The Janusfjellet Subgroup (Bathonian to Hauterivian) on central Spitsbergen: a revised lithostratigraphy

Abstract

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: Oppdalena fining upwards succession from conglomerates to shales; Lardyfjelletblack paper shales; Oppdalsitagrey shales with siltstones and sandstones; and SlottsmQyagrey shales and black paper shales. Within the Oppdalen Member three beds are recognised: Brentskardhaugenphosphoritic conglomerate; Marhogdaglauconitic sandstones; and Dronbreensiltstones and shales.The Rurikfjellet Formation (thickness up to 226 m) is composed of two members: Wimanfjelletgrey and partly silty shale sequence, containing the Myklegardfjellet Bed (of plastic clays) at its base; and Ullabergetsilty and sandy shales with siltstones and sandstones.

Integrating subsurface and outcrop data of the Middle Jurassic to Lower Cretaceous Agardhfjellet Formation in central Spitsbergen

“…Four of the wells have complete core coverage of the Agardhfjellet Formation (i.e., an average thickness of about 250 m). The successions in the wells were correlated with the classical sections of the Agardhfjellet Formation in the Janusfjellet area, c. 10 km to the northeast of the CO 2 wells (Dypvik et al, 1991a;Mørk et al, 1999;, as shown in Fig. 2.…”

“…Lower to Middle Jurassic, coarse-grained units have so far been the most prolific petroleum reservoirs in the southwestern Barents Sea (Henriksen et al, 2011). Middle Jurassic to lowermost Cretaceous, deeper shelf sediments are found in Svalbard at Hopen, Kong Karls Land, Wilhelmøya, Sørkapp and in the Isfjorden area, while in the Early Cretaceous the shallow shelf shallowed upward to deltaic and flood-plain deposits which were later flooded by Aptian shelf deposits (Dypvik et al, 1991a;Mørk et al, 1999;Krajewski, 2004;Midtkandal et al, 2016;Grundvåg et al, 2017). The Middle Jurassic to Lower Cretaceous Janusfjellet Subgroup comprises the Agardhfjellet and Rurikfjellet formations.…”

“…defined as including the Marhøgda Bed (Bäckström & Nagy, 1985;Krajewski, 1990;Dypvik et al, 1991b). The Oppdalen Member is normally an overall finingupward unit (Dypvik et al, 1991a). The lowermost part of the Bathonian-Oxfordian Oppdalen Member was deposited in a shallow-marine environment grading into the more distal, black shales of the Lower Kimmeridgian Lardyfjellet Member, which is seen as the lower finergrained part of a coarsening-upward unit (Dypvik et al, 1991a).…”

“…The Oppdalen Member is normally an overall finingupward unit (Dypvik et al, 1991a). The lowermost part of the Bathonian-Oxfordian Oppdalen Member was deposited in a shallow-marine environment grading into the more distal, black shales of the Lower Kimmeridgian Lardyfjellet Member, which is seen as the lower finergrained part of a coarsening-upward unit (Dypvik et al, 1991a). A 50 m-thick Upper Kimmeridgian to possibly Lower Volgian coarse-grained siliclastic wedge occurs as the Oppdalssåta Member in our study area.…”

New articulated asteroids (Echinodermata, Asteroidea) and ophiuroids (Echinodermata, Ophiuroidea) from the Late Jurassic (Volgian / Tithonian) of central Spitsbergen

“…1B). In the study area, this member is a ca 70-meter-thick succession of grey to black shales and siltstones irregularly interbedded with concretionary siderite and dolomite, deposited in an open-marine shelf environment with estimated water depths of 100-150 meters (Dypvik et al 1991;Mørk et al 1999;Hammer et al 2011Hammer et al , 2012Hryniewicz et al 2015). It has been interpreted as a transgressive unit with slightly dysoxic shelf conditions and periodic fluctuation in the oxygenation of the bottom water (Nagy et al 2009;Hammer et al 2011Hammer et al , 2012Hjálmarsdóttir et al 2012).…”