Record of palaeoenvironmental changes in a Lower Permian organic-rich lacustrine succession: Integrated sedimentological and geochemical study of the Rudnik member, Krkonose Piedmont Basin, Czech Republic
Abstract This study presents an integrated sedimentological and geochemical analysis of the Lower Permian Rudnik member — the most extensive lacustrine deposits in the Krkonose Piedmont Basin. Grey to black and variegated lacustrine mudstones, laminites and carbonates of the Rudnik member have a lateral extent of more than 400 km 2 . In the studied sections four facies associations were recognised: A) anoxic offshore, B) suboxic to oxic offshore, C) nearshore and mudflat, and D) slope deposits. In the northern part of the E–W elongated basin, anoxic to suboxic organic-rich offshore lacustrine facies dominate and form a succession up to 130 m thick. Fan-delta and turbidite facies occur locally along the faulted northern basin margin. The central part of the basin is occupied by anoxic to oxic offshore facies interfingering with nearshore carbonate and mudflat facies of the low-gradient lacustrine margin. In the central part of the basin, the thickness of the lacustrine deposits of the Rudnik member reaches up to 60–70 m. In the southern part of the basin fluvial and alluvial plain facies dominate and alternate with minor lacustrine nearshore facies. The lateral facies distribution indicates that subsidence along the northern basin fault was the main mechanism generating the asymmetric infill geometry in the basin's half-graben setting. The δ 1 8 O values of primary and early diagenetic calcite range between − 11.0‰ and + 1.3‰ (V-PDB) and δ 1 3 C values between − 5.1‰ and + 3.7‰; most of the data fall within the range of freshwater limestones. Coarser-grained pure microspar laminae show more positive δ 1 3 C values in comparison to clayey organic-rich micrite laminae, and are interpreted as a record of bioinduced precipitation during seasonal eutrophication. The obtained δ 1 3 C TOC values range from − 29.0‰ to − 24.0‰, the total organic carbon (TOC) content from 0.26% to 23%. Maceral analysis and Rock-Eval pyrolysis indicate that most of the samples contain a mixture of aquatic and terrestrial organic matter, but two minor, distinctive groups of samples with algally-dominated and terrestrially-dominated organic matter composition, respectively, were also found. The study of vertical changes in boron content in the clay fraction of the lacustrine mudstones shows that high lake level stages were periods of lower salinity with the lowest boron contents (from 73 to 268 ppm), and periods of falling lake level were followed by significant increases in salinity with much higher boron values (293–603 ppm). Lake-level fluctuations of the Rudnik lacustrine system, which are recorded by shallowing-up units of sedimentary facies within most of the sections throughout the basin, can also be traced within the monotonous black shale dominated sections, where no sedimentological evidence of these lake level changes exists. Good indicators for such changes seem to be the δ 1 8 O and δ 1 3 C values of primary calcite, δ 1 3 C TOC and HI. These lake level fluctuations are interpreted as driven by climatic oscillations in the order of tens of thousands years, which could reflect climatic changes connected with the last glaciation event of Gondwana.