The expected duration of upward-shallowing peritidal carbonate cycles and their terminal hiatuses

Abstract

Meter-scale accommodation cycles, which dominate the stratigraphic record of peritidal carbonate accumulation, might provide a time scale of unusually high resolution, but the duration of the hiatuses that cap the resulting cyclothems is difficult to determine. Thousands of rates of peritidal accumulation from many different carbonate sections have been combined, according to the length of the time span of measurement, to describe a hypothetical average section. Because all sections are interrupted by hiatuses, the average accumulation rate falls progressively with increasing time span and most steeply at the time spans that capture the highest proportion of hiatuses. The process of combining rates from many sections cancels local differences in the distribution of hiatuses. Only hiatuses that have similar spacing and duration in a majority of sections will stand out after compilation. The size and position of a pronounced inflection in the combined plot of accumulation rates against time span identifies a dominant cycle period of ∼100,000 yr, of which the terminal hiatus accounts for 80%-90%. The expected cyclothem is ∼10 m thick. Because peritidal accommodation cycles begin and end with the sediment surface at sea level, the expected cycle period can be estimated by finding the time span at which accommodation and accumulation rates balance. The maximum duration of the hiatus is the time span at which rates of subsidence and sea-level fall balance. Compilations of many empirical measurements were again employed to determine the relationship between time span and the rates of subsidence, sea- level change, and net accommodation. This second method confirms the results of the first and can be modified to predict the expected cyclothem in different climatic, tectonic, and depositional settings. Because Quaternary data dominate short-term rates, the hypothetical average section best describes a passive margin in an icehouse climate with strong glacio-eustatic fluctuations in sea level. After modification for an extreme greenhouse climate, devoid of ice caps, the second method predicts that 20,000-yr accommodation cycles will be preferentially recorded.The expected cyclothem is 2 m thick and accounts for about 50% of the cycle period. Increased subsidence rates, whether on active margins, in the initial phase of passive margin formation, or at the seaward edge of a platform, tend to extend the limit of cycle duration, increase cyclothem thickness, and partition less time to the bounding hiatus. Increased subsidence, however, raises the risk that the sediment surface will drown.