The Signal and the Noise: Forward Modeling of Allocyclic and Autocyclic Processes Influencing Peritidal Carbonate Stacking Patterns
Peter M. Burgess
Shell Internation Exploration and Production, Rijswijk, Netherlands
Recent carbonate sequence stratigraphic models suggest that the degree of order present in successions of carbonate parasequences is determined by temporal variations in eustatic amplitude (greenhouse versus icehouse) and by varying levels of autocyclicity, but new results from a three-dimensional forward model of Ginsberg-style parasequence generation suggest that this may be an over-simplification. Results from the modelling show that autocyclic parasequences, generated by shoreline progradation, form ordered vertical successions when accommodation changes, driven by a low-amplitude relative sea-level oscillation. This demonstrates that partly autocyclic strata may not be intrinsically any less ordered than entirely allocyclic strata. Addition of stochastic processes such as mosaic carbonate production and fluctuations in regional sediment transport direction tend to decrease the level of measurable order present in the autocyclic strata, as would be intuitively expected. However, increasing the frequency and amplitude of relative sea-level oscillations without any stochastic effects also tends to decrease the measurable order in stacked autocyclic peritidal parasequence thicknesses, even though all accommodation is filled by each parasequence. This effect is due to sensitive dependence in the model. These results suggest that controls on parasequence stacking patterns are likely to be more complex than suggested by recent sequence stratigraphic models with clear implications for outcrop interpretation as well as for accurate representation of parasequence stacking in static reservoir models of carbonate strata.