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

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Cryptic Disc Structures Resembling Ediacaran Discoidal Fossils from the Lower Silurian Hellefjord Schist, Arctic Norway RESEARCH ARTICLE Cryptic Disc Structures Resembling Ediacaran Discoidal Fossils from the Lower Silurian Hellefjord Schist, Arctic Norway Christopher L. Kirkland1*, BreandaÂn A. MacGabhann2, Brian L. Kirkland3, J. Stephen Daly4 1 Department of Applied Geology, (Centre for Exploration Targeting ± Curtin Node and Core to Crust Fluid Centre), Curtin University, Perth, Australia, 2 Department of Geography, Edge Hill University, Ormskirk, Lancashire, England, 3 Independent Researcher, Coleraine, N. Ireland, 4 UCD School of Earth Sciences and UCD Earth Institute, University College Dublin, Dublin, Ireland a11111 * [email protected] Abstract The Hellefjord Schist, a volcaniclastic psammite-pelite formation in the Caledonides of Arc- OPEN ACCESS tic Norway contains discoidal impressions and apparent tube casts that share morphologi- Citation: Kirkland CL, MacGabhann BA, Kirkland cal and taphonomic similarities to Neoproterozoic stem-holdfast forms. U-Pb zircon BL, Daly JS (2016) Cryptic Disc Structures geochronology on the host metasediment indicates it was deposited between 437 ± 2 and Resembling Ediacaran Discoidal Fossils from the Lower Silurian Hellefjord Schist, Arctic Norway. 439 ± 3 Ma, but also indicates that an inferred basal conglomerate to this formation must be PLoS ONE 11(10): e0164071. doi:10.1371/journal. part of an older stratigraphic element, as it is cross-cut by a 546 ± 4 Ma pegmatite. These pone.0164071 results confirm that the Hellefjord Schist is separated from underlying older Proterozoic Editor: Roberto Macchiarelli, Universite de Poitiers, rocks by a thrust. It has previously been argued that the Cambrian Substrate Revolution FRANCE destroyed the ecological niches that the Neoproterozoic frond-holdfasts organisms occu- Received: June 27, 2016 pied. However, the discovery of these fossils in Silurian rocks demonstrates that the envi- Accepted: September 19, 2016 ronment and substrate must have been similar enough to Neoproterozoic settings that frond-holdfast bodyplans were still ecologically viable some hundred million years later. Published: October 26, 2016 Copyright: © 2016 Kirkland et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Introduction Data Availability Statement: All relevant data are The Neoproterozoic Era includes the oldest known macroscopic fossil communities [1–5], within the paper and its Supporting Information including some suggested to have been early animals [6–13] and extinct lineages [14–17]. Dis- files. coidal impressions account for much of the preserved record of this life [18], but similar fossils Funding: Field and laboratory work was supported are comparatively rare in the Phanerozoic, in which the only reported examples are Cambrian by an IRCSET Basic Research Grant (SC/2002/248) in age [19,20]. awarded to J.S.D. and a Government of Ireland Here, we report the occurrence of discoidal fossils of Silurian age, which although simple, (IRCSET) Embark Initiative Postgraduate Research appear apparently indistinguishable in morphology from examples of Ediacaran age. Like all Scholarship awarded to C.L.K. The Institute for Geoscience Research at Curtin is acknowledged for discoidal impressions, these markings require caution in interpretation, since they are simple providing funding to C.L.K. The funders had no role in form. The fossils described in this article occur in the Hellefjord Schist Formation, within in study design, data collection and analysis, the Norwegian Caledonides in Finnmark, Arctic Norway (Fig 1). This unit contains PLOS ONE | DOI:10.1371/journal.pone.0164071 October 26, 2016 1 / 21 Silurian Discoidal Fossils decision to publish, or preparation of the volcaniclastic horizons and is cut by several granitoid sheets that precisely constrain its deposi- manuscript. tional age to the mid-Llandovery (early Silurian). Exposures of interbedded psammite-pelite Competing Interests: The authors have declared reveal discoidal impressions that are morphologically indistinguishable from Ediacaran exam- that no competing interests exist. ples Nimbia and Tirasiana. The depositional environmental for this unit is interpreted as mod- erate to deep-water pelagic sediments intercalated with distal turbidites, and includes volcanic outfall [21,22]. Here we use new field observations in conjunction with a SIMS U-Pb geochro- nology dataset in an effort to better constrain the temporal range of discoidal fossils and to refine the stratigraphy of the Arctic Caledonides. Geological Setting The Hellefjord Schist was originally considered a component of the Kalak Nappe Complex (KNC), a tectonically assembled