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Geologic Mapping of Salt Mines in Salt Diapirs: Approaches and Examples from South Louisiana
Gulf Coast salt diapirs have complex internal structure, which is shown by light and dark layering. The layering reflects variation in anhydrite content and is interpreted to represent highly stretched and sheared depositional bedding. Structure is dominantly vertical, consisting of repeatedly refolded isoclinal folds having vertical plunge.
Geologic mapping is conducted primarily as an aid to mine planning and development; separate maps are produced to record basic structure and display the distribution of clastic stringers, sylvite stringers and lenses, crystal size, wet areas, and gas. The latter characteristics, including coarse salt crystals, are anomalous and tend to be associated (in anomalous areas and trends). Gas is a mining hazard, causing Weeks Island and Cote Blanche Island to be classified by MSHA as gassy mines having many associated regulations. Five miners died in a gas explosion at Belle Isle mine in 1979, and blasting is now normally done while the mines are unoccupied (before the morning shift goes in, for example). Water influx, particularly if derived from outside the mine, is a cause of major concern for obvious reasons; at Avery Island a crew is constantly drilling and grouting to maintain control of leaks, and Jefferson Island mine has been lost as a result of flooding when penetrated by an oil rig. Sand reduces the salability of the commercial product and also increases mining costs.
The geologic maps provide structural trend information to help the mine engineers avoid undesirable or potentially dangerous areas including those in proximity to the edge of the diapir. They also cast light on the nature of salt diapirism; linear anomalous zones have routinely been interpreted (sometimes with minimal justification) as boundaries between separate salt spines.
Map examples from Weeks Island are included.
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