GSSP for the Ordovician-Silurian Boundary


Definition:

The base of the Silurian System and Rhuddanian Stage is defined at 1.6m above the base of the Birkhill Shale Formation, in a section at Dob's Linn, near Moffat in the Southern Uplands of Scotland, U.K., at the incoming of graptolites Parakidograptus acuminatus and Akidograptus ascensus.


Location:

The GSSP for the Silurian is located in a section at Dob's Linn in an artificial excavation created just north of the Linn Branch Stream (U.K. National Grid Reference NT 1962 1584).
click


Sedimentology:

Two lithological units (formations) occur near the boundary. The lower is the Hartfell Shale (48m thick), consisting chiefly of pale gray mudstone with subordinate black shales and several interbedded meta-bentonites. Above this is the 43m-thick Birkhill Shale, which consist predominantly of black graptolitic shale with subordinate gray mudstones and meta-bentonites.
click


Primary Markers:

Incoming of graptolites Parakidograptus acuminatus and Akidograptus ascensus at 1.6m above the base of the Birkhill Shale Formation.

Secondary Markers:

Chitinozoa and conodonts are known from the section, but are neither well preserved nor very age diagnostic.

References:

Cocks, L. R. M., 1985. The Ordovician-Silurian Boundary. Episodes, 8(2): 98-100.