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Syndepositional Folds Developed in Salt Wall Roof Strata, Paradox Basin, Utah and Colorado

Abstract

The Paradox Basin exposes of a series of salt walls that rose passively from the early Permian through late Jurassic time. Permian through latest Jurassic syndepositionally folded strata, are found in the diapir roof strata where the salt walls narrowed and became partially buried. These areas, termed salt shoulders, have been exhumed by incision of the Colorado Plateau at several of the salt walls where structure and sedimentology can be examined in detail. Folds include anticlines with curved axial planes separated by oval basins and elongate synclines with fold axis that parallel the margin of the salt wall. The folds vary in amplitude and wavelength, ranging from 50 m to over 2.5 km in width. Interlimb angles can be as close as 90°, but are commonly more open (160° to 130°). In some locations larger, longer wavelength folds are stratigraphically stacked on top of smaller folds, indicating continued deformation during deposition. The synclines are filled with growth strata displaying stratal thinning and onlapping onto adjacent anticlines.

Because of the widespread occurrence of this style of folding (5 of 8 dissected diapirs) within the Paradox Basin, syndepositional folding on salt shoulders seems to be common or fundamental model for deposition on mobile salt. The mechanism of initiating folding and subsidence on salt diapirs is a long-standing problem in salt tectonics. Proposed hypotheses include topographic differences, tectonic extension and contraction, migration of salt from beneath the basin, and subsalt deformation. Although the observed Paradox folds formed over a long period of time with, varying tectonic stresses, the fold axes are always subparallel to the diapir margin. This suggests, that at least some folds are not forced by regional tectonic stresses. The vertical stacking of larger folds on smaller folds indicates that the sediment, even though thin are influencing the style of folding, with thicker strata making larger wavelength folds. Incorporation of diapir-derived clasts indicates that the folds formed during passive diapirism, that included episodes of diapir exposure adjacent to the folds. We suggest possible causes of the folding are: 1) deep solution of underlying caprock or salt resulting in subsidence at the surface. 2) flow of salt from beneath the buried shoulder to the still-rising parts of the diapir, 3) compression of the shoulder areas due to lateral stresses from a topographic high on the rising diapir.