Two major unconformities beneath the Neoproterozoic Murchisonfjorden Supergroup in the Caledonides of central Nordaustlandet , Svalbard DAVID G . GEE and ALEXANDER M. TEBEN’KOV Gee D. G. & Teben’kov A . M . 1996: Two major unconformities beneath the Neoproterozoic Murchisonfjorden Supergroup i n the Caledonides of central Nordaustlandet, Svalbard. Polar Research l S ( I ) , X1-91. Two unconformities have been found in central Nordaustlandct. New mapping has located a major unconformity at the base of the Neoproteroaoic Murchisonfjorden Supergroup, with quartzites and basal conglomerates of the Djevleflota Formation unconformably overlying dark phyllites o f the Helvetestlya Formation and metavolcanic rocks of the Svartrabbane Formation. A second unconformity separates the Helvetesflya from the Svartrabbane formations. These rocks were isoclinally folded, metamorphosed in lower greenschist facies, and, apparently, syntectonically intruded by Grenville-age granites, prior to uplift, erosion and Neoproterozoic deposition. Caledonian tectonothcrmal activity, as recorded i n rhe Neoproterozoic srrara, appears t o vary very little across Svalbard’s Eastern Terrane from Ny Friesland, in the west, to Murchisonfjorden in western Nordaustlandet and, via Wahlenbergfjorden, to the central Nordaustlandet area, described here. Upright folds with associated high angle, usually E-dipping cleavages. charactcrise the Caledonian deformation over an east-west distance of about 100 km. This evidence allows the possibility that the pre-Devonian basement, to the east of Nordaustlandet, beneath the northern Barents Sea (Barentsia), may be composed of Grenville-age complexes little influenecd by Caledonian tectonothermal activity. Alternatively, Barentsia is dominated by Caledonian hinterland tectonics, with extensive middle Paleozoic tectonothermal reworking of a Precambrian basement. David G . Gee, Department of Geophysics, Uppsala University, Villauagen 1 6 , S-752 36 Uppsala, Sweden; Alexander M . Teben’kou, Polar Marine Geological Expedition, ul. Pobedy 24,189 510 Lomonosoo, Russia. Introduction Some of the most inaccessible, exposed areas of the Barents Shelf occur in northeastern Svalbard, where crystalline complexes of granites, augen gneisses and migmatites have long been known (Nordenskiold 1864, 1866) to be overlain by nearly unmetamorphosed sedimentary rocks (the Hecla Hoek formations) of Neoproterozoic and Early Paleozoic age (Kulling 1932, 1934). The relationships between these crystalline rocks and the fossiliferous successions have been much dis- puted, and only recently (Gee et al. 1995) has clear evidence been presented of a Grenville age for some of the granites. The latter intrude both the migmatites and some of the metasedimentary rocks (Fig. 1). In northwestern parts of Nordaustlandet, an unconformity, first documented by Ohta (1982a), separates low grade metasedimentary rocks (the Brennevinsfjorden Group) intruded by ca. 950 Ma granites (Gee et al. 1995), from an overlying volcanic and volcaniclastic sequence (the Kapp Hansteen Group); the latter underlies well-docu- mented Neoproterozoic sandstones, shales and carbonates of the Murchisonfjorden Supergroup (Kulling 1934; Flood et al. 1969; Ohta 1982a, b). A significant unconformity is inferred to also exist at the base of the Murchisonfjorden Supergroup, but the contact, occurring beneath Lady Franklin- fjorden, is apparently not exposed on land in western areas. Further east, in central Nordaustlandet, a similar unconformity (or unconformities) has been inferred by various authors, but not pre- viously demonstrated. Thus, Sandford (1950, 1956), on the basis of several expeditions to the area, concluded that a major unconforrnity should exist between underlying metamorphic complexes and overlying little-metamorphosed sedimentary and volcanic rocks; however, neither he nor Flood et al. (1969) were able to unambiguously demon- strate a regionally significant unconformity. This paper documents two major breaks in the stra- tigraphy; we now know that the problems of their identification relate to the existence of low grade metasedimentary rocks of both