The Proterozoic (1.85 Ga) Älvestorp Conglomerate, Bergslagen, Central Sweden

Abstract

The Älvestorp conglomerates, deposited in an alluvial fan setting, form part of the Svecofennian orogenic belt in west Bergslagen, south central Sweden and are estimated to be as old as c. 1.85 Ga. Reaching a thickness of one kilometre, their architecture and form suggest an alluvial origin. Along sections, massive conglomerates often grade into pebbly mudstones and greenschist facies slates, while pure slates with dolomite concretions and olistolites occur in dark slatey mudstones on the eastern shore of lake Brunnsjön. Inner fan trenches are filled with massive, clast-supported conglomerates that contain more than 80 percent epiclastic tuffaceous material. The Älvestorp conglomerate is therefore classified as the product of a Proterozoic stream-flow channel and debris flow, or alluvial fan. The Grythyttan Basin to the north originated by extension after the first of two orogenic stages of Bergslagen.

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

“…The volcaniclastic facies are in two places interrupted by glacial and periglacial deposits, the latter described by Kuipers et al (2013) and Vandenberghe et al (2020). The previously described alluvial fan south of Grythyttan (Kuipers & Beunk, 2014) is now recognised as part of a glacial sedimentary-facies assemblage. The evidence consists of the occurrence of (1) megaclasts and sporadic dropstones in a heterolithic diamictite, (2) flatiron-type clasts in the overlying debris-flow conglomerate, (3) mainly rounded clasts in an angular matrix and ( 4) occasional striatons on cobble surfaces in this Torrvarpen conglomerate.…”

“…The overlying Torrvarpen conglomerates follow discordantly to the east as a submember of the Saxhyttan member. The lens-shaped conglomerate units were previously interpreted as part of an alluvial-fan type sedimentary facies by Kuipers & Beunk (2014). The conglomerate lenses represent transverse crosssections through debris flows set in a poorly exposed sandy, pebbly and silty matrix.…”

“…This section concentrates on observations in the basal part of the pebbly slates forming the Saxhyttan Member of the Grythyttan Slate Formation, here named the Brunnsjön diamictite (Tucker, 2003); it provides the main argument of our contribution suggesting that we are dealing with a glacial sedimentary facies association previously described as an alluvial fan by Kuipers & Beunk (2014).…”

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

“…The Älvestorp region forms the southern part of the GSB, where a nearly two kilometre-thick proximal part of an alluvial fan conformably overlies the pebbly grey slates of the Saxhyttan member to the east between Hasselhöjden and Älvestorp, and extending to the south and north over 8 km (Fig. 2; Kuipers & Beunk, 2014). Reliable evidence for the overall younging of the sequence can be derived from an E-W transect in the southern part of the GSB, across the Torrvarpen conglomerates and volcanic rocks on either side, between lakes Brunnsjön and Sirsjön (Figs.…”

“…Kuipers (1987) identified the grey slates as volcaniclastic turbidites. The conglomeratic member consists of sandy and silty siliciclastics, interspersed with intrabasinal conglomeratic lenses, and was interpreted as an alluvial fan by Kuipers & Beunk (2014). The GSB displays a pronounced lateral lithological variation: grey slates in the north give way to black slates in the centre, and to the alluvial fan in the south (Fig.…”