The Origins of Shallow-Water Carbonate Lithofacies Thickness Distributions: 1D Forward Modelling of Relative Sea-Level, Production Rate and Erosion Rate Controls

The observation that shallow-marine carbonate strata often have exponential lithofacies thickness distributions is of fundamental importance. This is because it is an observation that can be tested for its repeatability in outcrop and subsurface examples, and also because it raises the possibility that strata may be well represented with stochastic models with no intrinsic organization or hierarchy present in the strata. Most importantly however, it is significant because it poses the fundamental question of what sedimentary processes lead to the formation of particular lithofacies thickness distributions. This in turn links to the significant issue of how carbonate strata record climatic change through geological time.

This work builds on previous work using a simple 1D numerical stratigraphic forward model of carbonate platform strata (Dougal) to investigate how various controls such as amplitude and period of eustatic oscillation, variations in production rate, different lag depths and variations in erosion rate can control the type of lithofacies distribution produced. Dougal records platform-top carbonate accumulation influenced by water-depth dependent sediment production in euphotic, oligophotic and aphotic production profiles with a lag-depth controlling onset of production.

Results from single model runs highlight the issue of non-stationary behaviour where statistical properties of the strata change with elevation up the section, and show that exponential lithofacies thickness distributions can be generated from an entirely deterministic model, in contrast to the stochastic models invoked previously. Simple model runs illustrate how different types of lithofacies distribution are constructed unit by unit as the depositional system responds to external forcing and internal autocyclic dynamics. These analyses are substantiated with hundreds of thousands of model runs that show in detail how different lithofacies distributions may arise under varying accommodation and production regimes likely to develop in different climate settings with different types of eustatic curves and different types of carbonate factory.

This 1D modeling lays important foundations for reproducing lithofacies thickness distributions using 3D models of fine-scale carbonate heterogeneity. An example of some initial results will be shown from CarboCAT, a 3D cellular automata model of carbonate facies migration and accumulation.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California