Preliminary account of the Silurian carbon isotope record (d13Corg) from the Barrancos region, Ossa Morena Zone, Portugal

Abstract

The Barrancos region provides one of the reference sections for the Silurian of the Ossa Morena Zone in Portugal. In this area the Silurian succession is condensed, with a maximum thickness of 50 m and the ages provided by graptolite faunas indicates that all the Silurian system is represented here. The lithologies are fairly homogenous throughout the succession consisting of black carbonaceous shales interbedded with black cherts, that were deposited in marine basins that developed at the northen margins of the Gondwana continent. This study is the first attempt to characterize the variation of 􀀙13Corg in this region, in order to assess well-documented Silurian climatic events. The studied section is located at Monte do Carreba, near Barrancos village, and consists of a 45 m thick succession of black shales and cherts with graptolite faunas that indicates a Llandovery to lower Ludlow age. The base of the section is faulted against Upper Ordovician greywackes and quartzites. In this section 􀀙13Corg shows a baseline of consistent low values ranging from -25.88 to -25.10%. This is interrupted by three positive excursions with maximum values of: -22.73% at the transition between the Llandovery (Telychian) and Wenlock (Sheinwoodian), -23.33% at the Homerian and -23.09% at the transition between the Wenlock and Ludlow (Gorstian). The excursions have positive shifts between +2.55 and +3.15% and are tentatively related to the three first global climatic events recognized for the Silurian (Ireviken, Mulde and Linde). Although this study is a preliminary account of 􀀙13Corg in this region, it could provide usefull data for the recognition and discussion of climatic global events in high latitude regions as was the OMZ located during Silurian times and for sections with high level of thermal maturation.