Oligocene cold-seep carbonates from the Carpathians and their inferred relation to gas hydrates

Abstract

The Krosno Formation of the Outer Carpathians is composed of synorogenic deposits laid down in the Silesian foredeep basin in front of an accretionary prism. The Oligocene shales of the Krosno Formation from Świątkowa Wielka (the Polish part of the Outer Carpathians) contain numerous authigenic carbonate rocks: concretions, a laminated limestone bed, and a carbonate build-up consisting of intraformational breccia. The application of stable carbon isotope analysis revealed that the formation of these carbonates was induced by methane oxidation. The presence of fossilized giant Beggiatoa-like filaments and large quantities of framboidal pyrite indicate that methane oxidation was microbially driven and coupled with sulfate reduction. A model of origin of these cold-seep carbonates in relation to hydrocarbon seepage is herein presented. Characteristic druses with clast-like appearance are thought to inherit their outlines from former clasts of gas hydrate that had been present within the build-up. It is proposed that thick gas hydrate deposits existed in the southern part of the Silesian basin at that time. This theory is consistent with regional, geotectonic, and palaeobathymetric data and provides a ready explanation of the major phenomena that have been recorded in the Krosno Formation.