Biostratigraphy of the Konkian (Middle Miocene of the Eastern Paratethys) deposits of Southern Ukraine based on foraminifera

Abstract

The Konkian (Middle Miocene) foraminiferal assemblages and molluscs from five Wells situated in Southern Ukraine were studied in order to correlate the palaeoecology and biostratigraphy of the coeval palaeobasins with different environmental conditions. The article contains comprehensive analysis of controversial issues of the Konkian stratigraphy of the Eastern Paratethys and additional keys for determination of Konkian development phases by foraminifera and molluscs. Five stenohaline normal-marine, two euryhaline and one mixed foraminiferal assemblage were identified in different levels in the investigated wells with the analysis of their accordance to different molluscs assemblages. The study defines an isochronous foraminiferal assemblage for some wells, suggests a palaeoecological and stratigraphic reconstuction of the middle Miocene sediments in the Eastern Black Sea Region and recognizes two models of development of the Konkian foraminiferal assemblages. These models show the differences between environmental conditions in the shallow-water basin of the Eastern Black Sea Region and deeper water basin of the Kerch Peninsula at that time. While the Eastern Black Sea Region was influenced by frequent and abrupt changes in environmental conditions during almost all Konkian time, a relatively deeper basin of the Kerch Peninsula had a successive phase. The late Konkian time created similar depositional conditions for both basins. certain species of mollusсs were traced in sections, then these deposits were defined as the same age and such mollusc associations were identified as "Beds with molluscs" (e.g. ANDRUSOV, 1917; ARKHANGUELSKY, 1930). Several "Beds with molluscs" were determined in the Konkian deposits: Beds with Ve nus konkensis in the Northern Black Sea Region of the Southern Ukraine (SOKOLOW, 1899), were named later as the Veselyanka Beds by MERKLIN (1953); the Pholadidae Beds on the Kerch Peninsula of the Southern Ukraine (ANDRUSOV, 1917); the Kartvel Beds in Georgia (DAVITASHVILY, 1930); the Sartagan Beds in Transcaspian (ZHIZHCHENKO, 1937a, b). The Kartvel Beds are characterized (Fig. 1) by Pholadidae (Barnea pseu doustjurtensis BOGATCHEV, B. ustjurtensis (EICHWALD), B. kubanica (ZHIZHCHENKO) B. scrinia (BOGATSCHEV) etc.); the Sartagan Beds are defined as deposits with a rich marine stenohaline Konkian mollusc fauna; the Veselyanka Beds are defined as deposits with euryhaline Konkian molluscs (MERKLIN, 1953; VERESHAGIN & MIRONOVA, 1982; MURATOV & NEVESSKAYA, eds., 1986). Later, such Beds with molluscs were found in the Konkian of other areas of the Eastern Paratethys (e.g. VARENTSOV, 1950; MERKLIN, 1953; MURATOV & NEVESSKAYA, eds., 1986). Shallow-water Konkian deposits often comprise Beds with Ervilia trigonula SOKOLOV or Beds with E. trigonula and Barnea pseudoustjurtensis, B. kubanica in the Southern Ukraine (MOLYAVKO, 1960; BARG, 1969). Therefore, they were called the "Ervilia-Pholadidae Beds" and were also considered as Kartvel Beds (BARG, 1969). The age of the Kartvel Beds has been under discus...