Mesoproterozoic and Neoproterozoic age. 82 D. G . Gee & A . M. Teben’kou Fig. I . Geological map of Nordaustlandet and N y Friesland (from Gee et al. 1995), with location of Fig. 2 . Previous work in central Nordaustlandet The Caledonian bedrock of central Nordaust- landet is exposed between two major ice-caps, Vestfonna and Austfonna, in the vicinity of Rijpfjorden and inner Wahlenbergfjorden and in the broad open N-trending valley connecting these two waterways. South of Wahlenberg- fjorden, flat-lying Carboniferous successions unconformably overlie the Caledonian rocks. Geological maps of the area have been published by Sandford (1956), Flood et al. (1969), Hjelle (1978), Ohta (1982~) and Lauritzen and Ohta (1984), and several Russian expeditions have pro- vided unpublished reports of the pre-Carbon- iferous geology. The regional structural relationships in central Nordaustlandet were established by Sandford (1956). H e showed the approximate distribution of the rock units, with migmatites and granites (his “metamorphic complex”) occurring in a major N-trending antiform extending through Rijpdalen, from innermost Wahlenbergfjorden to Two major unconformities beneath the Neoproterozoic Murchisonfjorden Supergroup 83 Rijpfjorden, and flanked to the east and west by synforms containing Neoproterozoic successions. The Rijpdalen Antiform plunges north in the south and south in the north, and, from the latter area, the migmatites and granites spread north- eastwards to occupy most of northeastern Nor- daustlandet . Sandford (1956) recognised several of the Neo- proterozoic formations that had been defined further west by Kulling (1934) in the classical Murchisonfjorden area. Kulling (1934) had shown that the various pre-Vendian units in that area - dolomites, limestones, shales and sandstones (his Murchison Bay Formation) - were underlain, on Botniahalvoya, by a succession of volcanic and low grade sedimentary rocks that he referred to as the Cape Hansteen Formation. In central Nordaustlandet, similar lithologies, though with only subordinate volcanites, were found by Sand- ford t o separate the sandstone units (Flora Series) of the Murchison Bay Formation from the under- lying “metamorphic complex”. Pink granites were recorded t o cut both the Cape Hansteen For- mation and the underlying rocks. Sandford com- mented (1956, p. 358), “It is pertinent, then, to consider whether the Cape Hansteen Formation is stratigraphically unconformable on the meta- morphic complex, o r is a part of a disharmonic mass between the surface of the complex and the competent beds of the Flora Series.” He noted (1956, p. 357) the presence of “sheets of meta- morphosed sediments” between the granitic rocks of the “metamorphic complex”, but no evidence of metamorphic transition into the low grade Cape Hansteen rocks; he therefore concluded that an unconformity must exist between them. The structural complexity of the migmatites and the granites, contrasting with the relatively simple more or less cylindrical folding of the Neo- proterozoic strata, also prompted this inter- pretation. Flood et al. (1969) carried out an extensive helicopter-supported mapping program of Nord- austlandet in 1965. In central Nordaustlandet, they recognised that there was a metamorphic transition from the underlying migmatites and foliated augen granites into the so-called Cape Hansteen Formation, with staurolite, andalusite and garnet crystallising syntectonically in the immediate contact zone. Metamorphic grade decreases rapidly to lower greenschist facies away from the contact. Younger reddish granites cut all these rock units. Flood et al. (1969) showed that the low grade metamorphosed sedimentary and volcanic rocks had a more complex structural history than the overlying Murchisonfjorden lithologies and they therefore preferred the hypothesis (Flood et al. 1969 pp. 95-96) that a major unconformity separated these two strati- graphic units. Nevertheless, the exact location and interpretation of the importance of the uncon- formity were disputed, being influenced not only by the field relationships, but also by the first isotope age-determination