THE EXPECTED DURATION OF UPWARD-SHALLOWING PERITIDAL CARBONATE CYCLESAND THEIR TERMINAL HIATUSES

Meter-scale accommodation cycles, which dominate the stratigraphic rec ord of peritidal carbonate accumulation, might provide a time scale of unusually high resolution, but the duration of the biatuses 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 measuremen t, to describe a hypothetical average section. Because all sections ar e interrupted by hiatuses, the average accumulation rate falls progres sively with increasing time span and most steeply at the time spans th at capture the highest proportion of hiatuses. The process of combinin g rates from many sections cancels local differences in the distributi on of hiatuses. Only hiatuses that have similar spacing and duration i n a majority of sections will stand out after compilation. The size an d position of a pronounced inflection in the combined plot of accumula tion rates against time span identifies a dominant cycle period of -10 0,000 yr, of which the terminal hiatus accounts for 80%-90%. The expec ted cyclothem is approximately 10 m thick. Because peritidal accommoda tion cycles begin and end with the sediment surface at sea level, the expected cycle period can be estimated by finding the time span at whi ch accommodation and accumulation rates balance. The maximum duration of the hiatus is the time span at which rates of subsidence and sealev el fall balance. Compilations of many empirical measurements were agai n employed to determine the relationship between time span and the rat es of subsidence, sea-level change, and net accommodation. This second method confirms the results of the first and can be modified to predi ct the expected cyclothem in different climatic, tectonic, and deposit ional settings. Because Quaternary data dominate short-term rates, the hypothetical average section best describes a passive margin in an ic ehouse climate with strong glacio-eustatic fluctuations in sea level. After modification for an extreme greenhouse climate, devoid of ice ca ps, the second method predicts that 20,000-vr accommodation cycles wil l 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 fo rmation, or at the seaward edge of a platform, tend to extend the limi t 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.