Incised submarine canyons governing new evidence of Early Triassic rifting in East Greenland

Palaeogeography, Palaeoclimatology, Palaeoecology 161 (2000) 267–293 www.elsevier.nl/locate/palaeo

Incised submarine canyons governing new evidence of Early Triassic rifting in East Greenland Lars Seidler * Geological Institute, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark

Abstract Regional studies of the uppermost Permian and lowermost Triassic successions in central East Greenland have shown that sedimentation took place in half-grabens during two phases of active rifting. The Permian–Triassic boundary locally contains submarine canyons, up to several kilometres wide and several tens of metres deep formed in latest Permian or earliest Triassic times. Sandy and conglomeratic turbidites filling the canyons are highly variable in architecture depending on the structural setting and the geometry of the submarine canyons. Bounding surfaces also change character because of an asymmetric subsidence pattern of tilted fault-blocks. The submarine canyons have been studied on Wegener Halvø, where exposures allow tracing of several canyons from the slope to the basin floor setting. The canyons are interpreted to have developed from fluvial incised valleys furthest updip on the faultblock and from turbidity-current erosion further downdip. The canyons have a SE–NW elongation, while palaeocurrents were unimodal towards the NW reflecting the dip of the rotated fault-block. The canyon fill turbidites were deposited by consequent drainage on a NW-dipping, ramp-like hangingwall slope within a half-graben. In areas between the canyons, the Permian–Triassic boundary is onlapped by Triassic mudstones of offshore marine origin. The studied succession is grouped into two facies associations: (1) submarine gravity-flow sandstones and conglomerates; and (2) suspension-deposited shales and mudstones. Traced down-dip, the submarine canyon fills display a change from chaotic, conglomeratic and coarse sandy high-density turbidites proximally to organised sandy turbidites basinwards. Farthest basinward, a remarkable basin-floor fan with well-developed giant-scale foresets is observed. This trend reflects canyon widening and a decrease in gradient at the basin-floor. Progradation of the turbidites was probably controlled by westward tilting of the fault-block causing a steeper slope and subaerial exposure of a larger part of the proximal hangingwall above the fulcrum. This resulted in erosion of the crestal area of the block, that is, Upper Permian and older successions, and led to an increase in sediment supply to the downdip area. Accommodation space was created below fulcrum and downdip transgression governed back-filling of the submarine canyons. The observed eastward onlap of ammonite zones in shales and mudstones between the canyons was contemporaneous with the filling of the canyons. A later drowning of the updip catchment caused decrease or shut-off of sediment supply leading to deposition of shales and mudstones blanketing the sandy turbidites in the canyons. The tilt of the studied fault-block is interpreted to have caused simultaneous transgression below the fulcrum and regression, with bypass or erosion, above the fulcrum. It is concluded, that the tilted fault-block setting has important consequences for sequence stratigraphic interpretations due to the asymmetric subsidence causing drastic changes in bounding surface character. Tectonism and sediment supply were the main controls on the stratigraphic architecture of the studied Lower Triassic succession on Wegener Halvø. © 2000 Elsevier Science B.V. All rights reserved. Keywords: rotated fault-blocks; submarine canyons; synrift sedimentation; turbidites * Present address: Institute of Energy Research, University of Wyoming, P.O. Box 3006, Laramie, WY 82071, USA. Tel.: +1-307-766-2843; fax: +1-307-766-2737. E-mail address: [email protected] (L. Seidler) 0031-0182/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 3 1 -0 1 8 2 ( 0 0 ) 0 0 12 6 - 7

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1. Introduction The marine part of the Triassic succession in East Greenland was first described by Koch (1929) and named the Wordie Creek Formation after a small creek at Kap Stosch, Hold With Hope to the north of the field area. Detailed palaeontological work has been carried out by several authors and resulted in biostratigraphical zonation based on ammonites and palynomorphs ( Koch, 1931; Nielsen, 1935; Spath, 1935; Grasmu¨ck and Tru¨mpy, 1969; Tru¨mpy, 1969; Piasecki, 1984, personal communication, 1999). Previous sedimentological studies have been focused mainly on the overlying Middle and Upper Triassic continental succession (Clemmensen, 1976, 1977, 1978a,b, 1979, 1980a,b; Kent and Clemmensen, 1996; Clemmensen et al., 1998), and although several studies have been carried out on the Wordie Creek Formation (Grasmu¨ck and Tru¨mpy, 1969; Birkelund and Perch-Nielsen, 1969, 1976; Haller, 1970; Callomon et al., 1972; Perch-Nielsen et al., 1972, 1974; Birkenmajer, 1977; Clemmensen, 1976, 1980a,b; Surlyk et al., 1981, 1984, 1986; Marcussen et al., 1987, 1988; Surlyk, 1990; Stemmerik et al., 1993a), only a few have emphasized sedimentological aspects and tectonism. The Lower Triassic Wordie Creek Formation crops out on a number of islands and peninsulas on the east coast of Greenland (Fig. 1). This paper is a sedimentological and sequence stratigraphic study of the lowermost part of the Wordie Creek Formation that was deposited within a tilted fault-block setting on Wegener Halvø in northern Jameson Land. The studied area covers ca. 300 km2 and the outcrops of especially the Upper Permian and Lower Triassic successions are excellent. A rich ammonite fauna allows the recognition of six regional zones, which provide the framework for the sequence stratigraphic interpretation (Grasmu¨ck and Tru¨mpy, 1969) ( Fig. 2). The field area offers an excellent opportunity to study the interplay between sea-level variations, tilting of fault-blocks, sediment supply and the resulting nature of the marine basin-fill and bounding surfaces. The main aim of this study is to describe and interpret the nature of several submarine canyons developed at the Permian–Triassic bound-

ary in an extensional half-graben setting. Secondly, the importance of eustasy versus tectonics and sediment supply is evaluated, using sequence stratigraphic and sedimentological methodology. Thirdly, it is investigated whether Early Triassic sedimentation in this part of Pangea took place during active rifting.

