δ13C chemostratigraphy of the Middle and Upper Ordovician succession in the Tartu-453 drillcore, southern Estonia, and the significance of the HICE; pp. 195–200

H Bauert,K Põldsaar,L Ainsaar,S Sepp

doi:10.3176/earth.2014.18

Abstract

The δ 13 C isotope data from the Tartu-453 core section in southern Estonia enabled creation of a continuous Ordovician carbon isotope record, ranging from the Floian to the end of the Hirnantian. Five positive δ 13 C excursions and one negative δ 13 C excursion were recognized. Most of the positive excursions correspond to those defined on the Baltoscandian carbon isotope chemostratigraphic scheme, except for the Paroveja and Rakvere ICEs which are not well expressed in the Tartu-453 curve. Besides the positive δ 13 C excursions, a distinctive negative δ 13 C excursion, the Lower Sandbian NICE, was recorded in the Tartu- 453 succession. The well-expressed HICE extends about 10 m higher than the traditional Ordovician-Silurian boundary (between the Porkuni and Juuru regional stages) into the Juuru Regional Stage, suggesting that the Ordovician-Silurian boundary is positioned significally higher than traditionally suggested for the East Baltic area.

Highlights

AND GEOLOGICAL BACKGROUNDThe Baltoscandian region is a well-known key area for global Ordovician biostratigraphic and chemostratigraphic research
The well-expressed Hirnantian δ13C excursion (HICE) extends about 10 m higher than the traditional Ordovician–Silurian boundary into the Juuru Regional Stage, suggesting that the Ordovician–Silurian boundary is positioned significally higher than traditionally suggested for the East Baltic area
The purpose of this paper is to describe the Ordovician δ13C chemostratigraphy in that core and to assess the relationships with coeval successions in Baltoscandia and elsewhere

Summary

Introduction

The Baltoscandian region is a well-known key area for global Ordovician biostratigraphic and chemostratigraphic research. Extensive stable carbon isotope chemostratigraphic studies of the Baltoscandian Ordovician– Silurian carbonate succession conducted in the last decade have revealed a series of positive δ13C isotope excursions (ICE) that have shown a great potential for both intrabasinal (in the Palaeobaltic basin) and global stratigraphic correlations. The log/description of the Tartu-453 drillcore with biostratigraphic data of some microfossil groups on certain stratigraphic units (chitinozoans: Lasnamägi to Porkuni regional stages (RSs); conodonts: Volkhov to Kunda and Lasnamägi to Keila RSs; ostracods: Volkhov to Kunda and Keila to Pirgu RSs) was published by the Geological Survey of Estonia (Põldvere 1998). The Ordovician carbonate succession in Tartu was deposited on the area separating the wide, inner ramp or shelf (North Estonian Confacies Belt) from the deep outer ramp or basin known as the Livonian Tongue The depositional environment in the entire Baltic area evolved from temperate carbonate ramp in Dapingian–Sandbian time to the tropical carbonate shelf in Katian–Hirnantian time (Nestor & Einasto 1997)

Objectives

Methods

Conclusion