Résumé
Les résidus palynologiques des argilites et grès silteux du Trias de la vallée de l’Azergues (nord-ouest de Lyon) fournissent des sporomorphes qui permettent de dater les faciès argileux noirs, riches en empreintes de halite et gypse de la limite Fassanien-Langobardien (Ladinien inférieur/Ladinien supérieur). Dominées par les grains de pollens Triadispora aurea, les associations microfloristiques des argilites noires sont le reflet d’un environnement de dépôt de type plaine deltaïque hypersaline à dominance de marées. Les argilites versicolores sous-jacentes, et celles vertes et lie-de-vin surincombantes sont interprétées comme des dépôts de plaine d’inondation. Outre les sporomorphes, les résidus palynologiques renferment des débris de bois (phytoclastes). Parmi les phytoclastes, les variations des quantités relatives des particules en aiguilles sont utilisées comme marqueur de la variation de la hauteur d’eau prévalante dans les milieux de dépôts considérés. La combinaison des paramètres typiques pris chez les sporomorphes et les phytoclastes permet de définir six types de palynofaciès. Deux d’entre eux caractérisent à la fois les environnements de plaine deltaïque et de plaine d’inondation et les quatre autres sont restreints à la plaine deltaïque. L’évolution des palynofaciès montre une bonne analogie avec la variation de la hauteur d’eau, telle qu’il est permis de l’apprécier à travers les variations des épaisseurs des unités génétiques sédimentologiques.
Mots-clés : Palynomorphe, Phytoclaste, Palynofaciès, Ladinien, Milieu deltaïque, Milieu hypersalin, Rhône, Vallée de l’Azergues.
Abstract
In the Triassic sediments of the Azergues valley (NW of Lyons, France; Fig. 1) kerogens of the green and reddish shales consist only of woody phytoclasts whereas the black shales and silty sandstones also contain sporomorphs. The sporomorph diversity in the black shales and associated sandstones is moderate (16 species of spores; 19 species of pollen; 1 species of lacustrine algae; 3 species of marine algae), with specimens of the bisaccate genus Triadispora (T. aurea and T. suspecta) being dominant among the spores and pollen associations (Fig. 2). Other types of spores and pollens are nowhere abundant (a few percent). Lacustrine palynomorphs (Plaesiodictyon mosellanum) and marine palynomorphs (Cymatiosphaera and Michrystridium) are rare. Abundant small smooth-walled palynomorphs are found locally; they are tentatively attributed to the genus Leiosphaeridia (probably a prasinophyte) and considered as marine elements. The simultaneous presence of Echinitosporites iliacoïdes, Heliosaccus dimorphus and Ovalipollis pseudoalatus and the absence of some characteristic palynomorphs, such as polysaccoid pollens, suggest that the black shales are Fassanian-Langobardian in age (Table 1). Triadispora aurea is known to be produced by halophytes and thermophytes growing in salinas and coastal sabkhas. Equisetophytic spores (Calamospora) and fern spores (Verrucosi-sporites) reflect a relative proximity of hygrophytic swamp or marsh vegetation, probably indicative of fresh-water-influenced delta ecosystems. The dominance of halophytic pollen, the scarcity of hygrophytic spores and the sporadic presence of marine elements reflect a wave-dominated hypersaline delta plain. Palynomorphs are very rare or absent in the green shales, below the black shales, and in the overlying reddish shales. When present they consist only of Triadispora, facies that are considered to be deposited in a floodplain environment. Woody phytoclasts are another important group of kerogen constituents in the shales (Fig. 3). The use of the slightly adapted classification of Whitaker (1984) allows the recognition of PM4E (black equidimensional particles; arithmetical mean: 58%) as the dominant class, followed by PM1E (brown equidimensional particles; 16%), PM4T (black lath-shaped particles; 15%), PM2 (yellow to brown woody tissue; 6%), PM1T (brown lath-shaped particles; 5%) and PM3 (yellow to brown cuticle; less than 1%). These different classes constitute the inertinite (73%) and vitrinite (27%) groups. From a morphological point of view, equidimensional phytoclasts (noted E) are more abundant (74%) than the lath-shaped ones (noted T; 26%). The different phytoclast and sporomorph associations permit the definition of six palynofacies types (Table 2) based on the presence/absence of leiospheres and the relative abundances of PM4T, PM2 and PM3. Two types (palynofacies A and C) characterize deposits from both the wave-dominated hypersaline delta plain and the flood plain; the other four types are exclusive to the wave-dominated type saline delta plain. The palynofacies types and their relative constituent abundances are: Palynofacies A: no leiospheres; PM4T less than 14%; PM2+PM3 less than 6%. Complementary characteristics: Triadispora from 0 to 100%; 0 to 7 spore taxa; 0 to 5 pollen taxa; vitrinite variable. Palynofacies B: no leiospheres; PM4T less than 14%; PM2+PM3 greater than or equal to 6%. Complementary characteristics: Triadispora greater than 94%; 0 to 3 spore taxa; vitrinite from 20 to 35%. Palynofacies C: no leiospheres; PM4T greater than 14%; PM2+PM3 greater than or equal to 6%. Complementary characteristics: Triadispora from 0 to 98%; vitrinite from 25 to 30%. Palynofacies D: no leiospheres; PM4T greater than 14%; PM2+PM3 less than 6%. Complementary characteristics: Triadispora greater than 85%; 6 to 12 spore taxa; 3 to 9 pollen taxa (this type shows the greatest specific diversity of sporomorphs); vitrinite from 28 to 36%. • Palynofacies E: presence of leiospheres; PM4T greater than 14%; PM2+PM3 less than 6%. Complementary characteristics: Triadispora from 57 to 88%; PM4E from 70 to 84%; high specific diversity of sporomorphs; vitrinite from 5 to 15%. This type has the greatest abundance of PM4E phytoclasts and the lowest quantity of vitrinite. Palynofacies F: presence of leiospheres; PM4T greater than 14%; PM2+ PM3 greater than or equal to 6%. Complementary characteristics: Triadi-spora from 58 to 82%; specific diversity of sporomorphs variable; vitrinite from 20 to 41%. Among the parameters used, the lath-shaped particles (PM1T and PM4T) are of particular interest because their relative abundance shows an increase from base to top. This increase is interpreted as indicative of increasing water depth. Consequently, the palynofacies with the lowest proportions of lath-shaped particles (A to D) are interpreted as the most continental deposits and those with the greatest abundance as the least continental deposits. The palynofacies trend thus reflects water depth variation in the delta plain ecosystem, a concept that is supported by the good analogy between the water depth curve established from both the coarsening-up/fining-up cycles of the genetic units and the palynofacies curve (Fig. 4). This demonstrates that the use of woody phytoclasts can be a pertinent tool in palaeoenvironmental reconstruction.
Key words: Palynomorphs, Phytoclasts, Palynofacies, Deltaic environment, Hypersaline environment, Ladinian, Rhône France, Azergues valley.
Dernière mise à jour le 02.07.2015