ABSTRACT

Bipartite piercement domes and arcuate diapiric ridges in the western Sverdrup Basin are composed of a lower unit of rock salt and an upper unit of anyhydrite with limestone interbeds. As revealed by analogue models of buoyancy-driven diapirs, a thick unit of dense imporous anhydrite could not have risen actively together with the rock salt. Yet the anhydrite rocks have neither been attenuated nor pierced by the salt. Structural field evidence demonstrates unequivocally that the CaSO₄ material (gypsum and/or anhydrite crystal mush) behaved as an active low-density medium in early states of diapirism. Apparently, the crystal water liberated by dehydration of buried sedimentary gypsum remained in the CaSO₄ unit and preserved its low density for a limited period. Gradually, the light CaSO₄ material turned into competent heavy anhydrite which formed a thick hood on the rising plugs and ridges of rock salt. After intensive shear along the flanks, the hood became a compact mass which was forced through the clastic overburden while breaking up internally. The competent anhydrite mass of at least one bipartite dome seems to have been exposed intermittently in the Upper Cretaceous.

Some anhydrite cores of diapirs in the Eureka Sound fold belt, eastern Sverdrup Basin, behaved in the same way as those of the bipartite domes in the western Sverdrup Basin, and may have grown geostatically. Other large diapirs in the eastern Sverdrup Basin seem to be related to major faults, and may have risen under horizontal tectonic stress. It is not known to what extent these diapirs were actually driven by horizontal compression as opposed to differential buoyancy. Neither do we know which diapirs, if any, were initiated before the Eurekan Orogeny on eastern Axel Heiberg Island. Most of the outstanding problems cannot be solved completely until gravity studies have been made with the object of estimating the volume of low-density material below large structures like the North Mokka anticline, the Mokka Fiord dome or the Whitsunday Bay diapir.