Sedimentology and architecture of a deep-water turbidite system; Kongsfjord Formation, Finnmark, northern Norway

Abstract

The Kongsfjord Formation is a Late Precambrian deep-marine sedimentary succession exposed along the northern coastline of the Varanger Peninsula, northern Norway. Several studies in the 1970s and 1980s led to the recognition of the Formation as an ancient submarine fan system and to the identification of several architectural elements, including lobes and channels. However, there are no published work documenting bed type variability in detail and recent investigations of the lobes are lacking. This study is based on outcrop data, including stratigraphic logs and digital outcrop models collected in three localities. The recognition of eight bed types suggests that the Formation was deposited by various sediment density flows including high- and low-density turbidity flows, transitional flows and cohesive debris flows. The investigated outcrop sections comprise stacked lobes that record deposition in unconfined settings. Lobes have a two-fold architecture with a thin-bedded lower part and a thick-bedded upper part that record deposition in lobe fringe, off-axis and axis settings. Lobes commonly show thickening upward trends interpreted as recording lobe progradation. In the most proximal investigated areas, scours record sediment bypass and deposition in confined and semi-confined settings. Hybrid event beds interpreted as being deposited by transitional flows that transformed from turbulent to laminar flow conditions, occur in lobe fringe and lobe off-axis settings of the most distal areas. Two sub-types of hybrid event beds are observed throughout the Formation, suggesting that different mechanisms led to flow transformation. The distribution of hybrid event beds throughout the Kongsfjord Formation may reflect periods of slope disequilibrium and the dominant stacking patterns of lobes. Differences in lobe dimensions and stacking patterns between the investigated outcrop sections indicate differences in flow evolution and feeder channel geometry and stability. Thin, aggradationally to compensationally stacked lobes are deposited in slope-proximal settings. Thick, aggradationally to compensationally stacked lobes are deposited in distal basin-floor settings. The Kongsfjord Formation accumulated in a rift basin as part of N–NE prograding slope to basin-floor system comprising the overlying fluvio-deltaic successions of the Båsnæringen Formation. High sedimentation rates and slope instability are likely to be the main controls for the deposition of the Kongsfjord Formation turbidite system as coalescent lobes on the basin-floor.