Graptolite faunas of the Llandovery “phtanites” at Les Fresnaies (Chalonnes-sur-Loire, southeastern Armorican Massif): Palaeontology and biostratigraphy

Abstract

In the SE of the Armorican Massif (France), the Ordovician-Silurian transition and the lower part of the Silurian are exposed at the locality Les Fresnaies at Chalonnes-sur-Loire. The Silurian graptolitic "phtanites" (black cherts) are richly fossiliferous. Detailed sampling allows a description of the graptolite faunas and a precise stratigraphical analysis of the succession that comprises the whole of the Rhuddanian and Aeronian stages. Both the Llandovery graptolite assemblages and the lithologies of Les Fresnaies are similar to those of other lower Silurian successions throughout peri-Gondwanan Europe,

Biostratigraphical and palaeobiogeographical implications of Lower Silurian Radiolaria from black cherts of the Armorican Massif (France)

“…Promising samples selected for laboratory processing were chemically treated as follows: samples were first washed, rinsed with distilled water, oven-dried, crushed into smaller fragments and then soaked in numbered plastic beakers containing diluted (4.8%) HF for 24 h. Then, the fragments were sieved with 63 µm and 630 µm sieves and rinsed. Fragments larger than 630 µm were soaked again in renewed 4.8% HF for 24 h, while the fraction Piçarra et al (2009) and the stratigraphic position of the radiolarian-bearing samples MT8, MT18, MT19 and MT20 (circles and squares represent SeM and thin section identifications, respectively).…”

“…The section exhibits in its northeastern part a c. 30 m thick succession of uppermost Ordovician diamictite (interval 'a' on Figs 1 and 2), followed by c. 2 m thick grey shale (interval 'b'), accumulated respectively during the Hirnantian glaciation and the subsequent deglacial period. Then, c. 10 m of 'phtanite' (cm-to dm-thick black bedded chert rich in organic matter, interbedded with black shale containing pyrite) corresponding to the interval 'c' overlie conformably the grey siltstone and shale (Piçarra et al, 2009). This interval represents the most complete Llandovery succession of the Ligerian domain, recording a continuous sedimentation during the Rhuddanian and the Aeronian, as testified by its graptolitic and chitinozoan content (Piçarra et al, 2009).…”

“…Then, c. 10 m of 'phtanite' (cm-to dm-thick black bedded chert rich in organic matter, interbedded with black shale containing pyrite) corresponding to the interval 'c' overlie conformably the grey siltstone and shale (Piçarra et al, 2009). This interval represents the most complete Llandovery succession of the Ligerian domain, recording a continuous sedimentation during the Rhuddanian and the Aeronian, as testified by its graptolitic and chitinozoan content (Piçarra et al, 2009). Interval 'd' corresponds to strata displaying deformed horizons, difficult to access (Piçarra et al, 2002(Piçarra et al, , 2009.…”

“…This interval represents the most complete Llandovery succession of the Ligerian domain, recording a continuous sedimentation during the Rhuddanian and the Aeronian, as testified by its graptolitic and chitinozoan content (Piçarra et al, 2009). Interval 'd' corresponds to strata displaying deformed horizons, difficult to access (Piçarra et al, 2002(Piçarra et al, , 2009.…”

Evidence for pre-orogenic, Early Devonian rifting in the Variscan belt: stratigraphy and structure of the Palaeozoic cover of the Mauges Unit (Upper Allochthon, Armorican massif, France)

“…The Monograptus Black Chert consists of a condensed euxinic sequence (5-10 m thick) of black cherts and black shales without any important influx of coarser siliciclastic material. This formation records sedimentation in a distal offshore depositional environment (Piçarra et al 2009). The graptolite fauna (Monograptus lobiferus) indicates a Llandovery age (Cavet et al 1971;Philippot 1950;Piçarra et al 2002).…”