Distinguishing Milankovitch‐Driven Processes in the Rock Record from Stochasticity Using Computer‐Simulated Stratigraphy

Repetitive patterns of facies recurrence are frequently reported from the shallow‐water sedimentary rock record and are postulated to have been driven by orbital forcing on eustatic sea level. Consequently, multiple statistical techniques have been developed to evaluate whether patterns of stratigraphic succession are more consistent with a periodic sea level signal or are stochastic. Previous studies focused on development and/or application of such methods to test empirical geological records. However, the character of such records cannot be known a priori, as deposition, erosion, and preservation influence the resultant composition, nor can those records be readily manipulated to explore the sensitivity, robustness, and overall validity of statistical methods. Here we simulate carbonate layers using computer‐modeled successions generated by periodic sea level changes. The resulting stratigraphic records were then evaluated statistically. Thickness distributions of simulated lithofacies were compared to distributions predicted for Poisson processes, which by definition are not driven by cyclical sea level changes. Our results suggest that periodic processes produce stratigraphic thickness frequencies that are difficult to distinguish from random frequencies except under high‐magnitude sea level fluctuations. Similarly, autocorrelation fails to correctly recognize cyclic patterns in such simulated records. Models with high‐magnitude sea level fluctuations (icehouse conditions) had thickness frequencies that are suggestive of orbital forcing, whereas low‐magnitude sea level fluctuations (greenhouse conditions) appeared independent even though they were modeled using Milankovitch orbital forcing. The increasing evidence based on spectral data from real rock successions suggests that Milankovitch drivers are common in both icehouse and greenhouse periods. Because statistical approaches are unsuccessful in recognizing the cyclic driver of these simulated records, we infer that it is difficult to disprove independence from real stratigraphy even when orbital forcing is controlling the rock composition. Even in the necessarily simplified world of computer simulations, the numerous factors involved in depositing stratigraphic successions work to complicate or mask any periodic signal, thus generating the appearance of stochasticity in some successions.