Tide-dominated offshore sedimentation, Lower Cambrian, north Norway

The Duolbasgaissa Formation, Lower Cambrian, of northern Norway consists of 550 m of mineralogically and texturally mature sandstones with subordinate siltstones, mudstones and conglomerates. Four facies are defined on the basis of grain size, bed thickness and sedimentary structures. Facies 1–3 consist of a variety of erosively-based, cross-stratified and parallel-stratified sandstones interbedded with siltstone and mudstone. Many of these sandstones show evidence of deposition from waning currents. Facies 4 consists of trough cross-bedded sandstones with sets up to 4 m thick. Symmetrical ripples and bioturbation are ubiquitous. Bipolar palaeocurrent distributions are common to all facies and one mode is usually strongly dominant. Lateral facies variations and sedimentary structures suggest that deposition took place in a tide-dominated, offshore, shallow marine environment in which maximum sediment transport probably occurred when storm generated waves enhanced tidal currents. The four facies are thought to represent the deposits of various parts of tidal sediment transport paths such as exist in modern seas around Great Britain. Small scale coarsening upward sequences may represent the superposition of facies independently of changing water depth. Lack of information prevents a detailed palaeogeographic reconstruction. It is suggested that sand body shape is not accurately predictable.