Glacial influence on Neoproterozoic sedimentation: the Smalfjord Formation, northern Norway
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ABSTRACT There is much debate regarding the intensity and geographic extent of glaciation during the Neoproterozoic, particularly in response to recent geochemical work suggesting that the Neoproterozoic earth was at times ice covered from equator to poles (the ‘Snowball Earth’ hypothesis). A detailed sedimentological analysis of the Neoproterozoic Smalfjord Formation of northern Norway was conducted in order to determine the extent and intensity of glacial influence on sedimentation. In the Tarmfjorden area, the Smalfjord Formation consists of a stacked succession of diamictites interbedded with fine‐grained laminated mudstones containing rare outsized clasts. Diamictites and interbedded mudstones are interpreted as the product of subaqueous mass flows generated along the basin margin. In the Varangerfjorden area, chaotically interbedded diamictites, conglomerates and sandstones are overlain by a thick succession of stacked sandstone beds; onediamictite unit at Bigganjargga overlies a striated pavement. The Varangerfjorden outcrops appear to record deposition on a subaqueous debris apron. Although diamictites contain rare striated and faceted clasts, suggesting a glacial sediment source, their origin as subaqueous mass flows prevents the interpretation of ice mass form or distribution. Rare lonestones may be associated with floating ice in the basin, which may be of glacial or seasonal origin. Glacial ice may have contributed poorly sorted glacial debris to the basin margin, either directly or through fluvioglacial systems, but there is no evidence of direct deposition by ice at Varangerfjorden or Tarmfjorden. The overall fining‐upward trend identified in the Smalfjord Formation and overlying Nyborg Formation is consistent with depositional models of rift basin settings. This fining‐upward trend, the predominance of mass flow facies including breccias associated with scarps and the evidence for extensional tectonic activity in the region suggest that tectonic activity may have played an important role in the development of this Neoproterozoic succession. The Smalfjord Formation at Tarmfjorden and Varangerfjorden does not exhibit sedimentological characteristics consistent with severe glacial conditions suggested by the snowball Earth hypothesis.
