Reference Sections, Fossil Ichthyofauna and Sr Isotope Signature of the Lower Frasnian Sediments of the Latvian Saddle and Orsha Depression, Belarus

Abstract—

Lithology of the Lower Frasnian (Upper Devonian) sections, including the Zhelon Regional Stage and the Saria Beds of the Sargaevo Regional Stage, exposed in the basin of the Saryanka River, the Latvian Saddle, is described. The complete lithological description of the Sargaevo and Semiluki deposits (Frasnian) in the Gralevo quarry of the Vitebsk Region, Orsha Depression is presented. Photographic images of macro- and micro-dimensional remains of heterostracans, placoderms, acanthodians, sarcopterygians, actinopterygians, as well as scolecodonts and conodonts found in the Lower Frasnian deposits are presented. For the first time, the Sr isotope composition was measured in remains of ichthyofauna from the Zhelon and Sargaevo deposits of Belarus. Paleoichthyological and taphonomic data, lithology and Sr isotope composition of ichthyofauna fragments, depositional environments in the Zhelon and Saria times were clarified, and the distant sections were correlated. The ⁸⁷Sr/⁸⁶Sr values in ichthyofauna samples from the Saria Beds in sections of the Latvian Saddle (sections of Kalyuty 5a and 6, Virenka 1) and the Orsha Depression (Gralevo quarry) lie within similar ranges: 0.70825–0.70842 and 0.70837–0.70856, respectively. The ⁸⁷Sr/⁸⁶Sr values in ichthyofauna from the Zhelon deposits in the Kalyuty 2 and Obukhovo 1 sections (Latvian Saddle) vary within 0.70898–0.70932 and 0.71065–0.71102, respectively. High ⁸⁷Sr/⁸⁶Sr values in the fossil ichthyofauna of both regional stages suggest that the Early Frasnian paleobasin was significantly desalinated, while the salinity in the Saria paleobasin was comparable to the salinity of the modern Baltic Sea.