Print this pageAbstract: Salt Solution-Collapse, Williston Basin: What, Where, Why, How, and When
A variety of structural forms is created when salt beds are leached. Some contain oil and gas. Seismic surveying can map them and some surface evidence has been observed. Dissolution of salt takes place only during periods of subaerial exposure by moving water. Small differential water densities combined with sufficient water-table elevations make dissolution possible thousands of feet below the surface. The same factors permit bypassing of some salt beds higher in the geologic column. Meteoric water in topographically high areas moves downward through sinks to the salt beds, and brines return to the surface as springs. In poorly drained areas, alkali lakes are evidence of fairly recent salt dissolution. Some collapse breccias are evidence of original sinks. As dissolutio moved outward from the sinks, sagging and settling with tension lowered the overlying rocks with little apparent disturbance. With the last dissolution of salt between the sinks, compression became a factor and crumpling and jamming occurred. This created the extra thickening of parts of the geologic column as seen at the Hummingbird and other structures. The abnormal thickening should not be interpreted as an indication of the time of dissolution. A study of paleodrainage may help localize favorable areas for other oil and gas traps.
In the Williston basin the most extensive salt dissolution took place during the Tertiary. The post-Paleozoic dissolution was of lesser extent but was important in the creation of oil and gas traps.
AAPG Search and Discovery Article #90971©1976 AAPG-SEPM Rocky Mountain Sections 25th Annual Meeting, Billings, Montana