Pyritization of dinoflagellate cysts : a case study from the Polish Middle Jurassic (Bathonian)

Bathonian mudstones and claystones were deposited in a shallow epicontinental sea of the Polish Basin (the Ore-Bearing Częstochowa Clay Formation). They are represented by two units, differing in stratigraphical and facies development. Dinoflagellate cyst taxa are dominated by Ctenidodinium. The effects of the environment and sedimentation rate on pyritization intensity were compared using two locations. Where the sedimentation rate was probably faster and grey mudstones were deposited under dysoxic conditions, the extent of cyst pyritization was relatively minor. In contrast, where the deposition of black claystones was slower with deposition under more suboxic conditions, very intense pyritization led to complete infilling of cysts by pyrite. Pyritization required transport of iron, probably from microbial iron reduction, and sulphide, from bacterial sulphate reduction, from the surrounding sediment into the open dinoflagellate cysts. The supply of sulphur was the limiting factor. Controls on pyrite crystal habit by the organic composition of the dinoflagellate cysts walls and cells are inferred by the observation that dinoflagellate cysts uniquely contain mainly cuboid pyrite crystals