Tidal circulation in an ancient epicontinental sea: The Early Jurassic Laurasian Seaway

We model tides and associated bed shear stress in the Early Jurassic Laurasian Seaway of northwest Europe. Sensitivity tests with different water depths highlight those regions that are least affected by paleobathymetric uncertainty. Results show that although the vast seaway was largely microtidal, the tides were still capable of affecting sediment transport more than 2000 km from the open ocean. Flow constriction associated with shallow platforms and straits produced elevated bed shear stresses (a direct proxy for the entrainment and transport of sediment) that were decoupled from tidal range and were capable of transporting sand. Areas of increased bed shear stress broadly correlate with published geological data. Varying the water depth in the basin provides an insight into the effects of an idealized transgression. Increasing the water level leads to a net reduction in the bed shear stress at the shelf edge, but promotes further penetration of tidal energy into the basin interior, although it is still focused into areas of flow constriction. Further drowning of the seaway eventually widens the straits to the point that they fail to constrict the flow and bed shear stress is reduced. Increasing water level counterintuitively leads to an increase in bed shear stress in a small number of locations.