The Palaeoproterozoic Grythyttan Field in the Svecofennian Orogen, West Bergslagen, Central Sweden: structure, stratigraphy and age

Gerrit Kuipers,Frank F Beunk,Frederik M Van Der Wateren,Keewook Yi

doi:10.17850/njg98-3-05

Abstract

A number of discrete slate belts of limited size occur in the Paleoproterozoic volcano-sedimentary Bergslagen Group of western Bergslagen in the Fennoscandian shield of south central Sweden. The Grythyttan Slate Belt, studied for more than a century, forms a single basin with the nearby Saxan Slate Belt. We use the lithostratigraphy of the Grythyttan Slate Formation of the Grythyttan belt, based on basin scale sedimentary facies associations of the volcaniclastic sediments in time and space, as a method to interpret overall tectonic structure. Contrary to traditional views, we reconstruct the GSB as a single overturned limb of a km-scale anticline with horizontal axis in the hanging wall of an east vergent thrust fault, reactivating a listric extensional fault. The fold connects the GSB to the neighboring SSB. Folding and thrusting were related to tectonic closure of a volcanic back- or intra-arc basin. The early folds were subsequently affected by strike-slip shearing and folding around vertical fold axes, which partitioned preferentially into the least competent lithologies (slates and marbles), significantly modifying the map appearance of the slate belt. The late shearing and folding resulted from accretion of Bergslagen onto the Fennoscandian continental margin during the late, Svecobaltic phase of the Svecofennian orogeny. The GSB forms a thin, intraformational wedge in the Bergslagen Group and represents a relatively short interlude with a conglomeratic alluvial fan and turbiditic volcaniclastics followed by more felsic volcanic rocks resembling those of the Bergslagen Group. We suggest that the GSB, and by inference the other slate belts, stem from calderas, as either terrestrial volcanic lakes or shallow submarine eruption centers. The conglomerates are pre-orogenic, not a post-orogenic molasse, as traditionally conceived, and their clast fabric has resulted from sedimentary processes only. Newly determined SIMS U-Pb zircon ages constrain the age of the GSB at ca. 1895 Ma.

Highlights

IntroductionThe Grythyttan Field (GF) in the western Bergslagen region of south-central Sweden is located in the southwestern part of the 1.9–1.8 Ga Svecofennian ( referred to as Svecokarelian) Orogen of the Fennoscandian Shield (Fig. 1)
The Grythyttan Field (GF) in the western Bergslagen region of south-central Sweden is located in the southwestern part of the 1.9–1.8 Ga Svecofennian Orogen of the Fennoscandian Shield (Fig. 1)
While Högbom (1910) and Sundius (1923) and their followers advocated a tectonic model of upright isoclinal folding whereby the Grythyttan and Saxån slate belts form the cores of synclinoria (Fig. 1), we find that the strata in the Grythyttan Slate Belt (GSB)

Summary

Introduction

The Grythyttan Field (GF) in the western Bergslagen region of south-central Sweden is located in the southwestern part of the 1.9–1.8 Ga Svecofennian ( referred to as Svecokarelian) Orogen of the Fennoscandian Shield (Fig. 1). Lundström (1995a) largely followed Sundius’ (1923) stratigraphy of the Grythyttan field, i.e., a one-dimensional, upward sequence from older greywackes to younger slates and conglomerates, not realising the importance of lateral facies changes between the grey and black slates in the northern and central parts of the GSB and the more felsic volcaniclastic sediments in its southern part, in the Lonnhöjden area (see Fig. 2). Rather than faulting (as in Wiklander & Lundström, 1991), we interpret the western boundary as a sedimentary contact and formational boundary between underlying well-bedded volcanic deposits exhibiting km-scale east-facing channels to the west, and stratigraphically overlying pebbly slates of the Saxhyttan member to the east (Fig. 3). The weighted mean 207Pb/206Pb age of sample Bk1307 (site 2, Figs. 3 & 8) could be taken as the most precise approximation of the depositional ages, at 1894.5 ± 2.8 Ma

Findings

Discussion

Conclusions