The Supanberget area - basement imbrication and detached foreland thrusting in the Tertiary fold-and-thrust belt, Svalbard WINFRIED K. DALLMANN AND HARMON D. MAHER. JR. Dallmann, W.K. & Maher, H.D., Jr. 1989: The Supanberget area - basement imbrication and detached foreland thrusting in the Tertiary fold-and thrust belt, Svalbard. Polar Research 7, 95-107. The Supanberget area, situated in the southern part of the Tertiary Spitsbergen fold-and-thrust belt, occupies a position mainly within the central zone of the belt, characterized by basement-involvedthrusting, but includes part of an eastern foreland zone of detached, thin-skinned thrust tectonics. The peculiar feature of Supanbergetis the thrust- ‘sliced’tectonic style that deforms a previously tightly folded basement- cover contact. Foreland structures indicate the additional presence of a subsurface detachment at a deeper structural level. Contrasts in magnitude and orientation of basement anisotropies may control lateral changes in tectonic style. Winfried K. Dallmann, Norsk Polarimtitutt, P. 0. Box 158, N-1330 Oslo Lufthavn, Norway; Harmon D. Maher, Jr., Dept. of Geography and Geology, University of Nebraska, Omaha, Nebraska 68182, U.S.A.; March 1989 (revised August 1989). The Tertiary fold-and-thrust belt of Spitsbergen, b; Dallmann & Maher 19881, dividing it into a Svalbard, is situated in an overall dextral trans- western zone of uplift of the Caledonian basement pressive, intracratonic plate regime. Models pro- with an uncertain, but varying amount of Tertiary posed and established by Harland (1969), Lowell deformation, a central zone of mainly thick- (1972), Birkenmajer (1972), Harland & Horsfield skinned, basement-involved thrusts, and an east- (1974), and Kellogg (1975) have been further ern foreland zone of detached, thin-skinned thrust constrained by evidence from the seafloor off systems with dkcollement and ramp-flat geo- the Barents Shelf (Myhre et al. 1982; Spencer metries. et al. 1984; Eldholm et al. 1987; Vignes et al. Lateral changes of tectonic style within these 1988). zones are common, and can partly be ascribed to Recent research in the fold-and-thrust belt pro- the different structural depths exposed (Dallmann vides a revised and expanded understanding of its 1988b; Maher 1988a; Dallmann & Maher 1988). structure. In spite of the overall transpressive The basement-involved thrust zones vary from regime, detailed investigations in the belt indicate simpler areas with large-scale folds cut by indi- that almost solely convergent motions are respon- vidual thrusts to complexly imbricated and tightly sible for the observed structures (Andresen et al. folded sections. The latter have been described 1988a, b; Bergh et al. 1988; Dallmann 1988a, b; from several places in Oscar I1 Land (St. Jonsfjor- Maher 1988a, b; Nottvedt et al. 1988a, b). The den-Winsnes & Ohta 1988). South of Isfjorden, eastward extension of the deformed foreland sig- similar structures have - as far as we know - only nificantly exceeds what previously was believed been observed in the Supanberget area (Fig. 1). (Andresen et al. 1988a, b; Haremo et al. 1988; The lack of more frequent observations may be Haremo & Andresen 1988). These results led to due to the extensive ice cover along the basement- the idea of decoupled transform and compressive cover contact in Wedel Jarlsberg Land. movements, the transform component being In this article, we describe the structure of the restricted to western offshore areas (Steel et al. Supanberget area, discuss its geometry and origin, 1985; Maher & Craddock 1988; Nottvedt et al. and consider a connection between the structural 1988a, b). A distinct zonation parallel to the trend style of the thrust zone and the basement aniso- of the belt has been worked out (Maher 1988a, tropies. 96 W. K. Dalltnann & H. D. Maher. Jr. Fig. 1. Location of the Supan- berget area and its position within the zone of strong Ter- tiary deformation in S Spits- bergen. the basement rocks, and are part of the ‘older Description of the Supanberget sequence’ of Bjornerud (1987). In the inves- area tigated area, the lithologies can be subdivided into two units. One consists of carbonate rocks, Location, accessibility and exposicre mainly massive grey to yellowish dolomites, with Supanberget is situated in the interior of Wedel subordinate banded marbles. The other unit con- Jarlsberg Land, 20 km S of Van Keulenfjorden, sists of dark grey phyllites interlayered with mass- and is part of the central NW-SE trending ridge ive quartzite layers up to several meters in separating the Caledonian basement in the SW thickness. A low angle discordance exists between from the post-Caledonian cover strata in the NE the lithologic banding of this metaclastic suc- (Fig. 1). The