Modern freshwater microbial carbonates: thePhormidium stromatolites (tufa-travertine) of southeastern Burgundy (Paris Basin, France)

The relationships of microstructure and vegetal remains, obtained by decalcification, were studied in Modern tufa from Burgundy, in order to try to link a given species with a particular crystal habitus. The edifices have various shapes (coatings on floors; encrusted pebbles, shells, vegetal shoots, mosses; oncolites; hydrodynamically shaped tufts). The biological content is rich in algae and animals, mainly at the proximity of springs, even if Phormidium incrustatum is the predominant species. It is associated with several species o fGongrosira, Schizothrix, and Oocardium stratum, the latter only known by its specific crystallizations. Among the animals, we point the galleries of Psychomiidae (Trichoptera= Phrygan) larvae. The algae and animals are associated within a “biological felt” (in the sense ofFOREL, 1901). Some species are encrusted by calcite crystals of typical habitus (micrite:Phormidium incrustatum, Gongrosira andSchizothrix, ssp; sparite:Oocardium and Batrachospermum), and there are very little diagenetic modifications. The fabric results in an alternation of seasonal light laminations composed of juxtaposed bundles of Phormidium incrustatum α, and dark laminations due to parallel filaments ofPhormidium incrustatum β. The influence of other algal species on shape and the internal fabric of the laminations is negligible.Phormidium incrustatum tufa are common in Western Europe, and probably have some fossil analogue in the Upper Cretaceous and Tertiary; the strongly differ from most older stromatolitic microstructures. Half of the studied tufa can suffer summer exposure and winter frost but related particular features do not seem to be preserved in the stromatolitic edifices.

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