GSSP for Toarcian Stage

 

Definition:

The base of the Toarcian Stage (lower Jurassic) is defined at the bed 15e in Ponta do Trovão, Peniche, Portugal. This level is characterized by mass occurrence of Dactylioceras ammonite species, namely by the first occurrence (FO) of Dactylioceras (Eodactylites) simplex co-occurring with D. (E.) pseudocommune and D. (E.) polymorphum. This species define the lower part of the Polymorphum Zone, Mirabile Subzone. The ammonite association of Dactylioceras (Eodactylites) ssp. and other species such as Protogrammoceras (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 of Lotharingius velatus, Biscutum intermedium (uppermost Pliensbachian), Discorhabdus ignotus and Carinolithus superbus (lower Toarcian) are recorded in Peniche as well as in other expanded Tethyan sections. Secondary global marker includes a 87Sr/86Sr ratio of 0.70706 (±2s) at the base of the Toarcian.

Location:

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:

The base of the Toarcian Stage at Ponta do Trovão, Peniche, is the uppermost part of the Lemede Fm corresponding to the Couches 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 the Couches 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 the Couches 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 ammonite D. (E.) simplex co-occurring with D. (E.) pseudocommune and D. (E.) polymorphum.

 

Secondary Markers: Ammonite association of D. (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 of L. velatus, B. intermedium (uppermost Pliensbachian), D. ignotus and C. 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 calcite 87Sr/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 of D. (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.

 



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