Abstract

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 slide's 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.

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