The significance of metre-scale allocycles in the quest for a fundamental stratigraphic unit

Abstract

The major ‘transgressive’ sequence in the lower Helderberg Group (Devonian) of New York State and a ‘regressive’ sequence of similar scale in the first Courceyan mesothem (Carboniferous) of South Wales are entirely divisible into metre-scale allocycles. The boundaries of these cycles are produced by non-sedimentological processes: larger-scale stratigraphic events associated with relative sea-level fluctuations. The existence of these boundaries as basin-wide discontinuities precludes the application of Walther’s Law except within cycles. Thus palaeogeographic reconstructions based on the assumption of contemporaneity of large-scale fades tracts (formations and members) must be abandoned. Instead stratigraphic interpretation requires the recognition and application of a genetic unit of analysis consistent with the actual internal allogenic fabric of such sequences. Application of the metre-scale allocycle, the smallest allogenic unit of stratigraphic accumulation, results in fundamentally different interpretations of the Helderberg and Courceyan sequences. Rather than being continuously and unidirectionally ‘transgressive’ or ‘regressive’, both sequences were discontinuously generated by episodes of rapid deepening and gradual shallowing at more than one scale.