Large microbial-foraminiferal oncoids from condensed Lower–Middle Jurassic deposits: a case study from the Tatra Mountains, Poland Michal Gradzin ´ski a, * , Jaroslaw Tyszka b , Alfred Uchman a , Renata Jach a a Institute of Geological Sciences, Jagiellonian University, ul. Oleandry Str. 2a, 30-063 Krako ´w, Poland b Institute of Geological Sciences, Polish Academy of Sciences, Krako ´w Research Center, ul. Senacka 1, 31-002 Krako ´w, Poland Received 18 September 2003; received in revised form 4 June 2004; accepted 6 July 2004 Abstract Microbial-foraminiferal macrooncoids have been found in the Toarcian–Aalenian limestones of the Krı ´z ˇna 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. D 2004 Elsevier B.V. All rights reserved. Keywords: Macrooncoids; Iron oxides; Biomineralization; Western Carpathians; Tethys 1. Introduction Most oncoids are formed by photosynthetic cyanobacteria and algae in shallow-water environ- ments (Peryt, 1981, 1983; Tucker and Wright, 1990). However, oncoids originated in deeper water envi- ronments are also known. They are relatively rare and much less investigated (but see e.g., Massari, 1983; Massari and Dieni, 1983; Dromart et al., 1994; Mis ˇı ´k and S ˇ ucha, 1997). Such oncoids are inter- preted as formed by non-phototrophic organisms in dim or completely dark conditions. They are commonly built of non-carbonate minerals, such as iron and/or manganese hydrooxides, phosphorites and chlorites. Large red oncoids occur in the condensed Lower– Middle Jurassic deposits of the Krı ´z ˇna unit in the 0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2004.07.010 * Corresponding author. Fax: +48 12 633 22 70. E-mail addresses: gradzinm@ing.uj.edu.pl (M. Gradzin ´ski)8 ndtyszka@cyf-kr.edu.pl (J. Tyszka)8 fred@ing.uj.edu.pl (A. Uchman)8 jach@ing.uj.edu.pl (R. Jach). Palaeogeography, Palaeoclimatology, Palaeoecology 213 (2004) 133– 151 www.elsevier.com/locate/palaeo