GSSP for Sinemurian Stage
The base of the Sinemurian
is exposed in the cliff north of the village of East Quantoxhead, around 6 km
east of Watchet, in the administrative district of West Somerset, southwestern
England. The boundary level is 0.90 m above the base of Bed 145 coinciding with
the first appearance of the genera Vermiceras (V. quantoxense, V. palmeri)
and Metophioceras (M. sp. indet. 1).
The basal ammonite assemblage of the Sinemurian in the GSSP is earlier than all other assemblages known elsewhere in Europe. In Europe, a stratigraphical gap between both stages is generally present. Correlation with other faunal provinces is possible using the first appearance of Metophioceras s. str.
The "East Quantoxhead" GSSP is situated in the West Somerset administrative district of the county of Somerset (in SW England, about 6km east of the town of Watchet; U.K. National Grid reference ST 137 443. The section is in a coastal cliff 500m north of the Court House, East Quantoxhead, where iron Steps ("Limekiln Steps") give access to the beach. A footpath (ca. 800m) leads from the car parking near the Court House to the locality. The boundary section is immediately below and on either side of the iron steps.
The boundary section comprises about 27m of strata which include the upper part of the angulata zone and the lower part of the bucklandi zone. This sequence, including the stage boundary, is entirely in Blue Lias facies. The sediments are fine-grained. The Blue Lias typically shows sedimentary cycles which, from the base upwards, comprise: bituminous shale - marl - limestone - marl.
The boundary level corresponds with the striking change in the ammonite fauna from the genus Schlotheimia (mainly Schlotheimia pseudomoreana) in the latest Hettangian to the genera Vermiceras and Metophioceras in the earliest Sinemurian. The GSSP coincides with the first appearance of the genera Vermiceras quantoxense, Vermiceras palmeri and Metophioceras sp. indet. 1.
Lingulina tenera plex. substriata is characteristic of the latest Hettangian and the consistent appearance of Planularia inaequistriata and the Frondicularia terquemi plexus group is characteristic of the basal Sinemurian.
Ammonite FAD Vermiceras quantoxense, Vermiceras palmeri
Notes on Derivation of Age:
Cycle-scaled linear Sr trend