Large deposits of elemental sulfur have been found in several areas in the southwestern part of the Isthmus Salt basin. Sulfur is being produced by the Frasch process from three of those deposits and at least one more probably will be put into production in the near future.

A large part of the Isthmus Salt basin has not yet been thoroughly explored for sulfur. There are good possibilities for finding additional commercial sulfur deposits in the southwestern part of the basin and also in other sectors. Ten private companies, financed largely by United States capital, currently are exploring for sulfur in the southwestern sector.

Stratigraphic conditions throughout the Isthmus Salt basin, and structural conditions in the eastern part, generally are similar to those in the Louisiana-Texas Gulf Coast region which is characterized by a thick section of Mesozoic-Tertiary sediments having a regional gulfward dip and pierced locally by salt domes. Structural conditions in the western part, however, differ markedly from those at comparable depths elsewhere in the Gulf Coast region: there are several large complex anticlines, or anticlinoria, with salt cores, and the salt is shallow in large areas along the crests of the anticlines. Present thickness of the salt in those areas probably exceeds 10,000 ft.

The sulfur deposits are present in so-called cap rock which overlies the salt. There is considerable lateral variation in composition and thickness of the cap rock; in some areas it consists of limestone, sulfate rocks (principally anhydrite and gypsum), and shale; however, in most of the region, insofar as is known, the cap rock consists only of sulfate rocks and shale. Thickness of the cap rock generally is 100-500 ft; in some places it is absent entirely, and in others its thickness reaches more than 1,600 ft (probably involving some duplication by complex folding and/or faulting).

The cap rock outcrops are covered by only a thin mantle of Quaternary continental sediments in several areas on the crests of the anticlines. Elsewhere it is overlain by marine clastic sediments of various ages ranging from Late Cretaceous to Miocene.

The age and origin of the cap rock, and also the age of the underlying salt, are controversial. In the writer's opinion most of the cap rock, including the limestone, consists of Late Jurassic and/or Early Cretaceous sediments deposited in a restricted marine environment. Other writers, following the prevailing hypothesis for the origin of the cap rock of Louisiana and Texas salt domes, have attributed the anhydrite beds to residual accumulation of disseminated anhydrite grains leached from the salt; the gypsum to alteration of the anhydrite; and the limestone and sulfur to alteration of the anhydrite and gypsum. However, the character of much of the cap rock in the large salt anticlines in the western sector of the Isthmus Salt basin indicates that it consists largely of primary sedi ents rather than of products of residual accumulation.

The elemental sulfur in the Isthmus deposits, like the elemental sulfur in Louisiana and Texas and other regions where it is present in carbonate and sulfate rocks, probably was formed by a complex process involving (1) reduction of the sulfates by bacteria and/or hydrocarbons, yielding hydrogen-sulfide; and (2) oxidation of the hydrogen-sulfide, yielding elemental sulfur.

Production of sulfur from deposits in the Isthmus Salt basin presently amounts to about 5,000 tons per day. Cumulative production to the end of 1967 was approximately 17.1 million tons. The remaining recoverable reserves in known deposits probably amount to more than 50 million tons.

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