Stratal Thickness Frequencies and the Prevalence of Orderedness in Stratigraphic Sequences

Bruce H. Wilkinson, Carl N. Drummond

Inference of sea level cyclicity invariably stems from perceived patterns of rhythmic sediment accumulation and, in this regard, the existence of discernable hierarchies of unit thickness and/or temporal recurrence in cratonic sedimentary sequences has become an increasingly-stated but rarely justified axiom of sequence and cyclostratigraphic studies. Multiple "orders" of stratigraphic "cyclicity" with ever-increasing integer ordination are commonly but uncritically linked with inferred duration and magnitude of rhythmic variation in global sealevel. Above and beyond the establishment of an informal parlance that serves to relate stratigraphic thickness to recurrence of eustatic change, assumption of stratigraphic orderedness also buttresses the commonly unstated inferenc that groups of sedimentary units with unambiguous modal dimensions in fact do occur with some generally-distinct frequency in typical cratonic sequences.

In contrast to a perception of episodic accumulation and resultant stratigraphic hierarchy, many groupings of stratal element vertical dimension and temporal duration exhibit frequency distributions readily described as exponentially-increasing frequency of occurrence with linearly-decreasing class size. Such distributions typify a wide range of sedimentary units, including thickness of individual lithofacies, thickness of formally-named stratigraphic units, thickness of epoch-interval sedimentary sections, thickness of "cyclic" peritidal lithofacies associations, durations of unconformity-bounded stratigraphic sequences, as well as magnitudes and durations of presumed change in global sealevel.

The nature of these distributions indicate that most (if not all) populations of variably-defined sedimentary units comprise an uninterrupted series of stratal element sizes, and indicate that most stratigraphic building blocks (regardless of criteria for their differentiation) comprise non-modal distributions of ever-increasing frequency with decreasing size. The importance of this statement lies in the fact that discrimination of stratigraphic orders and their designation as nth-order entities constitutes little more than the capricious subdivision of an uninterrupted stratigraphic continuum.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995