THE MESSAOUDENSIS-TRIFIDUM ACRITARCH ASSEMBLAGE AND CORRELATION OF THE BASE OF ORDOVICIAN STAGE 2 (FLOIAN)

Abstract

The Global Stratotype Section and Point (GSSP) for Stage 2 of the Ordovician System, now the Floian Stage and approximately equivalent to the lower and middle Arenig of England and Wales, is defined by the first appearance datum (FAD) of the graptolite Tetragraptusapproximatus in the Diabasbrottet Quarry section at Mount Hunneberg, Sweden. One of the issues this raises is how to correlate the base of Stage 2 at the GSSP with areas and successions that do not contain a correlative graptolite fauna. The distinctive Cymatiogaleamessaoudensis Stage 2 boundary there (the Tremadoctrifidum assemblage, notably that of Aureotestaclathratasimplex, which is considered to be close to the base of Stage 2 in NW England.