STRATAL THICKNESS FREQUENCIES AND THE PREVALENCE OF ORDEREDNESS IN STRATIGRAPHIC SEQUENCES

Assuming the existence of discernable hierarchies in the thickness and /or temporal recurrence of stratal units within sedimentary sequences has become an increasingly important axiom of sequence and cyclostrati graphic studies, and multiple orders of stratigraphic cyclicity are no w commonly associated with inferred durations and magnitudes of rhythm ic variation in global sea level. Beyond the desire to establish an in formal nomenclature relating stratigraphic thickness to periodicity of eustatic change, an assumption of stratigraphic orderedness also supp orts the inference that groups of sedimentary units with distinct moda l dimensions in fact do occur with some generally distinct frequency i n stratigraphic sequences. In contrast to such perceived patterns of s tratigraphic order, many groupings of sedimentary units exhibit lognor mal frequency distributions wherein most of the population has an expo nentially increasing frequency of occurrence with linearly decreasing class size. Such thickness distributions typify a wide range of sedime ntary entities, including individual lithofacies, formally named strat igraphic units, epoch-interval sedimentary sections, and cyclic periti dal lithofacies associations, as well as durations of unconformity-bou nded stratigraphic sequences and the magnitudes and durations of presu med change in global sea level. These distributions indicate that most natural populations of sedimentary units comprise a non-modal series of increasing frequency with decreasing size. The importance of this s tatement lies in the fact that discrimination of stratigraphic hierarc hies and their designation as nth-order cycles may constitute little m ore than the arbitrary subdivision of an uninterrupted stratigraphic c ontinuum.