2. Evolution of the East Greenland rifted margin The East Greenland Basin was initiated during the Devonian following the Caledonian Orogeny in the Silurian, and active plate separation started in Early Cenozoic times. In Late Carboniferous and Early Permian times, rotational block faulting took place along the basin margins (Surlyk, 1990). From the present study it has become evident that block faulting was reactivated during the Late Permian and Early Triassic and synsedimentary faults are seen at several localities ( Figs. 3 and 4). Before the Late Permian transgression and onset of sedimentation, the fault-blocks were peneplained. In the latest Permian most of the East Greenland Basin became exposed subaerially (Surlyk et al., 1986; Stemmerik et al., 1993a,b). The Permian–Triassic boundary is unconformable to conformable in nature, and variable and largely unknown amounts of the Upper Permian succession were eroded prior to the Griesbachian transgression (Stemmerik et al., 1997). Numerous Lower Triassic submarine canyons were eroded into the Upper Permian succession and reworked Permian sediments and fossils occur high in the Triassic succession. However, the deepest parts of the fault-bounded subbasins experienced a continuous marine sedimentation as no hiatus between the Permian and Triassic is seen (Piasecki, 1984). In latest Permian or earliest Triassic time the East Greenland Basin was again transgressed. On Wegener Halvø, this is shown by marine shales onlapping the top-Permian surface. The distribution of Triassic ammonites shows that successively younger marine shales gradually lap onto the underlying Permian succession towards the east (Grasmu¨ck and Tru¨mpy, 1969). Following the transgression, a large shelf embayment was established in the East Greenland Basin and this accom-

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Fig. 1. Locality map of Wegener Halvø. The Tvekegle Fault is downthrown towards east and is limiting the southern fault-block of Wegener Halvø towards east. Localities 3–6 are located within the course of the same submarine canyon.

modated a highly variable, mainly marine succession — the Wordie Creek Formation (Clemmensen, 1980a,b; Stemmerik et al., 1993a,b; Surlyk et al., 1986). It is evident that the thickness of the Wordie Creek Formation varies greatly depending on location in the subbasins relative to synsedimentary faults. The southern Triassic marine basin was bounded to the west by the southern extension of the Post-Devonian Main Fault of Vischer (1943) (Clemmensen, 1980a,b; Grasmu¨ck and Tru¨mpy, 1969). The eastern margin

of the basin is not well established but seems to have followed the N–S elongated Liverpool Land high. To the south, the embayment was closed around Scoresby Sund (Surlyk et al., 1981). It is likely that the basin had an open connection with the Boreal sea to the north, based on correlation of the Wordie Creek Formation fauna with faunas in Canada (Birkelund and Perch-Nielsen, 1976; Tozer, 1961, 1967, 1994). Towards the end of the Induan Stage, sedimentation gradually became continental, mainly in floodplain and coastal plain

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Fig. 2. Chronostratigraphic scheme for the Upper Permian and Lower Triassic at Wegener Halvø. The lithostratigraphic section is ca. 10 km across. The exact age of the two sequence boundaries is uncertain. Modified from Grasmu¨ck and Tru¨mpy (1969).

settings. Later in the Triassic, alluvial fan, fluvial, lacustrine and aeolian sediments were deposited in semi-arid environments (Clemmensen, 1980a).

3. Geological setting of Wegener Halvø In Griesbachian times, the subbasins now exposed on Wegener Halvø were made of two rotated fault-blocks which had rather different depositional environments. Both fault-blocks were tilted towards the W–NW, as is evident from the thickness variations, palaeocurrent directions and the distribution of facies in the Lower Triassic Wordie Creek Formation. This interpretation is supported by the facies distribution of the Upper Permian Wegener Halvø Formation (L. Stemmerik, personal communication, 1999). Grasmu¨ck and Tru¨mpy (1969) described the faults on Wegener Halvø to be of mainly Cenozoic age, but detailed sedimentological studies of the Upper Permian (L. Stemmerik, personal communication, 1999) and Lower Triassic successions reveal that some faults were active during Late Palaeozoic– Early Mesozoic times. The outcrops described here are situated on the southern fault-block of Wegener Halvø, which is structurally and sedimentologically

less complex than the northern fault-block ( Fig. 3). Depositional units can be followed for several kilometres along strike without any marked changes in thickness and facies. This contrasts with observations from the northern Wegener Halvø fault-block, where synsedimentary faults segregated the subbasin causing short-distance lateral changes in facies and thicknesses ( Fig. 4). The northern fault-block will not be described in further detail. On southern Wegener Halvø, the Wordie Creek Formation rests on the Upper Permian Ravnefjeld and Wegener Halvø Formations, which were eroded prior to Triassic deposition (Fig. 2) (Grasmu¨ck and Tru¨mpy, 1969). The sediments were deposited on the hangingwall dip-slope and were sourced from the relatively uplifted fault-block crest towards the east. This type of setting can structurally be compared to a ramp-like margin (Gawthorpe et al., 1994). Lateral tracing of depositional units in an E–W traverse across the hangingwall dip-slope has revealed considerable variation in thickness and facies. The thinnest and most proximal part of the depositional system is located on the relatively uplifted part of the hangingwall towards the E– SE, where ca. 270 m of Wordie Creek Formation sediments are present. These are dominated by

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Fig. 3. Topographic map of Wegener Halvø with observed NW-elongated submarine canyons filled with turbidite sandstones and conglomerates. The contour interval is 100 m. Synsedimentary faults have been observed farthest towards the north.

upper shoreface sandstones, coastal plain mudstones and minor fluvial sandstones, with evidence of periodic subaerial exposure. Thickening and fining towards the NW reflects a gradual change towards deeper water and basinal offshore depositional environments. In outcrops located furthest towards the NW in Fleming Fjord, >380 m of the Wordie Creek Formation is preserved. Only the lowermost part of the Wordie Creek Formation on southern Wegener Halvø is described here — the lowermost cycle between the Permian–Triassic

sequence boundary (SB) and the first flooding surface (FS), that is, the interval of ammonite zones 2 and 3 ( Figs. 2 and 5). This cycle varies in thickness between 17 m proximally and 55 m at the distal outcrops where it is thickest. The cycle is inferred to have been deposited during a period of ca. 0.5 my. This estimate is based on ammonite stratigraphy, but it is important to note that it is only an estimate, as the maximum time resolution of the ammonite stratigraphy is ca. 0.3–0.5 my (M. Bjerager, personal communication, 1998) and

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Fig. 4. Synsedimentary fault from the northern fault-block of Wegener Halvø. By the thickening of the wedge-shaped sediment package it can be seen that the right fault-block subsided during deposition in the Late Permian ( WH and Ra) and the earliest Triassic ( WC1–WC4). The fault died out at after deposition of unit WC4.

absolute age datings are absent. The Lower Triassic biostratigraphy in East Greenland is based on a framework of six ammonite zones defined by Koch (1931), Nielsen (1935), Spath (1935), Grasmu¨ck and Tru¨mpy (1969), Tozer (1967, 1994) and Tru¨mpy (1969). The zonation used here is the one by Grasmu¨ck and Tru¨mpy (1969) shown below: Zone 1: Glyptophiceras triviale. Zone 2: Glyptophiceras martini. Zone 3: Metophiceras subdemissum. Zone 4: Ophiceras commune Zone 5: Vishnuites decipiens Zone 6: Proptychites rosenkrantzi Because of a relatively uplifted topography of Wegener Halvø in the Early Triassic, this area was transgressed later than other lower lying areas in the East Greenland Basin. The oldest Triassic marine shales on Wegener Halvø thus belong to ammonite zone 2 (Grasmu¨ck and Tru¨mpy, 1969).

