Origin and significance of early-diagenetic calcite concretions and barite from Silurian black shales in the East European Craton, Poland

Abstract

The Silurian Pelplin Formation is a part of a thick, mud-prone distal fill of the Caledonian foredeep, which\xa0stretches 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\xa0source of shale gas. The Pelplin mudstones host numerous calcite concretions containing authigenic pyrite and\xa0barite. Mineralogical and petrographic examination (XRD, optical microscopy, cathodoluminoscopy, SEM-EDS)\xa0and stable isotope analyses (δ13Corg, δ13C and δ18O of carbonates, δ34S and δ18O of barite) were carried out in order\xa0to understand the diagenetic conditions that led to precipitation of this carbonate-sulfide-sulfate paragenesis and\xa0to see if the concretions can enhance the understanding of sedimentary settings in the Baltic and Lublin basins\xa0during the Silurian. Barite formed during early diagenesis before and during the concretionary growth due to a\xa0deceleration 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\xa0cementation led to preferential preservation of original sedimentary structures, faunal assemblages and\xa0early- diagenetic barite, which have been mostly lost in the surrounding mudstones during burial. These components\xa0allowed for the reconstruction of important paleoenvironmental conditions in the Baltic and Lublin basins,\xa0such as depth, proximity to the detrital orogenic source and marine primary productivity. Investigation of the\xa0concretions also enabled estimation of the magnitude of mechanical compaction of the mudstones and calculation\xa0of original sedimentation rates. Moreover, it showed that biogenic methane was produced at an early-diagenetic\xa0stage, whereas thermogenic hydrocarbons migrated through the Pelplin Formation during deep burial.