Significance of Swaley cross-stratification in the Carstone Formation, Hunstanton, Norfolk

Cross-bedding in the Lower Cretaceous Carstone Formation at Hunstanton cliffs in Norfolk was first reported almost a hundred years ago, but details on the nature and significance of the cross-bedding have been largely ignored. To redress this we made detailed field observations and measurements. Fourteen cross-bedded sets were suitable for accurate measurements of set thicknesses, visible set length and foreset orientation. Eleven cross-bedded sets had NE trending foreset dip directions, while three sets had S trending foreset dip directions. Circular statistics on the directional data gave a significant NE mean dip direction of 55°. Cross-bedding geometry is broadly of ‘trough-type’ but specifically ‘swaley’, characterised by concave-upward shallow scours between 0.5-2.0 m wide and a few centimetres deep. Swaley cross-stratification is thought to form below fair-weather wave base, but above storm wave base; our calculations using published physical equations suggest the Carstone bedforms were generated on a storm dominated shoreface in 30 to 40 m water depth. Northward palaeocurrent directions are strikingly different to the predominantly southward palaeocurrents recorded in older Lower Greensand deposits of southern England. A storm surge relaxation current from the land deflected right by Coriolis forces could have resulted in an alongshore NE trending flow forming the Carstone cross-bedding. This interpretation allows for predominantly southward ocean currents interrupted by episodic storms surges and resulting relaxation currents.