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Pyritization of microfossils: crinoid remains from the Middle Jurassic of Ogrodzieniec (Kraków-Częstochowa Upland, Poland)

Szczepanik P., Sawłowicz Z.

Studia Geologica Polonica

2005

Vol. 124

37-52

Pyritized crinoid skeletal elements have been found in the so-called "ore-bearing clays" of the Middle Jurassic exposed in Ogrodzieniec (Kraków-Częstochowa Upland, Poland). Their assem- blage consists of columnals, cirrals and brachials; calyx plates have not been found. Ossicles occur both as unpyritized and pyritized. Three main types of pyritization have been distinguished in investigated material: (a) original calcitic skeleton is not replaced by pyrite but its void spaces are infilled with pyrite; (b) calcitic skeleton is replaced by pyrite, with or without void infilling; (c) pyritization overwhelms the primary morphology of the ossicle. The first two types predominate in the studied material. The types of pyritization have been explained by several subsequent stages of this process. The main stage of crinoid pyritization happened probably in the sediment during early diagenesis and was limited to microenvironments of fossils. Different morphological forms of pyrite registered in the same ossicles (euhedra, framboids and massive pyrite, can be explained by different position and time of the fossil pyritization, the type and location of organic matter, porosity, several stages of pyritization, "openness" of skeleton, different assemblages of bacteria, and probably many other factors.

Pyrite framboids in pyritized Radiolarian skeletons (Mid-Cretaceous of the Pieniny Klippen Belt, Western Carpathians, Poland)

Szczepanik P., Sawłowicz Z., Bąk M.

Annales Societatis Geologorum Poloniae

2004

Vol. 74, No 1

35--41

Well preserved pyritized radiolarian skeletons have been found within the grey-green Mid-Cretaceous (Upper Cenomanian) shales in the Pieniny Klippen Belt (Carpathians, Poland). The skeletons contain numerous pyrite framboids in different positions, in channels and inside the abdomen of cryptothoracic forms, but their genetic context is not known. They were formed as a result of the reaction between dissolved iron and sulphide originated from the bacterial sulphate reduction. Two sources of organic matter, "post mortem" in situ decaying organic matter of radiolaria and disseminated organic matter from the surrounding sediment could be available for this process. Pyrite found in the radiolarians probably originates from different processes. It is suggested that pyritization of the radiolarian skeletons took place in the water column whereas pyrite framboids in the skeleton's free spaces could have been formed later during the diagenesis of the sediment. However, their simultaneous formation in the water column or in the sediment cannot be excluded.