Fluvial depositional environment evolving into deltaic setting with marine influences in the buntsandstein of northern vosges (France)

Abstract

The Buntsandstein in the Northern Vosges (France) originates mainly in an inland braidplain fluvial environment which passes in the upper part of the sequence into deltaic milieu in the coastal plain along the border of the sea, with the continental environment finally being drowned with the transgression of the shallow sea. The fluvial sedimentation is characterized by the presence of two facies throughout the Buntsandstein : channel facies and overbank plain facies. The channel facies comprises sandy and conglomeratic deposits forming within active streams by strong currents, whereas the overbank plain facies is built up of silty-clayey sandstones or silt/clay originating in stagnant water in abandoned watercourses, ponds, pools and puddles. The significance of particularly the floodplain sediments is subjected to considerable changes throughout the Buntsandstein sequence. There are all stages of transition between overbank plain deposits being only preserved in ghost-like facies as reworked clasts due to effective secondary removal of primarily occasionally formed suspension fines, and an abundance of autochthonous floodplain sediments in the depositional record resulting from favourable conditions of primary origin and secondary preservation. Reworked ventifacts within fluvial channel sediments testify to subordinate aeolian influences in the alluvial plain, with reasonable reworking, however, having removed all in situ traces of wind activity. Declining aridity of palaeoclimate towards the top is indicated by the appearance of violet horizon palaeosols in the Zone-Limite-Violette and the Couches intermediaires being accompanied by Brockelbank carbonate breccias originating from concentration of reworked fragments of pedogenic carbonate nodules. Biogenic traces are in the lower part of the sequence mainly present as Planolites burrows in the finer-grained sediments. Palaeosalinities as revealed from boron contents indicate progressively increasing supersaturation of stagnant waters with time. The fluvial environment persists up to the lower part of the Gres a Voltzia where the progression of the sea towards the west gives rise to a close intertonguing of fluvial and marine influences in a deltaic setting. Lenticular sandstone bodies are laid down as stream mouth bars at the end of the distributary channels and as river bars in the watercourses during both normal and flood discharge. Silty-clayey sediments settle out in stagnant water in restricted ponds, pools and puddles as well as in extensive veneers of shallow water in the overbank plain between the streams. Carbonate-bearing deposits originate in the coastal littoral mud flat, marsh seam, beach belt and tidal flat. The Gres a Voltzia has the greatest palaeoenvironmental and palaeoecological significance in the Buntsandstein of the Northern Vosges due to the occurrence of a wealth of extraordinarily well-preserved plant and animal fossils (having been recovered by Louis Grauvogel during almost 50 years and since abt. 25 years by Jean-Claude Gall). The rich suite of faunal and floral elements includes aquatic invertebrates, terrestrial animals and continental plants. The aquatic invertebrate fauna lives in fresh lakes and brackish ponds in the overbank plain and in brackish lagoons in the coastal seam as well as in hypersaline and euhaline marginal marine waters. The terrestrial plants colonize both dry and wet substrates, and the continental fauna consists of mainly arthropods, amphibians and reptiles inhabiting the levee zones of standing and flowing waters and strolling across the desiccated flats. The marine euryhaline association of invertebrates is with time replaced by a stenohaline community, and the deltaic plain of the Gres a Voltzia is finally inundated by a pellicular transgression representing the first stage of the Muschelkalk sea setting an end to Buntsandstein continental deposition.