Agglutinated benthic foraminifera in Ordovician and Silurian black mudrock facies of the Holy Cross Mountains (Poland) and their significance in recognition of oxygen content

The Upper Ordovician and Silurian dark to black shales and claystones in the northern Holy Cross Mountains (Poland) are transgressive facies developed in the early Sandbian, early Katian and early Sheinwoodian times, respectively. They were deposited under dysoxic to anoxic bottom waters interrupted, however, by short-term periods of oxygen increase. They contain lenticular and oval (and subordinately circular) structures composed of angular and well sorted micron-size quartz grains bound by cryptocrystalline silica cement. These structures are interpreted as remnants of agglutinated benthic foraminifera which have been members of opportunistic community inhibiting the subsurface sediment. They were able to survive of temporal anaerobic conditions at the bottom of sedimentary basin. Their high frequency in the Upper Ordovician black/dark claystones was facilitated by absence of grazing pressure from predators due to shift of the oxygen gradient above the sediment/water interface. However, in the case of the Sheinwoodian black shales, a high activity of predators and destructive impact of storm currents appear to be an important agents responsible for much lower frequency of foraminifera specimens.