Abstract: Subsalt Origin of Salt-Dome Cap Rock

Oscar L. Paulson, Jr.

Previous hypotheses for the origin of salt-dome cap rock have relied in varying degrees on the introduction of minerals and fluids from the strata surrounding the dome, whereas I propose a subsalt origin for the minerals. Cap-rock minerals beneath the salt of most evaporite basins are well-known, but the idea that these minerals become an integral part of the diapiric mass has not been broached. Studies of diapiric structures have revealed that materials underlying the principal diapiric constituent can be incorporated into the diapiric mass and this fact provides the basis for the present paper.

Irregularities in the thickness of salt-dome cap rock are associated with similar irregularities in the top of the salt. These irregularities are thought to result from spines of movement which serve as avenues for the expulsion of cap-rock minerals. The presence of hydrogen sulfide, sulfur, and pressured brine at depth in salt domes adds support to a subsalt origin for cap rock.

The species of minerals present in the cap rock of a particular dome generally is related to the amount of growth of the dome and its position in the evaporite basin. Domes which have undergone the greatest amount of upward growth are more likely to have a greater number of the minerals which precipitated early in the evaporite sequence. Metallic sulfides, such as those found in some domes, commonly are associated with sapropels which mark the beginning of the evaporite stage and therefore represent the oldest of the mineral species to reach the cap rock.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas