Chitinozoan biostratigraphy across the Katian (Late Ordovician) GICE event in the Borenshult-1 drillcore (Sweden)

Abstract

The Late Ordovician is a period of major global environmental changes. Its understanding requires an integrated stratigraphical approach. Here, chitinozoan assemblages are described from the Borenshult-1 drillcore from Motala (Sweden) and related to stable carbon isotope event stratigraphy. Fifty-six rock samples, corresponding to a stratigraphic interval ranging from the upper Sandbian to Llandovery, were dissolved and analysed for chitinozoans. This stratigraphic interval covers the Sandbian to Llandovery and includes, in ascending order, the Freberga, Slandrom, Fjäcka, Jonstorp, Loka and Motala formations.This study resulted in identification of two biozones the Spinachitina cervicornis Biozone (lower part of the Freberga Formation), as well as its Angochitina multiplex Subzone, and the Fungochitina spinifera Biozone (upper part of the Freberga Formation). The spinifera Biozone is characterized by the presence of Lagenochitina baltica, whereas the cervicornis Biozone is defined by the presence of its index species.Despite a low recovery of chitinozoans from Borenshult-1, we suggest that the Guttenberg Isotopic Carbon Excursion (GICE), as observed in the Borenshult-1 drillcore, correlates with the S. cervicornis Biozone. This fits well with chitinozoan and carbon isotope data from the Fjäcka section in central Sweden (Dalarna, Siljan region) and from the Smedsby Gård drillcore (Östergötland). The A. multiplex Subzone which is recognised in the Borenshult-1 and nearby Smedsby Gård drillcores, predates the GICE at both localities.