Messy Migrating Mosaics: A Numerical Forward Modelling Analysis of Carbonate Stacking and Bed Thickness Distributions

Data from modern shallow-water carbonate depositional systems indicate a complex spatial arrangement of sediment on platform tops, with little or no quantifiable relationship between sediment type across a water depth range of ~10 m, and large areas of non-deposition covering more than 20% of sea floor area. These observed distributions of modern shallow-water carbonate sediment are often referred to as a facies mosaic, and contrast markedly with some other models where simpler, essentially linear facies belts migrate under strong control of relative sea-level oscillations. Output from two numerical models, one with accommodation-controlled linear facies belts, and one with a shallow-water facies mosaic, produce vertical sections that can be compared with outcrop vertical sections to determine which of these two end-member models is more realistic. Quantitative comparison indicates the facies mosaic model produces a better match with observed outcrop lithofacies thickness distributions. These results have several important implications, most notably that many interpretations of outcrop carbonate strata as the product of linear facies belts migrating under simple accommodation control maybe unrealistic. More robust use of quantitative methods and modern analogues to ensure conceptual models are realistic, and a move away from “modelling in disguise” interpretations of outcrop strata, would perhaps be sensible.