MIDDLE MIOCENE (EARLY BADENIAN) AMPHISTEGINA (FORAMINIFERIDA) FROM OLIMPóW (CENTRAL PARATETHYS, POLAND)

摘要

Specimens of Middle Miocene (Early Badenian) Amphistegina from the Polish Carpathians were examined. A section, sampled at the Olimpów quarry, displays chloralgal carbonates, rich in rhodolith nodules. Symbiont-bearing foraminifera ( Amphistegina , Elphidium and Asterigerinata ) constitute 15–57% of the total foraminifera assemblage. Amphistegina hauerina d’Orbigny, 1846 is the only species of the genus in the deposits studied. The sensitivity of larger foraminifera to water turbulence and light availability was applied in a reconstruction of the community habitat. The diameter and thickness of the Amphistegina tests indicate that the habitat was a zone of low light penetration and weak energy – a palaeoenvironment, similar to that of the modern A. radiata (Hottinger et al., 1993). Changes in the sphericity of the Amphistegina tests reflect a temporal drop of bathymetry in the middle part of the section. This is supported by other environmentally controlled features of the foraminiferal assemblage the percentages of both planktonic and symbiont-bearing forms, as well as the proportions of robust and flat forms of Elphidium . It is possible that this variation in depth was due to eustatic changes in sea level, caused by Event Mi3 of the Middle Miocene climatic cooling. The spiral diameter of the test was at its lowest value then, indicating that maturity was reached at a faster rate. The period of shoaling offered the best life conditions for Amphistegina in this particular area.