Shear Zones Inside Gulf Coast Salt Stocks Help to Delineate Spines of Movement

Abstract

Following Balk's studies of the 1940s, 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 (present in four mines) can extend from the edge of the salt to the center. Shear zones of three types (external, internal, and boundary) define spines of 500 to 5,000 ft (150 to 1,500 m) in diameter. Every gradation exists from unfaulted but attenuated salt, through relatively simple, thin (15 ft; 5 m), linear sh ar 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 long have been recognized, and Balk and Muehlberger suggested these as the cause of the irregular salt motion. The present analysis suggests, however, that the salt spines cause the overlying strata disruptions. Flat caprock over the top of the entire salt stock thus indicates stability. The concept of pulsing spines of motion explains how salt can be moving continuously, as indicated by some lines of evidence,and yet still cause the discontinuities in the adjacent sediments that are explained best by pulsing movements. When moving, the salt as a whole remai s within a few thousand feet of the surface in agreement with the downbuilding hypothesis of Barton, and never is carried much below 5 mi (8 km).