Abstract

Many complex structures in the transition zone of central Utah can be explained by salt diapirism. Rock salt (halite), contained in the Twelvemile Canyon Member of the Arapien Shale (Middle Jurassic), has forced up the enveloping calcareous mudstones which in turn have bowed up the 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 the 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, sudden upward movements of the salt and its subsequent removal. These surges were separated by longer periods of much slower upwelling of the salt. Continued or renewed, nearly imperceptible upwelling of the salt after collapse and erosion of each fold is suggested by sedimentary thinning near and along the flanks of the diapiric folds. The salt probably has been moving since it was deposited; likely it is 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. The first episode began and ended during the Late Cretaceous. The second episode began and ended in the late(?) Oligocene. The third episode extends from the late(?) Oligocene to the Pliocene or Pleistocene; when it ended is not clear. Some tenuous evidence suggests a fourth, localized diapiric episode which began and ended during the Pleistocene.

Each episode is divisible into three interrelated stages: an intrusive stage during which the salt surges upward to form a diapiric fold, an erosional stage during which the fold either collapses or subsides and the remnants are then eroded to a surface of low relief, and a depositional stage during which younger sediments are spread across the newly formed surface. The depositional stage ends with a renewed upward surge of the salt, marking the onset of the intrusive stage of the next diapiric episode.

Field evidence at widely separated localities suggests that all the diapirs were reactivated nearly simultaneously during each diapiric episode.

Although autonomous, isostatic movement of the salt (halo-kinesis) may explain some aspects in the development of the diapiric folds, some form of tectonic influence (halo-tectonism) seems a more reasonable explanation for their uniform distribution and unusual length and straightness. Deep-seated fundamental normal faults in the pre-salt rocks are suggested as a controlling factor. When these faults reactivated and broke the post-salt beds, the deeply buried salt, under great static load, relieved the load by surging up the fault planes to form a series of parental salt diapirs whose trends and distribution reflect the buried faults. The rising salt forced the Twelvemile Canyon Member mudstones to intrude and bow up the overlying sedimentary rocks, forming diapiric folds. Subsequent reactivations of these fundamental faults caused the salt to surge upward time and again; each time, these younger folds, expectably, followed the same structural zones and had the same trends as the parental folds.