Abstract

Abstract

There are thick beds of gypsum in the Permian Zechstein sequence that forms the bedrock in the Darlington area. In this area, subsidence features attributed to gypsum dissolution are typically broad shallow depressions up to 100 m in diameter, and the ponds known as Hell Kettles [NZ 2814 1088] are the only recognized examples of steep-sided subsidence hollows around Darlington. We have acquired a loose grid of 11 2D seismic reflection profiles and a 3D seismic reflection survey in the area around the ponds. The 2D profiles have revealed evidence of foundering in the limestone of the Seaham Formation at depths of c. 50 m. The foundering is interpreted to have resulted from dissolution of gypsum in the Hartlepool Anhydrite Formation at c. 70 m depth. The reflection images of the gypsum itself are discontinuous, suggesting that its top surface has karstic topography. The 3D survey has revealed subcircular hollows in the Seaham Formation up to 20 m across, which are again attributed to foundering caused by gypsum dissolution. The results demonstrate the potential of the seismic reflection method for site investigation where there is a subsidence hazard because of the possible presence of subterranean cavities.

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