Ostracods of the Ordovician–Silurian boundary beds: Jūrmala core (Latvia) and its implications for Baltic stratigraphy

Ostracods of the Ordovician and Silurian transition interval in the Jūrmala section, Latvia, were studied. The diverse pre-Hirnantian ostracod assemblage is replaced by the Harpabollia harparum association that has been recorded across the Baltoscandian region and northwestern Poland and is confined to the Hirnantian. The first complete ostracod succession throughout the Kuldīga and Saldus formations reveals a remarkable diversity decline and rise of dominance of ostracods in this interval. The appearance level of the H. harparum association in other sections is drawn distinctly above the range of Spinachitina taugourdeaui, nearly coinciding with the appearance of Conochitina scabra and Noixodontus girardeauensis. The transition from the Saldus Formation into the Stačiunai Formation marks a complete ostracod turnover. The newly appearing assemblage, dominated by Longiscula smithii, is of low diversity. Slow diversity rise may in part be ascribed to the overall low recovery rate of post-glaciation faunal assemblages. The shape of the stable carbon isotopic curve in the Jūrmala core suggests that this section is one of the most complete successions across the Ordovician–Silurian boundary in the Baltoscandian area. The correlation of this curve with that of the Monitor Range, Nevada, and comparison with other curves of Latvia and Estonia suggest that the well-known appearance level of the ‘Silurian faunas’ is confined to the upper M. persculptus graptolite Zone. Judging from the chemostratigraphic correlation, the latter zone ranges for more than 10 m into the Stačiūnai Formation in the Jūrmala core.