A late Precambrian tidal shelf deposit, the Lower Sandfjord Formation, Finnmark, North Norway


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    Koninklijke Shell Exploratie en Produktie Laboratorium, Volmerlaan 6, Rijswijk (Z-H) The Netherlands.


The Lower Sandfjord Formation is a 1.5 km thick late Precambrian sandstone. It is a remarkably homogeneous unit consisting largely (98%) of cross-bedded, texturally and mineralogically mature, coarse or medium sandstone, and is interpreted as a shallow marine deposit. This interpretation is based on the maturity, the exclusively tabular bed geometry, occasional sets of herring-bone cross-bedding and most importantly, the abundance of sheet-like pebble layers only 1–5 grain diameters thick and sometimes overlain by thin siltstone drapes. Various different types of compound cross-bedding, all of which show evidence of reversing currents, are interpreted as sub-tidal sandwaves. These sets range in thickness from 0.5 to 14 m, and in conjunction with the overall abundance of cross-bedding probably indicate strong tidal currents. A tide-dominated current regime is also considered essential to explain the derivation of such large quantities of sand from the contemporary coasts. It is suggested that sand transport offshore took place during the erosional transgression of abandoned delta lobes. However, the predominance of a single, easterly, mode in the palaeocurrent patterns suggests that the tidal currents were reinforced by some other current system. The predominantly unimodal palaeocurrent patterns and the coarse, sand-rich nature of the succession, taken together with the thickness do not superficially seem likely characteristics for a shallow marine sequence. Nevertheless this study appears to demonstrate that such deposits were formed on tidal shelves in at least late Precambrian time.