Internal Kinematics of Salt Diapirs: DISCUSSION

Abstract

Iklbot and Jackson (1987) presented a very good sum­ mary of several aspects of salt-stock configuration and emplacement as applied to the United States Gulf Coast. One aspect of this emplacement is a process they describe as nested toroidal stream surfaces (Iklbot and Jack­ son, 1987, p. 1083. their Figure 11) or toroidal flow. The concept may have been originally suggested by the over­ turned and even convoluted salt structures observed at the Boulby mine in England (a highly deformed beddedsalt deposit). Concepts about these structures have been expanded upon by experiments, hydrodynamic theory, and further observations. Toroidal-type swirl struc­ tures develop early and are best displayed in salt in the early stages of deformation and diapir rise. My main purpose here is to note that Tiilbot and Jack­ son (1987) have apphed these toroidal shapes where they do not seem to apply—the high-rise diapirs of the Five Island salt stocks of south-central Louisiana. As a result, Iklbot and Jackson have suggested an unlikely origin for the spines in these stocks. They further infer that these toroidal structures should be present in mature-type salt diapirs of the Gulf Coast coastal prov­ ince; these diapirs have been well explored for petroleum. Tklbot and Jackson (1987, p. 1068) stated that mature diapirs can form crestal bulbs.... I suggest that these are unlikely assumptions. I have emphasized (for example, Kupfer, 1970a, p. 43, p. 55-56; 1988, p. 14) the distinctly different structural styles within low-rise salt diapirs exemplified by north German diapirs (with toroidal structures, for example Iklbot and Jackson's [1987] Figures 5 and 15) and the high-rise diapirs of coastal Louisiana. The Five Islands (from north to south: Jefferson, Avery, Weeks, Cote Blanche, Belle Isle) are over the deepest part of the Gulf Coast basin (60,000 ft or 18 km) and represent an extreme case of high rise above the present mother-salt hori­ zon. As Iklbot and Jackson have noted (1987, p. 1074, 1084), the salt at the top of these stocks has moved about 19 mi (30 km), is highly refolded, and the flow inside the diapirs is neither homogeneous nor simple. These stocks are complex and have lost or distorted nearly all of their early formed structures. Much of Tklbot and Jackson's experience (and evi­ dence cited in their paper) has been with salt glaciers, early diapirism, and with the intermediate-rise diapirs of eastern Texas (and northern Louisiana) .These interme­ diate stocks, as might be expected, retain much of the