Delimitation of the Frasnian

Abstract

A clearer definition of the Frasnian stage as an internationally recognized subdivision of the Devonian System is required. Following tradition the base of this stage should be taken as the base of the Upper Devonian. Pending international agreement on the definition of -the limits of the Frasnian, it is recommended that the base should continue to be defined as the base of the Assise de Fromelennes. This level agrees with the :boundary currently accepted by the Conseil geologique de Belgique; and seems close to the concept accepted by Gosselet when he proposed the Frasnian in 1879. This horizon appears, on present evidence, to fall within the varcus Zone of the conodont chronology: it may not be far from the base of the goniatite Lunulicosta Zone and Manticoceras Stufe. The serious repercussions of current proposals to change the definition to the base of the Assise de Frasnes are, outlined. The upper limit of the Frasnian is now defined as the base of the Assise de Senzeille and it is recommended that it should remain so until international agreement recommends a change. A case is argued for considering a slightly higher horizon within the conodont triangularis Zone and at the base of the Cheiloceras Stufe, since it is ,this boundary, in practice, which has widest international acceptance at present. The need for international discussion of correlations and definitions of these boundaries is emphasized. Selected aspects of these problems are considered in detail.