Chapter 58 Evidence of late Neoproterozoic glaciation in the Caledonides of NW Scandinavia

Abstract

Abstract The northwestern part of the Scandinavian Caledonides, formed by SE- to ESE-directed thrusting through the Neoproterozoic W. Baltica continental shelf, contains numerous small and often isolated outcrops of diamictite and associated strata. No precise biostratigraphic or isotopic data are available to constrain the age of these sediments, but, on the basis of their stratigraphic position, most are correlated with the Mortensnes Formation (Fm.) in E. Finnmark and also presumed to be of glaciogenic origin. The Mortensnes Fm. has been correlated with the 580 Ma Gaskiers glacial event on the basis of δ 13 C isotope studies. Structurally, the deposits occur in the Autochthon (below the Torneträsk Fm.), within an external imbricate zone (Lower Allochthon), within cover successions lying unconformably on allochthonous basement (Window Allochthon) palaeogeographically derived from below or outboard of the Lower Allochthon and, more rarely, within the Middle Allochthon, derived from outboard of the Window Allochthon. Evidence for a glaciogenic origin is typically poor or lacking. Only in the Komagfjord Antiformal Stack (Window Allochthon), where an up to 40-m-thick succession of three fining upwards cycles has been mapped, are the deposits comparable in thickness and complexity to the Mortensnes Fm. Other sequences are sometimes <1 m thick and unconformably overlain by post-‘glacial’ deposits. The Vakkejokk Breccia, a submarine slump in the Torneträsk area of the Autochthon closely underlies the correlative Precambrian–Cambrian lithostratigraphic boundary in E. Finnmark but overlies the first appearance of the boundary marker fossil Treptichnus pedum . Although sometimes interpreted as periglacial, this seems unlikely in view of the 30–50° palaeolatitude during deposition. Calcite nodules (<1 cm size) in the Vakkejokk Breccia have previously been interpreted as glendonite, but the microstructure and palaeolatitude makes this unlikely; they are likely a replacement of gypsum. Diamictites of uncertain origin have also been found in the Ediacaran Lower Siltstone Member of the Torneträsk Fm. and unconformably under the ?Lower Cambrian Lomvatn Fm. in the Komagfjord Antiformal Stack.