Tide‐ and wave‐dominated inshore and shoreline sequences from the late Precambrian, Finnmark, North Norway

Abstract

ABSTRACTThe Late Precambrian Ekkerøy Formation of Finnmark, northern Norway represents a 40 m thick coarsening upward, prograding deltaic sequence. The upper 6 m of this formation consists of two small‐scale fining upward sequences representing two contrasting shoreline environments. These two sequences have been the focus of a detailed facies analysis based on grain size, bed thickness and sedimentary structures. The study area comprises a series of horizontally bedded rocks which are continuously exposed for 3 km enabling individual facies and sedimentary structures be to traced‐out laterally.The lower sequence exhibits a wide variety of tidal features including directional bimodality of both large‐scale and small‐scale cross‐stratification, herringbone cross‐stratification, various sandstone‐mudstone alternations including lenticular and wavy bedding, and a general fining upward tendency. A detailed field analysis of the internal structures within the large‐scale cross‐stratified unit is presented and is interpreted as a sub‐tidal channel complex. The associated sandstone‐mudstone facies represents intertidal flats built up by tidal and storm‐generated currents.The upper sequence displays a wide variety of wave‐formed structures including wave‐ripples and swash‐lamination. Palaeocurrent data indicates a dominance of shoreward migrating megaripples and wave‐ripple crests oriented approximately parallel to the inferred shoreline with a distinct longshore component. The sequence is interpreted as a beach‐shoreface environment. In terms of sedimentary structures, palaeocurrent patterns and facies distributions, the sequence bears a remarkable similarity to the high‐energy, non‐barred shoreline of the modern Oregon coast U.S.A., described by Clifton, Hunter & Phillips (1971).