The signal and the noise: Forward modeling of allocyclic and autocyclic processes influencing peritidal carbonate stacking patterns

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 and by varying levels of autocyclicity. Ordered parasequence stacking is suggested to occur during greenhouse periods dominated by low-amplitude eustatic oscillations. Conversely, disordered vertical successions are said to occur when amplitudes of eustatic oscillations are high, or when autocyclic processes dominate and thus drown the periodic external signal. These assertions of causal mechanism are tested here by application of a three-dimensional stratigraphic forward model. The three-dimensional model generates parasequences via a Ginsburg-type mechanism of sediment transport and shoreline progradation. Parasequence stacking is controlled by a superimposed, longer-term eustatic oscillation and parasequences generated are analyzed using runs analysis and Durbin-Watson autocorrelation. Results from the modeling suggest that autocyclic parasequences generated by shoreline progradation form ordered vertical successions when accommodation changes, driven by a low-amplitude relative sea-level oscillation, suggesting that partly autocyclic strata may not be inherently 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. Increasing the frequency and amplitude of relative sea-level oscillations also tends to decrease the measurable order in stacked autocyclic peritidal parasequence thicknesses. These results suggest that controls on parasequence stacking patterns are likely to be more complex and varied than suggested by recent sequence stratigraphic models; more outcrop and modeling investigation is required, with appropriately circumspect interpretations applied.