4. Facies associations Facies associations 1 and 2 will be described only briefly because the detailed data on facies are summarised in Tables 1 and 2. Generally, FA1 comprise coarse-grained turbidites confined within submarine canyons and FA2 comprise the finegrained sediments in the areas between submarine canyons. 4.1. Facies association 1 (FA1), submarine gravityflow deposits 4.1.1.. General setting and geometry Submarine gravity-flow deposits comprise a large part of the basal Wordie Creek Formation on Wegener Halvø. In the proximal reaches of the hangingwall of the tilted block, they fill submarine canyons and are directly overlying Upper Permian carbonate reefs of the Wegener Halvø Formation

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Fig. 5. Schematic cross-section through the western part of the hangingwall illustrating deposition of the basin-floor fan and submarine canyon fill. The distance between localities 1 and 4 is ca. 4 km and between 4 and 6 ca. 1 km. The thickness of the Lower Triassic turbidite wedge is ca. 20 m and expected to thin and pinch out updip. Localities 1 and 4 represents confined deposition in a submarine canyon whereas locality 6 was largely unconfined. Not to scale.

(Fig. 6). Further basinwards, the sandy and conglomeratic turbidites are located in much wider canyons and overlie Lower Triassic shales with evidence of only minor erosion (Fig. 7). Individual sandy turbidite beds of FA1 are up to 23 m thick. In the proximal reaches the lateral bed continuity is only up to a few tens of metres and the bed geometry is markedly lenticular. Basinwards at locality 6 ( Fig. 1), the individual beds are more continuous and can be followed for many tens of metres ( Fig. 7). Palaeocurrents are towards the NW throughout the studied succession. 4.1.2. Interpretation The conglomerates and pebbly sandstones are interpreted to have been deposited by high-density turbidity currents and minor debris flows as argued in Table 1. In the proximal reaches on the faultblock, the turbidity flows were confined within submarine canyons. Further basinward, the canyons widened leading to an increase in bed continuity. Finer-grained turbidites (facies 1E) are observed in the areas between the submarine

canyons and deposition was by low-density turbidity flows and by suspension fall-out as argued in Tables 1 and 2 (cf. Mutti and Normark, 1987).

4.2. Facies association 2 (FA2): suspension deposited shales and mudstones 4.2.1. General setting Shales and mudstones dominate the Wordie Creek Formation on southern Wegener Halvø. They normally occur in overbank settings between the submarine canyons where the Permian–Triassic boundary is seen as a slight change in colour from greyish-black Upper Permian siltstones (Oksedal Member of Schuchert Dal Formation) to greenish Lower Triassic shales. In the overbank areas, the monotonous shaly successions can be several hundreds of metres thick, only interbedded with centimetre-thick, fine-grained turbidites. FA2 sediments locally fill the lower part of the basinward reaches of submarine canyons as observed at locality 6 ( Fig. 7).

Angular pebbles to boulders (>40 cm) of Permian carbonate reef origin. Rounded quartzite and granitic pebbles to boulders (<30 cm). Well-preserved laminated sandstone and shale clasts (<40 cm). Matrix is generally poorly sorted, arkosic sand to gravel. As facies 1A.

Arkosic medium-grained sandstone to gravelstone. Common granitic, quartz and limestone pebbles. Rip-up mudstone clasts and sandstone clasts also occur.

1A: Normally-graded, matrix or clast supported conglomerate with clast imbrication.

1C: Massive, medium-grained to pebbly sandstone to gravelstone.

1B: Inversely to normallygraded or normally-graded, matrix or clast supported conglomerate without clast imbrication

Lithology

Facies

Bed thickness, boundaries and trend

A-axis imbrication of clasts.

Beds up to ~4 m thick. Erosional base with up to >1 m of relief. Clasts and/or matrix are normally-graded. Several cycles of grading can occur within one bed. Can change laterally into facies 1B and is normally only distributed for tens of metres laterally. No imbrication of clasts. Beds up to 10 m thick. Erosional base with up to several metres of relief. Clasts and/or matrix are normally-graded. Individual beds can be traced laterally for tens of metres. No imbrication of clasts. Rare Beds up to 8 m thick averaging stratification outlined by 0.5–1 m. Erosional base with pebble layers. Flute marks and some decimetres of relief. scours common. Frequent Inversely-to-normally-graded og water-escape and load normally-graded. Non-graded structures. beds are rare. Beds can be traced laterally for hundreds of metres.

Sedimentary structures and trace fossils

Table 1 Facies descriptions and interpretations for facies association 1: submarine gravity flow deposits

High-density turbidity currents and sandy debris flows (Lowe, 1982). Pebble layers deposited from traction carpet. If stratification is present several episodes of traction carpet freezing (surging flow) (Hendry, 1973; Walker, 1975; Lowe, 1982; Surlyk, 1984). Graded upper part deposited from suspension fall-out from high-density turbidity current. Very thick beds likely deposited from sustained, quasisteady turbidity flows ( Kneller and Branney, 1995).

Deposition from high-density turbidity current, eventually with development of basal traction carpet.

Deposition by suspension fall out from high-density turbidity currents – eventually surging flows ( Walker, 1975; Lowe, 1982). A-axis imbrication indicates that dispersive pressure was important.

Depositional process

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Arkosic fine-grained to very coarse-grained sandstone. Common granitic, quartz and limestone pebbles. Rip-up mudstone clasts and sandstone clasts also occur.

Quartzitic siltstone to mediumgrained sandstone, locally with ripped up mud clasts.

Angular pebbles to cobbles (<20 cm) of Permian carbonate reef origin. Rounded quartzite and granitic pebbles to cobbles (<20 cm). Matrix is generally poorly sorted sand to gravel. Rare mud matrix also observed.

1D: Stratified, fine-grained sandstone to pebbly, very coarse-grained sandstone.

1E: Sharp-based, graded siltstone to medium-grained sandstone (‘classical turbidites’).

1F: Non-graded, matrix or clast-supported conglomerate.

Normally no imbrication of clasts, rare a-axis imbrication. Non-graded and no stratification.

No imbrication of clasts. Beds are non-graded or graded. Horizontal bedding and trough cross-bedding are weakly developed. Cross-bedded units are multi-storey with set thicknesses of 5–25 cm. Flute marks and scouring are frequent. Water escape structures (dish) are common. Flute and tool marks common. Normally-graded with sequential development from T (horizontal lamination) to B T (ripples) to T (weak C D horizontal lamination) to T E (suspension fallout). Most often only part of the sequence is developed. Water-escape structures common. Beds up to 5 m thick. Erosional base without significant relief, sharp upper boundary. Beds are laterally inextensive and often interbedded with facies 1A–D. Only observed as submarine canyon fill.

Beds 0.5–50 cm in thickness. Sharp to weakly erosional base, often interlaminated with mudstones/shales. Beds most often appear sheet-like with lateral extends of hundreds of metres.

Cross-bedded units up to 30 m thick. Erosional base of beds with up to ~1 m of relief. Rare gradational base. Fining-upward units most common.

