Influence of salt on the structural evolution of the Channel Basin

Abstract

Abstract The north margin of the Channel Basin is defined in Dorset and the Isle of Wight by Mesozoic extensional faults which were reversed during Tertiary contraction, causing forced folding of the Albian-Oligocene post-rift cover. The resulting contractional structures (the Purbeck and Isle of Wight Monoclines and the reversed Abbotsbury-Ridgeway Fault) have previously been used as type examples for inversion tectonics. The Jurassic-Cretaceous basin margin crosses the eastern edge of a Triassic salt basin (Dorset Halite, Upper Triassic Mercia Mudstone Group) near Swanage. Detachments in Triassic salt around Weymouth resulted in extension and inversion styles which differ from the better known basement-linked structures to the east. The Abbotsbury-Ridgeway Fault is listric and unlike the Tertiary hangingwall folds to the east the Weymouth Anticline formed as an extensional rollover, only tightened during inversion. Seismic and well data and structure maps are used to illustrate for the first time the increasing effects of salt on structural style as the salt thickens westwards. The onset of detachment on the basin margin is placed at Ringstead, where a north-stepping shift in the margin at pre-salt level defines a relay zone. Here extension was accommodated on many basement faults and the ratio of basement fault displacement to salt thickness was reduced sufficiently to allow the post-salt section to detach throughout extension (Abbotsbury-Ridgeway Fault). Tight Tertiary compressional folds in this area may result from linkage of the pre- and post-salt sections during inversion, a consequence of salt welds developed during the last stages of extension. In the western part of the Channel Basin (Lyme Bay) the Triassic salt is sufficiently thick to have detached the pre- and post-salt sections over a wide area. North-south shortening during basin inversion may have been accommodated in the Lyme Bay post-salt section by displacements on conjugate, northwest-southeast- and northeast-southwest-oriented, strike-slip faults such as the Mangerton Fault. The Dorset Halite is a complex sequence of halite and mudstone interbeds, different from the ‘pure’ halites seen in the North Sea Basin, and its influence on structural style differs accordingly. Variation in salt tectonics style between the Weymouth area (salt swells and rollers) and Lyme Bay (salt rollers only) may reflect lateral facies variation and hence rheology in the Dorset Halite Formation.