The onset of orogenic activity recorded in the Krosno shales from the Grybów unit (Polish Outer Carpathians)

Abstract

The Krosno shales were deposited synorogenically in front of an accretionary prism as interchannel flysch facies on a north-dipping slope that constituted the southern marginal part of the Silesian basin. The turbidite flows originated from channelised currents and probably also as separate sedimentation events from slow, dilute “sheet” flows derived from a linear source (probably a shelf-edge). In the background continuous hemipelagic deposition took place and there were two periods when pelagic sedimentation prevailed. Mean accumulation rate of the entire succession was moderate: from 8 to 11 cm/ky. The source rocks were Jurassic to Eocene sedimentary rocks. Geochemical data (La, Th and Sc contents) point to a continental island arc as the original source of the detrital material. Soft-sediment deformations and methane-related authigenic carbonates that are also present in this succession evidence the migration of methane-charged fluids through the sediment column. This fluid expulsion was probably provoked by orogenic activity, i.e. the formation of an accretionary prism.