Low-density turbidity currents (Bouma, 1962; Lowe, 1982). Traction sedimentation forms the T and T divisions, whereas T B C D is formed by near-bed traction and suspension. T represents E suspension fallout. Represents overbank and basin floor deposition (Mutti and Normark, 1987, 1991; Bouma et al., 1997; Lowe, 1982; Slatt et al., 1997). Sandy debris flows and grainflows with frictional freezing of plastic laminar flows (Lowe, 1982; Shanmugam, 1996).

High-density turbidity-currents. Traction transport with migration of dunes. Non-graded beds may indicate reworking by turbiditycurrents.

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Table 2 Facies descriptions and interpretations for facies association 2: suspension deposited shales and mudstones Facies

Lithology

Sedimentary structures and trace fossils

Bed thickness, boundaries and trend

2A: Black laminated shale.

Clay, often containing pyrite, limestone concretions and wellpreserved fossils.

No clearly defined beds. Deposited by ‘background’ Occur in successions up suspension fall-out in offshore to 50 m thick. settings.

2B: Silty mudstone.

Clay and silt, often interbedded with facies 1E. Varies in colour from grey-black to greenish.

Well developed horizontal lamination. Planolites type horizontal traces are rarely observed. Faint to well developed horizontal lamination in mudstones. Interbedded sandstones are graded and massive, current rippled, climbing rippled and horizontally laminated with Bouma T –T divisions. B D

4.2.2. Interpretation The shales and mudstones were deposited from suspension sedimentation mixed with the finest fractions of low-density turbidity currents of FA1 ( Table 2). The basal part of the Wordie Creek Formation in the distal area ( locality 6) consists of shales deposited following the Griesbachian

Depositional process

Form successions up to Mudstones deposited partly by approximately 24 m ‘background’ suspension fall-out, thick interbedded with partly by low-density turbidity facies 1E. Sandstone currents as the latest tail-stage of beds are 0.2–5 cm thick flows (Stow et al., 1996). Black and sheet-formed. mudstones deposited in dysoxic conditions. Sandstone beds were deposited as low-density turbidites. Represent offshore environments.

transgression (Grasmu¨ck and Tru¨mpy, 1969). The lack of bioturbation could be an indication of anoxia in the bottom waters during the initial stages of Triassic sedimentation. An upward-coarsening from clean shale to silty mudstone, and an increased frequency of interbedded turbidites are interpreted to reflect westward progradation of a

Fig. 6. Submarine canyon at locality 1 filled with turbidites of the Wordie Creek Formation. The canyon is SE–NW elongated and palaeocurrents were towards the NW. The canyon is throughout the entire width directly overlying the Upper Permian reef carbonates and pinches out towards the SW. The fill is ca. 50 m thick in the deepest part of the submarine canyon. The Permian–Triassic unconformity is outlined in black and the first FS in white.

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Fig. 7. Photo-profile and interpretation at locality 6, showing the basin-floor fan composed of foresets and topset underlain and overlain by Triassic mudstones. The profile only covers part of the ca. 2 km wide canyon. The Permian–Triassic boundary is a shale– shale contact. The foresets dip towards the west. See text for further discussion.

coarse clastic wedge of turbidites (see Fig. 8). The absence of shales and mudstones in the basal part of the proximal submarine canyon ( localities 1–5) is thought to be a result of a steeper gradient, resulting in erosion and the development of canyons acting as by-pass corridors for high-density turbidity currents.

5. Submarine canyons The Lower Triassic submarine canyons are thought to have formed by two main erosional processes in latest Permian or earliest Triassic times: 1. fluvial valley incision in the updip and crestal reaches of the tilted fault-block; and 2. submarine erosion by high-density turbidity currents in the downdip, down-faulted reaches of the fault-block below the fulcrum (Birkenmajer, 1977; Surlyk et al., 1986; Stemmerik et al., 1997). Locally, submarine canyons are observed to follow

synsedimentary faults (see Fig. 4). The observed submarine canyons and their fill can generally be classified as: 1. relatively narrow (<250 m wide) canyons with an upward-fining fill of conglomeratic to sandy turbidites; and 2. broad, up to ca. 2 km wide, canyons with an upward-coarsening fill of shale to sandy turbidites. The change from type 1 to type 2 is seen at the most distal outcrops associated with a marked change in depositional regime. In the following, a number of submarine canyon exposures are described. Localities 3–6 are sections through the same submarine canyon in dip-direction. 5.1. Submarine canyon sections and depositional processes 5.1.1. Locality 1 5.1.1.1. Canyon geometry. Two closely spaced submarine canyons have been observed in strike-

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Fig. 8. Measured sedimentary sections through the basin-floor fan at locality 6. See text for further discussion.

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Fig. 9. Inversely-to-normally-graded, matrix-supported conglomerate bed at locality 1, close to the base of the western submarine canyon. The matrix is very coarse-grained sandstone. Clast types are intraformational mudstone, Upper Permian carbonate reef fragments and Devonian rounded granitic pebbles. Clasts are not imbricated. Scale is 30 cm long.

sections (see Fig. 3). The western canyon is 250 m wide and 50 m deep and it is incised into Upper Permian limestone of the Wegener Halvø Formation ( Fig. 6). The eastern canyon wall is almost vertical, whereas the western limit of the canyon is low-gradient and can only be determined by the pinch-out of the canyon fill. The amount of erosion associated with incision of the canyon is unknown and the location may partially have been controlled by pre-existing topography of the underlying reef. The eastern submarine canyon at locality 1 is ca. 230 m wide and up to 24 m deep and cut into Upper Permian Ravnefjeld Formation basinal black shales ( Fig. 11). This locality was described by Birkenmajer (1977), who measured NE–SW canyon and palaeocurrent directions. Lateral tracing has shown, however, that the canyon is elongated SE–NW. 5.1.1.2. Canyon fill. In both submarine canyons, the coarse-grained sediments of FA1 directly overlie the canyon floor and have a high content of angular Wegener Halvø Formation carbonate fragments. The western canyon is filled with normally-graded, minor inversely-graded and

non-graded conglomerate beds at the base, succeeded by massive, normally-graded and faintly trough cross-bedded pebbly sandstone (facies 1A– D, 1F ) ( Figs. 6, 9 and 12, ). Beds are typically amalgamated and lensoid and can be traced only for a couple of metres. However, in the western canyon, the bed-continuity increases and the grain size decreases upwards in the canyon fill. Palaeocurrents were towards the NW. In the eastern submarine canyon, the fill consist of basal normally-graded conglomerates, overlain by massive, graded sandstones (facies 1A–C ) (Fig. 10). The canyon fills are conformably overlain by marine mudstone (facies 2B) and thin-bedded turbidites (facies 1E). 5.1.1.3. Interpretation. The two submarine canyons at locality 1 were likely formed by earliest Triassic submarine, high-energy turbidity current erosion as no evidence of fluvial erosion and subaerial exposure such as karstification of the carbonate reefs have been observed. However, the turbidity current erosion could potentially have removed evidence of a previous episode of subaerial exposure. The presence of trough cross-bedding

