Late Telychian (early Silurian) graptolitic shales and the maximum Silurian highstand in the NW Anatolian Palaeozoic terranes

Abstract

The late Llandovery included a period of global sea-level rise with a maximum in the spiralis graptolite Biozone, recorded by black graptolitic shales or red beds. Recent work in the less- known Silurian successions in the Zonguldak Terrane, NW Anatolia, has revealed the presence of Llandovery graptolitic black shales in the lower part of the Findikli Formation in the Camdag, Catak and Karadere areas. In the Camdag area, the “black shale member” yielded numerous well preserved graptolite rhabdosomes, including Oktavites spiralis, Barrandeograptus pulchellus and Retiolites geinitzianus. It is suggested that the graptolitic shales in this area were deposited during the spiralis–lower lapworthi zones. In the Catak and Ovacik areas, black shales intercalated with yellow shales yielded O. spiralis and R. geinitzianus. The deposition of black shales is related to the time of maximum Silurian sea level. The graptolitic shales in the Zonguldak Terrane are time-equivalents of the (late Llandovery) violet shales with green layers from a less deep basin in E Istanbul Terrane. In the W Istanbul Terrane, reddish sandstones with Fe-oolitic minerals and brachiopod-bearing carbonates were deposited during the late Llandovery maximum transgression. The early Silurian deposits in the Zonguldak Terrane are more akin to those of E Avalonian successions, whereas those of the Istanbul terrane resemble those of the Gondwanan periphery. Moreover, the studied sections enable the recognition of the regressive trend (or part of it) in this high level stand, as indicated by the deposition of pale and grey-greenish mudstone layers, carbonates and shell beds.