Report of the restudy of the defined global stratotype of the base of the Silurian System

Abstract

The Global Stratotype Section and Point (GSSP) for the base of the Silurian System was defined in 1985. Since that time, a number of researchers have suggested that this section has serious deficiencies for use as a GSSP. As a result, in 2000, the Subcommission Silurian Stratigraphy (SSS) proposed undertaking a formal restudy of this GSSP, which was subsequently approved by the International Commission of Stratigraphy (ICS). The result of this restudy was the formal proposal that the current GSSP, at 1.6 m above the base of the Birkhill Shale, at Dob's Linn, Scotland, should be maintained as the same locality and stratigraphic level, but the biostratigraphical definition of the boundary should be revised. The previously defined, basal Silurian graptolite zone, the Parakidograptus acuminatus Zone, should be subdivided into lower Akidograptus ascensus Zone and a higher P. acuminatus Zone. The base of the A. ascensus Zone, marked by the first appearance of A. Ascensus, should be regarded as the biostratigraphic mark for the base of the Silurian. The proposal has now been formally approved by the SSS and ICS, and ratified by International Union of Geological Sciences. This is the first GSSP to undergo formal restudy and redefiniton and the proposal also included a recommended procedure for restudy of GSSPs.