The Pliensbachian GSSP definition (Mesozoic, Lower Jurassic) : A case study
|Category||Paleontology and Stratigraphy|
|Location||International Geological Congress,oslo 2008|
|Author||Thierry, Jacques; Marie-Bianca, Cita|
|Holding Date||23 September 2008|
The Pliensbachian GSSP at Wine Haven section, Robin’s Hood Bay, Yorkshire Coast (UK) was ratified in 2006 by the IUGS following proposals and voting by the Pliensbachian WG of the ISJS and ICS (Meister et al., Episodes 29 (2) : 93-106). Third stage of the Lower Jurassic, its subdivisions are based on ammonites that provide high resolution biochronostratigraphy (5 zones, 15 subzones and 40 biohorizons). The original stage definition, based on outcrops near the village of Pliensbach, SE of Stuttgart (Baden-Württemberg; G) was proposed by Oppel to replace the Liasian of d’Orbigny. It is officialy subdivided in Carixian (Carixia, latin name for Charmouth, a Dorset coast locality; UK) and Domerian (type-section at Monte Domaro, Lombardian alps; I).
The Wine Haven section fulfills most of the necessary requirements listed in the guidelines of the ICS. The GSSP definition is complemented by integrated zonations and correlation tables (ammonites, belemnites, brachiopods, ostracods, foraminifera, dinoflagellate cysts, calcareous nannofossils), palaeomagnetostratigraphy and chemostratigraphy data (Sr and O isotope on belemnites). But, as the section has some weaknesses concerning foraminifera, ostracoda, and dinocysts record, a complementary section has been retained at Aselfingen, near the original type area of the Pliensbachian.
Historically, the Sinemurian-Pliensbachian boundary was located at an ammonite faunal turnover at the base of the Jamesoni Zone (Taylori Subzone). As some discrepancies in ammonite occurrence appeared, the smallest and lowest biostratigraphic unit at the base of the Pliensbachian, the donovani biohorizon was defined. Below, the tardecrescens biohorizon is the latest unit of the Sinemurian (Aplanatum Subzone, Raricostatum Zone).
The Sinemurian-Pliensbachian boundary at Wine Haven (base of the bed 73b, Redcar Mudstone Formation, Pyritous Shales Member) is located in a continuous exposure of about 30 m of pale grey and buff-coloured sandy mudstone which gradually upwards into silty dark shales without unconformities or sedimentary break. Calcite strontium-isotope ratio data indicates continuous sedimentation and oxygen-isotope data indicate a temperature drop of about 5°C across the boundary. A Trangressive System Tract, initiated in the uppermost Sinemurian (Aplanatum Subzone) continues into the Lowermost Pliensbachian (Taylori Subzone). The lithologic succession does not include potential chronometers for radiometric dating. It has a normal magnetic polarity with two discrete reversed polarity intervals; one of it spans much of the latest uppermost Sinemurian and terminates a few decimeters below the Sinemurian-Pliensbachian boundary; it could be a valuable chronostratigraphic marker. No structural complexity or metamorphism was observed. The site has easy accessibility and the section is well exposed on a cliff and forshore ; it is part of a Site of Special Scientific Interest and therefore it is protected under UK legislation.