Plate 176

Gypsum veins

The final two plates illustrate examples of structures that can be regarded as tectonic rather than sedimentary. One can thus have a taste of differences and analogies, especially in the fields of deformations (soft-sediment versus hard rock) and diagenetic processes (early vs. late diagenesis).

Veins are fractures and joints welded by crystals precipitated from circulating solutions. The mineral can vary depending on local factors: carbonates and quartz are the most common, but in this case there is gypsum, which is connected with sulfur-rich rocks.

Fracture and joints are produced in brittle rocks and sediments, either by the direct application of tectonic stresses or by a relaxation of masses  that are relieved from previous stresses and loads. In open fractures, normally caused by extension, mineral solutions can circulate freely, whereas pressure is needed to force fluids along surfaces of shear.

In the outcrop of laminated gypsum shown here, veins are marked by white lines and form two systems: one is parallel to the depositional laminae, the other cuts across them. Transversal veins represent the fill of open joints, whilst the others result from injection of sulfate solutions under pressure; this "hydraulic fracturing" chose the lines of lesser resistance in the rock. Can you, applying the principle of intersection, determine which system is older and which is younger?

Other gypsum (sericolite) veins can be seen in color photo 29: they cross a shale bed separating two beds of evaporitic gypsum, and are folded and displaced along a tectonic shear plane.

Veins cannot be easily mistaken for sedimentary features: they sharply truncate primary and diagenetic structures as well. There is another secondary structure that truncates previous ones: the stylolite,  or stylolitic surface  (see color photo 31). In rock sections, it appears as a jagged line, resembling a cranial suture and marked by red or brown coats of iron oxides (no crystals, then). It can be seen, in our example, that stylolites are antecedent to some vein systems, posterior to others. This suggests that stylolites can be either pretectonic or tectonic features, which seems confirmed by the fact that some of them are about parallel to bedding (i.e., perpendicular to the load exerted by the sediment pile), and some have various orientations that reflect tectonic stresses. Actually, the majority of stylolitic surfaces are related to loading effects, and can be regarded as full title sedimentary structures (of deformational type).

Stylolites are produced by dissolution of soluble materials (typically, carbonates) under a solid pressure (a process named pressure-solution ); their indented geometry indicates that the pressure is more focused on certain points, less on others. The oxides represent insoluble residues that remained in place (while the solutes were removed). A certain critical load and depth of burial are needed for stylolites to form; escape routes for solutions, through pores, fractures or the stylolitic surface itself, are also required.