MODELLING TIDAL CURRENT INDUCED BED SHEAR STRESSES AND PALEOCIRCULATION IN THE BOHEMIAN CRETACEOUS BASIN, CZECH REPUBLIC: AN EXAMPLE OF A MARINE CURRENT-MODIFIED COARSE-GRAINED DELTAIC SYSTEM

The Jizera Formation (Early Middle Turonian) of the Bohemian Cretaceous Basin, includes a series of coarse grained bedforms superimposed on a series of delta front clinoforms. The basin formed in a shallow epicontinental sea on the northern margin of Tethys just east of the Bohemian Massif. The mechanism generating the paleocurrents that transported the sands has been subject to debate. Critically the along-shore currents must have operated across the entire depth range (>35m) of the clinoforms. In addition, numerous reactivation surfaces, rare thin, silt drapes and occasional bi-directional trough cross-set axes suggest that tidal paleocurrents may be important. In the absence of more ubiquitous evidence of tides (e.g. tidal bundles), little can be concluded without a better understanding of the potential effects of tidal forcing on basin circulation.

The Imperial College Ocean Model (ICOM), a non-hydrostatic, finite element model has been used to test the hypothesis that tides were capable of generating the paleocurrents suggested by field observations. The model itself has been previously validated by comparing model results with that of the global ocean and a variety of coastal seas.

Bed shear stresses were extracted from the model results in order to better understand sediment transport. The maximum bed shear stresses and their orientations are consistent with paleocurrent orientations of the coarse grained bedforms (ripples and dunes). In addition, mean bed shear stress vector plots indicate dominant paleocurrent directions in agreement with the largely unidirectional dip direction of the bedform foresets. This supports the notion that the Bohemian Cretaceous Basin contained a vigorous tidal circulation that extensively modified coarse-grained Gilbert deltas.