Michel Robardet

The Armorica ‘microplate’: fact or fiction? Critical review of the concept and contradictory palaeobiogeographical data

Abstract The problem of an ‘Armorica microplate’, detached from Gondwana and having had during part of the Palaeozoic an independent latitudinal evolution, is reconsidered in terms of a critical review of the palaeomagnetic data that were the very roots of this concept and through an alternative approach based on palaeoclimatic and palaeobiogeographical data. Palaeomagnetic data for the Silurian and the Devonian of the south European regions supposedly constituting the Armorica microplate remain rare and ambiguous. Those from Gondwana are more numerous but contradictory enough to give rise to diverging models regarding the latitudinal evolution of this continent. Consequently, the reality of an Armorica microplate cannot be considered as established. On the contrary, lithological indicators of palaeoclimate and palaeobiogeographical data are in total harmony and indicate that, in actual fact, the southern European regions remained permanently closely connected with Gondwana, of which they composed the northern margin. Although repeatedly maintained for more than 20 years, the concept of an Armorica microplate can thus be considered a fiction. This conclusion should lead to the dismissal of geodynamical models proposed for the Variscan Belt to which this concept was integrated and are contradicted by inescapable palaeobiogeographical constraints.

Early Palaeozoic palaeobiogeography of the Variscan regions

Abstract The palaeobiogeographic method appears to be an attractive alternative for reconstruction of the palaeogeography of the Variscan regions during the Palaeozoic. The evolution of faunal assemblages and climatic characteristics from the Cambrian up to the end of the Devonian, demonstrates that Baltica and North Gondwana represent two major and independent palaeogeographic entities separated by a mid-European Rheic Ocean which opened as early as the Cambrian and closed progressively from the Devonian. The South Armorican Ocean, whose closure generated the so-called Ligerian Orogen, is regarded as a branch of the Rheic Ocean. Both a Tornquists Sea, separating Baltica from its western extension (Avalonia) during the Ordovician and a Proto-Tethys Ocean, extending between southwestern Europe and north Africa during the Devonian, are excluded from our reconstructions.

δ18O and REE contents of phosphatic brachiopods: a comparison between modern and lower Paleozoic populations

δ18O values of both carbonate (18.7–25.1‰) and phosphate (15.1–16.7‰) components of the francolite from the fossil brachiopod shells are interpreted as the result of a partial resetting of pristine isotopic compositions. The similarity of hat-shape REE patterns of some lower Ordovician linguloids with those of extant littoral specimens suggests the preservation of a 500 Myr-old record of REE marine chemistry. In agreement with the known paleogeography, the phosphatic brachiopods have been deposited in a coastal environment under shallow and oxygenated marine waters. However, the contents and distribution of REE in some strongly altered brachiopod shells reveal an alteration of the original negative Ce anomalies and both Nd and Sm enrichments that transformed the initial hat-shape patterns of linguloids into strongly convex or bell-shape patterns.

The late Ordovician diamictic formations from southwestern Europe: North-Gondwana glaciomarine deposits

Abstract This paper presents a review and a discussion of the clast-bearing formations (diamicts) which occur in the uppermost part of the Ordovician succession of numerous regions in Southwestern Europe. Lithology, facies, stratigraphic setting and age are discussed. The strong similarities of these formations with contemporaneous glaciomarine deposits of the northern and northwestern margins of the African ice sheet and the Late Ordovician paleogeographic pattern based upon paleomagnetic data lead to the conclusion that the south European clast-bearing formations are glaciomarine deposits supplied with clasts by floating icebergs released from the African ice sheet. The proposed paleogeographic model confirms the close connection between southwestern Europe and Gondwana during Early Palaeozoic times.

Deciphering “temperature” and “salinity” from biogenic phosphates: the δ18O of coexisting fishes and mammals of the Middle Miocene sea of western France

Abstract This study examines the possibility of estimating the oxygen isotopic composition and temperature of seawater by using a combination of δ18O values of phosphate from coexisting fishes and mammals of the Middle Miocene sea of western France. Teeth of different species of teleosteans and selachians, and rib fragments of Metaxytherium medium (related to modern Dugongs) were sampled in the Langhian-Seravallian calcareous sediments of eastern Brittany and Touraine. δ18O values of fishes range from 21.4 to 22.7‰ while those of mammals range from 20.1 to 21.7‰. The variability in δ18O values of Miocene fishes is not related either to sampling localities or taxa. Oxygen isotope analyses were also carried out on living Dugong dugon from Djibouti ( δ 18 O = 19.6−20.3‰ ) and on Hydrodamalis gigas which lived two hundred years ago in the low salinity waters of the Bering Strait ( δ 18 O = 17.3‰ ). The results show that the slope of the oxygen isotope fractionation curve between the dugongids and water is probably close to 1. δ18O variations within Miocene fish and mammal populations are similar and close to or slightly higher than 1‰. This variation may be attributed mainly to a δ18O change in the ambient seawater composition. The Miocene shallow water masses in western France were thus characterized by varying δ18O values (as well as average salinities) equal to or higher than the contemporaneous open world ocean. Calculated average temperatures of 20±2°C are compatible with a sub-tropical climate. Local and global causes for this variation in the oxygen isotope composition of Miocene seawater are discussed

Sedimentary and Faunal Domains in the Iberian Peninsula During Lower Paleozoic Times

The purpose of this chapter is to define, characterize and illustrate the distinct domains that composed the Iberian Penisula during the Lower Paleozoic (from Cambrian-Ordovician to Devonian), and to discuss and evaluate the affinities or the differences that existed between these domains and to compare them with other regions of southwestern Europe. The Upper Devonian and Carboniferous periods have not been considered here: in those times the different regions were on the verge of assembly in the Hercynian Belt, and their distinctive sedimentary and faunal characteristics were most probably partially or totally obliterated. Consequently, the South Portuguese Zone, which consists entirely of Upper Devonian-Carboniferous, is outside the scope of the present chapter.