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5.1.2. Locality 2 5.1.2.1. Canyon geometry. Approximately 5 km north of locality 1, a markedly lensoid submarine canyon, 18 m deep and 38 m wide, incises into Permian siltstone (see Fig. 3). The outcrop is cut obliquely to the strike of the canyon that has a NW elongation and can be traced into localities 3 and 4 (Fig. 1). The base of the canyon shows no evidence of subaerial exposure. The canyon base (Permian–Triassic boundary) has been traced to the time-equivalent interfluve areas where no erosion is observed. 5.1.2.2. Canyon fill. The canyon fill consist of thin, normally-graded, matrix-supported conglomerates and amalgamated, normally-graded, massive or faintly stratified pebbly sandstones with numerous angular Permian carbonate clasts (facies 1B–D). Beds are lensoid and amalgamated throughout the fill and palaeocurrent directions were towards the NW. Where traced to overbank areas, the Permian–Triassic boundary appears as Upper Permian intensely bioturbated siltstones (Oksedal Member) conformably overlain by Triassic black shales containing ammonites.

Fig. 10. Thick sandstone bed at locality 1 in the upper half of the submarine canyon fill. The bed is massive to faintly laminated and no grading is observed. The bed is interpreted as deposited by sustained high-density turbidity currents. Backpack is 50 cm high.

in the western canyon fill probably suggests hyperpycnal flow and the presence of a nearby inflowing river governing sustained flow (see also Kneller, 1995; Kneller and Branney, 1995). However, the presence of the shoreline is not known so no strong evidence supports this. The sediments filling both canyons are therefore thought to primarily have been deposited by high-density turbidity currents with minor debris flows initially filling the basal part of the canyons. The upward-fining and decreasing amalgamation observed in the western canyon is thought to reflect an increase in baselevel rise and a gradual decrease in sediment supply.

5.1.2.3. Interpretation. This canyon is interpreted to have been formed by submarine erosion by high-density turbidity currents as no evidence of subaerial exposure is seen. The canyon-fill sandstones and conglomerates were deposited mainly from high-density turbidity currents. Overbank areas were dominated by suspension fall-out sedimentation of shales and mudstones with minor low-density turbidity currents, probably partly sourced from overbank spilling from the turbidity currents confined within the canyon (facies 1E and 2B). 5.1.3. Locality 3 5.1.3.1. Canyon geometry. This outcrop shows an ca. 120 m wide and 20 m deep submarine canyon elongated NW, exposed in a strike-section ( Fig. 13). The canyon is incised through the Upper Permian siltstone (Oksedal Member) into the underlying Ravnefjeld Formation shales. The canyon is relatively steep-sided (see Fig. 13). No

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Fig. 11. The eastern submarine canyon at locality 1. The Permian–Triassic unconformity is outlined in white. The canyon is ca. 230 m wide. See text for description.

Fig. 12. Inversely-to-normally-graded conglomerate at the base of the eastern canyon at locality 1. The conglomerate is matrixsupported by coarse-grained sandstone to gravelstone and the clasts are mainly derived from the underlying Upper Permian Martinia limestone. Scale is 2 m long.

evidence of subaerial exposure has been observed. Here, the erosive base of the canyon can also be traced to the overbank area, where Upper Permian bioturbated siltstones are conformably succeeded by Triassic shales. This canyon can be followed to

more distal sections through the same canyon outcropping at localities 4–6 (see Figs. 1, 3 and 5). 5.1.3.2. Canyon fill. The lower canyon fill consists of normally-graded, massive to weakly stra-

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Fig. 13. An approximately 120 m wide submarine canyon at locality 3 elongated SE–NW. The thickness is ca. 15–20 m. The canyon is incised into Permian siltstone (Oksedal Member) and the canyon fill consists of normally-graded pebbly sandstones with numerous clasts of Permian reef limestones ( Wegener Halvø Formation).

Fig. 14. Close up of the fill in the canyon at locality 3. The beds are lensoid in shape and can only be traced a few metres. Each pebbly sandstone bed is normally-graded and massive or faintly laminated. Hammer for scale is 30 cm long. Clasts are mainly quartz granules and Permian limestone pebbles.

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tified pebbly sandstones (facies 1D) with numerous angular clasts of Permian reef carbonates ( Wegener Halvø Formation) as well as up to 50 cm long, deformed shale clasts of the Ravnefjeld Formation (see Fig. 14). The upper part of the canyon fill is finer-grained and consists of coarse to very coarse, massive to weakly stratified sandstone beds containing granule size Permian carbonate fragments (facies 1D). In the upper part of the canyon fill beds have a higher degree of continuity than in the basal part of the fill. Palaeocurrent directions were parallel to the canyon axis towards the NW. 5.1.3.3. Interpretation. The canyon is thought to have been formed by turbidity current erosion in earliest Triassic times. The canyon fill was deposited by high-density turbidity currents and the previously described ( locality 1) increase in base level rise and decrease in sediment supply is thought to account for the observed vertical changes in grain size and amalgamation. 5.1.4. Locality 4 5.1.4.1. Canyon geometry. This submarine canyon is 17 m deep and eroded into Upper Permian siltstone (Oksedal Member). The width is not determinable because the outcrop is diporiented. Judging from nearby exposures, the canyon at this locality is elongated to the NW. The submarine canyon can be traced traced basinward to locality 5. 5.1.4.2. Canyon fill. The submarine canyon fill consists of a basal, 1 m thick, ungraded, clastsupported conglomerate (facies 1F ), overlain by normally-graded, faintly stratified pebbly sandstones (facies 1C and 1D). Also here an upward decrease in grain size and degree of amalgamation is observed (see Fig. 5, locality 4). The few measurable palaeocurrent indicators show directions towards the NW. 5.1.4.3. Interpretation. Canyon incision is thought to have been submarine. The basal ungraded conglomerate was likely deposited from a debris flow, probably initiated by local collapse

Fig. 15. Measured section at locality 5 showing the conglomeratic and coarse-grained sandy turbidites filling the canyon.

or slumping of the canyon fill. The overlying pebbly sandstones were deposited from high-density turbidity currents. The previously described increase in base level rise and decrease in sediment supply is also thought to have influenced the deposition in this section of the canyon. 5.1.5. Locality 5 5.1.5.1. Canyon geometry. This section is situated 1170 m down-dip (N–NW ) of locality 4 and the dip-oriented outcrop is continuous between the two sections ( Fig. 5). Here, the submarine canyon is 20 m deep, while the width cannot be determined due to the dip-orientated outcrop. The canyon is