studies- Rb/Sr whole rock and mineral ages of ca. 600 Ma ages on schists of the “metamorphic complex” (Hamilton & Sandford 1964). Support for the interpretation that the Murchisonfjorden Neoproterozoic strata were underlain by a basement complex of mig- matites, granites and low grade volcano-sedi- mentary rocks was found locally, for example, east of Bengtssonbukta, where a quartzite con- glomerate was recognised (Winsnes in Flood et al. 1969, p. 37) to discordantly overlie low grade metasediments. In general, the lithologies below the Mur- chisonfjorden Supergroup in central and western Nordaustlandet are similar. However, whereas in western Nordaustlandet Flood et al. (1969) recognised that the sub-Murchisonfjorden rocks composed two major mappable units, the Bren- nevinsfjorden and Kapp Hansteen formations (together comprising their Botniahalvoya Group), in central Nordaustlandet they were unable to distinguish these formations. And in the latter area, an independent quartzite and dark phyllite unit, with subordinate intercalations of carbonates, was identified - the Austfonna For- mation, overlying the other formations of the Botniahalvoya Group in the Rijpdalen Antiform. Subsequent work on Botniahalvoya (Gee et al. 1995) has demonstrated that the Brennevinsfjor- den and Kapp Hansteen units are both sub- divisible into formations and therefore should have group status. Because a major unconformity separates these units on Botniahalvoya (Ohta 1982c), the term Botniahalvoya Group was aban- doned (Gee et al. 1995). The stratigraphy of the Murchisonfjorden Supergroup (Table 1) has also been modified since its early definition by Kulling in 1934 (the Murchison Bay Formation) and correlation from the type areas, eastwards into central Nord- austlandet is not without problems. In the Murchisonfjorden area, below V e d i a n tillites, Kulling (1934) distinguished six series (for- T ab le I . St ra ti gr ap hi c co rr el at io n of t h e M es o- a nd N eo pr ot er oz oi c su cc es si on s of c en tr al a nd w es te rn N or da us tl an de t W E S T E R N (O th a 19 82 a & b) N O R V IK F m F LO R A F m K A P P L O R D F m C E N T R A L (O ht a 19 82 c) N O R V IK F m F LO R A F m K A P P L O R D F m M E Y E R B U K T A F m st ' su bo rd . 1s t) A U S T F O N N A G ro up (I N N V IK H B G D A D JE V LE F LO T A an d B A S A L Q ZI TE B R E N N E V IN S - F JO R D E N F or m at io n B R E N N E V IN S - F JO R D E N F or m at io n N O R D A U S T L A N D E - I CE N T R A L (T hi s pa pe r) W E S T E R N I C E N T R A L (F lo od et a l. 19 69 ) W E S T E R N (K ul lin g 19 34 ) N O R V IK S er ie s (s ha le s) C E N T R A L (S an df or d 19 56 N O R V IK S er ie s W E S T E R N (G ee et a l. 19 95 N O R V IK F m N O R V IK F m N O R V IK F m FL O R A F m F LO R A F m F LO R A F m K A P P L O R D F m F LO R A S er ie s F LO R A S er ie s (q ua rt zi te s) S uc ce ss io ns be lo w re pe at ed re cu m be nt fo ld in g. by C A P E H A N S T E E N F or m at io n (P hy lli te s, qz p or ph yr ie s an de si te s ag gl om er at es & co ng lo m er at es ) K A P P L O R D F m (s ha le s) W E S T M A N - B U K TA F m (s ha le s) K A P P L O R D F m W E S T M A N - B U K T A F m W E S T M A N - W E S TM A N . B U K T A F m 1 B U K T A F m W E S T M A N - E O K T A F m W E S T M A N - B U K T A F m P E R S B E R G E T Fr n (q ua rt zi te s) P E R S B E R G E T F rn P E R S B E R G E T F m P E R S B E R G E T F rn F m I M E Y E R B U K T A F m ig fe z re _ d - ,. ,. .. * . I - - - - K A P P H A N S T E E N G ro up 7 0 0 ~ 0 0 ~ - B R E N N E V IN S - F JO R D E N G ro up (I nt ru de d by ca . 9 5 0 M a gr an ite s) D JE V LE F LO T A F or m at io n w ith ba sa l c on gl om . W ? u n o B R E N N E V IN S - F JO R D E N (m et as ed .) & K A P P H A N S T E E N (v ol ca ni te ) fo rm at io ns o rm ity ? J V L A U S T F O N N A 8 K A P P P LA T E N fo rm at io ns K A P P H A N S T E E N 8 B R E N N E V IN S - F JO R D E N U nd iff . (m ig m ita te s & gr an ite s in tr ud e B O TN IA - H A LV 0Y A G ro up ) fo rm at io ns ) H A N S T E E N H A N S T E E N F or m at io n G ro up S V A R T - R A B B E N E F or m at io n 2 0 0 0 0 0 ~ w H E LV E T E S - F LY A F or m at io n (I nt ru de d by ca . 