GEOMETRY AND GROWTH KINEMATICS OF SALT PILLOWS IN THE SOUTHERN NORTH-SEA

Low-amplitude salt pillow structures can potentially reveal important details of the physical behaviour of salt and overburden from their growth histories. This is because the initial characteristic wavelengths art likely to be preserved, beds are more easily traceable across the tops of the structures, and original salt thicknesses can be accurately estimated. For these reasons, a detailed analysis of depth-converted seismic data was undertaken to gain insight into the formation, growth and spatial distribution of salt pillows in the southern North Sea basin (around Quadrant 44). This area contains a Permian Zechstein salt interval which has undergone widespread halokinesis to the pillow stage since Triassic times, but with the main movement during the Tertiary, perhaps due to inversion. The wavelength of the pillows decreases northwards from over 24 to 1 km, and estimated original salt thickness also decreases northwards from 1300 to 800 m, and there is a strong positive relationship between salt thickness and pillow wavelength. Pillows appear to be spaced at a characteristic wavelength, as they are not directly related to faults and the wavelength varies smoothly through the area. However, there is a strong alignment of some pillows parallel with the underlying NW-SE fault trend, suggesting that their axis of elongation was controlled by faulting. Average vertical growth rates of the pillows vary from 40 m/Ma in the north to less than 10 m/Ma in the central part of the area, measured over vertical distances up to 1250 m. Most of the growth occurred in the Tertiary, although a few pillows initiated in the Triassic in the north of the area. The northern pillows appear to have slowed or even stopped growing during the Jurassic and Cretaceous, and then restarted in the Tertiary. Assuming Rayleigh-Taylor instability controlled the initial pillow wavelengths and using the calculated original salt/overburden thicknesses ratios, the viscosity ratio between the overburden and the salt can be estimated across the area, and was found to vary from 6000 to less than 100. The pillows which moved earliest had the lowest inferred viscosity ratios, which is probably because the sediments were less lithified at the time of movement.