Résumé
Du point de vue stratigraphique, nous rencontrons dans les éléments palynologiques de la région lyonnaise un assemblage de taxons, qui compte tenu de leurs extensions stratigraphiques connues (extinction ou apparition) est caractéristique du Rhétien. Ces taxons sont Anaplanisporites echinatus, Dapcodinium priscum, Inaperturopollenites dubius, Retitriletes clavatoides, Retitriletes semimuris, Rhaetipollis germanicus, Rhaetogonyaulax rhaetica, Ricciisporites tuberculatus, Suessia swabiana, Tigrisporites microrugulatus, Tsugaepollenites pseudomassulae et Zebrasporites thuringiacus. Cet assemblage est attribuable à la zone d’assemblage à Rhaetipollis germanicus. Les associations palynologiques, multiples, sont caractérisées par l’absence ou la grande rareté d’éléments typiques des associations européennes comme Aratrisporites, Densosporites ou Triancoraesporites. On note occasionnellement une forte fréquence de Rhaetogonyaulax rhaetica. Ceci souligne des différences significatives observées entre les associations palynologiques lyonnaises et celles qui sont observées dans d’autres secteurs géographiques très proches. De la même manière, cela confère un cachet particulier à ces associations lyonnaises par comparaison à celles du Jura, du Bas-Dauphiné et de l’Ardèche. Il est donc fondé d’envisager des différences paléoenvironnementales pour expliquer ce caractère palynologique typé du Rhétien lyonnais, qui est décrit pour la première fois.
Mots-clés : Palynomorphe, Biostratigraphie, Rhétien, Rhône, Belmont, Saint-Didier-au-Mont-d’Or.
Abstract
The Rhaetian Stage is still an enigma because of the absence of a formal stratotype. The aim of this paper is not to air our “sentiments” on this fact – we follow the point of view of Dagys and Dagys (1994) and Tozer (1994) and adopt the time scale for the Triassic period of Gradstein et al. (1994) – but to present an analysis based on detailed palynological investigation of Rhaetian deposits in two boreholes and an underground gallery near Lyons (southeastern France; Fig. 1). The lithologic units intersected by the boreholes are composed, from the base up, of alternations of siltites, sandstones and limestones overlain by black sandy shales with nodules of limestone; the siltites are rich in echinid and ostracod bioclasts and are associated with storm deposits. The same lithological composition was identified in the underground gallery, although with more limestone levels, in places with a high content of bivalve shells. The top of the section consists of oolitic limestones, mainly grainstones. Palynological results and dating The succession, from a palynological standpoint, is distinguished by 34 spore and 31 pollen taxa, and a rich dinocyst and acritarch microflora (10 species) including acanthomorph acritarchs, herkomorph acritarchs, peridinial dinophycae, chlorophycae, and undetermined Leiosphaeridae (see systematics list in the French text). It should be noted that although the presence of the classical taxa R. tuberculatus and R. germanicus is not indicative of the Rhaetian stage (Fig. 2), the associated presence of palynomorphs such as Anaplanisporites echinatus, Dapcondinium priscum, Inaper-turopollenites dubius, Retitriletes clavatoides, Retitriletes semimuris, Rhaetipollis germanicus, Rhaetogonyaulax rhaetica, Ricciisporites tuberculatus, Suessia swabiana, Trilites microrugulatus, Tsugaepollenites pseudo-massulae and Zebrasporites thuringiacus is significative of a Rhaetian age. Thus, the studied sections are Rhaetian in age (Figs. 3, 4A, 4B and 5). Local evaluation of palynological associations The spores and pollen assemblages from the boreholes and the underground gallery constitute local associations that can be defined on the basis of the combined occurrence of dinocysts, abundant Circumpolles, and some species of spores. • Borehole C1 at Belmont (Fig. 6): only one association is defined from the occurrence of D. priscum. Other characteristics are an abundance of Circumpolles (C. granulata and C. meyeriana; 22 to 71%) and spores, mainly K. reissin-geri and L. argenteaeformis (5 to 45%). The most striking feature is the absence of R. rhaetica and the rarity of pollen grains other than Circumpolles. • Borehole X1 at Belmont (Fig. 7): the assemblage presents three associations. 1 - The X1A association (17.20 m - 10.55 m) characterized by an abundance of Circumpolles (C. torosus and C. meyeriana; 15 to 85%) and spores (10 to 34%). Marine elements are dinocysts (B. caminuspina, B. delicata, B. langii, C. hexagonalis, D. priscum), Leiosphaeridae and acantho-morph acritarchs (Micrhystridium). R. rhaetica is absent or very rare. 2 - The X1B association (25.75 m) dominated by R. rhaetica (95%) in association with B. delicata, C. polypartita, D. priscum and rare Botryococcales. 3 - The X1C association (29.20 m - 26.10 m) composed of Circumpolles (C. torosus and C. meyeriana; 6 to 50%), disaccates (O. pseudoalatus; 4 to 45%) and spores (6 to 37%). One notes a scarce-ness of R. rhaetica and more generally of dinocysts and acritarchs. • Underground gallery at Saint-Didier-au-Mont d’Or (Fig. 8): the assemblage presents two associations. 1 - The SA association (S11-S09), fairly similar to the X1B association with dominant R. rhaetica (31 to 63%) and a comparable composition in sporomorphs (mainly C. granulata, O. pseudoalatus, R. germanicus, R. tuberculatus). 2 - The SB association (S08-S01) with similarities to the X1C association in high percentages of Circumpolles and O. pseudoalatus. Regional comparisons Considering the general aspect of the palynological assemblages recovered from boreholes C1 and X1 at Belmont and the underground gallery at Saint-Didier-au-Mont-d’Or in southeastern France, we can correlate them to the Orbell’s Rhaetipollis zone. In detail, however, one notes the absence or rarity of common European genera of spores (Aratrisporites, Densosporites, Limbosporites, Perinosporites, Triancoraesporites or Zebrasporites) and pollen (Granuloperculatipollis, Lunatisporites or Ovalipollis). Also of note is the strong variations of R. rhaetica, which is in places dominant. These features are local, but original. Publications on recent palynological studies in relatively nearby areas, i.e. the Largentière Basin, deposits in the Jura Mountains, the Bas-Dauphiné (a geo-graphical sector in the Alps), show that none of these sites presents consistent and strong palynological similarities with the Lyons area, particularly where marine elements are concerned. General conclusions This is the first time that the Rhaetian has been biostratigraphically character-ized and truly recognized in the Lyons area that we have studied. In the past, it was tentatively located below the first levels in which Psiloceras planorbis (lowermost Hettangian) is found and above the Upper Triassic facies. It is also the first time that a complete Rhaetian section is described in this area. Correlating the studied sites through the defined palynological associations (Fig. 9) indicates that the Rhaetian deposits developed between near-shore and open marine conditions.
Key words: Palynomorphs, Biostratigraphy, Rhaetian, Rhône, France, Belmont, Saint-Didier-au-Mont-d’Or
Dernière mise à jour le 02.07.2015