Sedimentology of Late Precambrian storm-influenced pro-deltaic successions: Varanger, eastern Finnmark

Abstract

This study investigates the sedimentology of the Klubbnasen and Andersby formations of the Late Precambrian (Neoproterozoic) succession in Varanger, eastern Finnmark. Additionally, the lower parts of the Fugleberget and Paddeby formations overlying the Klubbnasen and Andersby formations, respectively, were superficially investigated in order to provide stratigraphic context. The investigated formations are part of the Vadsø Group, which have previously been interpreted to represent a syn-rift succession of Late Riphean age. The present study is based on extensive field work along the northern coastline of the Varangerfjorden and document storm-bed (i.e. tempestite) variability within the Klubbnasen and Andersby formations, and interpret their depositional environments and sequence stratigraphic development. Ten lithofacies were identified based on outcrop investigations, which further are grouped into four lithofacies associations reflecting outer shelf deposits (FA1), storm-influenced prodelta deposits (FA2), storm-influenced delta front deposits (FA3) and braided river deposits (FA4; only occurring in the Fugleberget and Paddeby formations). The numerous tempestite beds of Klubbnasen and Andersby formations were mainly governed by wave-enhanced turbidity currents and storm waves producing a variety of storm-bed architectures (e.g. wave-modified turbidite). Deposition of a typical tempestite (e.g. wave-modified turbidite) was typically preceded by erosion of strong, but decelerating, offshore-directed turbidity currents, which were followed by a gradually more oscillatory-dominated flow governed by storm waves. The beds are characterized by high aggradation rates where the bed-architecture was controlled by the temporal and spatial evolution of the two transporting agents. Seafloor topography may also have affected the deposition. The abundancy of soft-sediment deformation structures, including the presence of a laterally extensive slump deposit, suggest a trigger mechanism related to seismic activity. The cyclic stacking trend of that the Klubbnasen-Fugleberget and the Andersby-Paddeby formations and the sharp, erosive boundaries within these stratigraphic couplets, may suggest deposition within a seismically active rift-basin where the basin fill evolution was largely governed by hanging wall-subsidence and limited sediment supply (due to small catchment area).Periods of increasing subsidence rates may have led to the formation of marine flooding surfaces (as observed in the Klubbnasen and Andersby formations), while sudden uplifts of the footwall resulted in episodes of extensive erosion of the underlying deltaic units and the formation subaerial unconformities (as observed at the bases of the Fugleberget and Paddeby formations). Tectonic uplift and exposure of larger areas eventually promoted the generation of extensive braided river systems (as represented by the cross-bedded sandstone of the Fugleberget and Paddeby formations).