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Le Paléozoïque De La Presqu'île De Crozon, Massif Armoricain

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Le Paléozoïque De La Presqu'île De Crozon, Massif Armoricain GÉOLOGeology of GFranceIE and surrounding areas de la France 2011 N° 1 n Le Paléozoïque de la presqu’île de Crozon, Massif armoricain (France), M. Vidal, M.-P. Dabard, R. Gourvennec, A. Le Hérissé, A. Loi, F. Paris, Y. Plusquellec, P.R. Racheboeuf ............................................... 3 Le Paléozoïque de la presqu’île de Crozon, Massif armoricain (France) SOMMAIRE COUVERTURE Trimestriel ÉditéÉdité par par le BRGMle BRGM et la et SGF la SGF photo de couverture Géologie de la France est analysée dans la base des données PASCAL - GÉODE (BRGM/INIST). Le Paléozoïque de la presqu’île de Muriel Vidal* (1), Crozon, Massif armoricain (France) Marie-Pierre Dabard (2), Rémy Gourvennec (1), Alain Le Hérissé (1), Le Paléozoïque de la presqu’île de Crozon, Massif armoricain Alfredo Loi (3), (France) Florentin Paris (2), Yves Plusquellec (1), Patrick R. Racheboeuf (1) Géologie de la France, n° 1, 2011, p. 3-45, 18 fig. Mots-clés : Massif armoricain, Gondwana, Ordovicien, Silurien, Devonien, terrain. Keywords : Armorican Massif, Gondwana, Ordovician, Silurian, Devonian, fieldtrip. Résumé Ordovician to a sub-equatorial position in the Carboniferous. La presqu’île de Crozon offre des affleurements de qualité en falaise et sur l’estran permettant l’étude des Geological outline of the Armorican Massif is briefly séries paléozoïques du Massif armoricain. L’évolution exposed and integrated in a climatic, eustatic and sédimentaire et faunique de cette région s’intègre dans tectonic global framework. Afterwards, the second part l’histoire de la marge gondwanienne sous le contrôle de deals with formations of the Crozon Peninsula since the l’ouverture de l’océan Rhéic, des variations eustatiques Ordovician to Upper Devonian with their faunal and et de sa migration paléogéographique depuis les hautes sedimentological characteristics. In the third part, the latitudes à l’Ordovicien jusqu’à une position sub- main outcrops visited during the fieldtrip of the Paleozoic équatoriale au Carbonifère. French Group (GFP) during the Congress « Strati 2010 » are described; these outcrops illustrate some aspects of La première partie présente l’évolution du Massif the armorican geological history. armoricain intégrée dans un cadre global, climatique, eustatique et tectonique. Les formations sédimentaires 1. Introduction de la presqu’île de Crozon depuis l’Ordovicien jusqu’au Dévonien Supérieur sont ensuite décrites avec leurs caractéristiques fauniques et sédimentologiques. Dans Le Massif armoricain occupe l’Ouest de la France et la troisième partie, les affleurements visités lors d’une correspond au segment occidental de la Chaîne varisque excursion réalisée pour le Groupe Français du (Fig. 1) qui s’étend vers l’Est jusqu’en Europe centrale. Paléozoïque à l’occasion du congrès « Strati 2010 », Caractérisé par l’absence des séries mésozoïques, il s’é- sont décrits. Ces affleurements illustrent certains points tend depuis le Cotentin au Nord jusqu’en Vendée au Sud. Il clefs de l’histoire géologique armoricaine. est constitué d’un socle protérozoïque, représenté par des formations d’âges Icartien (Protérozoïque basal) à Abstract Briovérien (Protérozoïque terminal voire Cambrien) et d’une couverture paléozoïque. Au cours de son histoire, Geological outcrops are of good quality along the deux orogenèses l’ont façonné, l’orogenèse cadomienne foreshore and cliffs in Crozon Peninsula allowing the (environ 750-540 Ma) puis l’orogenèse varisque (à partir du study of Paleozoic series of the Armorican Massif. The Dévonien Supérieur). sedimentary and faunal evolution of this area is part of the history of the Gondwana margin and records the Dès le Paléozoïque inférieur, l’évolution sédimentaire et opening of the Rhéic Ocean, eustatic variations and faunique du Massif armoricain s’intègre dans l’histoire de la paleogeographic migration from high latitudes in the marge nord-gondwanienne qui regroupe notamment le 28-29 Août 2010 - Excursion du GFP 2010, Excursion pré-congrès “STRATI 2010”, Sous l’égide du Comité Français de Stratigraphie 1 Université de Brest, CNRS, IUEM, Domaines Océaniques UMR 6538, 6 avenue Le Gorgeu, CS93837, F-29238 BREST Cedex 3, France. 2 Géosciences-Rennes, Université de Rennes 1, CNRS UMR 6118, Campus de Beaulieu, 35042 RENNES Cedex, France. 