1 0 5 0 M a gr an ite s) C A P E H A N S T E E N F or m at io n - M et am or ph ic co m pl ex (m ig m at ite s & gr an ite s) Two major unconformities beneath the Neoproterozoic Murchisonfjorden Supergroup 85 mations) grouped into two main units, and inter- preted the lower part of the succession to be repeated by recumbent folding. Flood et al. (1969) placed the upper units (dominated by dolomites and limestones) into the Roaldtoppen Group and the lower (shales and quartzites) into the Celsiusberget Group. They rejected Kulling’s (1934) “recumbent fold” interpretation and recognised beneath the Celsiusberget Group a third cycle of shales and quartzites, the Frank- linsundet Group. Subsequently, Ohta (198213) showed that the lowermost quartzites of the Franklinsundet Group (the Persberget Formation) were underlain by another, somewhat different shale-dominated unit, which he named the Meyerbukta Formation. In central Nordaustlandet, the general Murchisonfjorden Supergroup relationships were established by the mid 1950s (e.g. Sandford 1956), with the Vendian tillites exposed on the northern side of inner Wahlenbergfjorden, underlain by the carbonates of the Roaldtoppen Group and shales and quartzites of the Celsiusberget Group. Flood et al. (1969) recognised the formations of the Franklinsundet Group, northeast of Vestfonna (north and south of Bengtssonbukta) and further south in inner Wahlenbergfjorden (north of Etonbreen). The basal quartzites of the Franklinsundet Group were correlated with the Persberget quartzites and shown to directly over- lie the Austfonna Formation along the eastern margin of Vestfonna. Further east, quartzites and overlying shales in the Venesjcben Synform were shown to be similar to the Persberget and Westmanbukta formations. However, in these eastern areas, they rest on the undifferentiated volcano-sedimentary rocks of the “Botniahalv~y a Group” and unconformable relationships were therefore inferred. Ohta’s (1982~) map of central Nordaustlandet revised the stratigraphic relationships between the Murchisonfjorden Supergroup and underlying rocks. H e interpreted the Austfonna Formation lithologies t o be equivalent t o his newly defined Meyerbukta Formation (from western Nordaustlandet) of the basal Murchisonfjorden succession. He upgraded the Austfonna unit to group status and included within it three for- mations, a Basal Quartzite, a middle shale- dominated section (the Djevleflota Formation) and an upper sandstone-shale unit (the Innvik- hagda Formation). H e also documented the local “Basal Quartzite” (Ohta 1982c, p. 5 2 ) , enter- taining the possibility of a regional break beneath the Murchisonfjorden Supergroup. However, both granite intrusions and migmatisation were thought to influence the Austfonna Group, and these were regarded therefore as Caledonian. New mapping of central Nordaustlandet In 1994, a camp was established on Helvetesflya, in the watershed between Rijpdalen and Flaum- dalen. This allowed a ten day investigation of the central part of the area between Vestfonna and Austfonna and a closer examination of the relationships between the Murchisonfjorden Supergroup strata and underlying volcano-sedi- mentary rocks. The new mapping (Fig. 2 ) extended from the Murchisonfjorden basal contact along the margin of Vestfonna in the western limb of the Rijpdalen Antiform, eastwards via the Venesjcben Synform to the Kvartsitthaugen Synform, exposed along the edge of Austfonna. The type area for the Austfonna Group was not within the range of the 1994 operations, but was examined briefly in 1995. Similar major Caledonian folds, further west along Wahlenbergfjorden, involve Vendian til- lites and, in the vicinity of Hinlopenstretet, Cam- bro-Ordovician strata; in central Nordaustlandet, these are upright structures, with a cleavage in the shales dipping generally at high angles east- wards. A variety of faults complicate the fold geometry of the major N-trending Rijpdalen Antiform and flanking synforms. Described below are the lithologies and struc- tures of the Helvetesflya area, with