Abstract
The article deals with grain size distribution of the insoluble residue of Upper Ordovician carbonate rocks of the Baltoscandian Basin in the West Latvian Aizpute-41 core. Great fluctuations in mean grain size in the sections of the Pirgu (uppermost Katian) and Porkuni (Hirnantian) stages are considered as indications of eustatic sea level fluctuations, pointing to the glaciation in the Hirnantian and its prelude in Pirgu time. An alternative cause of grain size fluctuations might be the up-and-down tectonic movements. Nevertheless, the synchronicity of cessation of fluctuations with the end of the Hirnantian glaciation supports the glaciation-related reasons.
Highlights
IntroductionThe start of the end-Ordovician glaciation at the South Pole is usually related to the beginning of the Hirnantian at 445.2 Ma (www.stratigraphy.org)
THE GEOLOGICAL BACKGROUNDThe start of the end-Ordovician glaciation at the South Pole is usually related to the beginning of the Hirnantian at 445.2 Ma
As the carbonate component was removed before grain size analysis, our results are based on the insoluble residue
Summary
The start of the end-Ordovician glaciation at the South Pole is usually related to the beginning of the Hirnantian at 445.2 Ma (www.stratigraphy.org). This time exhibits the greatest faunal changes, disappearance of many taxa and spreading of cosmopolitan Hirnantian fauna. The δ18O measured from marine fossils increases, indicating a rapid decrease in temperature at the beginning of the Hirnantian (Trotter et al 2008). The δ18O excursion coincides with the positive excursion of carbon isotopes (Brenchley et al 1994; Kaljo et al 2007; Bergström et al 2010, 2014). The Hirnantian glaciation lowered the sea level about 150 m compared to the Middle Katian, whereas several eustatic fluctuations preceded the Hirnantian culmination
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