Modelling tidal current‐induced bed shear stress and palaeocirculation in an epicontinental seaway: the Bohemian Cretaceous Basin,Central Europe

Lower to Middle Turonian deposits within the Bohemian Cretaceous Basin (Central Europe) consist of coarse‐grained deltaic sandstones passing distally into fine‐grained offshore sediments. Dune‐scale cross‐beds superimposed on delta‐front clinoforms indicate a vigorous basinal palaeocirculation capable of transporting coarse‐grained sand across the entire depth range of the clinoforms (ca 35 m). Bi‐directional, alongshore‐oriented, trough cross‐set axes, silt drapes and reactivation surfaces indicate tidal activity. However, the Bohemian Cretaceous Basin at this time was over a thousand kilometres from the shelf break and separated from the open ocean by a series of small islands. The presence of tidally‐influenced deposits in a setting where co‐oscillating tides are likely to have been damped down by seabed friction and blocked by emergent land masses is problematic. The Imperial College Ocean Model, a fully hydrodynamic, unstructured mesh finite element model, is used to test the hypothesis that tidal circulation in this isolated region was capable of generating the observed grain‐size distributions, bedform types and palaeocurrent orientations. The model is first validated for the prediction of bed shear stress magnitudes and sediment transport pathways against the present‐day North European shelf seas that surround the British Isles. The model predicts a microtidal to mesotidal regime for the Bohemian Cretaceous Basin across a range of sensitivity tests with elevated tidal ranges in local embayments. Funnelling associated with straits increases tidal current velocities, generating bed shear stresses that were capable of forming the sedimentary structures observed in the field. The model also predicts instantaneous bi‐directional currents with orientations comparable with those measured in the field. Overall, the Imperial College Ocean Model predicts a vigorous tide‐driven palaeocirculation within the Bohemian Cretaceous Basin that would indisputably have influenced sediment dispersal and facies distributions. Palaeocurrent vectors and sediment transport pathways however vary markedly in the different sensitivity tests. Accurate modelling of these parameters, in this instance, requires greater palaeogeographic certainty than can be extracted from the available rock record.     

Recognition of strong seasonality and climatic cyclicity in an ancient,fluvially dominated,tidally influenced point bar: Middle McMurray Formation,Lower Steepbank River,north‐eastern Alberta,Canada

Inclined heterolithic stratification in the Lower Cretaceous McMurray Formation, exposed along the Steepbank River in north‐eastern Alberta, Canada, accumulated on point bars of a 30 to 40 m deep continental‐scale river in the fluvial–marine transition. This inclined heterolithic stratification consists of two alternating lithologies, sand and fine‐grained beds. Sand beds were deposited rapidly by unidirectional currents and contain little or no bioturbation. Fine‐grained beds contain rare tidal structures, and are intensely bioturbated by low‐diversity ichnofossil assemblages. The alternations between the sand and fine‐grained beds are probably caused by strong variations in fluvial discharge; that are believed to be seasonal (probably annual) in duration. The sand beds accumulated during river floods, under fluvially dominated conditions when the water was fresh, whereas the fine‐grained beds accumulated during the late stages of the river flood and deposition continued under tidally influenced brackish‐water conditions during times of low‐river flow (i.e. the interflood periods). These changes reflect the annual migration in the positions of the tidal and salinity limits within the fluvial–marine transition that result from changes in river discharge. Sand and fine‐grained beds are cyclically organized in the studied outcrops forming metre‐scale cycles. A single metre‐scale cycle is defined by a sharp base, an upward decrease in sand‐bed thickness and upward increases in the preservation of fine‐grained beds and the intensity of bioturbation. Metre‐scale cycles are interpreted to be the product of a longer term (decadal) cyclicity in fluvial discharge, probably caused by fluctuations in ocean or solar dynamics. The volumetric dominance of river‐flood deposits within the succession suggests that accumulation occurred in a relatively landward position within the fluvial–marine transition. This study shows that careful observation can reveal much about the interplay of processes within the fluvial–marine transition, which in turn provides a powerful tool for determining the palaeo‐environmental location of a deposit within the fluvial–marine transition.