CALCITE TWINNING STRAIN ANALYSIS OF THE ALLOCHTHONOUS JURASSIC SUNDANCE, SOUTH FORK DETACHMENT, NORTHWEST WYOMING

Calcite twinning strain analysis was used to gain a better understanding of the emplacement and deformation of the South Fork detachment located to the southwest of Cody in Park County, Wyoming. The South Fork detachment, a 10 km by 40 km allochthon exposed in the valley of the Shoshone River, consists of deformed, folded and faulted, Jurassic Sundance through Cretaceous Cody strata. Sixteen oriented samples were collected along the decollement surface in the Jurassic Sundance Formation. Calcite twinning analysis was conducted using a petrographic microscope with a 4-axis universal stage to determine the stress and strain associated with mechanical calcite twins. These samples were analyzed in conjunction with sixteen previously collected samples. All data were analyzed in the context of recent remapping of the South Fork detachment in the Belknap Creek Quadrangle. A low percentage of negative expected values (avg. 12%), or lack of a strain overprint, indicates that there was only one major twinning event. The calcite twins do not appear to correspond to regional and contemporaneous Sevier or Laramide orogenic activity. Almost all samples preserve layer-parallel shortening. However, the orientation of the shortening axes is variable, and can be attributed to rotation and tilting of the bedding during transport of the allochthon.