area is extensively glaciated. The cession and the contact with the carbonate suc- easiest access by foot is from Van Keulenfjorden cession. The stratigraphic relationship between along the glacier ‘Penckbreen’. Exposures along the individual units has not been investigated. the mountain sides and ridge crests are mostly excellent, and only a few small areas are covered Post-Caledonian cover strata. - Middle Car- with moraines or scree. The majority of the ridges boniferous through Lower Jurassic strata of the can easily be set foot on from the glaciers, but Supanberget area lie within the larger area in only a minor part of them can be fully surveyed which Rozycki (1959) describes the post-Cale- without mountaineering or helicopter support. donian stratigraphy. However, Cutbill & Chal- linor (1965) revised the Carboniferous and Permian stratigraphy, and age relations were Stratigraphy and lithology again reconsidered by Steel & Worsley (1984). Caledonian basement strata. - The Caledonian Upper Carboniferous through Permian sections basement of the study area is assigned to the have also been described by Nysather (1977) from MagnethOgda sequence (‘Magneth~gdaGroup’ Polakkfjellet, 18 km SE of Supanberget. The of Harland (1978)) of Precambrian, possibly stratigraphic column, Fig. 2, is based on these Middle Proterozoic age (Flood et al. 1971; Y. descriptions, but modified by our own obser- Ohta pers. comm. 1988). No detailed descriptions vations. of the lithologies at Supanberget have been pub- The Carboniferous and Permian stratigraphy is lished, but similar rocks from the same strati- complexly deformed and partly cut by thrusts at graphic sequence have been described by Hauser Supanberget. In order to decipher the stratigra- (1982 - ‘Antoniabreen Succession’). These strata phy, it is therefore important to observe the undis- lie below a major angular unconformity within turbed Zittelberget section to the north. In The Supanberget area 97 continuously into the Reinodden Formation at Van Keulenfjorden. It also seems to be an equi- valent of the Drevbreen beds at Polakkfjellet (Nyszether 1977) and of the Treskelodden For- mation at Hornsund (Birkenmajer 1964). Steel & Worsley (1984) assume that these three Upper Carboniferous to lowermost Permian successions are correlatable. The formation is thinly layered, interbedded with minor dolomites and limestones, and numerous shaly layers as well as clast-supported conglomerate beds with rounded clasts and a porous matrix. These parameters are consistent with a relatively incompetent behav- iour. The overlying Lower Permian Gipshuken For- mation (c. 80-100 m) consists mainly of light grey to yellowish weathering dolomitic limestones and dolomites with intercalated cross-bedded sand- stones, polymict conglomerates, and shales. The facies of the lower part becomes richer in con- glomerates to the south, where the boundary towards the underlying Reinodden Formation is not well defined. The Upper Permian Kapp Starostin Formation (c. 150m) is dominated by fossiliferous, partly siliceous limestones. Cherts that are typical for this formation further north are here mostly sub- stituted by shales, silt- and sandstones which occur preferentially in the upper part. The Kapp Sta- rostin Formation still forms the most competent unit of the post-Caledonian succession in this area. The Lower and Middle Triassic formations of the Sassendalen Group consist of sandstonel shale alternations, representing four main coar- Fig. 2. Stratigraphic column of the post-Caledonian sequence of the Supanberget area. Thicknesses are approximate. sening upward cycles which total approximately 550 m thickness. The upper unit of the group, the Bravaisberget Formation, contains c. 120 m of addition, a complicating factor is that the Lower incompetent, bituminous shales and a c. 40 m Permian strata (Gipshuken Formation) undergo thick capping sandstone. The overlying Upper fairly rapid lateral facies changes towards more Triassic to Lower Jurassic Kapp Toscana Group clastic dominated deposits to the south. is made up of c. 120 m marginal marine sand- and The lowermost unit is a red, polymict con- siltstones (M~rket al. 1982). Clastic sedi- glomerate succession with intercalated, discon- mentation continues into the Jurassic/Lower Cre- tinuous red sandstones of probably Middle (or taceous Janusfjellet Formation, consisting of Upper?) Carboniferous age, and it represents a highly bituminous siltstones and shales. facies equivalent of the Hyrnefjellet Formation (Birkenmajer 1964) at Hornsund. The rocks vary in thickness from 0 to c. 150m, and appear to Structure of the Supanberget thrust be deposited in Carboniferous graben and half- graben structures (Figs. 3, 4 - section D, and 7). system The overlying,