The processes and alteration of evaporites in the presence of undersaturated water. Unaltered evaporite is shown on the left, initial alteration in contact with undersaturated water in the middle, and intense alteration after prolonged contact with undersaturated water is shown on the right.
The processes involved in salt dissolution are phased depending on how long the evaporites have been in contact with invading undersaturated ground water. These phases were identified during examination of suites of cores across the Palo Duro and Hollis Basins (Hovorka and Granger, 1988). Initial alteration at the base of the salt-dissolution zone where undersaturated downward-moving water encounters halite is dominated by halite dissolution. Halite is removed from halite beds, forming beds of insoluble residue. Halite cements are also removed from other lithologies, increasing porosity and greatly enhancing permeability. This increase in porosity allows recognition of salt dissolution on resistivity logs.
In evaporite-residue sections that have been in contact with undersaturated brines for longer, gypsum alteration is important in creating textures. Anhydrite is hydrated to gypsum in undersaturated brines (Gustavson and others, 1994). Accompanying density change requires that volume-for-volume hydration of anhydrite to gypsum release large amounts of calcium sulfate to solution. Observed textures in core indicate that volume-for-volume hydration of anhydrite to gypsum is the dominant replacement mechanism, and show that gypsum cement is precipitated as fracture and void fillings. Sulfate is also removed in solution. This alteration is characteristic of the dissolution zone from several feet above the top of the uppermost salt to near land surface. Near land surface and in high-flow, high-transmissivity intervals, gypsum has been extensively dissolved, producing gypsum karst. The phased nature of evaporite dissolution is important for understanding log relationships observed in cross sections. Anhydrite and gypsum beds are commonly well preserved in areas where halite has been dissolved and can be traced through the dissolution zone to their depositional or erosional edge.
Regional Low Angle DissolutionPassive Let Down
Commonly, undersaturated ground water moves downward at recharge areas, horizontally for long distances through aquifers, and upward at discharge points. Where salt has been dissolved in this kind of ground-water regime, the upper surface of the salt approximately parallels the flow lines and lies at a low angle to the land surface.
One example of this
geometry is seen in the Palo Duro Basin, where the top of salt lies at
800 to 1,000 ft in depth and approximately parallels the low-relief Southern
High Plains surface. The top of salt forms a low-relief surface paralleling
the regional hydrologic gradient. Cross section based on data from Hovorka,
Fisher, and Nance, 1988. Cross section location is shown on the general
Permian Basin index map. This salt-dissolution
surface regionally crosscuts stratigraphy, so that in the northwest, the
Seven Rivers Formation is the uppermost salt-bearing unit and overlying
salts have been slowly dissolved; down hydrologic gradient to the southeast
Salado halite is partly preserved.