ABSTRACT: Anomalous Zones (Domal)

Donald H. Kupfer

Each zone contains several anomalous salt properties ("anomalous features"). Zones cannot be characterized by any single property. Zones are highly variable, lenticular, and discontinuous in detail; however, once established, they commonly have a predictable trend. The individual "anomalous features" can occur alone (locally in pairs) over areas of various sizes and shapes. These "alone" occurrences are not anomalous zones. Anomalous zones may be of any origin, and origin is not part of the definition. Typical origins include: primary (sedimentary), external "sheath" zone, separating two spines of salt, or caused by toroidal flow.

The major importance of an anomalous zone is that it consists of various "anomalous features" distributed discontinuously along the zone. Thus, if three or more anomalous properties are observed together, one should look for others.

The anomalous zones observed in the Gulf Coast thus far are vertical, linear, and semicontinuous. Most are reasonably straight, but some bend sharply, end abruptly, or coalesce.

Textures in salt involve grain size, color (white to dark gray), grain shape, or grain distribution of the salt. Typical anomalous textures are

coarse-grain, poikiloblastic, and friability. A change in color is commonplace and seldom anomalous.

Structural anomalous features, broadly defined, account for most of the rest of the anomalous features. Not uncommonly they cause mining problems. Among the structural anomalous features:
INCLUSIONS: Sediments, hydrocarbons, brine, gases. Common gases are air (as N₂), CH-compounds, CO₂, and H₂S.
STRUCTURES: Sheared salt, undue slabbing or jointing, voids (crystal-lined pockets), permeability, increased porosity.
COMPOSITION: High anhydrite content, visible anhydrite as grains or boudins, very black salt = disseminated impurities such as clay.

Layering in salt is a combination of composition, color, and structure and is the original bedding in the salt as modified by metamorphism. Layers in the salt may emphasize other anomalous features such as impurities and shearing, but as all types of layering are commonplace, layering or lack of layering is seldom an anomalous feature.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990