Distinguishing Milankovitch-Driven Processes in the Rock Recordfrom Stochasticity Using Computer-Simulated Stratigraphy

摘要

Repetitive patterns of facies recurrence are frequently reported from the shallow-water sedimentary rock record andare postulated to have been driven by orbital forcing on eustatic sea level. Consequently, multiple statistical techniqueshave been developed to evaluate whether patterns of stratigraphic succession are more consistent with a periodic sealevel signal or are stochastic. Previous studies focused on development and/or application of such methods to testempirical 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 thesensitivity, robustness, and overall validity of statistical methods. Here we simulate carbonate layers using computermodeledsuccessions generated by periodic sea level changes. The resulting stratigraphic records were then evaluatedstatistically. Thickness distributions of simulated lithofacies were compared to distributions predicted for Poissonprocesses, which by definition are not driven by cyclical sea level changes. Our results suggest that periodic processesproduce stratigraphic thickness frequencies that are difficult to distinguish from random frequencies except underhigh-magnitude sea level fluctuations. Similarly, autocorrelation fails to correctly recognize cyclic patterns in suchsimulated records. Models with high-magnitude sea level fluctuations (icehouse conditions) had thickness frequenciesthat are suggestive of orbital forcing, whereas low-magnitude sea level fluctuations (greenhouse conditions) appearedindependent even though they were modeled using Milankovitch orbital forcing. The increasing evidence based onspectral data from real rock successions suggests that Milankovitch drivers are common in both icehouse and greenhouseperiods. Because statistical approaches are unsuccessful in recognizing the cyclic driver of these simulatedrecords, we infer that it is difficult to disprove independence from real stratigraphy even when orbital forcing iscontrolling the rock composition. Even in the necessarily simplified world of computer simulations, the numerousfactors involved in depositing stratigraphic successions work to complicate or mask any periodic signal, thus generatingthe appearance of stochasticity in some successions.