards along the Iapetus margin (Bryant & Smith, 1990; Smith et al. 1999). The thrusts overlying the carbonates appear to be sub-parallel to the underlying stratigraphy in the windows and must, as observed in Kronprins Christian Land, follow long flats in the Lower Palaeozoic carbonates, the youngest sediments deposited prior to the Caledonian orogeny. These carbonates are thus a relatively ductile glide horizon, utilized by the Caledonian thrusts as they propagated to the west-northwest. Underlying the carbonates in the foreland windows are quartzites which contain Skolithos ichnofossils, the so-called pipe rock of John Haller and others. These Lower Cambrian developments are represented by the Kap Holbæk Formation of Kronprins Christian Land (and Danmark Fjord), the Zebra series of Dronning Louise Land and the Slottet Formation of the Eleonore Sø and Målebjerg windows. Skolithosbearing quartzites have now been found in situ in East Greenland over a N S distance of 750 km, but must once have extended further south since glacial erratics are common throughout the orogen as far south as the Scoresby Sund region (70 72 N). An instructive reconstruction of source areas of the Skolithos-bearing quartzites based on erratic finds was provided by Haller (1971, fig. 48). Diamictites interpreted as tillites have been found in the Gåseland, Charcot Land and Målebjerg windows, in all cases unconformably overlying crystalline basement. At Målebjerg the tillites are immediately overlain by the Lower Cambrian Slottet Formation; there seems no reason to doubt the proposed correlation of the tillites in all three windows with the Tillite Group of the fjord region (Fig. 8). There are thus broad similarities in the Neoproterozoic Lower Palaeozoic parautochthonous foreland developments along the entire length of the

11 Foreland stratigraphy of East Greenland Caledonides 153 W Alfabet Nunatakker Tillit Nunatak CHARCOT LAND HINKS LAND E a T CHARCOT LAND WINDOW 0 10 km Allochthonous thrust sheets Lower thrust units Archaean orthogneisses b PAUL STERN LAND GÅSELAND WINDOW Lower thrust units mainly metasediments GÅSELAND Upper thrust units high grade metasediments and granite Charcot Land window Palaeoproterozoic gneisses and granites Gåseland window Palaeoproterozoic gneisses and amphibolites Palaeoproterozoic Charcot Land supracrustal sequence Varangerian to Cambro- Ordovician tillite, carbonate and phyllite T Tillite Post-Caledonian Palaeogene basalt Figure 7. (a) W E cross-section of the Charcot land window, redrawn after Higgins (1982). (b) W E cross-section of the Gåseland window, redrawn after Wenk (1961), showing flat trajectory of the thrust above the thin calcareous sedimentary unit, and the abrupt downturn of the thrust on the east side of the window. Ma 400 Devonian Silurian Kronprins Christian Land Lambert Land Nørreland Dronning Louise Land Eleonore Sø Hamberg Gletscher Målebjerg Charcot Land Gåseland NW Scotland foreland East Greenland allochthon 500 Ordovician Cambrian KH ZS M S M S D E KOF 600 Vendian Sturtian TG Riphean HF EBS T ZZ IF HS IF IF TS ES HG CL Dolomite Limestone Sandstone Mudstone/sandstone Tillite Volcanic rocks Figure 8. Comparisons of the Proterozoic Lower Palaeozoic stratigraphy in the parautochthonous windows from north to south along the 1300 km length of the East Greenland Caledonides, with columns depicting the foreland stratigraphy in Northwest Scotland and the allochthonous East Greenland stratigraphy of the fjord zone for comparison. CL: Charcot Land supracrustal sequence (includes volcanics). D: Durness Group. E: Eriboll and An t-sròn formations. EBS: Eleonore Bay Supergroup. ES: Eleonore Sø volcano-sedimentary complex. HF: Hagen Fjord Group. HG: Hamberg Gletscher foreland complex. HS: Hekla Sund basalts. IF: Independence Fjord Group (cut by metadolerite dykes). KH: Kap Holbæk Formation. KOF: Kong Oscar Fjord Group. M: Målebjerg Formation. S: Slottet Formation. T: Torridonian Supergroup. TG: Tillite Group. TS: Trekant series (cut by metadolerite dykes). ZS: Zebra series. ZZ: Zig-Zag Dal Basalt Formation.

