In mid-April, 1983, an old landslide near Thistle, in Utah County, Utah, began to move, and within days had blocked Spanish Fork Canyon. As the slide9s movement continued, construction crews gradually converted the toe of the slide into an earth-filled dam—Thistle Dam—that impounded northwest-flowing Spanish Fork River. The resultant reservoir, known as Thistle Lake, was subsequently drained because of uncertainty about the stability of the dam. Recently, officials of Utah County have explored various alternatives for a water-retention structure in the area, including utilization of Thistle Dam. The Thistle Slide Committee, established by the State of Utah to evaluate the suitability of using Thistle Dam, suggests that construction of a new dam upstream from the present one might be a more reasonable and cheaper solution than investigating the stability of the present dam. Two potential geologic hazards that could impact a dam site upstream from the Thistle Dam, however, may be in the area: the Thistle Canyon fault and the Thistle Creek diapiric fold. Uncertainty shrouds the existence of both. The Thistle Canyon fault is a postulated high-angle normal fault that trends about N20°E through Thistle. The fault, downthrown on the east, separates an erosional escarpment formed on the Charleston-Nebo thrust plate from younger overlying Cretaceous and Tertiary sedimentary rocks. The Thistle Creek diapiric fold theoretically trends about N30°E through the area. Tenuous evidence suggests that the Middle Jurassic Arapien Shale, an evaporite-rich intrusive sedimentary unit that forms the core of the fold, was overridden by the upper plate of the Charleston-Nebo thrust fault. Since then, the Arapien has welled upward, arching both the thrust plate and the overlying younger sedimentary cover. Additional field investigations should be completed to determine the existence of these and other geologic hazards prior to any final decision about a new dam. The presence of either or both of these hazards, however, does not necessarily preclude the construction of a safe and stable dam that would impound a multi-purpose reservoir.
Two major opposing monoclines trend northward through the Sanpete-Sevier Valley area, central Utah. The Wasatch monocline, along the eastern edge of the area, faces west and forms the west flank of the Wasatch Plateau. Some 20 km (12 mi) to the west is the Valley Mountains monocline which faces east and forms the east flank of the Valley Mountains. Canyons cut in each monocline expose similar Cretaceous and Tertiary units that focally are complexly deformed. In both monoclines the same structural pattern has been impressed on these Cretaceous and Tertiary rocks, implying that both monoclines were formed approximately contemporaneously by the same geologic processes. We attribute the structural complexity of the Cretaceous-Tertiary sequence to the repeated growth and collapse of compound salt diapirs. We postulate at least three such diapiric episodes. The linearity, trend, and some of the height that mark each monocline are due partly to widespread fate Tertiary and Quaternary basin and range block faulting and partly to the dissolution of salt from individually distinct diapirs. This dissolution of salt removed the support for the overlying beds which then progressively sank. The sinking of discrete compound salt diapirs (one underlies Sanpete Valley and another underlies part of Sevier Valley) resulted in the full-scale formation of the Wasatch and Valley Mountains monoclines, respectively.
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------------
