Quantification of Tertiary erosion in the Inner Moray Firth using sonic velocity data from the Chalk and the Kimmeridge Clay

Abstract

Sonic velocities for the Chalk and Kimmeridge Clay were analysed to determine the importance and magnitude of Tertiary erosion for 26 wells in the Inner Moray Firth. Apparent erosion (height above maximum burial depth) was derived by the displacement, along the depth axis, of a given velocity-depth trend from the normal (undisturbed) trend. The similarity in apparent erosion results derived from the Cretaceous Chalk and Late Jurassic Kimmeridge Clay successions of the Inner Moray Firth suggests that burial beyond present depths caused the observed overcompaction rather than any sedimentological and/or diagenetic process. Resultant apparent erosion estimates are variable across the Inner Moray Firth. Such values reach approximately 1 km in the western part of the basin and decrease progressively to zero in the eastern Inner Moray Firth. However, the actual magnitude of erosion (equal to the sum of apparent erosion and post-erosional Tertiary burial) was approximately 1 km throughout the whole Inner Moray Firth area, indicating that Early Tertiary erosion was regional and not simply restricted to the inverted basin margins. However, a well defined mechanism sufficient to have generated the regional event is still sought.