12 154 A. K. HIGGINS, A. G. LESLIE & M. P. SMITH marginal zone of the East Greenland Caledonides (Fig. 8). It is noteworthy that there is a great contrast between these relatively thin sequences, punctuated by periodic and substantial hiatuses, and the c km of almost continuous Neoproterozoic Ordovician sedimentation present in the allochthonous outer fjord region of East Greenland. Although somewhat disrupted by Caledonian thrusting, it is clear that a series of roughly N S trending facies belts define the orientation of the Laurentian passive margin from Scoresby Sund (70 N) to Kronprins Christian Land (81 30 N). Older Proterozoic sedimentary sequences are variably developed (Fig. 8). As noted above, the Trekant series quartzites of Dronning Louise Land and the quartzites in the Nørreland window have been correlated with the Palaeoproterozoic Mesoproterozoic Independence Fjord Group quartzites of Kronprins Christian Land as both are cut by similar dyke swarms (Midsommersø dolerites). The Palaeoproterozoic Eleonore Sø volcano-sedimentary rift sequence appears to represent accumulations in a rift basin of limited extent; the succession is not represented at Målebjerg only 35 km to the east, but there are similarities with the Palaeoproterozoic Charcot Land supracrustal sequence of the Charcot Land window 150 km to the south, and possibly also the Hamberg Gletscher complex, which has not yet been dated. Once again, depositional tracts at this time seem to have been relatively more elongate along a N S direction. 4. Structural setting of the foreland windows The transition between the Caledonian orogen and undisturbed foreland is only completely preserved in Kronprins Christian Land. To the south, all of the windows through Caledonian thrusts that are intermittently exposed along the margin of the Inland Ice reveal rock units that show some degree of Caledonian deformation. With respect to the Charcot Land and Gåseland windows, it has been argued that the evidence of Caledonian deformation shows that the areas are not autochthonous foreland, and are but parts of lower thrust sheets with a further thrust or thrusts at depth (Moncrieff, 1989; Manby & Hambrey, 1989). However, the contrast between the stratigraphical developments within the windows and that in the overlying allochthonous thrust sheets suggests to us that the sequences preserved in the windows are probably representative of the autochthonous foreland now hidden beneath the Inland Ice. The similarities in stratigraphical developments from window to window along the length of the fold belt and with the undisturbed foreland of western Kronprins Christian Land (Fig. 8) support this viewpoint, as do the occurrences of erratic blocks of Skolithos-bearing quartzites (and of Lower Palaeozoic carbonates: authors observations) identical to lithologies in some of the windows. We would argue that the sequences within the windows are not far displaced from their original positions, and can be viewed as parautochthonous foreland. Their structural setting can be compared to that of the Lower Palaeozoic sequence in the thinskinned fold-and-thrust belt west of the main thrust front in Kronprins Christian Land, which passes westwards into undisturbed foreland (Higgins et al. in press b). In Kronprins Christian Land (Fig. 2), it is envisaged that the Caledonian sole thrust coincides with the shallow floor thrust that underlies the thin-skinned fold-and-thrust belt west of the Vandredalen thrust front (Fig. 4). Eastwards it is presumed to continue at a shallow depth beneath the flat-lying Vandredalen thrust. North of Sæfaxi Elv (Fig. 2) a thrust duplex involves Proterozoic metasedimentary and volcanic rocks beneath the Vandredalen thrust but, as crystalline basement units have not been incorporated, the thrusting can still be designated as thin-skinned. The transition from thin-skinned to thick-skinned thrust geometry in Kronprins Christian Land is placed at the prominent steeply dipping shear zone traceable from western Amdrup Land to western Holm Land and Hovgaard Ø (Fig. 2), where Proterozoic quartzites and volcanic rocks occur in the footwall and crystalline basement gneiss complexes in the hangingwall. The thin-skinned thrust belt in Kronprins Christian Land is thus up to 90 km wide. The scattered windows that occur along the margin of the Inland Ice to the south of Kronprins Christian Land are, in broad terms, characterized by arched Caledonian thrusts concordant with the footwall stratigraphy (Figs 4, 6, 7); they are clearly located within the marginal thin-skinned thrust belt. Based on the argument that the various windows expose parautochthonous foreland, the Caledonian sole thrust envisaged as the western border of the marginal thrust belt is placed on Figure 1 a few tens of kilometres west of the innermost windows, but on the east side of the nunataks preserving the Hamberg Gletscher foreland. This position is not markedly different from the limit of the Caledonian orogen depicted by Haller (1971, fig. 14) and Henriksen & Higgins (1976, figs 172, 173), although these limits were simply positioned west of the then known windows (Gåseland, Charcot Land). South of Kronprins Christian Land, the feature marking the transition between the western marginal and thick-skinned thrust belts (Fig. 1) appears to continue along the Storstrømmen Shear Zone (Strachan et al. 1992), a prominent N S trending sinistral deformation zone extending from at least N down to 76 N, that runs east of the Nørreland window and along the east margin of Dronning Louise Land. The southward continuation is conjectural, but has been drawn close to the line of the Fjord Region Detachment of Andresen, Hartz & Vold (1998).

