Abstract

John Cope writes: Hallam's (1990) article on the Triassic–Jurassic boundary raises many questions on stratigraphical practice and correlation. In addition to his arguments, there are also errors of fact which require a response.

The first observation to be made, with which Hallam would no doubt agree, is that it is good stratigraphical practice to define units only by their base, so that their tops are fixed automatically by the base of the succeeding unit of the same rank. This practice has been endorsed since 1967 in the various Geological Society guides to stratigraphical procedure. In fixing the Triassic-Jurassic boundary, therefore, we should be aiming to fix the base of the Jurassic System and not the top of the Triassic.

For international acceptance, a System boundary should be provided with a Global Boundary Stratotype Section and Point (GSSP) (Cowie et al. 1986). Such points have been fixed for some Systems (e.g. Silurian and Devonian) and have been ratified by the International Commission on Stratigraphy (ICS) and the International Union of Geological Sciences (TUGS) (Bassett 1985). The essential point about a Global Boundary Stratotype Section and Point is that it must provide an internationally acceptable reference point from which accurate correlations can be made.

It may well be that we have not yet reached a point where such a decision can be made with respect to the base of the Jurassic System, but Hallam’s article contributes nothing to this choice, as he proposes a lithological boundary at which there is no

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