The Janusfjellet Subgroup (Bathonian to Hauterivian) on central Spitsbergen: a revised lithostratigraphy H. DYPVIK, J. NAGY, T. A. EIKELAND, K. BACKER-OWE, A. ANDRESEN, P. HAREMO, T. BJAZRKE, H. JOHANSEN, and A. ELVERHBI Dypvik, H.,Nagy, J., Eikeland, T. A., Backer-Owe, K., Andresen, A,, Haremo, P., Bjrerke, T.,Johansen, H. & Elverhl, A.: The Janusfjellet Subgroup (Bathonian to Hauterivian) on central Spitsbergen: a revised lithostratigraphy. Polar Research 9(1), 21-43. The Janusfjellet Subgroup is a marine shelf to prodeltaic succession dominated by shales with subordinate siltstones and sandstones. The subgroup comprises a lower Agardhfjellet (Upper Bathonian - Berriasian) and an upper Rurikfjellet (Berriasian - Hauterivian) formation. Based on field work in central Spitsbergen the following subdivisions of the formations are proposed (units listed in ascending order). The Agardhfjellet Formation (up to 290 m thick) contains four members: Oppdalen - a fining upwards succession from conglomerates to shales; Lardyfjellet - black paper shales; Oppdalsita - grey shales with siltstones and sandstones; and SlottsmQya - grey shales and black paper shales. Within the Oppdalen Member three beds are recognised: Brentskardhaugen - phosphoritic conglomerate; Marhogda - glauconitic sandstones; and Dronbreen - siltstones and shales. The Rurikfjellet Formation (thickness up to 226 m) is composed of two members: Wimanfjellet - grey and partly silty shale sequence, containing the Myklegardfjellet Bed (of plastic clays) at its base; and Ullaberget - silty and sandy shales with siltstones and sandstones. H. Dypvik, J. Nagy, T.A. Eikeland, K. Backer-Owe, A. Andresen, and A. Elverh@i, Department of Geology, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo 3, Norway; P. Haremo, Norsk Hydro AIS, P.O. Box 200, N-1321 Stabekk, Norway; T. Bjerke, Storgata 47, 3290 Stauern; H. Johansen, Norsk Polarinstitutt, Rolfstangveien 12. Snar@ya, P.O. Box 158, N-1330 Oslo lufthavn, Norway. In light of the latest stratigraphical information graphical correlation, to increase the resolution generated by recent structural, sedimentological potential of the structural geology and to act as an and paleontological investigations (Andresen et improved framework in ongoing sedimentological al. unpubl.; Dypvik et al. 1988; Nagy et al. discussions (Figs. 1 and 2). The study is con- 1990; Dypvik et al. in press), a revision of the centrated around the region between Sassen- Jurassic/Cretaceous stratigraphy of central Spits- fjorden, Kjellstr0mdalen and Agardhbukta. bergen appears to be required. The revised strati- designated here as central Spitsbergen. graphical subdivision proposed below aims to The Bathonian to Hauterivian time-span in facilitate detailed geological mapping and strati- Spitsbergen is represented by an approximately 400 m thick sedimentary succession composed New or revised definitions of stratigraphic units in Norway mainly of shales with subordinate siltstones and (including the Norwegian polar areas) are to be registered sandstones deposited in shelf environments and confirmed by the ‘Norwegian Committee on Stratigraphy’ (Dypvik 1985). This succession is the Janusfjellet (NSK). A subcommission, the ‘Committee on the Stratigraphy of Svalbard’ (SKS), has recently (1990) been established to unit (Fig. l), which was ranked originally as sub- develop a formal stratigraphic nomenclature for the bedrock group (Parker 1967) but changed subsequently to stratigraphy of the mainland of Svalbard. Until this work has formation. The present contribution revises the been completed, SKS will not accept individual applications. stratigraphy of the Janusfjellet unit, and the main Authors are requrested not to submit formal definitions of stratigraphic units in Svalbard during this time without first items of this revision are: redefining the Janusfjel- consulting SKS (c/o Norwegian Polar Research Institute, P.O. let unit as subgroup; elevating the rank of its two Box 158, N-1330 Oslo Lufthavn). subunits (the Agardhfjellet and Rurikfjellet) from The present article was submitted prior to the establishment member to formation; providing a new and of SKS and has therefore been treated as an exception. The detailed subdivision of these formations. stratigraphy presented has not yet been confirmed by NSK and will be considered along with other proposed stratigraphic The basis of the present stratigraphical study is subdivisions. extensive field work carried out at Bohemanflya, 22 H.Dypvik et al. Fig. I. The Janusfjellet Subgroup at Wimanfjellet, seen from Isfjorden (WI-Wilhelm~yaFormation, JA-Janusfjellet Subgroup). Festningen, Janusfjellet, Wimanfjellet, Knor- FJELLE? ringfjellet, numerous sections in the Advent- FM. FESTNINGEN MEMBER dalen-Reindalen