A new Late OrdovicianHirnantiabrachiopod Fauna from NW Turkey, its biostratigraphical relationships and palaeogeographical setting

Abstract

AbstractLate Ordovician fossils, including the distinctiveHirnantiabrachiopod Fauna, have been found in Lower Palaeozoic successions in Istanbul and Bolu (Yığılca), western Pontides, NW Turkey. The Middle Ordovician (Sandbian) faunas belong to the cooler-water Mediterranean Province, and they are followed by Katian brachiopods includingSulevorthis,Nicolella,Hesperorthis,Glyptorthis,SaukrodictyaandKullervoand ostracods such asPiretella,EochilinaandKlimphores, which represent deposition in warmer waters; however, the Mediterranean Province usually cooler-water brachiopodsDraboviaandLeptestiinaalso occur. The Pendik Formation includes thin bryozoan-rich limestones which probably represent the Boda Global Warming Event. The overlying turbidites contain aHirnantiaFauna, developed within a brachiopod–diplograptid association. Above them there are characteristic Llandovery (Rhuddanian–Aeronian) brachiopods, such asLeangella,Eoplectodonta,StricklandiaandHindellawith the coralsHalysites,PaleofavositesandStreptelasma. In the Bolu area, Katian brachiopods such asMcewanella,Dalmanella,Glyptorthis,Christiania,Oligorhynchia,Nicolella,HowellitesandDrabovinellaalso occur, but there the overlyingHirnantiaFauna is developed within aHirnantia–Mucronaspisassociation. The fauna and sediments indicate that the western Pontides were not very cold during the latest Ordovician. Despite Turkey being placed in higher latitudes by previous authors, it seems more probable that the Pontides were at somewhat lower palaeolatitudes, perhaps at about 40°S in those times; however, the precise palaeogeographical position of the terrane remains uncertain: there are no Hirnantian glaciogenic rocks there, such as are found in the Taurides of southern Turkey.