Quantitative tests for stratigraphic cyclicity

Periodic Milankovitch (M‐) orbital forcing provides an explanation for subjectively recognized short‐term repetition of lithofacies‐‘cycles’‐in the stratigraphic record. Tests of this explanation often find no order in the lithofacies and/or no regularity in the recurrence of lithofacies. This does not disprove the influence of M‐forcing, but a sedimentary response in terms of irregular M‐forced ‘cycles’ is indistinguishable from one in which repetition of facies is not M‐forced. Use of such cycles in time calibration is correspondingly suspect. Stricter, dimensional cyclicity invokes Sander's Rule, which suggests periodicity in sedimentation, for which M‐forcing provides an obvious explanation. Time calibration on the basis of strict cyclicity thus appears more dependable. Objective tests for regular M‐forced stratigraphic cyclicity commonly depend upon spectral analyses. Such tests are not unambiguous. Bilogarithmic thickness/frequency plots derived from objective layer thickness inventories (LTI) provide an alternative. Commonly, such plots show power‐law relationships that preclude dimensional M‐cyclicities. By contrast, a model data series that perfectly encodes the M‐cyclic fluctuations in terrestrial insolation generates a strongly inflected, non‐power‐law LTI plot. Power‐law plots result where the model data series is decimated by random hiatuses, with numbers and durations tuned to M‐cycle frequencies. It seems improbable that natural data series record such tuning. The general absence of strict cyclicity in the M‐frequency range is more likely to reflect the nonlinear response of sedimentary systems to cyclic M‐forcing of insolation. Interestingly, when applied to the classically cyclic lacustrine Triassic sediments of the Newark Basin, USA, the LTI test suggests a decimated record, preserving some evidence of M‐cyclicity. Copyright © 2008 John Wiley & Sons, Ltd.