A sedimentological study of black shales from the Spekk Formation, Norwegian Sea

Abstract

Black shales from the Late Jurassic-Early Cretaceous Spekk Formation have been investigated by using a detailed sedimentological approach. Four IKU drilling cores from the Norwegian Sea have been used as study material to perform facies analysis based on core logging together with XRD analysis and optical microscopy of thin sections. Focussing on mudstone fabric and small-scale features, black shales of the Spekk Formation have been shown to represent a fluctuating depositional environment in terms of circulation and activity of currents, in addition to have intermittent oxic periods. Eight facies, including seven mudstone facies and one sandstone facies, have been defined based on lithology, sedimentary structures and textures, colour variations and grades of bioturbation. The facies have been used to infer the depositional environment, reflecting an overall dysoxic-anoxic bottom-water setting with recurring current activity represented by coarser mudstones, having intermittent oxic periods in the form of bioturbated mudstones. Lenticular lamination together with bigradational grading of coarser mudstones and presence of discontinuous wavy lamination, are all supportive observations of the presence of bottom current activity at the depositional site. In addition, flume experiments described in the literature have shown parallel laminated mudstones to represent mud transported and deposited at velocities capable of transporting and depositing sand. Combining all these features, the depositional environment of the mudstones in the cores seems to reflect rather dynamic conditions in terms of changing flow activity at the sediment-water interface. In addition, published TOC data for core 6307/07-U-02 have shown a correlation can be made between the TOC content and the facies distribution in terms of how coarse-grained the mudstones are. A characteristic depositional pattern of cyclic sedimentation in the form of characteristic depositional patterns can be recognised in the mudstones of core 6307/07-U-02, reflected in the various facies present in different core sections.