ASGP (2005), vol. 75: 287-308

ORIGIN OF CHERT NODULES FROM THE POLISH MUSCHELKALK, MIDDLE TRIASSIC

Stanisław KWIATKOWSKI

Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Kraków, Senacka 1, 31-002 Kraków, Poland

Kwiatkowski, S., 2005. Origin of chert nodules from the Polish Muschelkalk, Middle Triassic. Annales Societatis Geologorum Poloniae, 75: 287-308.

Abstract: In the Muschelkalk of Poland, carbonate nodules and chert nodules (siliceous and silico-carbonate ones) occur in two regions: the Upper Silesia and the Holy Cross Mountains. The chert nodules were formed by silicification of carbonate deposit, what is proved by silicified skeletal remains of carbonate fauna. The primary deposit formed in aerobic and sometimes evaporitic conditions was permeable, not lithified, and contained some per cent of SiO2.
The silicification advanced either radially from the centre outwards, or from the outer surface inwards. In the latter case, a precursor of chert nodule was probably a carbonate nodule. The formation of chert nodules proceeded in two stages. In the first stage, the primary fabrics of rock were silicified but preserved, while in the second one they were destroyed. Simultaneously, the organic matter and iron were removed. Lithification advanced from the outer surface inwards. At first, the whole nodule was in a semi-plastic state, what is proved by the occurrence of plastic deformations. Some deformations have been induced by earthquake tremors. With aging of the nodule, only its outer, indurated part underwent cracking. The cortex was developed mostly after nodule's lithification at the expense of the outer part of the nodule, and it entered into the nodule's interior along the cracks. The transformations in particular nodules were stopped in various stages of their development.