Bent chert tubes in the Phosphoria Formation of Wyoming: Pressure solution strain markers

Abstract

Tubular chert in the Phosphoria Formation in the Gros Ventre Mountains of Wyoming appears to have been deformed by pressure solution during the flexural-slip folding of the Gros Ventre uplift. We suggest that these chert tubes can be used as strain markers. The tubes are 0.4 to 1.2 m in length and 5 to 12 cm in diameter, and are bent where they occur in two concentrated zones which parallel bedding. The orientation of the bent chert tubes is regionally consistent. The tubes plunge to the northeast in the direction of bedding-plane dip at an angle greater than dip. This is the orientation that would be produced by shear parallel to bedding on the northeast limb of the northwest-trending Gros Ventre anticline. Pressure solution is indicated by the truncation of concentric bands by the walls of the tubes observed in cross sections. The tubes are not bend by soft-sediment deformation or the action of paleocurrents. A value of maximum shear strain of {gamma} {approximately} 1.5 and a maximum volume loss of 46% is obtained by treating the zones of bent chert tubes as shear zones.