High-resolution petrography of marls from Goleszow (Polish Outer Carpathians, Upper Jurassic, Vendryne Formation)

In the Ghibaudo (1992) classification, the Upper Jurassic Lower Cieszyn Shales (Vendryne Formation) exposed in Goleszow (southern Poland) show sedimentological features typical of the MyG (muddy gravel beds) facies. Two lithic components of this facies have been petrographically studied: calcareous shales, which are principal constituent of the olistostrome matrix, and the rocks displaying field characteristics of marls, which occur as olistoliths. Two factors controlling the mode of intrabasinal redeposition have been recognized: the primary depositional environment, and the presence of calcareous nannofossils in the original sediments. Because the rocks occurring as olistoliths and those forming the matrix are compositionally similar but differ in grain size and clay abundance, it is reasonable to assume that the matrix rocks were originally deposited in quieter water conditions than the future olistoliths. Despite post-sedimentary modification, FESEM/BS imagery of the marls from the olistoliths reveals coccoliths in their groundmass, which are hold together with calcite overgrowth cement; this feature is eogenetic. Thus, the presence of coccoliths appears to be the crucial factor that made possible early hardening of the sediments and subsequently their redeposition as lithic blocks. This process in the starting sediments for the olistostrome matrix was inhibited by clay. Thus, they remained unconsolidated and were then redeposited as muds.