a focus on the evidence for major unconformities beneath the Murchisonfjorden Supergroup strata and within the underlying volcano-sedimentary succession. Lithologies and stratigraphy The stratigraphy of central Nordaustlandet is treated here in two parts: underlying units, equivalent to Flood et al’s (1969) Botniahalvcbya Group (referred to here as the Helvetesflya and Svartrabbane formations), and unconformably overlying units in the basal part of the Murchisonfjorden Supergroup. With regard to the latter. one of the formations included bv Ohta I development of an unconformity beneath his FIR 2 Geological map of the area between Vestfonna and Austfonna. central Nordaustlandet. (1982~) in the Austfonna Group dominates on Helvetesflya - the Djevleflota Formation. We therefore distinguish only the Djevleflota For- mation on the new geological map (Fig. 2), with underlying thin basal quartzites in eastern areas and basal conglomerates outcropping along the margin of Vestfonna. Djevleflota Formation and basal quartzites and conglomerates Flood et al. (1969) and Ohta (1982~) showed the distribution of the Persberget Formation quartz- ites in the eastern and western limbs of the Rijpda- len Antiform and their relationship to underlying strata. Ohta (1982~) recognised that, in the Venesjoen Synform on Djevleflota, these quartz- ites were underlain by a varied packet, estimated to a little over 1000 m in thickness, of shales and subordinate calcareous sandstones, quartzites and minor limestones; he therefore defined a new formation in this area. The succession is well preserved and primary structures are abundant (Fig. 3). Two major unconformities beneath the Neoproterozoic Murchisonfjorden Supergroup 87 Fig. 3 . Sedimentary structures in the sandstones of the Djev- Fig. 4 . Basal conglomerates of the Murchisonfjorden leRota Formation. A . Ripple marks. B. Mud cracks. Supergroup: Locality close to the eastern margin of Vestfonna (see Fig. 2). Clasts of quartzite, vein-quartz and dark meta- volcanites in both A and B . In the western limb of the Rijpdalen Antiform, only about 100 m of the Djevleflota Formation is exposed beneath the Persberget quartzites (thick- ness probably reduced by normal faulting), and basal conglomeratic beds are locally preserved. Conglomerates, described by Winsnes, in Flood et al. (1969, p. 37) from west of Rijpfjorden, in the base of the Persberget Formation, may also be part of this Murchisonfjorden basal unit. Ohta (1982c, p. 47) referred to a “basal breccia” at the base of the Persberget Formation, west of Brina, in southwestern Rijpdalen and inferred that it marked the base of a separate sedimentary cycle within the Murchisonfjorden Supergroup. This unit probably also coincides with our basal Murchisonfjorden conglomerate. Beneath these basal conglomerates, in the west- ern limb of the Rijpdalen Antiform, the litho- logies are dominated by dark phyllites of the Helvetesflya Formation. In this area, the new mapping does not support the correlation (Ohta 1982c) of these metasediments with the Djevleflota Formation. In the eastern limb of the Venesjcien Synform, the basal part of the Djevleflota Formation is poorly exposed; a few quartzite outcrops occur and the base is not seen. Further to the northeast, this formation is exposed again beneath the Persberget quartzites in the Kvartsitthaugen Syn- form. Here, the succession beneath the quartzites contains much green-grey cross-laminated silt and graded fine sandstone with quartzites at the base. However, the contact to the underlying Helvetesflya dark phyllites and Svartrabbane andesites is tectonic. The basal conglomerates of the Murchison- fjorden Supergroup are of particular interest. In the field, a variety of clasts were recorded, includ- ing red quartzites, vein quartz, dark phyllites, dark greenish mafic volcanic rocks, and light acid volcanics (Fig. 4). In thin section, the meta- volcanic rocks in the clasts are similar to those in the Svartrabbane Formation. Relic fluidal and porphyritic textures are recognisable; quartz and feldspar are altered to quartz-sericite aggregates and the glass in the acid rocks has