ECHELON VEINS AND PRESSURE SOLUTION SEAMS IN LIMESTONES AT RAPLEE AND COMB FOLDS, UTAH

This study is a characterization of echelon vein and pressure solution seam array sets in limestones and an interpretation of their development relative to regional folding. We present a conceptual model of echelon vein and seam formation over time, supported by deformation results from 2D mechanical models and by petrographic observations and measurements of geometric attributes and relationships. Two sets of systematic bed-perpendicular, echelon vein arrays with ~orthogonal echelon pressure solution seams are observed in Pennsylvanian to Permian limestone across Raplee anticline and the neighboring portion of Comb monocline, Utah. Their occurrence is exclusive to limestones and array orientations are uniform across the folds. Veins and pressure solution seams coexist in each of the 240+ measured arrays. The limestone strata consist of micritic carbonate matrix and approximately 10% quartz grains, fossil fragments, and pores. Veins are filled with mm-sized, twinned calcite crystals that formed during several events of fracturing. Pressure solution seams consist of curvilinear, mm-thick zones of quartz grains, micritic carbonate, and clay. Intersection relationships of veins and seams imply that they developed concurrently. Seam shortening is calculated based on the relative abundance of quartz grains. Using dimensional analysis, we identify five physical attributes that contribute to vein shape: coordinate position, initial fracture spacing, initial fracture-array angle, array-remote stress angle, and seam shortening. Model results, guided by rock mechanics data and burial history, suggest that vein shape is most influenced by seam shortening, however, an inelastic constitutive relationship is required to achieve the large aperture/length ratios observed in the field. We estimate the remote stress state during deformation, and we infer the occurrence of echelon vein and seam arrays to have developed during an early stage of folding.