Can Deformation Bands Indicate Paleostress Orientations?

Carter Mayland, Iowa State University
Jacqueline Reber, Iowa State University
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Abstract

Deformation bands are common planar structures created by localized deformation in porous rocks such as sandstones. Several studies suggest their use as paleostress indicators. The Bighorn Basin in Wyoming is an ideal location to test this hypothesis. The widely studied Laramide Orogeny significantly deformed the Bighorn Basin, forming Sheep Mountain Anticline among many other structures. Fracture orientations indicate NE-SW compression, perpendicular to Sheep Mountain Anticline's fold axis. Greybull Sandstone lenses outcrop at varying distances from Sheep Mountain Anticline and were not uniformly impacted and inclined during folding. The orientation of deformation bands in the Greybull sandstone can therefore be used to test whether the deformation bands formed under Laramide compressional directions. If the deformation bands have common orientations in all Greybull Sandstone outcrops, they formed under Laramide stress directions. Alternatively, if the deformation band orientations are similar between Greybull Sandstone lenses only after correcting them for bedding, their formation was unrelated to the Laramide Orogeny.
Greybull Sandstone is composed of well-sorted, rounded quartz grains, poorly cemented and scattered with deformation bands, forming up to four sets of orientations. We measure the in-situ strike and dip of the Greybull Sandstone's bedding planes and deformation bands. We correct the deformation band orientations at each outcrop for bedding, then determine their maximum horizontal stress orientations, as we would for fractures. We then compare the resulting stress orientations with the stress orientation of the Laramide Orogeny. Additionally, we use optical microscopy to image deformation bands in thin section. We measure porosity, grain orientation, grain size, and the number of neighbors each grain has, both in the band and in the host rock. Through this, we will gain insight into micro-scale characteristics of the deformation bands and investigate whether they are related to band orientation. The results of this study will help determine whether deformation bands are viable paleostress indicators.

Session

Deformation in the upper crust