13 Foreland stratigraphy of East Greenland Caledonides 155 Between 74 and 72 N, displacements on this linear feature have been shown to be dominantly extensional (Hartz & Andresen 1995; Andresen, Hartz & Vold 1998; Hartz et al. 2000), but this now extensional feature appears to lie close to the location of a major change in thrust geometry. The broad region of the East Greenland Caledonian orogen east of the western marginal thrust belt is interpreted as a thick-skinned thrust belt in which extensive areas of crystalline gneiss terrain represent the deep-seated portions of major thrust sheets. The frontal parts of these thrust sheets formerly projected westwards over the relatively thin-skinned marginal thrust belt. Thrusts and deformation zones within this thick-skinned belt are, for the most part, steeply inclined. The ubiquitous east-southeast to west-northwest Caledonian thrusting probably involved cumulative displacements of the order of km, representing a shortening across the orogenic belt of about % (Higgins & Leslie, 2000). Two main thrust levels are distinguished: lower thrust units comprising Archaean Palaeoproterozoic gneiss complexes and Mesoproterozoic metasedimentary rocks, and upper thrust units dominated by high-grade metasediments intruded by early Neoproterozoic and Caledonian granites, and in their uppermost part preserving the Neoproterozoic Eleonore Bay Supergroup and Tillite Group, and Lower Palaeozoic carbonates. The youngest units involved in thrusting within the East Greenland Caledonides are early Wenlock limestones and overlying turbidites in Kronprins Christian Land (Fig. 8). The onset of deformation is probably reflected by the initiation of clastic supply to the deep water trough of the Franklinian Basin of North Greenland during the spiralis Biozone (late Llandovery) (Hurst & Surlyk, 1982). Evidence that orogenic uplift continued into the mid-wenlock is provided by the chert conglomerates and interbedded sandstone turbidites of the Nordkronen Formation (Hurst & Surlyk, 1982; Higgins et al. 1991). The pebble-sized chert clasts within the unit were derived either from uplifted Ordovician basinal sequences (Hurst & Surlyk, 1982) or from Ordovician Silurian shelf carbonates. Further to the south, in the fjord region of Northeast Greenland, the youngest units seen to be involved in thrusting are mid-ordovician (Smith & Bjerreskov, 1994), but there is no evidence to suggest that thrusting pre-dates that in Kronprins Christian Land. There is thus a markedly different Caledonian deformation history in comparison with that of Scotland, with a single Scandian phase, rather than the Scandian deformation being superimposed on an earlier Grampian/Taconic phase. In both the marginal relatively thin-skinned and eastern thick-skinned segments of the East Greenland Caledonides, extensional faults are important and have been associated with collapse of the over-thickened orogen and initiation of the Devonian basins (Larsen & Bengaard 1991; Strachan, 1994; Hartz & Andresen, 1995; Strachan et al. 1995; Andresen, Hartz & Vold, 1998; Hartz et al. 2000). 5. Comparison with the Caledonides of Northwest Scotland The continuation of the East Greenland Caledonides is concealed beneath Palaeogene flood basalts south of Scoresby Sund (70 N). The most proximal sector of the Laurentian margin with Caledonian deformation is Northwest Scotland which, in palinspastic terms (Cambridge Paleomap Services, 1998), may have lain as little as 500 km to the south of Scoresby Sund (in comparison with the 1300 km length of the exposed East Greenland Caledonides). Thus, in broad terms, the foreland succession of Northwest Scotland corresponds to the parautochthonous foreland windows of the nunatak region of East Greenland and the thinskinned (parautochthonous) fold-and-thrust belt of western Kronprins Christian Land (Figs 8, 9). The orthotectonic rocks of the Northwest Highlands and Central Scottish Highlands should, in turn, be equivalent to the lower and upper thrust levels of East Greenland. Consideration of the tectonic history and the palinspastic restoration of this sector provides significant insights into viable models for Neoproterozoic basin geometry in East Greenland as well as for more widespread patterns of basin evolution on the developing Iapetus margin (Fig. 9). The stratigraphy of the foreland and Moine thrust sheet in Northwest Scotland is fairly well understood, although controversies and uncertainties endure. On the foreland (Fig. 8), sediments of the Torridonian Supergroup rest unconformably on crystalline basement deformed and metamorphosed intermittently between 2900 and 1100 Ma (Gibbons & Harris, 1994). This supergroup is predominantly composed of terrestrial sediments assigned to three groups. The oldest unit, the Stoer Group (max. 2 km), is dominated by alluvial fan and lacustrine sediments deposited in a half-graben bounded by an east-facing fault (Stewart, 1991, 1993). Stoer Group deposition was terminated by a period of tilting and uplift, during which most of the group, and part of the underlying basement, was eroded. The succeeding Torridon Group rests with angular unconformity on the Stoer Group and marks a significant expansion of the basin, as extension stepped westwards by over 50 km to a new east-facing bounding fault (Blundell, Hurich & Smithson, 1985; Stewart, 1991, 1993). The Torridon Group (up to 7 km) is predominantly composed of arkosic pebbly sandstones deposited in alluvial fans and braided rivers flowing from northwest to southeast. A rather different relationship is preserved beneath the Torridon Group on Skye and the adjacent mainland, where the Sleat Group (3.5 km) was again deposited in a relatively small graben dominated by an east-facing