Program > By author > Oliva-Urcia Belen

The different development of Lower Devonian strata and faunas lead to the extensive
use of two different scales of difficult adjustment during the XIX and XX centuries (and
even today). This situation prompted the IUGS and the corresponding International
Subcommission on Devonian Stratigraphy (SDS) to establish an unified
chronostratigraphic scale, which initially combined both scales. The resulting
subdivision into three Stages: Lochkovian, Pragian and Emsian presented numerous
difficulties, being the Emsian Stage one of the more complex and controversial.

Based on the extensive work carried out by Yolkin and collaborators, the SDS decided
to establish the base of the Emsian in 1994 with the entry of the conodont
Polygnathus kitabus in the Zinzilban section (Kitab Geological Reserve, Uzbekistan).
Numerous reports by Carls and Valenzuela-Ríos demonstrated the inaccuracy of this
decision and the need to revise, as soon as legally possible, the boundary criterion.

In the SDS field conference held in the Kitab Geological Reserve in 2008, two major
agreements were reached: 1) P. kitabicus is a good index but not for the base of the
Emsian and we shall look for a different index. The proposed index was Polygnathus
excavatus 114. 2) The intention to maintain the GSSP in the Zinzilban section, but in
higher strata (those showing the entry of Polygnathus excavatus 114 within the
phylogenetic branch of the Polygnathus excavatus group). Consequently, two
international expeditions (2008 and 2015) searching for the new position of the
boundary were launched in the Kitab Reserve. Unfortunately, results were not as
expected and the SDS decided to expand the investigation to other potential areas.

One of them is the Spanish Pyrenees, were Pragian and Emsian strata crop out in
several places. Two of these sections are Isábena 1 and Baliera 6 in the province of
Huesca (Spanish Pyrenees). Both of them contain a rich conodont record spanning
through the Pragian/Emsian boundary in both senses, the current and the proposed
boundary (entries of Pol. kitabicus and Pol. excavatus 114 respectively). Besides, they
also record taxa of the genus Icriodus, which allows direct correlations with those
sections without polygnathids.

Taking into account these favourable conditions, we have started a combined
comprehensive and multidisciplinary study of these two sections. In this report we
present pioneer data of magnetic susceptibility aligned with the biostratigraphic ones.

Magnetic susceptibility trends and excursions are well characterised and dated and
enable to start the construction of a multidisciplinary matrix of Pyrenean data, which
in turn will confer the required stability for future decisions concerning the Devonian
Time Scale and, in particular, the definition of the Emsian stage and its further
subdivision.

This work is a contribution to IGCP652, to ICS program Regenerating the Time Lords:
Towards the completion, calibration, digitization and outreach of the geological
timescale, and the Research groups GIUV2017-395, GEOTRANSFER E32_17R and
PERIGONDWANA UCM 910231. MIU-Next Generation EU (ZA21-005) supports J-C Liao research.