THE FIRST RECORD ON THE PALEOBACTERIAL OBJECTS FINDINGS ON THE WALLS OF FORAMINIFERAL TESTS FROM THE LOWER OLIGOCENE OF SOUTHERN UKRAINE AND THEIR IMPORTANCE FOR THE RECONSTRUCTION OF SEDIMENTOGENESIS CONDITIONS

Abstract

The article presents the first information about the paleobacterial objects which have been found on the walls of the foraminiferal tests from the Borysthenian (Planorbella) Regional Stage of the Lower Oligocene of Southern Ukraine. 145 agglutinated and calcareous tests have been selected and studied from the Lower Oligocene (Borisphenian) sediments opened by boreholes in the South-Western Crimea and the Northern Black Sea region. Microscopic objects have been established on the walls of 50 tests and attributed to paleobacterial formations on the basis of a number of characteristics. Most of the bacterial objects are represented by biogenic sulphides, as well as bacterial films and mats found on foraminifera tests of various taxonomic affiliation. Coccoid clusters are often observed on the tests of agglutinating foraminifera. Chlorophytes of cyanobionts (?) are frequently seen on the calcareous and finely agglutinated tests. Actinomycetes (?) are found on calcareous and agglutinating foraminifera, mainly on infauna.
An analysis of the infauna-epifauna ratio in foraminifera associations suggested that gas regime was fluctuating in the Early Oligocene (Borysthenian) basin of Southern Ukraine mainly from low oxic to sub- and dysoxic conditions. Rare episodes of oxygenation increasing contributed to the development of epifauna with a large test (representatives of the genera Neogyroidina, Baggina, Heterolepa, Cibicidoides), as well as the appearance of planktonic foraminifera. Sometimes, the gas regime changed to anoxic, as indicated by the intervals without the fauna of foraminifera. At the end of the Borysthenian basin, anoxic conditions were presumably established. A significant difference between the initial composition of foraminifera in biocenoses and their composition in oryctocoenoses is assumed. Frequent findings of remains of paleobacterial communities and traces of their vital activity in the foraminiferal tests suggested these communities could have an important influence on the reservation and completeness of the oryctocoenoses, being involved in the processes of diagenesis. 

Key words: microbioliths, foraminifera, Borysthenian Regional Stage, Maikop Series, Lower Oligocene, Southern Ukraine.