A 1.9 Ga glacial sedimentary-facies association at low palaeolatitude in the Bergslagen Group, Grythyttan Field, Central Sweden

Gerrit Kuipers,Keewook Yi,Frank F Beunk

doi:10.17850/njg101-1-1

Abstract

© the authors.Glacial sedimentary facies are recognised in the Palaeoproterozoic (Orosirian) Bergslagen Group in central Sweden, described previously as post-orogenic conglomerates in the Grythyttan Field. The evidence is provided by the presence of macro boulders in a megaclastic to conglomerate rock formation or diamictite around lake Brunnsjon, south of Grythyttan. Pebble to cobble size conglomerates discordantly overlie a megaclastic rock formation, with facetted clasts resembling flatirons. The sedimentary facies assemblage resembles an ice-front resedimented sequence, previously interpreted as an alluvial fan by the writers. A glacial interpretation is supported by evidence of permafrost and related periglacial structures in coeval, 1895 ± 5 Ma (U–Pb zircon age), metavolcaniclastic rocks in the Bergslagen Group. The association of glacial sedimentary facies, permafrost and banded ironstones in the low-latitudinal Bergslagen Group suggests its similarity to better known ‘Snowball Earth’ deposits from Palaeoproterozoic and particularly Neoproterozoic times.

Highlights

Severe, occasionally global (‘Snowball Earth’) glaciations occurred at several times during the Proterozoic Eon, but were conspicuously absent during a long Palaeo–Mesoproterozoic non-glacial interval (c. 2.3 Ga to 750 Ma; Eyles, 2008), known as the ‘Proterozoic glacial gap’ (Fig. 1)
We suggest that the entire Grythyttan Slate Formation can be interpreted as a progressive sequence of facies from ice-proximal to ice-distal, bearing a close similarity to the Neoproterozoic glacigenic olistostrome complexes reported from Death Valley, California, where debris flows, underlying diamictite and overlying sandstone channels, form an ice-contact depositional system or fan (Le Heron et al, 2014, 2017)
We offer a glacigenic interpretation for the megaclastic sedimentary-facies association in the Palaeoproterozoic Bergslagen Group of the Grythyttan Field

Summary

Introduction

Occasionally global (‘Snowball Earth’) glaciations occurred at several times during the Proterozoic Eon, but were conspicuously absent during a long Palaeo–Mesoproterozoic non-glacial interval (c. 2.3 Ga to 750 Ma; Eyles, 2008), known as the ‘Proterozoic glacial gap’ (Fig. 1). 2.3 Ga to 750 Ma; Eyles, 2008), known as the ‘Proterozoic glacial gap’ (Fig. 1). 1.9 Ga old, Orosirian, volcanosedimentary Bergslagen Group (Beunk & Kuipers, 2012) in the Svecofennian orogen of the Fennoscandian Shield in Sweden hints at an as yet little known glaciation during an apparent glacial gap. Previous paleomagnetic studies have assigned a tropical palaeolatitude to the Fennoscandian Shield as a whole (Pesonen et al, 2012; Mertanen & Pesonen, 2012), along a planetary scale, active and accretionary continental margin which assembled the supercontinent Columbia. & van der Wateren, F.M. 2021: A 1.9 Ga glacial sedimentary-facies association at low palaeolatitude in the Bergslagen Group, Grythyttan Field, Central Sweden.

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