Regional Velocity-Depth Anomalies, North Sea Chalk: A Record of Overpressure and Neogene Uplift and Erosion

Abstract

A normal velocity-depth trend for the Upper Cretaceous-Danian Chalk Group is determined by identifying interval-velocity data that represent maximum burial in areas unaffected by overpressuring; these data are derived from 845 wells throughout the North Sea Basin. Data from pelagic carbonate deposits on a stable plateau constrain the trend for shallow depths. Positive velocity anomalies relative to the trend are mapped along the western and eastern margins of the North Sea Basin, and reflect regional Neogene uplift and erosion of up to 1 km along the present-day limit of the Chalk. A hiatus at the base of the Quaternary increases in magnitude away from the basin center, where a complete Cenozoic succession is found. This hiatus is consistent in size with the missing section estimated from Chalk velocities when allowance is made for the Quaternary reburial of the Chalk. Negative velocity anomalies in the central and southern parts of the basin outline an area within which overpressures in the Chalk exceed 10 MPa, equivalent to a burial anomaly greater than 1 km relative to the normal trend. The Chalk pressure system is primarily dependent on overburden properties because retention of overpressure generated by the load of the upper overburden depends on the thickness and sealing quality of the lower overburden; therefore, the Chalk is considered to represent a regional aquitard, and the hydrodynamic model of long-distance migration within the Chalk is rejected. The Neogene uplift and erosion of the margins of the North Sea Basin and the rapid, late Cenozoic subsidence of its center fit into a pattern of late Cenozoic vertical movements around the North Atlantic.