Cryptic Disc Structures Resembling Ediacaran Discoidal Fossils from the Lower Silurian Hellefjord Schist, Arctic Norway

Abstract

The Hellefjord Schist, a volcaniclastic psammite-pelite formation in the Caledonides of Arctic Norway contains discoidal impressions and apparent tube casts that share morphological and taphonomic similarities to Neoproterozoic stem-holdfast forms. U-Pb zircon geochronology on the host metasediment indicates it was deposited between 437 ± 2 and 439 ± 3 Ma, but also indicates that an inferred basal conglomerate to this formation must be part of an older stratigraphic element, as it is cross-cut by a 546 ± 4 Ma pegmatite. These results confirm that the Hellefjord Schist is separated from underlying older Proterozoic rocks by a thrust. It has previously been argued that the Cambrian Substrate Revolution destroyed the ecological niches that the Neoproterozoic frond-holdfasts organisms occupied. However, the discovery of these fossils in Silurian rocks demonstrates that the environment and substrate must have been similar enough to Neoproterozoic settings that frond-holdfast bodyplans were still ecologically viable some hundred million years later.

Anticlockwise metamorphic pressure–temperature paths and nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision

“…460 to 450 Ma) was followed by early Silurian (ca. 445-435 Ma) granulite facies metamorphism and magmatism (Gromet et al, 1996;Majka et al, 2012;Klonowska et al, 2013Klonowska et al, , 2017 at similar conditions to those recorded in the Nordmannvik Nappe. However, no evidence for Ordovician high-pressure metamorphism has been observed in the RNC (sillimanite inclusions in garnet suggest pre-Silurian high temperatures instead).…”

“…Intrusive rocks of early Silurian age are widely recognized along the length of the Caledonides (e.g. Tucker et al, 1990;Gromet et al, 1996;Vaasjoki and Sipilä, 2001;Andréasson et al, 2003;Kirkland et al, 2005;Corfu et al, 2011;Majka et al, 2012;Klonowska et al, 2013Klonowska et al, , 2017. In north-ern Norway, they are found in the Narvik Nappe Complex (mafic Rånå intrusion; 437 ± 0.5 Ma; Tucker et al, 1990), the Vaddas or Kalak Nappe (mafic Halti Igneous Complex; 434 ± 5 and 438 ± 5 Ma; Vaasjoki and Sipilä, 2001;Andréasson et al, 2003), and granitic and gabbroic rocks on Magerøy (intruded between 440-435 Ma; Kirkland et al, 2005;Corfu et al, 2011).…”

An anticlockwise metamorphic P-T path and nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision

“…12D, 13B), recording stacking of the Vaddas Nappe over the KNC. The age of S2 metamorphism is 15 consistent with ages for Caledonian metamorphism in the equivalent Magerøy Nappe (Andersen et al, 1982;Corfu et al, 2006Corfu et al, , 2011Kirkland et al, 2005Kirkland et al, , 2016, Narvik Nappe Complex (Augland et al, 2014) and upper KNC (Kirkland et al, 2007a;Gasser et al, 2015).…”

“…Caledonian metamorphic or deformation history. The Magerøy Nappe, considered equivalent to the Vaddas Nappe, was sedimented in the earliest Silurian based on fossil evidence and an age of 438 ± 4 Ma for volcaniclastic rocks, indicating deposition almost synchronously with emplacement of mafic-ultramafic and felsic magmas, and implying rapid burial of sediments (Andersen, 1981;Kirkland et al, 2005Kirkland et al, , 2016. Although no depositional ages for the Vaddas rocks associated with Solid Earth Discuss., https://doi.org /10.5194/se-2018-74 Manuscript under review for journal Solid Earth Discussion started: 31 July 2018 c Author(s) 2018.…”

“…1B; Gayer & Roberts, 1973;Andersen, 1981;Lindahl et al, 2005;Corfu et al, 2006Corfu et al, , 2011Kirkland et al, 2005Kirkland et al, , 2016. Volcaniclastic and intrusive rocks in the Magerøy Nappe give early Silurian ages (442-435 Ma) suggesting 25 almost synchronous deposition and intrusion (Robins, 1998;Corfu et al, 2006Corfu et al, , 2011Kirkland et al, 2005Kirkland et al, , 2016.…”