Name: La Voulte-sur-Rhône
Location: Ardèche, France
Age: Lower Callovian, Jurassic


Taphonomy


The fossils at La Voulte-sur-Rhône show special preservation where the soft tissues have been preserved. They are found in two places, in concretions and in the shales surrounding them.

Concretions

Organisms are preserved three dimensionally and the soft tissues were replaced by a suite of minerals: - apatite, calcite, gypsum, pyrite, chalcopyrite, galena.

These form a diagenetic sequence where the minerals replace and/ or coat the previous ones. As diagenesis progressed, more and more anatomical detail was lost.

 

 Diagenetic sequence

Surrounding Shales

The organisms are flattened and their soft tissues are replaced mainly by apatite and pyrite with the intervening areas filled with calcite.

Thin layers of sediment found between the arms of cephalopods indicate that the allochthonous fauna was introduced by turbidity currents from an oxygenated area.

The in situ bivalve (Bositra buchi) is still articulated. If they had been transported any distance the articulation would be broken. An anaerobic event killed them all off at the same time.

Diagenetic minerals

Apatite is the commonest mineral and is responsible for preservation of the greatest detail (individuals cells are still visible). It is only found on soft tissues that decay easily, such as muscle tissue. There is a decreasing gradient of phosphatization (apatite formation) from the outside of the muscle to the inside.

Calcite replaces phosphatized muscle, obliterating the fine details of the muscle fibres and retaining only the gross morphology. Calcite also fills fractures in phosphatised muscle fibres.

Gypsum sometimes replaces phosphatised muscle tissues instead of calcite. Replacement occurs in irregular patches unlike the smooth coats formed by pyrite.

Pyritization occurs after phosphatisation, either at the same time or after the earliest stages of calcite precipitation. If there is too much pyritisation, detail is obscured. If there is too little pyritisation, the specimen disintegrates. Pyrite coats gut, gills, carapace and the phosphatised thick muscle fibres in crustaceans. Pyrite is the most important mineral for soft part preservation in squid and plants, and it replaces the valves of Bositra buchi.

Chalcopyrite is a minor phase associated with pyrite.

Galena replaces earlier mineralised tissues (especially if phosphatised) and preferentially develops on pyrite. It is only found in specific zones around fossils, and only preserves the gross morphology.