NORWEGIAN JOURNAL OF GEOLOGY Petrography and sedimentology of the Slottsmøya Member at Janusfjellet, central Spitsbergen 89 Petrography and sedimentology of the Slottsmøya Member at Janusfjellet, central Spitsbergen Marine Collignon & Øyvind Hammer Collignon, M. & Hammer, Ø.: Petrography and sedimentology of the Slottsmøya Member at Janusfjellet, central Spitsbergen. Norwegian Journal of Geology, Vol. 92, pp. 89-101. Oslo 2002. ISSN 029-196X. The Slottsmøya Member (Agardhfjellet Formation) in Svalbard is composed mainly of marine grey shales deposited on a slightly dysoxic and shallow shelf (offshore transition), in Volgian (Late Jurassic) to Ryazanian and possibly earliest Valanginian (Early Cretaceous) time. We present detailed lithostratigraphic, mineralogical, chemical and magnetic susceptibility logs of the Slottsmøya Member at Janusfjellet, Central Spitsbergen , providing a high-resolution stratigraphic framework for the ongoing palaeontological and geological investigations in this area. A condensed section has been recognised in the upper part of the member, based on chemical, mineralogical and palaeontological evidence. Marine Collignon, Natural History Museum, University of Oslo, P.O. Box 1172 Blindern, NO-0318 Oslo, Norway. Present address: ETH Zürich, Geologisches Institut, Sonneggstrasse 5, NO E21, 8092 Zurich, Switzerland. E-mail: [email protected]. Øyvind Hammer, Natural History Museum, University of Oslo, P.O. Box 1172 Blindern, NO-0318 Oslo, Norway. E-mail: [email protected] Introduction most of the vertebrate fossils have been found. Following the recent discoveries and studies of numerous remains of marine reptiles (Hurum et al. 2012, this vol- Geological setting ume) and fossil-rich methane seep carbonates (Hammer et al. 2011) in the shales of the Slottsmøya Member at The Middle Jurassic to Lower Cretaceous in Spitsbergen Janusfjellet, Central Spitsbergen (Fig 1a), it was decided forms a single major stratigraphic unit, the Adventdalen to study this section in more detail with respect to dep- Group (Parker 1967), which comprises two sequences ositional conditions and thus to provide a local, high- of shelf deposits (the Janusfjellet Subgroup and the resolution, stratigraphic framework. Caroline fjellet Formation) interrupted by the fluvial and nearshore deposits of the Helvetiafjellet Formation (Fig. The Slottsmøya Member constitutes the uppermost unit 2). The lower sequence (with the Janusfjellet Subgroup) of the Agardhfjellet Formation (Fig. 2) and is composed consists of several hundred metres of organic-rich, clayey mainly of grey to black (often paper) shales, thought to and silty material whilst the upper sequence (with the represent alternating oxic to anoxic bottom conditions Carolinefjellet Formation) contains more sandstones and (Dypvik 1980, 1985). The shales record total organic shales and is poorer in organic matter. content (TOC) up to 11% (Hvoslef et al. 1986). The Agardhfjellet Formation extends from the Late Batho- The Janusfjellet Subgroup (Parker 1967; Dypvik et al. nian to the Ryazanian and possibly earliest Valanginian, 1991a, Dypvik et al. 1991b), with a variable thickness based on macrofossils (mostly ammonites: Parker 1967; of 280-420 m in Eastern and Central Spitsbergen, rests Ershova 1983; Wierzbowski et al. 2011), foraminifera on shallow-shelf to marginal marine sandstones and (Nagy & Basov 1998) and palynology (Løfaldli & Thusu shales of the Knorringfjellet Formation, and is overlain 1976; Bjærke 1978). The middle part of the Slottsmøya by coarse-grained, light-coloured, fluviodeltaic sand- Member is of particular interest, as it records most of the stones of the Festningen sandstone member of the Hel- vertebrate fossils found until now (Hurum et al. 2012, vetiafjellet Formation. The subgroup comprises the this volume). Moreover, Hammer et al. (this volume) Agardhfjellet and the Rurikfjellet formations (Dypvik et recorded, in this part of the member, indications of c. 400 al. 1991a). The dark shales of the Agardhfjellet Forma- kyr periodicity, which can be interpreted as a result of tion (90-350 m, 242 m in the stratotype at Agardhfjellet, orbital forcing (long eccentricity). Sabine Land) are interpreted as having been deposited in an oxygen-deficient shelf environment with fine-grained We will here present mineralogical and chemical ana- sedimentation (clays and organic matter), but the for- lyses through the Slottsmøya Member at Janusfjellet with mation also contains some siltstone and sandstone beds a special focus on the middle part of the member where (Dypvik 1980, 1985; Dypvik et al. 1991a; Dallman 1999). 90 M. Collignon & Ø. Hammer NORWEGIAN JOURNAL OF GEOLOGY 1 km Diabasodden Figure 1. Geological map of the Sassenfjorden area in Field localities: Central Spitsbergen (modified 1: Janusfjellet Deltaneset Elveneset 2: Marhøgda from Dallman et al. 2001). 