Les formations coralliennes de Saint-Germain-de-Joux (Ain)

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Abstract
Coralline formations in the Saint-Germain-de-Joux area (France) are dated as Kimmeridgian (upper Jurassic) based on their position in relation to ammonite-bearing horizons. Only the lower formation is truly coralline, characterized by the juxtaposition of columnar coral build-ups and limestone detritus. The columnar accumulations reach a height of 50-60 meters and never exceed a few meters in diameter. They are composed of branching corals grouped in 'bunches' and toward the top contain massive colonies. The pericoralline sediments exhibit no bedding and constitute the major part of the formation. An interval of compact limestone and algal nodules separates the lower and upper formations. The upper formation contains no coral structures and is composed almost entirely of pseudo-oolitic limestones, more well-bedded than the lower formation. The coralline formations resemble the Valfin reef but surpass them in size, completeness and preservation. The parallel and linear disposition of these chronologically equivalent zones suggests development in a barrier reef or channel.

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