Salt tectonics in the Norwegian—Danish Basin

Abstract

A study is presented of salt tectonics in a 28,000 km 2 area in the Norwegian-Danish Basin of the North Sea. The basin is mainly of Permian to Early Jurassic age and contains sediments which locally reach 8 km in thickness. Zechstein (Upper Permian) salt occurs widely in the basin and has given rise to widespread and intense salt tectonics. Using seismic reflection sections with a total length of 14,000 km, as well as borehole data, 86 salt structures have been mapped within the study area. The floor of the basin (that is, the base of the Zechstein salt) and the faults affecting this floor have also been mapped. It is therefore possible to investigate the association of faults and salt structures and hence to ascertain to what extent faulting of the basin floor may have initiated salt movement. Various lines of investigation and reasoning have been developed to arrive at an estimate of the number of structures for which salt movement is likely to have been initiated by faulting of the basin floor. It is found that only for a limited number of salt structures (probably about five out of a total of 86 structures) the salt movement is likely to have been initiated by faulting. The very large majority of the salt structures originated independent of faulting. The mechanism responsible for the initiation of salt movement in this majority has also been studied. It is found that these salt structures can be divided into two groups. One group comprises about 37 salt structures which are randomly distributed. Nearly all of them are situated where the present basin floor is relatively flat and at its deepest. They are thought to be the result of classical halokinesis, that is, the autonomous upward movement, under the influence of gravity, of mobile salt at locations where the sedimentary overburden at the time of initial movement was locally less heavy. A second group comprises about 43 individual structures which are aligned along nine so-called “trend lines”. It is first shown that these trend lines are independent of faults and fault trends in the basin floor. The origin of these trends is then discussed by considering two alternative hypotheses for the mechanism responsible for the initiation of salt movement along these trend lines. The first hypothesis assumes that salt ridges in the subsurface are initially pushed up by a prograding sedimentary overburden. The second hypothesis assumes that initial ridges of mobilized salt are formed along a palaeo-depth contour which migrates up the flank of the subsiding basin. Both hypotheses assume that the salt ridges, thus formed, can subsequently develop into individual salt structures which produce the observed pattern of trend lines. A critical analysis of these two alternative hypotheses leads to the conclusion that the second one, involving the concept of a palaeo-depth contour which migrates up-flank, is the more acceptable. The mechanism involved may be characterized as a special type of halokinesis, namely as halokinesis constrained to operate along a line.