APERIODIC ACCUMULATION OF CYCLIC PERITIDAL CARBONATE

Tabulation of thickness data on nearly 3000 Proterozoic and Paleozoic peritidal carbonate cycles indicates that metre-scale facies associati ons exhibit exponential thickness frequency distributions. Because car bonate deposition occurs during infilling of available space between s ea level and the surface of the preceding cycle, and because the rate of generation of accommodation space is ultimately determined by rates of platform subsidence, thicknesses of individual cycles must record the duration of time since deposition of underlying units. Exponential thickness distributions therefore require either that upward-shoaling carbonates record apetiodic accumulation, or that any periodic forcin g manifest during sedimentation has been masked by the vagarious natur e of depositional processes. Such a conclusion is contrary to interpre tation of such carbonate units as originating from high-frequency eust atic change. In addition, exponential thickness distributions and sequ ence thickness structures are replicated when assuming a random probab ility of carbonate deposition under conditions of constant subsidence. The nature of cycle thickness distributions therefore invalidates vir tually any endeavor to derive an average depositional period from mean cycle thicknesses, and refutes the use of such estimates as proxy rec ords of past sea-level oscillation frequency, whether they are related to Milankovitch-band climate forcing or to any other periodic process .