recrystallised to 88 D . G. Gee & A . M . Teben’kou form a fine-grained matrix. Tourmaline-bearing clasts are conspicuous in these basal conglom- erates. Helvetesflya and Svurtrabbane volcano- Sedimentary rocks Two mappable formations occur below the Mur- chisonfjorden Supergroup. The one (Helvetesflya Formation) is dominated by dark phyllites, locally, with volcanic intercalations; the other (Svartrabbane Formation) is mainly composed of andesites with subordinate rhyolites. The Helvetesflya metasedimentary rocks are’ often relatively well preserved and “way-up” has been recorded locally. It may prove possible, with more detailed mapping, to beiher control the stra- tigraphy, but the occurrence of tight to isoclinal folding and local inversion do not allow this on the present data base. The Svartrabbane volcanic rocks are widespi-ead in the upper structural levels of the Rijpdalen Antiform in the eastern limb where they overlie the sedimentary units. Simi- larity of the latter to the Brennevinsfjorden For- Fig. 5. Svartrabbane Formation volcanic rocks: A . Agglom- erates cut by granite sheet (bottom left). B. Rhyolites with sedimentary clasts, probably derived from the underlying Hel- vetesflya Formation. mation and the former to the Kapp Hansteen Formation, noted by previous investigators, sup- ports correlation with Botniahalv~ya. A major unconformity separates these units. Helvetesflya Formation (dark phyllites) .-Mon- otonous dark phyllites, some of them black (1- 2% graphite), dominate this phyllite formation. Many of these lithologies have been described previously as shales or slates, but all that we have mapped have a strong, penetrative, fine-grained, sericitic schistosity and a superimposed crenu- lation that is usually conspicuous. Thin (a few centimetres) sandy intercalations are frequent and usually graded. Thicker quartzites are rare in this formation. Isoclinal folding makes thickness estimates very uncertain, but the formation is at least a few hundred metres thick; it is interpreted to be largely of turbidite origin. Suartrabbane Formation (uolcunites) .-The Svar- trabbane volcanic and associated sub-volcanic intrusions of central Nordaustlandet have been described in some detail by Teben’kov (1983) and Ohta (1985). The fine-grained penetrative schistosity makes it often difficult to be sure whether the massive, volcanic rocks are extrusive or intrusive. However, in many outcrops, good exposures of transitions from volcaniclastic rocks to massive volcanites occur; in addition, the pres- ence of sedimentary clasts in rhyolites and the intercalation of agglomerates with andesites, pro- vide good evidence of extrusion (Fig. 5 ) . The massive mafic rocks have been previously referred to as diabase and basalt, but have been shown by geochemical studies to be andesitic in composition (Teben’kov 1983; Ohta 1985). Interbedded dark phyllites occur locally and sub- ordinate quartzites are also present. Ohta ( 1 9 8 2 ~ ) reported thin carbonate units in the contact between the Svartrabbane and Helvetesflya for- mations. In 1995, a major unconformity was found to separate Helvetesflya turbidites from Svartrab- bane volcaniclastic rocks. Ohta (1982c, p. 50) referred to the presence of conglomerates sep- arating these volcanic and sedimentary rocks at Kjedevatna and inferred local unconformity. Our 1995 mapping shows that the Helvetesflya metasediments, with well-preserved sedimentary structures (e.g. graded bedding and cross-lami- nation), are locally inverted and overlain by a thin (up to ca. 5 m) quartzite pebble conglomerate, Two major unconformities beneath the Neoproterozoic Murchisonfjorden Supergroup 89 passing up into rhyolites and andesites of the Svartrabbane Formation. Granites (Rijpdalen-Winsnesbreen) Red granites occur in a main massif in Rijpdalen, developing a conspicuous thermal aureole in the host rocks to a distance of about 100 m from the contact. This Rijpdalen Granite (Hjelle 1966; Flood et al. 1969) occurs in the core of the Rijpda- len Antiform and appears to be connected from the northern outcrop areas of inner Rijpfjorden, southwards to the Winsnesbreen Granite near