THE SALT PILLOW TO DIAPIR TRANSITION - EVIDENCE FROM UNROOFING UNCONFORMITIES IN THE NORWEGIAN-DANISH BASIN

Salt diapirs in the Norwegian Danish Basin are characterized by the oc currence of marked angular unconformities which dip towards the diapir s. These unconformities developed above salt pillows, showing that the pillows grew close to the surface, or even reached the surface, in th e time interval predating diapir formation. By contrast, the unconform ities above present-day pillows show that these pillows have never rea ched close to the surface, strongly suggesting that pillow growth to ' near unroofing' was a necessary condition for a transition to diapiris m. Evidence from pillows indicates that unroofing in some cases was an asymmetric process involving growth of the pillow on the footwall sid e of a fault which soles out on the top surface of the salt. A consequ ence of this mechanism is that a separate stage of salt structure grow th, with a combination of both pillow and diapir features, must be inc luded in a general model for the 'classical' Zechstein basin salt stru ctures.