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incised into Permian siltstone of the Oksedal Member ( Fig. 15) and the local erosional relief of the canyon base is up to 2 m between localities 4 and 5. 5.1.5.2. Canyon fill. The canyon fill is here generally coarser-grained and more amalgamated than updip at locality 4 (Fig. 15). It consist of amalgamated beds of inversely-to-normally-graded and normally-graded, pebbly, massive sandstones fining-upward to mainly massive sandstones with common dish structures (facies 1C ). The sandstones are overlain by a 2 m thick lens of laminated black shale which is eroded away both updip and downdip. The shales are succeeded by a 2.5 m thick bed of inversely-to-normally-graded boulderconglomerate (see Fig. 15). 5.1.5.3. Interpretation. Sedimentation was dominated by high-density turbidity flows. Locally debris flows may have existed, as indicated by thin,

non-graded conglomerates. The coarser grain size and higher amalgamation compared to locality 4 is thought to reflect local flow variations and topography within the canyon (see also Mastalerz, 1995). The shale lens suggests that the entire downdip part of the canyon experienced a period of shale deposition and that the shales were later eroded away by succeeding high-density turbidity currents depositing the overlying conglomerate bed. 5.1.6. Locality 6 5.1.6.1. Canyon geometry. The proximal sections of the submarine canyon described above at localities 3, 4 and 5 can be walked out basinwards to locality 6, which is a strike-section through the same canyon (Figs. 7 and 16). Locality 6 represents the most distal depositional environment of submarine canyon sections 3–6 and is situated ca. 800 m NW of locality 4. The measured profile is

Fig. 16. Dip section at locality 6, NW is towards left. Localities 3–5 can be traced into this section from further east (right). The cliff-forming turbidites thicken towards the left caused by the change in depositional regime from within the submarine canyon to the largely unconfined submarine fan. Fan foresets are only observed at left end of the outcrop, where the cliff is thickest. The Permian–Triassic shale–shale contact is outlined in white.

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565 m long but the entire canyon width is probably close to 2 km (Fig. 7). The canyon cuts into Upper Permian Ravnefjeld Formation shales and the erosion surface displays up to 18 m of relief along the coastal profile (Fig. 7). 5.1.6.2. Canyon fill. 5.1.6.2.1. Lower infill. The total thickness of the canyon fill at locality 6 ranges between 30 and 60 m ( Fig. 7). Lower Triassic shales fill the basal part of the canyon, and these are succeeded by silty mudstone with interbedded, thin, low-density turbidites (see Fig. 8). 5.1.6.2.2. Foreset unit. The mudstones are succeeded unconformably by coarse to very coarse sandstones with a granule or pebble layer at the base of each bed. The sandstones are arranged in up to 16 m high foresets with a direction of progradation towards the west, which is a prominent change in transport direction of 45° compared to what is observed in the updip narrower canyons (Fig. 17). The base of the foreset unit is slightly erosional. The foreset unit is 5.1 m thick in the NE end of the profile, thickening to 16 m at the SW end of the exposure ( Fig. 8). The foresets dip 10–15° towards the W–SW and are composed of a basal granule or pebble layer one or two grains thick, grading to coarse to very coarse sandstone. Some foresets also contain a basal layer of reworked Permian brachiopods. Each foreset is 5– 50 cm thick without internal stratification (facies 1C ) (Fig. 17). Towards the NE, the foreset unit displays a coarse-tail upward-coarsening, whereas it further towards the SE shows a coarse-tail finingupward trend. The clast size increases in the direction of progradation, while the matrix grain size is constant. 5.1.6.2.3. Upper infill. Towards the NW end of the outcrop, the foreset unit is sharply overlain by a coarsening-upward, fine to medium-grained sandstone bed composed of <0.5 m thick, convoluted beds with dish structures (facies 1D) (Fig. 18). Farther towards the SW, the foreset unit is sharply overlain by horizontally bedded pebbly sandstone (Fig. 8). Pebble size is constant through-

Fig. 17. W–SW dipping foresets at locality 6. Each normallygraded foreset is 5–50 cm thick without internal stratification. At places the base of foresets are overlain by granules or pebbles. The grain size is constant throughout the fan foresets. Encircled backpack is 50 cm high.

out the topset and each bed is graded similarly to the internal structure of the foresets. No offlap break (Myers and Milton, 1996) between the topset and the foreset bed has been observed in the profile, but such a break is inferred to be situated farther towards the west. A unit of fine to coarse-grained, convoluted sandstone is overlying the topset sharply but not erosionally (Fig. 8). This unit thins from 5.5 m in the NE end of the exposure to 3.8 m towards the SW end, and the basal part of this sandstone bed displays prominent flute marks and load structures. Some low-angle trough-cross sets and horizontal stratification occur in this bed. The sandstone bed is cut erosion-

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Fig. 18. Flute rolls from the uppermost fine-grained sandstones at locality 6. Load structures are present at the base of the same bed. Lens cap is 5.5 cm across.

ally by a normally-graded or inversely-to-normally-graded pebbly sandstone/conglomerate bed thickening from 0.5 m in the NE end of the profile to 3 m in the SW end ( Fig. 8). The conglomerate bed appears massive, but faint horizontal stratification and large-scale trough cross-bedding is also observed. Pebble size and matrix grain size changes are non-systematic. The FA1 canyon fill is capped by a FS overlain by mudstones of FA2 (Figs. 7 and 8). Shales and mudstones with minor interbedded turbidite sandstones dominate the remaining part of the Wordie Creek Formation on the southern fault-block of Wegener Halvø. 5.1.6.3. Interpretation. 5.1.6.3.1. Lower infill. The upward-coarsening of the basal shales to siltstone with thin, interbedded turbidites represents initial suspension deposition in the submarine canyon with a gradual increase in deposition from low-density turbidity currents. The absence of bioturbation in the laminated basal shales and mudstones probably reflects anoxia in the bottom waters or, possibly that rapid turbidite deposition induced a stressed environment with no burrowing animals.

5.1.6.3.2. Foreset unit. The foreset unit together with the overlying sandstones and conglomerates are as a whole interpreted as a basin-floor fan for the following reasons: 1. a change in submarine canyon geometry from 38 to 250 m wide steep-sided canyons at localities 1–5 to an ca. 2 km wide and largely unconfined canyon at locality 6; 2. a basinward decrease in grain size and amalgamation simultaneous with an increase in the bed-continuity; and 3. a marked ~45° change in the transport direction from the narrow canyon sections to the wide basinward section. The presence of giant foresets in submarine fans is to not much described in the literature, but is known to occur in channelised, steep-headed fan deltas on the west margin of the Triassic East Greenland Basin ( F. Surlyk, personal communication, 1999). However, these turbidites were footwall-sourced from the Greenland craton and were interpreted by Surlyk et al. (1986) as steep-headed fan deltas comparable to those described by Surlyk (1978) and Hwang et al. (1995). A decrease in slope-gradient and turbidity-current confinement are thought to have caused rapid deposition on

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the basin-floor from waning high-density turbidity currents exiting the slope-canyons (see also Lowe, 1982; Middleton, 1993; Kneller and Branney, 1995), resulting in a delta-like unit of turbidites. The change in transport direction compared to localities 1–5 is also thought to reflect decreasing flow confinement as well as a change in the seafloor gradient and dip-direction from the slope to the basin-floor. Turbidity currents are thus thought to have been redirected to axial flow when leaving the slope canyons — a phenomenon commonly observed in submerged half-graben basins (Ravna˚s and Steel, 1997, 1998; R. Steel, personal communication, 2000). 5.1.6.3.3. Upper infill. The observed decrease in grain size in the beds succeeding the foreset bed and topset bed and change to partly tractional deposition were most likely caused by a change from high-concentration to diluted turbulent currents and a decrease in current velocity (e.g., Lowe, 1982). This is interpreted to be caused by a decrease in slope from deposition on the steep fan front (the foresets) to deposition on a gentler sloping fan top. The presence of strongly convoluted sandstone beds suggests one of two mechanisms: 1. rapid deposition on an unstable slope where

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small-scale slumping and liquefaction occurred; or 2. periods of active fault-block tilting generating earthquakes shortly after deposition.