sequence of overthrust units in the Norwegian Caledonides [23]. However, more recent work demonstrates the Hellefjord Schist is a component of the overlying Magerøy Nappe, a package of metasedimentary rocks intruded by gabbroic and gra- nitic bodies of Silurian age [21,24]. The underlying basement to the Hellefjord Schist, the KNC, was initially regarded as constructed from a single, conformable package, the Sørøy Succession, Fig 1. Locality and stratigraphy of the fossils. Geological map of the LangstrandÐHellefjord area, Sørøy (modified after [21]). Fossil locality indicated by star. Inset left: main tectonic units of Finnmark with overview map of Norway. Inset right: simplified tectonostratigraphy of KNC and overlying Hellefjord Schist. doi:10.1371/journal.pone.0164071.g001 PLOS ONE | DOI:10.1371/journal.pone.0164071 October 26, 2016 2 / 21 Silurian Discoidal Fossils which comprises (metamorphosed) shallow marine siliciclastic rocks, limestone, deeper marine pelagic rocks and turbidites [22,25,26]. However, U—Pb dating of granitic intrusions and detri- tal zircons indicates that the KNC metasediments form several age-distinct lithotectonic pack- ages [27,28]. The rocks in the lower nappes of the KNC (Svaerholt Succession) have a depositional age of c. 980–1030 Ma [27] and were affected by a c. 980 Ma tectonomagmatic event [29]. Uncon- formably overlying the Svaerholt Succession is the Sørøy Succession, which consists of the Klubben Psammite and Storelv Schist that were deposited between c. 840 and 910 Ma [27]. On the island of Sørøy, a sequence of metamorphosed limestone and pelite (the Falkenes Lime- stone and Åfjord Pelite) overlies the Sørøy Succession (Fig 1). Chemostratigraphy on the car- bonate rocks suggests a depositional age of 760–710 Ma, with the contact to underlying units interpreted to be tectonic [30]. The Hellefjord Schist was originally regarded as the youngest component of the KNC [22,31]. It is intruded by 438 ± 2 Ma granitoid sheets and contains detrital zircons as young as 438 ± 4 Ma [21]. These temporal constraints are consistent with correlation of the Hellefjord Schist with the fossiliferous Middle Llandovery Juldagnes Formation within the Magerøy Nappe [32–35]. The Magerøy Nappe is recognized as a structure that developed during the c. 420 Ma Scandian Orogeny [36]. Krill and Zwaan [37] linked the Magerøy Nappe to the KNC suggesting deformation occurred during the Scandian throughout these units a finding consis- tent with Ar-Ar geochronology [38]. Lithological characteristics of the Hellefjord Schist and its regional correlation The Hellefjord Schist crops out extensively over NE Sørøy, but is also found in limited expo- sures on Porsangerhalvøya [36] (Fig 1). It is a monotonous sequence of medium- to fine- grained quartz-, plagioclase-, amphibole- and biotite-bearing schist (psammite) with finer- grained biotite-, quartz-, and plagioclase-bearingphyllite (pelite) [22]. Pelitic beds contain gar- net, and the unit has been metamorphosed to amphibolite facies. Sedimentary structures include load casts and flame structures, with rare current ripples indicating flow from the NNW [22]. The Hellefjord Schist reflects a flysch deposit in that it is a sequence of deep marine sedimentary rocks deposited in a back-arc during an early stage of orogenesis [21]. At the con- tact with the underlying Falkenes Limestone, the Gamnes Conglomerate is traditionally con- sidered as a primary basal feature of the Hellefjord Schist [22,39]. The Hellefjord Schist is intruded by c. 438 Ma granites that are compositionally similar to coeval granites within Laurentian-derived allochthonous units elsewhere in the Norwegian Caledonides [21,24]. Similar metasedimentary rocks on Magerøy are intruded by c. 438 Ma gabbros [40], from which paleomagnetic constraints imply an equatoral position on the Lau- rentian margin during the Silurian [24]. These mafic intrusions reflect development of volcanic arcs and back-arcs in the northern Appalachian segment of the margin of Laurentia [41]. Fossils in the Hellefjord Schist Several discoidal positive hyporelief casts have been found on the lower bedding surface of steeply dipping pelite—psammite interbeds of the Hellefjord Schist at Pikfjellet, on northern Sørøy (gravity cast-style preservation; Fig 2). The rock in which the fossils are found is fine- grained and metamorphic grain size coarsening has not significantly affected this specific unit. In addition it lacks a pervasive metamorphic foliation. A 3D photogrammetric reconstruction of the material is provided in S1 Video. These specimens are characterized by a depressed, low- relief disc, 2–20 mm in diameter, with a central boss that is less than one tenth of the diameter PLOS ONE | DOI:10.1371/journal.pone.0164071
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