Benjamin T. Cardenas, Benjamin P. Smith

Preserved cross-set thicknesses are powerful tools for unravelling past environmental conditions. The relative rate of bedform aggradation to migration (climb angle) is encoded into the distribution of cross-set thicknesses. In siliciclastic settings, climb angles have been used to reconstruct properties of the depositional system, including ancient topography, which exerts a control on local aggradation rates. Cross-set thickness distributions in carbonate environments should prove equally useful. Carbonate sediments are often bound by early cements or microbes, both of which influence sediment transport. If cross sets record these interactions, then they may contain information about local—and possibly global—changes to sediment cohesion.
To test this idea, we analyzed the distribution of cross-set thicknesses in a grainstone interval of the Cretaceous Glen Rose Formation at an outcrop in Austin, TX, USA. Bedform climb angles inferred from the distribution of cross-set thicknesses were on the order of 0.5° to 5°. In siliciclastic systems, climb angles this high are typically driven by the filling of local relief; relief is minor in this carbonate system. We interpret this as evidence for rapid bed aggradation driven by early cements or organic binding, a boundary condition of potential significance to carbonate depositional settings. We suggest that at geologic time scales, global trends in carbonate bedform preservation should be sensitive to both carbonate chemistry and biotic innovations. If so, our results provide a quantitative method for exploring these topics in deep time.