area, Lardyfjellet in Kjell- ULLABERGET strcbmdalen and sections at Agardhbukta (Figs. 3 and 4). The preliminary results of these in- vestigations have partially been published else- where (Dypvik et al. 1988; Nagy et al. 1990), and the present paper is devoted to the more general features of stratigraphical relevance. A proposed sedimentological model based on the new data from the subgroup will be presented in a separate 1 SLOTTSMBYA MEMBER paper (Dypvik et al. in press). z P tia General stratigraphy of the P Janusfjellet Subgroup c W The Middle Jurassic to Lower Cretaceous suc- d LARDYFJE~LET MEMBER d cession in Spitsbergen forms a single major strati- W v 3 graphic unit, the Adventdalen Group originally 0 OPPDALEN proposed by Parker (1967), which contains a L MARHBGDA BED fluviodeltaic and three marine formations. The MEMBER : BRENTSKARDHAUGEN BED Fig. 3. Geological map of Spitsbergen showing position of Fig. 2. Revised stratigraphical scheme of the Janusfjellet Suh- localities discussed in text. Central Spitsbergen, hoxcd in. is group for central and eastern Spitsbergen. enlarged in Fig. 4. The Janusfellet Subgroup (Bathonian to Hauterivian) on central Spitsbergen 23 @ Syltoppen @ Bohemanflya @ Billefjorden @ Sassenfjorden @ lsfjorden @ Festningen @ Van Keulenfjorden @ Ullaberget @ Kvalvagen @ Jurakammen @ Wedel Jarlsberg L.= @ Torell Land @ Hornsund @ Serrkapp Land @ Keilhaufjellet /19O 11 5O I180 24 H. Dypuik et al. Janusfjellet @) Aasgaardfjellet I 1 GLACIERS 0 @ Marhragda @ Bergmraya @ Oppdalen @ Knorringfjellet @ OppdalsAta I_=] JURASSIC @ Marmierfjellet @ VrAbreen - CRETACEOUS Roulettegga Lardyfjellet - @ @- l;T,?j TRIASSIC @ Arctowskifjellet @ Glitrefjellet @ Juvdalskampen @ ~~ottsmraya II I CARBONIFEROUS Ll PERMIAN @ Brentskardhaugen @ Lundstrramdalen @ Drranbreen @ Agardhfjellet \ FAULTS @ Fleksurfjellet @ Myklegardfjellet @ Fonnhetta @ Klementievfjellet @ Tronfjellet @ Rurikfjellet Fig. 4. Geological map of Central Spitsbergen. Position of localities discussed in text indicated. group unconformably overlies the large hiatus 1940). The unit was formally termed the Janus- which comprises much of the Middle Jurassic, fjellet Subgroup by Parker (1967), and this usage while the top of the group corresponds to an was shared by Flood et al. (1971). Later on the extensive Late Cretaceous hiatus. The lower, unit was currently ranked as formation. The sub- shaley part of this succession has long been recog- group rests on shallow shelf to marginal marine nized as a major single unit variously designated sandstones and shales of the Wilhelmoya Forma- as Aucellensichten (Nathorst 1897; Hoe1 & Orvin tion. It is overlaid by fluviodeltaic sandstones of 1937) or Aucellen shale (Hagerman 1925; Orvin the Helvetiafjellet Formation. The Janusfjellet Subgroup (Bathonian to Hauteriuian) on central Spitsbergen 25 The Janusfjellet Subgroup is composed of the Maher (1989) discusses a storm-related origin for Agardhfjellet Formation of Late Bathonian to the Brentskardhaugen Bed. Directly on top of Berriasian age, and the Rurikfjellet Formation of this conglomerate, the oolitic sediments of the Berriasian to Hauterivian age. These two units Marh~gdaBed follow in an upward fining de- are separated by a thin interval (usually <2 m) of velopment. yellowish weathering clays. The lower part of the Agardhfjellet Formation is dominated by somewhat silty and sandy sedi- ments, grading upwards into black (often papery) Agardhfjellet Formation and grey shales forming the main part of the unit. This unit was originally defined as formation (Par- The age of this formation is Late Bathonian to ker 1967), but became changed in rank to member Berriasian as deduced from the literature includ- in several papers (Major & Nagy 1972; Bir- ing Pchelina (1965a, b); L~faldli& Nagy (1983); kenmajer et al. 1982; LGfaldli & Nagy 1983). Parker (1967); Erschova (1983); Kopik & Wierz- Detailed studies in central Spitsbergen demon- bowski (1988) Wierzbowski (1989); Nagy et al. strated that the unit has a varied lithology, which (1990). The thickness of the Agardhfjellet For- together with its large thickness warrants its rank- mation varies between 165 m at Keilhaufjellet to ing as formation. 255-290 m in the Van Keulenfjorden and Festnin- The lower part of the Janusfjellet Subgroup is gen areas. The dark shales of the Agardhfjellet made up of the few decimeter to some meter thick Formation contain some siltstone and sandstone Brentskardhaugen Bed, containing pebbles of beds, but generally represent deposition in oxy- phosphate, chert, rock fragments, etc. (Back- gen-deficient shelf environments characterized by strom & Nagy 1985). The position of the Brent- sedimentation of fine-grained material such as skardhaugen Bed with regard to higher category