Conodont Biostratigraphy of the Upper Frasnian-Lower Famennian Deposits in the İstanbul-Zonguldak Terrane, NW Turkey: Evidence for the Upper Kellwasser Event

Abstract

This study focuses on the limestone successions of the Middle Devonian-Lower Carboniferous Yılanlı formation of the Istanbul-Zonguldak Terrane in Bartın in northwestern Turkey. The succession comprises an alternation of grey, dark grey, black, medium to thick-bedded limestones, dolomitic limestones and dolomites intercalating with thin-bedded, black, calcareous shales. A set of seven K- bentonite beds having thicknesses ranging between 20-60 cm have been documented within the studied section. The section characterizes a shallow marine, nearshore facies setting that contains low abundant and less diversified conodont associations mainly represented by the species of Icriodus and Polygnathus. The reason of the lower abundance in the studied unit should be due to the changes in the global sea levels and the Upper Kellwasser Event (UKE) around the Frasnian/Famennian boundary. Despite this low diversity and abundance, the samples yielded important upper Frasnian lower Famennian conodont elements including the species of Icriodus aff. subterminus and Icriodus cornutus, Icriodus iowaensis iowaensis, Polygnathus brevilaminus, Polygnathus xylus?. Based on the defined conodont species, the lowermost part of the unit corresponds to Frasnian and upper part exemplifies the middle triangularis zone due to presence of Icriodus cornutus and the depositional age of the formation is proposed as upper Frasnian - Lower Famennian. This age data is the first evidence for the presence of UKE in northern Turkey and will help for a better palaeogeographic correletion of the Istanbul –Zonguldak Terrane with further terranes in Laurasia and Peri-Gondwana.