Le Silurien et le Dévonien basal dans le Nord de la province de Seville (Espagne)

Resume Dans la region etudiee («Zone dOssa-Morena) le Silurien et la base du Devonien (Lochkovien =approximativement Gedinnien) sont constitues par environ 130 m de schistes ampeliteux a graptolites separes en deux unites (Schistes a graptolites inferieurs et superieurs) par lintermede carbonate des «Calcaires a Orthoceres et Scyphocrinus du Ludlow terminal et du Pridolien. 21 zones de graptolites ont ete reconnues, depuis la base du Llandovery (zone de A. acuminatus) jusquau Lochkovien superieur (zone de M. hercynicus). Par sa lithologie et par ses faunes, cette succession est etroitement comparable a celles qui caracterisent le domaine de la Paleotethys (Thuringe, Alpes Carniques, Sardaigne, NW de lAfrique).

Palaeomagnetic constraints upon the palaeogeographic position of the Baltic Shield in the Ordovician

The palaeomagnetic analysis of the Ordovician limestones in Southern Scandinavia (75 sites) has given the following results: (1) in the southern part of the Oslo Region, all Ordovician sediments were remagnetized during the Lower Permian, by a thermo-chemical mechanism triggered by the emplacement of the Permian volcanic and intrusive suite; (2) in the northern part of the Oslo Region, the Upper Caradoc Mjosa Limestone carries a high-temperature component (decl. = 53°, incl. = −40°, α95 = 25°, 4 sites) with corresponding palaeopole at 4°S, 143°E; (3) in southern Sweden, all Lower Ordovician “Orthoceras limestones” sampled in Dalarna, Narke, Scania, Vaster-gotland, Oland and Oster-gotland display the same well-defined southeast steep high-temperature components, leading to a palaeopole at 30° N, 55° E, A95 = 9°; in areas with late Palaeozoic volcanic activity (Scania, Vaster-gotland), this high-temperature component is partially overprinted by a remagnetization associated with dolerite emplacement. According to these results, Baltica was situated in temperate southern latitudes (50° S) in the Early Ordovician and has drifted northwards at a latitudinal drift rate of 4 to 7 cm/year to reach tropical latitudes (20°S) by the Late Ordovician to Late Silurian. This interpretation is in agreement with previous models based on the occurrence of Ordovician reef structures, and sheds new light on the early Palaeozoic position of Baltica relative to other continental blocks.

Le silurien du synclinoriumde Moncorvo (NE du Portugal): Biostratigraphie et importance paléogéographique

Resume Dans la succession silurienne du synclinorium de Moncorvo (NW de la Zone Centre Iberique, Portugal), des lentilles calcaires ont livre, dans deux localites distinctes, les premiers conodontes siluriens du Portugal. Dans la premiere localite, Kockelella cf. variabilis, K., cf. absidata, Ozarkodina confluens, Oz. excavata et Pseudooneotodus beckmanni indiquent le Ludlow s.l. (ou peut-etre le Wenlock superieur ou terminal). Dans la seconde, Oulodus elegans, O. cf. cristagalli et Ozarkodina ex gr. remscheidensis indiquent le Pridoli, ce que confirme la presence, dans le meme gisement, de scyphocrinoides du genre Scyphocrinites, et en particulier les lobolithes a cirrhes. Dans les schistes noirs a nodules sous-jacents aux calcaires, des graptolites montrent lexistence de niveaux du Llandovery moyen et superieur (Aeronien et Telychien) et du Wenlock. La succession silurienne de Moncorvo apparait ainsi comme une sequence condensee, analogue, en particulier, a celles qui existent dans la Zone dOssa Morena, en Sardaigne et dans certaines regions dAfrique du Nord. Ces successions sont bien differentes de celles qui caracterisent les regions centrales et meridionales de la Zone Centre Iberique ou, dans leur partie superieure, les depots siluriens, de faible profondeur, sont beaucoup plus epais et plus riches en materiel terrigene grossier. Ces donnees permettent denvisager que, au Silurien, le NW de la Peninsule Iberique (y compris une partie de la Zone Asturo-Leonaise) appartenait a un domaine paleogeographique particulier presentant des caracteres de plate-forme distale plus profonde.

First articulated Silurian sponges from the Iberian Peninsula (Spain and Portugal)

The first-described articulated Silurian sponges from Spain and Portugal include a moderate assemblage of hexactinellids and a single monaxonid demosponge. The sponges were collected from a thin layer at the top of the Cyrtograptus lundgreni-Monograptus testis graptolite biozone, in a possible volcanic ash of latest Homerian (Wenlock) age. The sponges are from southeastern Portugal and southwestern Spain in the Ossa-Morena Zone of the Hesperian Massif. The hexactinellid collection includes several specimens of the new species, Protospongia iberica, and fragments of Diagoniella species and Gabelia (?) sp. Specimens of the latter two taxa are too small for species identification. Demosponges are represented by a single described specimen of a probably new genus and species preserved as a “wreath” of monaxon spicules. Dermal and gastral layers are of very fine spicules developed over the moderately coarse, aligned, principal body spicules.