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Preliminary account of the Silurian carbon isotope record (d13Corg) from the Barrancos region, Ossa Morena Zone, Portugal
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Abstract(s)
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.
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Citation
Lopes, Gilda; Fernandes, Paulo; Goodhue, Robbie; Pereira, Zélia; Piçarra, J.M.Preliminary account of the Silurian carbon isotope record (d13Corg) from the Barrancos region, Ossa Morena Zone, Portugal, Trabalho apresentado em Goldschmidt Conference 2011, In Mineralogical Magazine, Praga, 2011.