Abstract

I. G. Goodall, G. M. Harwood, A. C. Kendall & T. McKie write: In his recent reappraisal of the stratigraphy of the English Zechstein, Tucker (1991) presents an elegant model of the evolution and stratigraphical relationships of the various formations in this section. However, some aspects of the interpretation deserve further discussion; namely the duration and relative completences of the sequences, the relationships of the Trow Point Bed (and underlying megabreccia-slide) and the Edlington Formation within the alternative stratigraphy, and the relative contributions of climatic and eustatic controls on evaporate mineralogies and sequence development.

Duration and completeness of sequences. The 5 Ma timespan of the Zechstein (Menning et al. 1988) falls within the periodicity range of third order ‘eustatic’ cycles (1–10Ma), implying that the sequences described by Tucker are high frequency fourth order sequences (sensuMitchum & Van Wagoner 1991). Given a period of 0.1–0.2 Ma for fourth order cycles it is evident that the seven sequences identified by Tucker do not represent all the potential eustatic cycles during Zechstein deposition. The stratigraphical record is likely to be most complete during the third order sea-level rise i.e. within the more carbonate-dominated lower Zechstein, where Tucker’s sequence model is most applicable. In contrast, the upper part of the Zechstein section must have been largely deposited during a third order sea-level fall. It is within this more evaporite-dominated section (sequences 5, 6 and 7) that the record of the ‘hidden/missing’ fourth order eustatic cycles lie. This interval may contain the

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