GSSP for Toarcian Stage
Definition:
Toarcian Stage (lower Jurassic) is defined at the bed 15e in Ponta do Trovão, Peniche, Portugal. This level is characterized by mass occurrence ofDactylioceras ammonite species, namely by the first occurrence (FO) ofDactylioceras (Eodactylites) simplex co-occurring withD. (E.) pseudocommuneand D. (E.) polymorphum. This species define the lower part of the Polymorphum Zone, Mirabile Subzone. The ammonite association ofDactylioceras (Eodactylites) ssp. and other species such asProtogrammoceras (Paltarpites) cf. paltum, Lioceratoides aff.balinense and Tiltoniceras aff.capillatum is particularly significant for the boundary definition. Secondary biostratigraphic events are represented by a succession of calcareous nannofossil first occurrences observed across the Pliensbachian/Toarcian boundary, namely the FOs ofLotharingius velatus,Biscutum intermedium(uppermost Pliensbachian),Discorhabdus ignotus andCarinolithus superbus (lower Toarcian) are recorded in Peniche as well as in other expanded Tethyan sections. Secondary global marker includes a87Sr/86Sr ratio of 0.70706 (±2s) at the base of the Toarcian.
Location:h4>
The GSSP section at Ponta do Trovão is located near the Peniche town (Peniche peninsula), approximately 80 km north of Lisbon (Portugal) at a latitude of 39º22’15’’ N, and a longitude of 9º23’07’’ W.
Sedimentology:h4>
The base of the Toarcian Stage at Ponta do Trovão, Peniche, is the uppermost part of the Lemede Fm corresponding to theCouches de passage(Transition beds). These beds are bioturbated, centimetre-thick marlstones enriched in bioclasts alternating with decimeter-thick bioclastic limestones with a wackestone texture. The highest bed of theCouches de passage (15e) has yielded a characteristic association of dactylioceratids that is classically interpreted as marking the beginning of the Toarcian. As a consequence, the chronostratigraphical boundary at the base of bed 15e is distinct from the lithologic boundary, this latter being situated between theCouches de passage (top of bed 15e, topmost part of Lemede Fm) and the base of the Cabo Carvoeiro Fm (bed 16a), which is represented by decimetre-thick alternations of marls and centimetre-thick limestones.
Primary Markers:
The GSSP is marked by the FO of the ammoniteD. (E.) simplex co-occurring withD. (E.) pseudocommuneand D. (E.) polymorphum.
Secondary Markers:
Ammonite association ofD. (Eodactylites) ssp. and other species e.g. Protogrammoceras (Paltarpites) cf. paltum, Lioceratoides aff.balinense and Tiltoniceras aff.capillatum is particularly significant for the boundary definition. A succession of calcareous nannofossil first occurrences includes the FOs ofL. velatus,B. intermedium(uppermost Pliensbachian),D. ignotus andC. superbus (lower Toarcian). A prominent (~2‰) negative excursion of d13C measured on bulk rock, belemnites and brachiopod calcite is also recorded just above the boundary level.
Strontium Isotopes:
Analysis of belemnites yields a calcite87Sr/86Sr ratio for the boundary level of 0.70706.
Magnetostratigraphy:
Magnetostratigraphy was not conclusive in the Peniche section. A very precise correlation with the Almonacid de la Cuba section (Iberian Range, Spain), based upon ammonite and Sr isotopes, shows that the Pliensbachian/Toarcian boundary nearly corresponds to the transition between a normal and a reversed polarity.
Correlation Events:
Ammonite association ofD. (Eodactylites) ssp. and other species e.g.Protogrammoceras (Paltarpites) cf. paltum, Lioceratoides aff.balinense and Tiltoniceras aff.capillatum is particularly significant for the boundary definition.
Notes on Derivation of Age:
Correlation to the Karoo volcanic rocks provided absolute age. Cyclostratigraphy was performed on the basal Toarcian of Peniche section.