3: Knorringfjellet The dashed line indicates the presence of one or more thrust Marhøgda faults causing stratigraphic 1. K onusdalen Wimandalen repetitions or excisions. 2. Janusfjellet Wimanfjellet Deg Konusen ee r dalen Carolinefjellet 3. 80°N Paleogene units Knorring- Louisfjellet Nordaustlandet Carolinefjellet Fm. fjellet Helvetiafjellet Fm. Spitsbergen Rurikfjellet Fm. 78°N Agardhfjellet Fm. Knorringfjellet Fm. Helvetiafjellet Fm. (BarrEdgeøyaemian) De Geerdalen Fm. ogdalen Hanask Tschermakfjellet Fm. 12°E 20°E 28°E Diabasodden Suite Age GP. Cen. Spitsbergen Cen. E’ Spitsbergen Figure 2. Upper Jurassic- Lower Cretaceous lithostrati- ? Schönrockfjellet member graphic units in East ern and ? Central Spitsbergen (modified Albian Zillerberget member ? from Dallman 1999 and Wier- Langstakken Member zbowski et al. 2011) Carolinefjellet Formation Innkjegla Member Aptian ACEOUS Dalkjegla Member T Early Helvetiafjellet Formation CRE Barremian Festningen sandstone member Ullaberget Member Hauterivian Rurikfjellet Formation Valanginian Wimanfjellet Member ? Myklegardfjellet Bed ? Ryazanian Slottsmøya Member ADVENTDALEN GROUP Volgian Janusfjellet Subgroup Agardhfjellet Formation Oppdalssåta Member Kimmeridgian Late Oxfordian Lardyfjellet Member JURASSIC Callovian Oppdalen Member ? ? Middle Bathonian K.T. Knorringfjellet Formation K.T. = Kapp Tosccana Group. Bituminous intervals of black paper shales dominate in feature of this formation is the occurrence of diagenetic the Oxfordian and Kimmeridgian parts of the succes- carbonates (ankerite/dolomite and siderite) at specific sion (Pchelina 1965a, 1965b; Nagy & Løfaldli 1981). The horizons (Dypvik 1978; Krajewski 1992; Bausch et al. shales of the Agardhfjellet Formation are easily weath- 1998). ered and a yellow to red jarosite/gypsum coating can be observed at the surface (Birkenmajer 1980). A particular The Slottsmøya Member is 70 to 100 m thick (84 m in NORWEGIAN JOURNAL OF GEOLOGY Petrography and sedimentology of the Slottsmøya Member at Janusfjellet, central Spitsbergen 91 the stratotype at Slottsmøya, western Sabine Land). Material and methods Its base is defined as the renewed onset of black shales above the silt and sandstones of the Oppdalssåta Mem- The described lithology is based on field observations ber, and its top by the last silt and very fine sandstones and magnetic susceptibility measurements. Conspicu- below the Myklegardfjellet Bed (i.e., base of the Rurik- ous beds (Fig. 3) consisting of siltstones or carbonate fjellet Formation). The member consists of dark-grey beds were used as markers in the field due to the homo- to black shales and paper shales, with local occurrences geneous nature of the shales. The zero metre level in our of red to yellowish siderite concretions as well as sider- logs is defined by the top of a thin, yellow, silty bed, c. 27 ite and dolomite interbeds. The middle to upper part of m below the Dorsoplanites marker bed, which is a richly the member grades into a shale-silt-very fine sandstone fossiliferous sideritic horizon. Sample positions were succession, commonly rich in ammonites (Dorsoplanites) recorded with a Leica TCR 110 total station, with error < and bivalves (Dallman 1999). The member, dated from 1 cm at 100 m distance, and corrected later with respect Volgian to Ryazanian and possibly earliest Valanginian to dip. The magnetic susceptibility was measured directly time (Nagy & Basov 1998; Wierzbowski et al. 2011) was in the field with a KT-10, hand-held, magnetic suscepti- deposited in an open marine shelf environment. A hia- bility meter (Terraplus Inc.). Three measurements were tus in the uppermost part of the Slottsmøya Member a taken every 5-30 cm in the central part of the member few metres below the Myklegardfjellet Bed has also been and averaged to reduce noise. The instrument was sys- recorded (Smelror 1994; Nagy & Basov 1998; Wierz- tematically calibrated in free air before and after every bowski et al. 2011). measurement. The Myklegardfjellet Bed (0.5-11m in thickness), whose XRD (Siemens D5000 apparatus) using Ni-filtered CuKα base is defined by the onset of reddish-yellow or green radiations was used for identification of mineral phases plastic clay, overlying dark shales of the Slottsmøya and semiquantitative determination of the mineral com- Member (Dallman 1999), is an important marker in position. The analysis was carried out on 52 untreated Central and Eastern Spitsbergen, and has been assigned and non-oriented samples at the Department of Chemi- to the lowermost Valanginian based on ammonite evi- stry, University of Oslo. Thirty samples were sent to Act- dence (Wierzbowski et al. 2011).