New insights into the early evolution of horizontal spiral trace fossils and the age of the Brioverian series (Ediacaran–Cambrian) in Brittany, NW France

Gougeon, Romain; Néraudeau, Didier; Loi, Alfredo; Poujol, Marc

doi:10.1017/s0016756820001430

Cited by 5 publications

(1 citation statement)

References 88 publications

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“…The French Geological Survey maps it as Early Ordovician in age, based on correlation to stratigraphy at Crozon that lies 125 km to the south across the North Armorican Shear Zone in the central Armorican domain (Egal et al 1995). This correlation is consistent with the Port Lazo Formation being bounded by underlying Brioverian metasediments, which have recently yielded zircon U-Pb radiometric dates of 551 ± 7 Ma and 540 ± 5 Ma (Gougeon et al 2021), and a 472 ± 5 Ma age from overlying andesites (Fig. 2a) (Auvray et al 1980).…”

Section: Arumberia In the Port Lazo Formationsupporting

confidence: 85%

Enigma variations: characteristics and likely origin of the problematic surface texture Arumberia, as recognized from an exceptional bedding plane exposure and the global record

et al. 2021

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Arumberia is an enigmatic sedimentary surface texture that consists of parallel, sub-parallel or radiating ridges and grooves, most commonly reported from upper Neoproterozoic – lower Palaeozoic strata. It has variably been interpreted as the impression of a small metazoan, a ‘vendobiont’, a physical sedimentary structure formed on a substrate with or without a microbial mat covering, or a non-actualistic microbial community. In this paper we contribute new insights into the origin of Arumberia, resulting from the discovery of the largest contiguous bedding plane occurrence of the texture reported to date: a 300 m2 surface in the lower Cambrian Port Lazo Formation of Brittany, NW France. We compare the characteristic features of Arumberia at this locality with 38 other global records, revealing four defining characteristics: (1) the three-dimensional (3D) morphology of exposed Arumberia lines (either positive relief ‘ridges’ or negative relief ‘grooves’) records fully preserved cords within clay laminae; (2) lines may transition laterally into reticulated patterns; (3) characteristic parallel and sub-parallel Arumberia lines can become modified by desiccation on emergent substrates prior to interment; and (4) Arumberia are streamlined with palaeoflow in successions showing evidence of unidirectional currents, but are organized parallel to ripple crests where strata were sculpted by oscillatory flows. These characteristics indicate that Arumberia records a 3D entity, distinct in material properties from its host sediment, which occurred in very shallow water settings where it was prone to passive reorganization in moving water, and desiccation when water drained. A literature survey of all known Arumberia occurrences reveals that the most reliable examples of the form are stratigraphically restricted to a 40 Ma interval straddling the Ediacaran–Cambrian boundary (560–520 Ma). Together these characteristics suggest that Arumberia records the remains of extinct, sessile filamentous organisms (microbial or algal?) that occupied very shallow water and emergent environments across the globe at the dawn of the Phanerozoic Eon.

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Section: Arumberia In the Port Lazo Formationsupporting

confidence: 85%

Enigma variations: characteristics and likely origin of the problematic surface texture Arumberia, as recognized from an exceptional bedding plane exposure and the global record

et al. 2021

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The Lower Cambrian deposits of the Le Rozel Formation (Normandy, NW France): Insight into a newly described ichnofossil assemblage

Coutret,

Néraudeau,

Gendry

et al. 2024

Palaeogeography, Palaeoclimatology, Palaeoecology

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Zircon and apatite U-Pb geochronology of the Paleoproterozoic (Eburnean) basement and late Neoproterozoic (Pan-African) metamorphism and magmatism from Port-Béni, Armorican Massif (France)

Poujol,

Hallot,

Abiven

et al. 2024

BSGF - Earth Sci. Bull.

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By re-examining the historical outcrops of Port-Béni located in the Trégor unit of the North Armorican Cadomian belt, the present work delivers four new ages that provide additional constraints on the Proterozoic history of northern Brittany. It is established that granitic, porphyritic rocks crystallized at the end of the Rhyacian (Paleoproterozoic), 2038 ± 12 Ma ago, before being transformed into orthogneisses at a late Neoproterozoic (Ediacaran) age of 621 ± 2 Ma, which is a minimum age, given the retrograde alteration these rocks underwent. The age of ca. 1.8 Ga previously proposed for the protolith of the Port-Béni orthogneiss should be discarded, and these two new ages are consistent with most of those yielded so far by the other Icartian (i.e., Eburnean) basement relics from the Armorican Massif. The gneissic basement was then intruded and disrupted into xenoliths by a granodioritic magma that crystallized 604.5 ± 2.0 Ma ago. This age, slightly younger than previously thought, corresponds to the emplacement age of one of the main units of the North Trégor batholith − the Pleubian-Talbert unit −, part of the Trégor volcano-plutonic complex, which may have built up over a longer period than that indicated by the uncertainty associated with this age. Caution should be exercised in extrapolating this age to that of the whole complex. Finally, doleritic dykes, possibly resulting in two swarms previously thought to be Paleozoic in age, have crosscut this complex. One of the latest yielded an age of 597 ± 15 Ma, indicating that the Trégor doleritic dyking episodes also occurred during the late Neoproterozoic, in between ca. 605 Ma and ca. 580 Ma. As the doleritic dykes are of tholeiitic composition, which distinguishes them from the earlier calc-alkaline magmas, they suggest that the intra-arc extension, documented in the southern, adjacent Saint-Brieuc unit of the belt, also affected the Trégor unit. They may likely have fed northern equivalents of the lava flows from the Paimpol Formation (exposed in between the Saint-Brieuc and the Trégor units), when magma production became moderately influenced by the Cadomian (i.e., Pan-African) subduction and mostly dominated by extension, possibly as a result of a steepening of a north-dipping subduction slab. Indeed, a re-examination of the available geochemical and geochronological data in the light of our new results documents that arc-magma production moved progressively from north (Trégor unit) to south (Saint-Brieuc unit) over time, in the interval 605-580 Ma.

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New insights into the early evolution of horizontal spiral trace fossils and the age of the Brioverian series (Ediacaran–Cambrian) in Brittany, NW France

Cited by 5 publications

References 88 publications

Enigma variations: characteristics and likely origin of the problematic surface texture Arumberia, as recognized from an exceptional bedding plane exposure and the global record

Enigma variations: characteristics and likely origin of the problematic surface texture Arumberia, as recognized from an exceptional bedding plane exposure and the global record

The Lower Cambrian deposits of the Le Rozel Formation (Normandy, NW France): Insight into a newly described ichnofossil assemblage

Zircon and apatite U-Pb geochronology of the Paleoproterozoic (Eburnean) basement and late Neoproterozoic (Pan-African) metamorphism and magmatism from Port-Béni, Armorican Massif (France)

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