Facies and fluvial architecture of a high-energy braided river: the Upper Proterozoic Seglodden Member, Varanger Peninsula, northern Norway

Abstract

The Seglodden Member (Late Proterozoic), as studied in two areas of the Varanger Peninsula, northern Norway, is dominated by sand-rich braided river deposits. A quantitative summary of described facies emphasises the dominance of structures deposited under high-energy conditions, and a general absence of fine-grained deposits and visible small-scale sedimentary structures. The sigmoidal and concave-up cross-stratified, planar horizontally stratified and massive sandstone facies represent 80–95% of the total vertical thicknesses of the measured sections. Architectural elements are defined as ‘sheet sandstone bodies’ and ‘channel forms and fills’ and comprise simple bars, transitional simple bar-dune complexes, dune complexes, downstream accretion complexes and various types of channel deposits. The Seglodden fluvial system was operating in a non-vegetated setting with well exposed sediment sources, probably of a sedimentary origin. This situation gave a very erratic discharge system with rapid runoff, high denudation rate and very high sediment discharge. Increased flow within channels was compensated for by a broadening of the channels, and it is inferred that the channels were very wide and shallow relative to their depths. Bounding surfaces of at least six different orders (0th–5th) are interpreted within the studied successions, with the third-order surface as the lowest-order surface that bound architectural elements as defined in this study. The highest-order surface (5th order) is probably the result of channel belt switching due to a combination of sedimentary processes and tectonic subsidence of the basin. The east and northeasterly palaeocurrent trend in the Seglodden area compared to the southeasterly trend in the Smellror area indicates that the axis of the depositional system changed significantly in direction basinwards over the 15–20 km distance between the two study areas. The controlling mechanisms for this may be autocyclic as well as allocyclic processes, although the tectonic influence on sedimentation was probably significant.