Maceral and calcareous nannofossil assemblages as proxies of late Rupelian (Oligocene) environmental changes in the Paratethys: An example from a section of the Menilite Formation in the northern Outer Carpathians

The Menilite Formation (Oligocene) of the Skole Nappe in the Polish Carpathians was deposited in the Para-tethys. Deep-sea unbioturbated fine-grained carbonates of the Dyn ' ow Marl Member contain cold-water calcareous nannoplankton from Zone NP23 (Rupelian), including Reticulofenestra ornata and Pontosphaera fibula, which are typical of brackish waters in the Paratethys. It is likely that the water column was brackish in the upper part and fully marine and anoxic in the lower part. The Dyn ' ow Marl Member records the maximum isolation of the Paratethys from the oceanic circulation (the Upper Solenovian Event). The overlying green mudstones (probably the Krepak Member) are shallowly bioturbated. They contain fully marine, warm-water calcareous nannoplankton from Zone NP24. The environmental change (marine/brackish) is reflected in the quantity and type of organic matter. The abundance and preservation of organic matter are very high (TOC varies from 2.7 wt % to 26 wt %; HI values are >300 mg HC/g TOC) in brown, laminated shales, which were accumulated in saline anoxic conditions. The macerals in the shales are predominantly composed of the liptinite group, including lamalginite, telalginite, liptodetrinite, and bituminite. These macerals are primarily derived from planktonic and benthic algae as well as bacteria. The percentages of vitrinite and inertinite, originating from terrestrial sources, range up to 10% and 0.5%, respectively. The macerals are much less abundant in marls of the Dyn ' ow Marl Member because of dispersion in the carbonate groundmass during the calcareous nannoplankton blooms in brackish surface waters. The overall liptinite (mainly liptodetrinite and lamalginite) contribution is 77%-99.7%. Vitrinite and inertinite contents are in average 4.6% and 0.3%, respectively. Macerals are also infrequent in the overlying green mudstones, likely because of oxidation and consumption by organisms in warmer waters.

Automated summary

This study investigates the geological environment and organic matter characteristics of the Skole Nappe in the Polish Carpathians. The research reveals that the water column in this area during the Oligocene period was likely divided into tidal water in the upper part and fully marine and anoxic water in the lower part. The brown shale contained abundant organic matter, while the green mudstones had lower organic matter content, potentially due to oxidation and consumption by organisms in warmer waters.