Morphology and Origin of Salt Domes of Isthmus of Tehuantepec1
The Isthmian Salt basin occupies the whole northern part of the Isthmus of Tehuantepec. Identified strata range from probable Permian to upper Miocene. Post-Miocene deposits unconformably overlie all other strata, but no study has been made of them. The Miocene section has been studied most closely because it contains petroleum-productive sandstones. The minimum thickness of the entire Miocene section is in the western part of the basin, where it is only up to 400 m thick, and the maximum thickness is near Ogarrio field, where more than 5,000 m may be present in synclines.
Petroleum and sulfur exploration have yielded considerable data on the salt in the basin. However, the drilling depths generally are not sufficient to define the flanks of individual domes and salt masses. The salt is distributed in at least five huge masses, ranging in width from 10 to 25 km, and in length from 12 to 30 km. These masses lie within 50 m of the surface, and individual domes rise from the larger masses of salt. Some single domes—not obviously related to salt masses—also are present in the area. Most of the large salt structures, as well as the individual domes, have a cap rock composed mainly of anhydrite and smaller quantities of gypsum. Some of the domes, however, have no cap rock. The thickness of the cap rock is greatly varied, and sulfur is present in the cap rock of some domes.
A palynological study of three salt cores showed that the upper part of the salt is of latest Jurassic or earliest Cretaceous age. The lower age limit is not known, but may be as old as Triassic. The thickness of the salt in the massifs may be more than 5,000 m, but the original depositional thickness is unknown. The salt was put into motion as a result of the isostatic effect of the overlying Upper Cretaceous, Paleocene, and Eocene sediments. However, the thick Oligocene sediments probably constituted the main load which induced upward movement of the salt, especially in the eastern part of the basin where Jurassic folding had little effect.
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“Diapir” and “diapirism” come from the Greek diapeirein, which means “to pierce.” Diapirism sensu lato is a process by which earth materials from deeper levels have pierced, or appear to have pierced, shallower materials; it is divided into magmatic intrusion and diapirism sensu stricto on the basis of the temperature at which piercement occurs. Diapirs s.s. are composed of evaporites, argillaceous sediments, coal, peat, ice, serpentine, or other earth materials which have the critical characteristics of low equivalent viscosity and low density. These materials range in age from Precambrian to Recent. Diapirs are found in all parts of the world except the shield areas. They have many forms, ranging from smoothly rounded pillows to complexly injected laminae, are either connected with or disconnected from the “mother” bed, and are present either at the surface, where they form distinctive features, or at considerable depth. Diapirs have well-developed internal structures indicative of an origin by flow. Strata around a diapir may be strongly affected structurally and/or stratigraphically by the diapir, or they may be unaffected. Field and model studies indicate that diapirs have developed as a result of horizontal compression, gravitational instability, or both. Diapiric structures of various types contain large quantities of oil and gas, sulfur, salt, and potash and are important for underground storage and nuclear testing.