Base of the Toarcian Stage of the Lower Jurassic defined by the Global Boundary Stratotype Section and Point (GSSP) at the Peniche section (Portugal)
Bordalo da Rocha, R
de Oliveira, LCV
Episodes: Journal of International Geoscience
International Union of Geological Sciences
© International Union of Geological Sciences, All Rights Reserved.
The Global Stratotype Section and Point (GSSP) for the base of Toarcian Stage, Lower Jurassic, is placed at the base of micritic limestone bed 15e at Ponta do Trovão (Peniche, Lusitanian Basin, Portugal; coordinates: 39°22'15''N, 9°23'07''W), 80km north of Lisbon, and coincides with the mass occurrence of the ammonite Dactylioceras (Eodactylites). The Pliensbachian/ Toarcian boundary (PLB/TOA) is contained in a continuous section forming over 450m of carbonate-rich sediments. Tectonics, syn-sedimentary disturbance, metamorphism or significant diagenesis do not significantly affect this area. At the PLB/TOA, no vertical facies changes, stratigraphical gaps or hiatuses have been recorded. The base of the Toarcian Stage is marked in the bed 15e by the first occurrence of D. (E.) simplex, co-occurring with D. (E.) pseudocommune and D. (E.) polymorphum. The ammonite association of D. (Eodactylites) ssp. and other species e.g. Protogrammoceras (Paltarpites) cf. paltum, Lioceratoides aff. ballinense and Tiltoniceras aff. capillatum is particularly significant for the boundary definition and correlation with sections in different basins. Ammonites of the PLB/ TOA are taxa characteristic of both the Mediterranean and Northwest European provinces that allow reliable, global correlations. The PLB/TOA is also characterized by other biostratigraphical markers (brachiopods, calcareous nannofossils, ostracods and benthic foraminifers) and by high-resolution stable carbon and oxygen isotopes, and 87Sr/86Sr ratios that show distinctive changes just above the PLB/TOA, thus providing additional, powerful tools for global correlations. The PBL-TOA lies at the end of a second (and third) order cycle of sea-level change, and the top of bed 15e is interpreted as a sequence boundary. Cyclostratigraphy analysis is available for the Lower Toarcian of Ponta do Trovão. Detailed correlations with the Almonacid de la Cuba section (Iberian Range, Spain) provide complementary data of the ammonite succession in the Northwest European Hawskerense and Paltum Subzones, and magnetostratigraphical data that allow supraregional correlations. The proposal was voted on by the Toarcian Working Group in June, 2012, and by the International Subcommission on Jurassic Stratigraphy in September, 2012, approved by the ICS in November, 2014, and ratified by the IUGS in December, 2014. With this Toarcian GSSP, all international stages of the Lower Jurassic have been officially defined.
Several scientists have been members of the Toarcian Working Group. We would like to acknowledge all of them. We are also grateful to the ISJS and ICS members who have made valuable comments on a previous version of this manuscript. We warmly thank Marc Philippe for his help with the literature on Pliensbachian/Toarcian continental successions. We warmly thank Christian Meister and Jim Ogg for their helpful review. Constructive remarks by Jim Ogg on an early version of the paper were greatly appreciated. We also acknowledge the precious help of David Besson for providing the ammonite specimens from the Mouterde collection (Musée des Confluences, Lyon). Ammonite photographs were taken by Emmanuel Robert (Collections de Géologie de Lyon). This paper is dedicated to the memory of Abbé René Mouterde and Serge Elmi, who died in 2007 after having been for years the main supporters of the Peniche section as GSSP of Toarcian Stage. Calcareous nannofossil slides are curated at the Collections de Géologie de Lyon (No. FSL 766535-766617). This work has been supported by the BIOSCALES Project (POCTI/ 36438/PAL/2000), coordinated by the Universidade NOVA de Lisboa; R. B. Rocha thanks the support of A. F. Soares, J. C. Kullberg, P. S. Caetano and P. H. Verdial. Financial support was provided to L. V. Duarte, S. Pinto and M. C. Cabral by Projects PDCTE/CTA/44907/2002 and PTDC/CTE-GIX/098968/2008.
This is the final version of the article. Available from the publisher via the DOI in this record.
Vol. 39 (3), pp. 460 - 481