5.1.6.4. Summary of submarine canyon architecture. The downdip change is canyon width is accompanied by a marked change in canyon fill architecture, which reflects the structural position of the localities on the slope. The disorganised, conglomeratic and coarse sandy turbidites are restricted to localities 1–5 within narrow canyons and represent deposition on the slope, likely associated with significant bypass of finergrained sediment (see also Walker, 1992). At these proximal positions, the canyons acted as sediment conduits to the basin floor. The organised turbidites at locality 6 represent the basinward extension of the disorganised slope turbidites and image a change in depositional regime and flow direction. Here, the updip confined flows changed to largely unconfined flows, which spread out laterally and changed direction ca. 45° (see Fig. 19). This also resulted in generally finer-grained beds with a higher, lateral continuity. A fan developed at the basinward end of the slope canyon with development of giant-scale foresets reflecting a decrease in

Fig. 19. Schematic model for deposition of hanging-wall sourced basin-floor fans in the Lower Griesbachian on the southern faultblock of Wegener Halvø. The submarine slope-canyons are elongated W–NW. The basin-floor fans prograded towards the W–SW indicating that turbidity currents changed direction (~45°) when reaching the basin floor, as indicated by the thick arrows. Probably, the individual basin-floor fans were connected basinward as a function of axial transport. Tilt of the fault-block induced transgression at the lower part of the block below fulcrum contemporaneously with uplift and regression in areas updip of the fulcrum. See text for further discussion.

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flow velocity and deposition from waning turbidity currents.

6. Sequence stratigraphy Lateral tracing of depositional units and their bounding surfaces has made it possible to interpret the presented data within a sequence stratigraphic context. The overall structural setting of the rotated fault-block is important, as it has important consequences for stratigraphic interpretations. In half-graben basins, tectonism and sediment supply are likely to be the most important controls in accommodation creation and sequence architecture (Surlyk, 1989; Gawthorpe et al., 1994, 1997; Ravna˚s and Steel, 1997, 1998). Fault-block movements and sediment supply can outpace variations in eustatic sea-level depending on the location on the hangingwall/footwall (Helland-Hansen and Gjelberg, 1994; Helland-Hansen and Martinsen, 1996). Correlation of bounding surfaces is thus not as straightforward as in basins with a simpler structural setting because changes in creation of accommodation space and sediment supply may occur within short distances and because downdip deepening and updip uplift occur simultaneously. 6.1. Bounding surfaces The Permian–Triassic boundary generally marks a period of subaerial exposure and erosion associated with submarine erosion and bypass, where part of the Upper Permian succession was removed. This surface can be correlated all over the East Greenland region (Grasmu¨ck and Tru¨mpy, 1969; Perch-Nielsen et al., 1974; Surlyk, 1990; Surlyk et al., 1986) and is therefore used as a SB. In East Greenland, a profound change in fauna and flora is observed between the Late Permian and Early Triassic, which is used to define the Permian–Triassic boundary (Spath, 1935; Tru¨mpy, 1969; Grasmu¨ck and Tru¨mpy, 1969; S. Piasecki, 1984, personal communication, 1999). The variable and largely unknown amount of erosion on Wegener Halvø created a topography of the pre-Triassic ‘basement’, that to some degree controlled position of the submarine canyons and

thus sedimentation (S. Piasecki, personal communication, 1999; L. Stemmerik, personal communication, 1999). The most extensive erosion took place on the SE uplifted part of the faultblock where submarine canyons cut into the Upper Permian reef build-ups. Apparently, erosion decreased towards the NW on the relatively subsided part of the fault-block, but the amount of erosion is unknown. The nature of the Permian– Triassic SB varies both along strike and dip. No ravinement surface was developed during the Early Triassic transgression, probably because of a combination of a very rapid rise in relative sea level and low wave energy. Within the submarine canyon sections at localities 1–5, the Permian– Triassic SB is overlain by coarse-grained turbidite sandstones and conglomerates of FA1. Distally, towards the NW at locality 6 (Figs. 7 and 8), the SB is developed as a shale–shale contact within the wide and largely unconfined basinward portion of the submarine canyon. The slightly erosional base of the basin-floor fan succession (FA1) is in this distal setting not coincident with the SB, but is separated by 34 m of Triassic shales and therefore offlapping. Pre-deposition erosion of turbidity currents seems to have been of minor importance. When traced to the overbank areas, both in the proximal and distal outcrops, the Permian–Triassic boundary appears as a subtle, conformable change from greyish black Permian shales to Triassic greenish, silty shales of FA2 and, here the boundary can only be safely positioned by biostratigraphy. The Lower Triassic deposits onlap the Permian–Triassic SB both in the overbank areas and within the incised valleys as these were backfilled, as seen by eastward onlap of ammonite zones in the overbank areas and by the offlapping geometry of the turbidites ( Fig. 5).

7. Discussion 7.1. Canyon incision One of the main problems concerning the Permian–Triassic boundary on Wegener Halvø is the almost total absence of fossils and palynomorphs in the upper part of the Permian succession

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(S. Piasecki, personal communication, 1999). This makes it impossible to determine the amount of erosion of the Permian succession in both proximal and distal areas of the fault-block. Generally, the Permian–Triassic unconformity is regional in extent and recognised all over East Greenland (Surlyk et al., 1984, 1986; Marcussen et al., 1987). The submarine canyons were formed by the connection and linkage of updip subaerial valley incision (only observed at one locality) and downdip turbidity-current incision of slope canyons during a major eustatic fall in latest Permian times ( Tru¨mpy, 1969; Teichert and Kummel, 1976; Birkenmajer, 1977; Surlyk et al., 1986). In some places, especially on the northern fault-block of Wegener Halvø, the most extensive erosion coincides with normal faults governing zones of weakness. In the most proximal exposures, the Permian– Triassic boundary is associated with up to 50 m of incision as at locality 1, where Lower Triassic turbidites directly overlie Upper Permian reefcarbonates. The depth of canyon incision decreases from 50 m at locality 1, to 17 m at locality 3 situated ca. 5 km further basinward (see Fig. 3). The general trend observed is a decrease in canyon incision depth towards the NW, that is, basinwards. The incision depth may have been controlled by a combination of the topography of the Upper Permian limestone build-ups, the resistance to erosion of the substratum, as well as time available for erosion. The Upper Permian reef limestones ( Wegener Halvø Formation) were forming on the relatively uplifted crest of the faultblock towards the E (Stemmerik et al., 1993b; Surlyk et al., 1986). This, together with deeper canyon incision towards the east, indicates, that this area both in Permian and Triassic times was uplifted and located above the fulcrum, that is, the axis separating the uplifted and subsided parts of a fault-block. Further distally towards the NW, the Permian reef limestones either thin drastically or disappear, which suggests that waterdepth on the western edge of the fault-block was too large for reef forming organisms, and therefore that the fault-block was tilted towards W–NW in Late Permian times. This structural setting apparently still controlled sedimentation in the Early Triassic.