Wahlenbergfjorden. The mineralogy of the granite is dominated quartz, albite and microcline with subordinate muscovite (up to 10%) and minor biotite. Tourmaline is frequently present, particularly in association with pegmatites. The aureole mineralogy in the metasedimentary rocks is characterised by garnet and biotite, locally with staurolite; tourmaline also occurs commonly in the metasediments near the granite contact. Shear zones and a semi-penetrative foliation with accompanying brittle deformation are generally present. Augen gneisses (Ringgisdalen) In the northern part of Fig. 2, augen gneisses and augen granites are in contact with the Helvetesflya rocks. The ductile foliation in the these igneous rocks is cut by the Rijpdalen granite. Contact relationships between the augen gneisses and the Helvetesflya Group have been observed in the area of Fig. 2, north of Svartrabbane and, further north (Flood et al. 1969, pp. 105-112), in Ring- gisdalen (Fig. I); in both areas, garnet, straurolite and andalusite crystallise in narrow thermal aureoles. These gneisses have been interpreted (Flood et al. 1969) to be derived from granites that were intruded during deformation; the latter was probably of Grenville age, based on recently presented U/Pb zircon age-determination studies (Johansson pers. com. 1994). Deformation of the Helvetesflya and Svartrabbane Formations Flood et al. (1969) described the deformation of the Helvetesflya and Svartrabbane formations (their Botniahalvoya Group) to be more complex than that in the Murchisonfjorden Supergroup Fig. 6 . Folding in central Nordaustlandet. A.Upright S-plunging Caledonian anticline in the DjevleflotaFormation. B. Sideways- closing Precambrian folds in graded Helvetesflya turbidites (length of hammer shaft 40 cm). strata; this conclusion is supported by our new work and Ohta’s unpublished data ( Y . Ohta pers. com. 1996). Major N-trending folds, such as the Rijpdalen Antiform and the Venesjoen and Kvartsitthaugen synforms, with their related high angle E-dipping cleavages, are apparently the only major structures to deform the Neo- proterozoic strata; they clearly fold an earlier generation of tight to isoclinal folds in the Hel- vetesflya and Svartrabbane formations (Fig. 6). These older folds, as shown by Flood et al. (1969, pp. 85-87), have generally transverse axes (ca. WNW-ESE) and a well-developed axial surface cleavage, with associated fine-grained schistosity. Crenulation of the latter is generally due to the refolding by the N-trending Rijpdalen Antiform and related structures. Beneath the basal Mur- chisonfjorden Supergroup unconformity of east- ern Vestfonna, the axial surface schistosity dips nearly concordantly beneath the overlying con- glomerates, implying that the folds in the older rocks were recumbent prior to Caledonian deformation. 90 D . G . Gee & A . M . Teben’kov Regional metamorphism A fine-grained schistosity, well defined by sericite and chlorite, characterises the Helvetesflya metasedimentary rocks in most of the area of Fig. 2. Crenulation of this fabric resulted in partial reorientation of these phyllosilicates, but only minor recrystallisation. The latter is compatible with the evidence in the overlying shales of the Murchisonfjorden Supergroup of only incipient growth of sericite and chlorite in the cleavages. Discussion and conclusions It can be concluded that a major unconformity separates the sedimentary rocks of the Helvetesflya Formation from the volcanic Svar- trabbane Formation. A second unconformity occurs at the base of the overlying Neoproterozoic strata of the Murchisonfjorden Supergroup. Syn- tectonic intrusion of early granites, during recum- bent folding of the Helvetesflya and Svartrabbane formations was followed by post-tectonic intrusion of the Rijpdalen-Winsnesbreen granite. The latter is not seen to cut the Murchisonfjorden Supergroup sediments. The possibility exists that this granite may be Caledonian in age; however, the presence of tourmaline in the clasts of the basal Murchisonfjorden Supergroup conglom- erate and the association of this mineral with the aureole of the Rijpdalen granite favours