The two fracture types that cross on this bedding surface formed at different times and under different environmental conditions


This bedding surface is in Arrow Canyon, S. Nevada. It is the unconformity at the top of the Mississippian Battleship Wash Formation, which is composed of mixed carbonates. The fractures visible here are perpendicular to the unconformity surface. The white fracture in the center of the photo is filled with calcite cement. Careful examination shows that it also exhibits a gray medial suture running along the middle of the cement filling. The orange echelon fractures, which are commonly known as tension gashes, are filled by an iron-stained cement. In addition to different cement species, the fractures have a very different geometric character.

The calcite-filled fracture is continuous along its length and has a fairly uniform aperture (low aperture/length aspect ratio). The orange fractures are a right-stepping set whose individual geometry is lens-shaped, with wide central aperture that tapers off to tips over a distance of 10s of cms (high aspect ratio). The aspect ratio of the orange echelon fractures indicates they formed when the rock behaved plastically, whereas the white fracture formed when the rock was more brittle. Does the plasticity behavior indicate the rock was deeply buried and very hot when the gashes formed, or did the plastic behavior occur soon after deposition when the rock had not yet become fully lithified? The white fracture offsets the orange fracture by 0.5 cm of right-lateral shear indicating unquestionably that the orange fracture pre-dates the white one.Thus we know the relative timing but not whether the tension gashes indicate very high, or very low temperature of formation.