Origin and Significance of the Wasatch and Valley Mountains Monoclines, Sanpete-Sevier Valley Area, Central Utah

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.