the interpretation that the Rijpdalen-Winsnesbreen granite is of Grenville age. The Ringgisdalen augen gneisses, derived from the syntectonic granites intruding the Helvetesflya Formation, have not as yet been investigated isotopically, but their age is probably pre-Neo- proterozoic. Recent work by Johansson (pers. com. 1994) has shown that a variety of megacrystic augen granite/gneiss, outcropping along the northwestern margin of Austfonna (Fonndalen, Fig. 1) is of Grenvillian (ca. 1050 Ma) age; it may be related to the syntectonic intrusions referred to above. Although granite clasts have not been found amongst the greenschist facies pebbles in the base of the Murchisonfjorden Supergroup of central Nordaustlandet, it is worth noting that they are present in the Vendian tillites, higher in the Neo- proterozoic succession (Kulling 1934). A wide range of low-grade metasedimentary and meta- volcanic rocks together with granite boulders have been described by Krasil’SEikov (1967, translated by Harland et al. 1993, pp. 48-49) and by Edwards (1976). An attempt was made by Edwards and Taylor (1976) to date these granites by the Rb/Sr method. No isochron was obtained, but an indication of a Mesoproterozoic age was suggested; new studies are in progress. These granite clasts have been said by Hjelle (in Edwards & Taylor 1976, p. 256) to be similar to the Rijpdalen-Winsnesbreen Granite. The presence of granite boulders (up to 1 m in diameter) in the tillites of eastern Wahlen- bergfjorden and their smaller size further west prompted Edwards (1976) to suggest that the source area lay within a few kilometres distance from the outcrops. Without evidence for major facies changes in the Murchisonfjorden Supergroup, from the type area of western Nord- austlandet t o inner Wahlenbergfjorden, this would suggest that latest Proterozoic extensional faulting locally defined an eastern margin to the Neoproterozoic basin, exposing basement to Vendian erosion. The new work in central Nordaustlandet, reported here, supports previous conclusions (Gee et al. 1995) that Caledonian tectonothermal activity on Nordaustlandet is of a fundamentally different character than that found in western Ny Fnesland. Within the Neoproterozoic suc- cessions, from eastern Ny Friedand, via Murchisonfjorden and Wahlenbergfjorden to Rijpdalen, the folding is upright to westerly inclined and the recrystallisation in the associated cleavages is generally lower greenschist facies, with growth of fine-grained sericite and chlorite. Only in the immediate contact to the Planetfjella Group and in the contact aureole t o the Chy- deniusbreen Granite massif in southern Ny Fries- land are higher greenschist facies conditions recorded in the Neoproterozoic strata. The Cale- donian lower greenschist facies tectonothermal overprint may increase a little in grade towards the east, as suggested by K/Ar and Rb/Sr mica ages (Ohta 1994) in the underlying migmatites, but the possibility remains that the pre-Devonian basement of the northern Barents Sea is essen- tially a Grenvillian complex, partially covered by a slightly deformed blanket of Neoproterozoic and early Paleozoic strata. Ackttowledgements. -Our field party in eastern Nordaustlandet in 1994 included three students, J . Gustafsson, J . Nilsson and S. Sandelin and geornorphologist L. Sernenova. I n 1995, we were assisted by E. Tagesson. W e thank them all for their Two major unconformities beneath the Neoproterozoic Murchisonfjorden Supergroup 91 contribution to the field investigations, at times under difficult wintery conditions. Our work on Nordaustlandet has been supported by the Swedish Polar Research Secretariat i n Stock- holm, thc Polar Marine Geological Expedition (PMGE) in St. Pctersburg and the Swedish Natural Science Research Council. We also greatly appreciate excellent communication with Norsk Polarinstitutt and PMGE colleagues and, particularly, discus- sion and review of this paper by A. Krasil’SEikov and Y. Ohta. We also thank P. Witt-Nilsson and M. Friberg for help with the final preparation of the diagrams. References Edwards, M. 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