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

Abstract

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.

Giant cross-beds in the Neoproterozoic Port Askaig Formation, Scotland: implications for snowball Earth

“…Levell, 1980;Kamp et al, 1988;Smith and Taverner-Smith, 1988;Ashley, 1990;Dalrymple, 1992;Smith, 1992). Herring-bone cross-bedding or smaller-scale cross-stratification that dips upstream relative to the larger-scale slip faces on which it developed (indicative of reverse flow) were not observed in the Port Askaig Formation.…”

“…Thick successions of mature cross-bedded sandstone have been documented in other Late Precambrian marine successions (Anderton, 1976;Levell, 1980;Lindsay, 1999;Tirsgaard, 1993). Based on modern analogues, the cross-bedded sand accumulated under the influence of unidirectional currents with speeds of between 0.5 and 3 m s À 1 (Ramsay et al, 1996;Ikehara, 1998;Berné et al, 1998).…”

A proposed continental margin in the precambrian of Western Kenya

“…Their initial position in the sedimentary sequence is uncertain. Similar substantial thicknesses of mature elastic sediment such as the Trias quartzarenite of the western Alps (RlcaaRDS, 1981), the Lower Palaeozoic Peninsula formation of South Africa (HoBDAY and TANKARD, 1978), the Cambrian quartzite of NW Scotland (SWETT et al, 1971), the late Precambrian Jura quartzite of Scotland (ANDERTON, 1976) and most of the lower Sandford formation, Finnmark (LEvELL, 1980) have all been interpreted as shallow-marine sediments deposited in a tide-dominated environment. Although no eonfirmat0ry sedimentary structures can be recognised in the highly deformed quartzite a similar interpretation is suggested for the Cherangani sequence.…”

Precambrian basin-margin fan deposits: Mesoproterozoic Bagalkot Group, India

“…Subfacies E B at the distal section depicts pulsatory transgression of the standing body of water, and the granular lag at its base presumably formed when the standing body of water encroached upon the fan. Laterally discontinuous granular lags within the subfacies are possibly of wave-winnowing origin ( [Clifton, 1973] and [Levell, 1980]). Fig.…”