Chapter 15 - The Position of the Ordovician–Silurian Boundary in Estonia Tested by High-Resolution δ13C Chemostratigraphic Correlation

Abstract

Correlation of the base of the Silurian System, which is defined by graptolite biostratigraphy, with the non-graptolitiferous carbonate shelf successions has been complicated due to rapid environmental changes and widespread unconformities of glacioeustatic origin in the end-Ordovician interval. The Hirnantian carbon isotope excursion (HICE) occurring in the Hirnantian Global Stage at a short distance below the Ordovician/Silurian boundary has been widely used in global and regional chemostratigraphic correlation of this boundary interval. In the present study we analyzed the δ13C trends in carbonates of the Ordovician/Silurian boundary interval from the drillcore sections of the Pandivere area in central Estonia. Comparison of the high-resolution Hirnantian and Rhuddanian δ13C curves suggests some changes in the traditional stratigraphic correlation scheme of Estonia. The new data demonstrate that the HICE crosses the traditional Ordovician/Silurian boundary, which should therefore be placed higher up in the Baltoscandian succession, in the lower part of the Juuru Regional Stage. The base of the Juuru Stage, traditionally correlated with the system boundary, has been defined by shelly fossil data. The second phase of the global Hirnantian extinction was followed by a slow faunal recovery with the appearance of postextinction biota in late Hirnantian that obviously included some “Silurian-type” shelly fossil groups. For that reason, the traditional position of the Ordovician/Silurian boundary in many regions with non-graptolitiferous successions, established by shelly fauna should be critically reevaluated.