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

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ABSTRACT

The 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).

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