Modelling Tidal Current-Induced Bed Shear Stress and Palaeocirculation in a Shallow Epicontinental Seaway: The Bohemian Cretaceous Basin, Central Europe

The Lower-Middle Turonian of the Bohemian Cretaceous Basin (Central Europe) preserves a series of coarse-grained deltaic sandstone bodies grading offshore into fine-grained sediments. Dunes superimposed on the deltaic foresets give evidence of a vigorous basinal palaeocirculation capable of transporting coarse-grained sand across the entire depth range of the clinoforms (~35m). Bi-directional, longshore oriented, trough cross-set axes, silt drapes and reactivation surfaces suggest tidal currents to be the main driving mechanism. This scenario therefore lends itself as a suitable case study for the validation of numerical ocean models at predicting tidal palaeocirculation in ancient seas.

We used the Imperial College Ocean Model (ICOM), a fully hydrodynamic, next generation finite element model to test the hypothesis that tidal circulation was capable of generating the observed facies, grain-size distribution and sediment transport pathways within the Bohemian Cretaceous Basin. The ability of the model to predict bed shear stress and sediment transport pathways is validated by comparing simulated results for the North Sea with grain-size maps and previously published work.

Predicted maximum bed shear stress magnitudes are consistent with the observed grain-size and bedforms across a wide range of sensitivity tests. However, the mean and maximum bed shear stress vectors fluctuate considerably between sensitivity tests involving very subtle palaeobathymetric variations. This degree of palaeobathymetric subtlety cannot be determined in the geological record suggesting that current vectors and associated sediment transport pathways cannot be modelled in this instance.

ICOM predicts a vigorous tide driven palaeocirculation within the Bohemian Cretaceous Basin that would have indisputably influenced deposition and facies distribution. Our results do not exclude the possible influence of wind driven and storm currents but show that tides alone are capable of generating bed shear stresses consistent with outcrop data.