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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: 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.

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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.

The Billefjorden Fault Zone represents a major lineament on Spitsbergen with a history of tectonic activity going back into the Devonian and possibly earlier. Recent structural, sedimcntological and stratigraphical investigations indicate that most of the stratigraphic thickness variations within the Mesozoic strata along the Billefjorden Fault Zone south of Isfjordcn are due to Tertiary compressional tectonics related to the transpressive Eocene West‐Spitsbergen Orogeny. No convincing evidence of distinct Mesozoic extensional events, as suggested by previous workers, has been recognized. Tertiary compressional tectonics are characterized by a combined thin‐skinned/thick‐skinned structural style. Decollement zones arc recognized in the Triassic Sassendalen Group (tower Décollement Zone) and in the Jurassic/Cretaceous Janusfjellet Subgroup (Upper Décollement Zone). East‐vergent folding and reverse faulting associated with these decollement' zones have resulted in the development of compressional structures, of which the major arc the Skolten and Tronfjellct Anticlines and the Advcntelva Duplex. Movements on one or more high angle east‐dipping reverse faults in the pre‐Mesozoic basement have resulted in the development of the Juvdalskampcn Monocline, and are responsible for out‐of‐sequence thrusting and thinning of the Mesozoic sequence across the Billefjorden Fault Zone. Preliminary shortening calculations indicate an eastward displacement of minimum 3‐4 km, possibly as much as 10 km for the Lower Cretaceous and younger rocks across the Billefjorden Fault Zone.