Preliminary radiolarian biostratigraphy across the Jurassic-Cretaceous boundary from northwestern Turkey

This is the first study from Turkey where radiolarians from calcareous sequences of the Sakarya Continent are tested in the delineation of the Jurassic-Cretaceous boundary. The Sakarya Continent, which comprises the southern part of northwest Turkey, was a Jurassic-Early Cretaceous carbonate platform micro-continent distant from terrestrial sediment influx. It was a site of pelagic carbonate deposition with abundant calcareous and siliceous microfossil input. Among 44 measured stratigraphic sections of the Jurassic-Lower Cretaceous deposits of the Sakarya Continent, four continuous sections (MK, KEL, AÇ, ÇD) which encompass the Tithonian-Berriasian boundary were selected for study. However, due to poor preservation and calcification of radiolarian faunas, only section KEL produced rich and well-preserved radiolarians. This section is 600 meters in thickness and spans the upper Tithonian-lower Berriasian interval. In addition to radiolarians, section KEL contains abundant calpionellids, calcareous nannofossils, benthic foraminifera and calcareous algae. The upper part of this stratigraphic section is made up of the Sogukçam Limestone which lithologically resembles the Biancone and Maiolica Formations in the Umbria-Marche region of Italy. The best preservation and richest radiolarian faunas were observed from the Sogukçam Limestone in section KEL. This is significant because many radiolarian studies as well as studies on magnetostratigraphy, calcareous nannofossils and calpionellid biostratigraphy in the Tethyan realm have been carried out on the Biancone and Maiolica Formations. The rapid sedimentation rate, pelagic microfacies and well preserved non-calcified radiolarian faunas make section KEL ideal for the investigation of the Jurassic-Cretaceous boundary with radiolarians. The correlation between radiolarian faunas and co-occurring calpionellids and calcareous nannofossils in section KEL can be compared with similar biostratigraphic studies from the Biancone and Maiolica Formations in Italy. The calcareous nannofossils in section KEL sometimes attain rock forming abundance, and the first appearance of nannoconids as well as their first abundance peak are recognized in this section. Throughout the present study the boundary between Calpionellid standard zones A and B is considered as the Tithonian-Berriasian boundary. The close sampling interval, 15-20 meters, provides excellent resolution in documenting the first occurrences of numerous Cretaceous radiolarian species. These first occurrence datum points for radiolarian taxa are not concurrent. Due to the refined resolution of dating, a stepwise appearance of Cretaceous radiolarian faunas in section KEL is clearly observed; however, radiolarians do not delineate the Jurassic-Cretaceous boundary because a marked synchronous faunal turnover does not exist.