Les palynomorphes du Rhétien du bassin de Lodève : stratigraphie et environnements de dépôts

Abstract

Palynology is a valuable stratigraphic tool for dating the Rhaetian in West European continental series lacking ammonoid fauna. The Rhaetian stage, of course, still remains imperfect because of the absence of a formal stratotype. In addition, its definition through palynology varies according to different authors. Here, adopting the time scale of Gradstein et al. (1994) for the Triassic period, we present new palynological data based on two sections (Pégairolles and Parlatges) of Rhaetian deposits in the Lodève Basin (southeastern Massif Central, France). At the top of the Triassic, a series has been termed the Rhaetian Formation owing to the presence of Avicula concorta. It is a significant lithostratigraphic unit (Lopez, 1992) composed of silicoclastic-calcareous sediments that overlie Keuper facies and are overlain by Hettangian mudstone from which Psiloceras planorbis had previously been recovered. Lopez (1992) subdivided this Rhaetian Formation into two members: a lower member representative of a littoral sedimentary complex with lagoonal deposits (shale and dolomitic mudstone) and fluvial argillaceous sandstone, and an upper member composed of calcareous shoal deposits (grainstone to wackestone) that represent an important transgressive period. The overlying Hettangian mudstone reflects a more stable sedimentation system that grades into restricted marine conditions. Palynological results and paleoenvironments As regards the paleoenvironmental setting, 52 samples were collected from the Keuper deposits through to the Hettangian carbonates. Four palynological assemblages were found. Association 1 is poorly calibrated because organic matter is rare; it is represented only by Ovalipollis pseudoalatus and thermophilic pollens such as Circulina meyeriana and Circulina granulata. This assemblage belongs to the Keuper, although the same microflora has been found locally in the upper member of the Rhaetian Formation in deposits reflecting hypersaline lagoonal conditions. The next two associations contain Rhaetian palynomorphs. Association 2 is representative of the lower member of the Rhaetian Formation (sensu Lopez, 1992), which contains more fossiliferous assemblages than the Triassic and corresponds to a less arid climat. Circulina spp. and O. pseudoalatus everywhere dominate the assemblages, but low percentages of other continental species are common, such as T. pseudomassulae, R. germanicus, O. cultus, Inaperturopollenites, Cycadopites, Abietineaepollenites, A. varius, A. echinatus, B. comaumensis, C. spiniger, D. mesozoïca, L. argenteaeformis, R. tuberculatus, T. microrugulatus and T. microverrucosus. Spores are poorly represented (>5%) and microplanktons, with predominant D. priscum and R. rhaetica, are not common (average 10%); the lagoonal or bay conditions indicated by the sedimentological data did not favour their development. Exceptionally, R. rhaetica dominates (50 to 90%) in lagoonal shaly siltstone and beach silty sandtone, and other phytoplankton species such as Beaumontella langi, B. delicatum, Micrhystridium lymensis, M. exilium and Veryahium formosum ancorostratum are present in places. Association 3 reflects an important microfloral change characterized by an enrichment in spores (with the first appearance of Kraeuselisporites reissingeri, Camarozosporites rudis, Retitrilites semimuris and Zebrasporites interscriptus) to the detriment of Circulina spp. and O. pseudoalatus. Marine palynomorphs such as D. priscum become dominant, indicating more open marine conditions. This last association is associated with high-energy carbonates of the upper member and is located just above a transgressive surface (Lopez, 1992; Haudiquet, 1997). The landward encroachement seems to have been detrimental to R. rhaetica, which becomes rare in carbonates. Association 4 is found either in the Hettangian carbonates or at the top of the Rhaetian Formation (sensu Lopez, 1992). This interval shows an increase in pollen grains such as Circulina spp. and Pollenites verrucosus. The spores have a proportionally lower presence, except for K. reissingeri and L. argenteaeformis (2 to 4%). Quantitatively, Michrystridium spp. replaces D. priscum, indicating more restricted marine conditions (Wall, 1965) in line with the sedimentological data. No typical Hettangian taxa have been recorded; although Todisporites, L. cerebriformis and L. adrienniformis only appear here for the first time, they are known elsewhere from older deposits.

Key words: Palynomorphs, Rhaetian, Paléoenvironment, Hérault France, Lodève France.