On the lower boundary of the Floian Stage in Estonia

H Pärnaste,V Viira

doi:10.3176/earth.2012.4.02

H Pärnaste, V Viira

Open Access

https://doi.org/10.3176/earth.2012.4.02

Copy DOI

Abstract

The lower boundary of the Second Stage of the Lower Ordovician Series, the Floian Stage, is defined by the first appearance of the graptolite Tetragraptus approximatus. In the stratotype section of Diabasbrottet at Hunneberg, southern Sweden, the boundary falls within the Megistaspis planilimbata trilobite Zone and within the Oelandodus elongatusAcodus deltatus deltatus conodont Subzone of the Paroistodus proteus Zone, and within the Hunneberg Regional Stage. Graptolites, including the index species T. approximatus, are missing in the terrigenous sediment of Estonia. The earliest trilobites are poorly preserved in this terrigenous succession. They represent an interval older than the Megalaspides dalecarlicus Zone but probably younger than the M. planilimbata Zone, and are correlative to the Prioniodus elegans conodont Zone. Thus the lower boundary of the Second Stage can be detected using mainly conodonts. In accordance with conodont occurrence, the base of the Floian (i.e. somewhat higher than the base of the O. elongatusA. d. deltatus Subzone) falls within the Joa Member of the Leetse Formation. Probable levels have been suggested in some localities. Thus the lower part of the Leetse Formation belongs to the Tremadoc Stage, and to the Hunneberg Regional Stage.

Highlights

The lower boundary of the Floian Stage in Estonia has been set at different levels
In the stratotype section of Diabasbrottet at Hunneberg, southern Sweden, the boundary falls within the Megistaspis planilimbata trilobite Zone and within the Oelandodus elongatus–Acodus deltatus deltatus conodont Subzone of the Paroistodus proteus Zone, and within the Hunneberg Regional Stage
The lower part of the Leetse Formation belongs to the Tremadoc Stage, and to the Hunneberg Regional Stage

Summary

Introduction

The lower boundary of the Floian Stage in Estonia has been set at different levels. 2) drew that boundary at the lower boundary of the Prioniodus elegans Zone. In a recent overview of conodont diversity in the northern Baltic, Männik & Viira This position, is incorrect or too approximate, considering the graptolite, trilobite and conodont distribution and the correlation data in the stratotype section in Sweden. In some other recent reports on the Ordovician stratigraphy of Estonia, the resolution is lower and a particular level is not bonded with lithostratigraphical units (e.g. Nõlvak et al 2006, 2007).

Objectives

Conclusion