Large microbial-foraminiferal oncoids from condensed Lower–Middle Jurassic deposits: a case study from the Tatra Mountains, Poland

Microbial-foraminiferal macrooncoids have been found in the Toarcian–Aalenian limestones of the Krížna unit in the Tatra Mountains, Western Carpathians. The oncoids were formed by the consortium of microbes (?bacteria and fungi) and encrusting foraminifers dominated by Nubecularia aff. mazoviensis, Dolosella sp. 1 and Tolypammina sp. The microbial bodies and biofilms are mineralized with iron hydrooxides and/or oxides. The association of foraminifers with microbes is supposed to be an adaptation of foraminifers to oligotrophic conditions on the sea floor. It is very likely that the biofilm served as a continuous food source for the encrusting foraminifers. Formation of the oncoids was possible under conditions of low rates of deposition and periodic, weak water agitation. Deposition of the oncoid-bearing limestones took place during gradual deepening of the basin and associated with increasing oligotrophy at the sea floor.