ABSTRACT

Ever since Balk's studies of the 1940's, the idea has been growing that the salt in a stock moves vertically upward in several unit cylinders called spines. Observations in the salt mines of the Five Islands trend in south Louisiana confirm this interpretation. In the most obvious cases the spines are defined by sedimentary gouge (shale, sand, limestone), which becomes included in the salt core and clearly marks the spine boundaries. These boundary shear zones (found in four mines) can extend from the edge of the salt to the very center of the stock. Shear zones of three types (external, internal and boundary) define spines of 500-5000 feet (150-1500 m) in diameter. Every gradation exists from unfaulted but attenuated salt, through realtively simple, thin (15 ft., 5 m), linear shear zones to much wider ones that have been twisted and sheared into the salt in complicated patterns by later salt movements. Oil, gas, and other impurities are common associates.

The shear zones can be traced upward through the salt to surface irregularities in the salt-stock surface, and into topographic irregularities on the ground surface (generally valleys). Disruptions of the overlying domal sediments have long been recognized, and Balk and Muehlberger suggested these as the cause of the irregular salt motion. The present analysis suggests, however, that it is the salt spines that cause the overlying strata disruptions. The concept of pulsing spines of motion explains how salt can be continuously moving, as indicated by some fines of evidence, and yet still cause the observed discontinuities in the adjacent sediments that are best explained by pulsing movements. When moving, the salt as a whole remains within a few thousand feet of the surface in agreement with the downbuilding hypothesis of Barton, and never is carried much below 5 miles (8 km).