New data on conodonts, brachiopods, and ostracodes from the stratotypes of the Ilmen and Buregi Beds (Frasnian, Main Devonian Field)

Abstract

The authors restudied biostratigraphyically the stratotypes of the Ilmen Beds and Buregi Beds distinguished by R. Hecker in the Fransian of the central part of the Main Devonian Field (Lake Ilmen region, Russian Platform). Abundant brachiopod, ostracode, and conodont associations were otbaiend and one new ostracode species Milanovskya bicristata was described. Both the Ilmen (mainly terrigenous) and Buregi (carbonate) beds correspond to relatiely shallow-water facies. Conodont associations permit us to correlate these beds with the Middle-Upper asymmetricus Zone and to recognize two local zones (the Polygnathus pollocki Zone and the P. efimovae Zone). Ostracode, brachiopod, an dconodont faunas observed in the Ilmen and Buregi beds resemble the Semilukian ones of the Central Devonian Field. Facial, taphonomical, and some palaeoecological observations allow for the reconstruction of the palaeolandscape succession which was caused by regional sea-level changes.