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7.2. Deposition of canyon fill The overall shale-dominated nature of the entire Lower Triassic succession suggests that the subbasin was sediment-underfilled following the classification by Ravna˚s and Steel (1998). Triassic deposition is interpreted to have been initiated with transgression of the top Permian erosion surface without development of a ravinement surface. The eastward transgression resulted in the onlap of successively younger ammonite zones across Wegener Halvø from west to east. In the Fleming Fjord region the lowermost zone is the Glyptophiceras martini ammonite zone and farther eastwards the Metophiceras subdemissum and Ophiceras commune zones are transgressive (Grasmu¨ck and Tru¨mpy, 1969). In the westernmost downfaulted areas, the oldest sediments are shales, also in the submarine canyon at locality 6 (Figs. 7 and 8). Farther towards the east at localities 1–5, the Permian–Triassic boundary is directly overlain by coarse-grained turbidites in the submarine canyons. The turbidite systems consist of relatively small canyon fills and submarine fans offlapping the hangingwall as seen from localities 5 to 6 and encased in basinal shales similar to types described by Ravna˚s and Steel (1998) ( Fig. 5). In the overbank areas, the entire Wordie Creek Formation consists of eastward transgressive shales and mudstones with centimetre-thick, interlaminated low-density turbidites. This suggests that progradation of turbidites in the submarine canyons took place during the eastward transgression of the fault-block as evidenced by ammonites. A westwards tilt episode of the faultblock could contemporaneously create accommodation space and transgression in the basinal (below fulcrum) western areas and uplift and increased sediment-catchment in the proximal eastern reaches (above fulcrum). An increase in hangingwall gradient and sediment influx is therefore interpreted to have been the main controls on the formation of the offlapping canyon fills. Sediment delivered from the transverse submarine canyons was spread out laterally and redirected ca. 45° to axial transport when reaching the basin floor. This is seen at the basin-floor fan foresets at locality 6 ( Fig. 8), and is thought to reflect a decrease in

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slope and probably deposition close to the centre of the subbasin (Fig. 19). Flooding of the coarsegrained basin-floor fan took place in the Ophiceras subdemissum/commune zone. It has not been possible to show whether the FS (on Figs. 5 and 8) is isochronous. Flooding most likely took place in response to cessation of the eastern sediment sources due to tectonic quiescence. Another explanation could be an increase in the rate of relative sea-level rise inducing retrogradation of the coarse clastic depositional system.

8. Summary and conclusions The studied subbasin is comparable to the sediment-underfilled rift basin setting of Ravna˚s and Steel (1997, 1998). Regional erosion along the Permian–Triassic boundary is seen on Wegener Halvø, where a number of incised valleys and submarine canyons were eroded during sea-level lowstand. The Griesbachian transgression first created accommodation space in the deepest parts of the half-graben basin towards the NW, initiating deposition of the FA2 offshore mudstones in the basin and in the distal ends of the submarine canyons ( Fig. 5). The eastward onlap of ammonite zones onto the Permian–Triassic boundary (Grasmu¨ck and Tru¨mpy, 1969) shows that the relative rise in sea-level gradually created accommodation space farther eastwards on the hangingwall dip-slope below the fulcrum. In early stages of the transgression, the relative uplifted parts of the fault-block above the fulcrum were the zones of bypass or minor erosion. A renewed westward tilting of the fault-block created a steeper slope and led to exposure of a larger part of the proximal hanging-wall above the fulcrum. This initiated renewed erosion of the underlying Upper Permian and older successions. At several localities on the northern part of Wegener Halvø, contemporaneous syn-depositional fault-block rotation supports this view. The increasing supply of coarse clastic material and the steeper slope forced the westward progradation of the turbidite systems ( FA1) in the submarine canyons. This early stage of rifting is seen as the slightly coarsening upward succession from the Permian–Triassic boundary to the base

of the coarse fan-succession at locality 6 (Fig. 8). Tectonic subsidence was larger than sediment supply in downdip areas below the fulcrum, while initial emergence of the updip areas took place. The coarse-grained basin-floor fan succession images the initial rift climax stage where tectonic subsidence outpaced sediment supply on the part of the fault-block below fulcrum. The tilt of the block led to enlargement of the emerged source areas resulting in increased sediment catchment and thereby feeding of the turbidite system. Deposition of out-building slope canyon turbidites and basin-floor fans took place at the end of the hangingwall dip-slope ( Fig. 19). Here, valleywidths were greater and flows became less channelised which together with gentler slope is inferred to have controlled deposition of basin-floor fans. Later during the rift-climax stage and during late stage rifting, increased tectonic subsidence below fulcrum outpaced sediment supply resulting in drowning of the source areas and shut-off the sediment supply to the basin resulting in formation of a FS (on Fig. 5) and subsequent deposition and blanketing of mudstones (see also Ravna˚s and Steel, 1998). Basin-floor fan progradation, backfilling of submarine canyons and the succeeding retrogradation of the system are assigned to the transgressive systems tract below fulcrum. However, above the fulcrum an erosional unconformity was formed simultaneously with the downdip transgression, which is explained by the tilting of the fault-block. This subsidence pattern hampers use of sequence stratigraphy in tilted fault-block settings, a generally acknowledged problem (Steel, 1993; Ravna˚s and Steel, 1997, 1998). Basin-floor mass gravity deposition during transgression is also known from other areas as described by Galloway et al. (1991), Helland-Hansen (1994), Ravna˚s and Steel (1997, 1998) and is likely to occur at tectonically active basin margins (Surlyk, 1989).

Acknowledgements The author wishes to thank M. Bjerager, M. Kreiner-Møller, L. Stemmerik and S. Piasecki for assistance and companionship in the field. The

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authors also wants to thank L.B. Clemmensen and L. Stemmerik for early improvements of the manuscript. Reviews by O.J. Martinsen, A.F. Embry, F. Surlyk, R. Steel and S.J. Porebski are greatly acknowledged. Lastly, funding by the Danish Research Council through the project ‘Resources of the Sedimentary Basins of North and East Greenland’ and the University of Copenhagen is appreciated.

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