Abstract

A 30m succession of late Precambrian quartzose sandstones at Skallneset, Finnmark, north Norway, is subdivided into five sequences which coarsen upward and contain three facies which occur in a preferred vertical order of deposition A --> B --> C. Overlying and underlying strata indicate a shallow marine environment. Facies A (siltstone facies) consists of horizontally bedded siltstones and sandstones, with abundant symmetrical ripples trending predominantly NW-SE. It is interpreted as resulting from alternating periods of deposition from suspension during fair weather, and of wave oscillation during storms. Facies B (major foreset facies) consists of sandstones with small-scale trough cross-bedding directed northward, separated by westward inclined surfaces which are largely erosional in origin. East-west trending symmetrical ripples sometimes occur on these surfaces. This facies is interpreted as the product of a westward buildout of sandbars by deposition from northward flowing semi-permanent currents, interrupted by occasional storms which truncated the western margins of the bars. Facies C (small-scale trough cross-bedded facies) consists of sandstones containing undulatory scoured surfaces, trough cross-bedding dipping northwestwards, and ripples striking N-S and NW-SE. It is interpreted as due to northward flowing semi-permanent currents interrupted by occasional storms. The characteristics of the individual sequences appear to be the result of two distinct regimes, one dominated by fair weather (constructional) and one by storms (destructional). The fair weather regime is indicated by the northward dipping trough cross-beds deposited by semi-permanent currents. The storm regime is indicated by the large-scale westward inclined surfaces. The most likely modern comparison is with the second order ridge and swale system which occurs on large submarine ridges. It is suggested that these too might, in some cases, be created by a combination of semi-permanent currents and occasional storms.

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