3 Dipartimento di Scienze della Terra, Università degli studi, 51, Via Trentino, 09127, Cagliari, Italie. *Auteur correspondant : Muriel Vidal - Université de Brest, Domaines Océaniques UMR 6538, 6 avenue Le Gorgeu, CS93837, F-29238 BREST Cedex 3, France. [email protected] * Manuscrit présenté le 2 août 2011, accepté le 15 octobre 2011. GÉOLOGIE DE LA FRANCE, N° 1, 2011 3 LE PALÉOZOÏQUE DE LA PRESQU’ÎLE DE CROZON Fig. 1 - Le Massif armoricain dans la chaîne varisque (Ballèvre et al., 2009). Fig. 1 - The Armorican Massif in the Variscan belt (Ballèvre et al., 2009). domaine Ibéro-Armoricain, la Bohême et l’Afrique du Nord. (associations benthiques, migrations, apparitions/dispari- Plusieurs facteurs globaux vont contrôler l’évolution strati- tions…). graphique de ces domaines : 1) l’ouverture de l’océan Rhéic entre le Sud d’Avalonia et le Nord de Gondwana dès D’un point de vue paléogéographique (Fig. 2), le Massif le Cambrien ; 2) les variations eustatiques ; 3) la migration armoricain est subdivisé en deux domaines, le Domaine du super continent Gondwana depuis les hautes latitudes à Médio-Nord Armoricain (DMNA, réunion des Domaines l’Ordovicien jusqu’à une position sub-équatoriale au Nord-Armoricain et Médio-Armoricain) qui inclut le Cotentin, Carbonifère ; 4) l’épisode glaciaire fini-Ordovicien. Ces évé- le Sud de la Normandie, la Sarthe, la Mayenne et la nements sont à l’origine de variations sédimentaires (litho- Bretagne centrale et occidentale, et le Domaine Sud logies, faciès, environnements de dépôt) et fauniques Armoricain (DSA) qui comprend la partie sud de l’unité de 4 GÉOLOGIE DE LA FRANCE, N° 2, 2011 LE PALÉOZOÏQUE DE LA PRESQU’ÎLE DE CROZON N Cotentin Normandie DOMAINE NORD ARMORICAIN May/orne Urville zone bocaine Brest Mortain-Domfront Sées CNA Menez-Bélair Crozon Châteaulin Pail DOMAINE MEDIO-ARMORICAIN Coëvrons CSA Rennes Laval (branche Nord) Réminiac Martigné-Ferchaud DOMAINE St-Julien-V Lanvaux ouvantes - Angers SUD ARMORICAIN CSA St-Georges/Loire (branche Sud) Ancenis 0 100 Km Nantes Vendée formations protérozoïques, Cadomien formations paléozoïques formations mésozoïques et roches ignées varisques Fig. 2 - Carte du Massif armoricain avec les affleurements paléozoïques dans les différents synclinaux du Domaine Médio-Nord Armoricain (Cotentin, May-sur-Orne, Urville, la zone bocaine, Sées, Mortain-Domfront, Pail, Coëvrons, Châteaulin, Menez-Bélair, Laval, Martigné-Ferchaud) et du Domaine Sud Armoricain (Saint-Julien-de-Vouvantes – Angers pro parte, Lanvaux, Saint-Georges-sur-Loire et Ancenis). Les synclinaux de Châteaulin, Menez-Bélair et Laval sont aussi réunis sous le nom de synclinorium médian armoricain. CNA et CSA : Cisaillement Nord et Sud Armoricain. Fig. 2 - Geological map of the Armorican Massif with Paleozoic outcrops in the synclines of the Medio-North Armorican Domain (Cotentin, May- sur-Orne, Urville, la zone bocaine, Sées, Mortain-Domfront, Pail, Coëvrons, Châteaulin, Menez-Bélair, Laval, Martigné-Ferchaud) and the South Armorican Domain (Saint-Julien-de-Vouvantes – Angers pro parte, Lanvaux, Saint-Georges-sur-Loire, Ancenis). CNA et CSA: North and South Armorican shear zone. Saint-Julien-de-Vouvantes, les synclinaux de Saint- Chauvel, 1969 ; Darboux et Plusquellec, 1985). Dans un Georges-sur-Loire et Ancenis, la Bretagne sud et la premier temps, nous rappellerons les grandes lignes de Vendée. Les successions paléozoïques du DMNA sont l’évolution stratigraphique anté-varisque pour le Massif relativement homogènes et connaissent une évolution sem- armoricain. Puis l’évolution des environnements de dépôt et blable à celle enregistrée dans les domaines Centre- associations fauniques en presqu’île de Crozon sera pré- Ibérique ou Bohémien. En revanche, l’évolution du DSA est sentée, suivie de la description des différents affleurements plus singulière. Ces deux domaines sont actuellement visités. séparés par la branche nord du Cisaillement Sud Armoricain (CSA). 2. L’histoire paléozoïque du Massif armoricain La presqu’île de Crozon est située à l’ouest du DMNA. La succession paléozoïque est bien représentée, sans dis- continuité majeure jusqu’au Dévonien Supérieur. Elle repo- La succession paléozoïque armoricaine repose sur des se en discordance sur le socle constitué dans la région par granites cadomiens (comme en Normandie) ou en discor- des formations sédimentaires briovériennes (Le Corre et dance sur des séries briovériennes constituées de sédi- GÉOLOGIE DE LA FRANCE, N° 2, 2011 5 LE PALÉOZOÏQUE DE LA PRESQU’ÎLE DE CROZON Fig. 3 - Évolution paléogéographique au cours du Paléozoïque (Paris in Dabard et al., 2009). Représentation des aires océaniques en bleu foncé, des plateformes épicontinentales en bleu clair, des terres émergées en orange, de la calotte glaciaire au Pôle Sud à l’Ordovicien Supérieur, des chaînes calédonienne et varisque en blanc. Fig. 3 - Paleogeographic evolution during the Paleozoic times with ocean areas in dark blue, platforms in light blue, lands in orange, ice cap at the South pole during the Upper Ordovician, and Caledonian and Variscan belts in white (Paris in Dabard et al., 2009). 6 GÉOLOGIE DE LA FRANCE, N° 2, 2011 LE PALÉOZOÏQUE DE LA PRESQU’ÎLE DE CROZON ments terrigènes dans lesquels s’intercalent localement des Les datations de niveaux
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