Origin and significance of early-diagenetic calcite concretions and barite from Silurian black shales in the East European Craton, Poland
Authors
Maciej J. Bojanowski
ING PAN – Institute of Geological Sciences, Polish Academy of Sciences, Twarda 51/55,
PL-00-818 Warszawa
Artur Kędzior
ING PAN – Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Cracow, Senacka 1, PL-31-002 Kraków
Szczepan J. Porębski
AGH University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, Mickiewicza 30, PL-30-059 Kraków
Magdalena Radzikowska
ING PAN – Institute of Geological Sciences, Polish Academy of Sciences, Twarda 51/55,
PL-00-818 Warszawa
Keywords:
Carbonate concretions, Stable isotopes, Cathodoluminoscopy, Sedimentation rate, Marine productivity, Biogenic gas production, Bacterial sulfate reduction, Shale gas
Abstract
The Silurian Pelplin Formation is a part of a thick, mud-prone distal fill of the Caledonian foredeep, which stretches along the western margin of the East European Craton. The Pelplin Formation consists of organic carbon-rich mudstones that have recently been the target of intensive investigations, as they represent a potential source of shale gas. The Pelplin mudstones host numerous calcite concretions containing authigenic pyrite and barite. Mineralogical and petrographic examination (XRD, optical microscopy, cathodoluminoscopy, SEM-EDS) and stable isotope analyses (δ13Corg, δ13C and δ18O of carbonates, δ34S and δ18O of barite) were carried out in order to understand the diagenetic conditions that led to precipitation of this carbonate-sulfide-sulfate paragenesis and to see if the concretions can enhance the understanding of sedimentary settings in the Baltic and Lublin basins during the Silurian. Barite formed during early diagenesis before and during the concretionary growth due to a deceleration of sedimentation during increased primary productivity. The main stages of concretionary growth took place in yet uncompacted sediments shortly after their deposition in the sulfate reduction zone. This precompactional cementation led to preferential preservation of original sedimentary structures, faunal assemblages and early- diagenetic barite, which have been mostly lost in the surrounding mudstones during burial. These components allowed for the reconstruction of important paleoenvironmental conditions in the Baltic and Lublin basins, such as depth, proximity to the detrital orogenic source and marine primary productivity. Investigation of the concretions also enabled estimation of the magnitude of mechanical compaction of the mudstones and calculation of original sedimentation rates. Moreover, it showed that biogenic methane was produced at an early-diagenetic stage, whereas thermogenic hydrocarbons migrated through the Pelplin Formation during deep burial.
