A micromorphological perspective on the Neoproterozoic Smalfjord and Mortensnes Formation diamictites—Varangerfjord, Norway

Abstract

Many Neoproterozoic diamictites are glacial in origin (Young, G.M. 2018. Precambrian glacial deposits: their origin, tectonic setting, and key role in Earth evolution. In Past glacial environments. 2nd ed. Edited by J. Menzies, and J.J.M. van der Meer. Elsevier. Chapter 2, pp. 17–45. doi:10.1016/B978-0-08-100524-8.00001-4). However, many are thought to be nonglacial mass movement deposits. Discussion on the nature of Neoproterozoic climate change focuses on the controversial origin of diamictite-bearing strata and the criteria to determine the extent and nature, or not, of glacial influence on their deposition. At the macroscale, sediments (diamictites) deposited beneath palaeo-ice masses or debris flows are massive, lack few visible signs of stratification, or deformation signatures. Microscopic investigations have resulted in specific sedimentary structures being identified, indicative of stress during glacial deposition, or developed in transit within mass movements. Case examples derived from the Neoproterozoic in Varanger, Norway have been investigated that provide strain signatures of “unique sequences” of microstructures. Micromorphology, relatively new to Precambrian research, can deliver far greater detail on the depositional and deformation histories recorded by these diamictites than obtained from macroscale studies alone. The technique provides evidence of composite deformation histories of potential glacigenic and nonglacigenic sequences. Many microstructures can be observed in most diamictites. It can be demonstrated that diamictites, both glacigenic and non-glacigenic, contain most of these microstructures in varying levels of abundancy and can be separated on specific “sets” of summative structures present. At Varanger, 12 samples were analyzed for thin sectioning. As illustrative of the thin sections as a whole, 6 are presented here that offer a microscale perspective on the diamictites from this area and establish that both the Smalfjord and Mortensnes Formation diamictites are of glacial terrestrial or subaqueous origin.