About This Item
Share This Item
In central Utah, many complex structures in the transition zone between the Colorado Plateau Province and the Great Basin can be explained by salt diapirism. Flowage of rock salt (halite) in the Arapien Shale (Middle Jurassic) has forced up the enveloping mudstones, which in turn have bowed up younger consolidated strata to form elongate, linear diapiric folds, fan-shaped in cross section. Removal of salt, by extrusion, solution, or lateral flowage, has resulted in partial destruction of these folds, either by collapse along faults or by general subsidence. Field evidence suggests that these diapiric folds grew and failed repeatedly, presumably as a result of sporadic, rapid, upward movements of the salt and its subsequent removal. These surges were separated by longer pe iods of much slower upwelling of the salt. Continuous, nearly imperceptible upwelling of the salt after collapse and erosion of each fold is suggested by sedimentary thinning near the flanks of the diapiric folds. The salt has been moving probably since it was deposited; it is probably moving today. As a result of this episodic diapirism, younger daughter folds occupy the same structural zones as the older parental folds. At least three diapiric episodes are reflected in the country rocks.
Although autonomous isostatic movement of the salt (halokinesis) may explain some aspects in the development of the diapiric folds, external tectonic stresses (halotectonism) seem a more reasonable explanation for the uniform distribution and the unusual length (as much as 125 km, 75 mi) and straightness of the folds. Movement along deep-seated fundamental normal faults in the pre-salt rocks provides a plausible mechanism for controlling the timing and location of the folds.
End_of_Article - Last_Page 1362------------