Tidally influenced clastic epeiric sea at a Mesoproterozoic continental margin, Rjukan Rift Basin, southern Norway

Abstract

Precambrian epeiric sea paleoenvironments record information about ancient tidal conditions and nearshore sedimentation patterns. The Mesoproterozoic Brattefjell Formation, part of the Rjukan Rift Basin in southern Norway, presents diagnostic criteria of an ancient tidal influence in a shallow clastic epeiric sea environment during the post-rift stage of the sedimentary basin. Eight different sedimentary lithofacies/subfacies and two genetically related lithofacies associations were recognized from the formation. The conglomerate lithofacies records a transgressive ravinement surface due to wave scouring. The sandstone lithofacies consists of six subfacies: ripple-marked, planar cross-bedded, trough cross-bedded, low-angle cross-bedded, and horizontal laminated sandstones. The sandstone lithofacies records sedimentation by shallow current flows or oscillatory waves of lateral migrating straight- or slightly sinuous-crested sandbars and channel-fill deposits. The associated bi-polar cross-bedding, slump-folds, ladder-back type ripples, shrinkage cracks, double-mud drapes, iron ooids, load casts, and flame structures indicate variation of the flood-ebb cycles in tidal sandbars, rapid sedimentation, and sediment compaction. Tidal bundle measurements suggest that 16–18 bundles were formed during the neap-spring cycle, indicating mixed semidiurnal tides. A mudstone lithofacies (two subfacies) records waning current strength deposits, channel abandonment, and either vertical aggradation of the channels or coastal plain deposits. The lithofacies associations reflect fluctuating energy conditions in a shallow clastic shoreline influenced by tides and waves. At least 23 shallowing upward cycles caused by autocyclic tidal water level changes were recognized from the lower part of the formation.Based on the lithofacies assemblages, the Brattefjell Formation can be subdivided into sandy subtidal, lagoon-estuary, and shallow marine shoreface-offshore environments deposited above the fairwater wave base. The Brattefjell Formation records millions of years of fluctuating water levels with an overall transgressive trend, indicating allocyclic patterns caused by regional subsidence. The formation is interpreted to represent a tidally influenced epeiric sea in which shallow water levels extended over part of a continent and it was associated with marine transgression. The shoreface was fed sediment by alongshore and coastal current drifts during shoreface transgressive erosion, in a low shoreline gradient with reduced sediment input and lack of major braided-fluvial channel(s).