Pressure solution seams have various shapes. They can be planar, sinusoidal or wavy, and suture (Figure 1). The terms planar and wavy describe on a larger scale the enveloping surfaces of pressure solution seams.

Figure 1. Geometry of pressure solution seams, sutured and non-sutured. Non-sutured may be planar, wavy, and anastamosing. From Engelder and Marshak (1985).

Suture describes the 2D cross section of pitted architecture (Figure 2 and 3). Those with sutured shape are called stylolites. Individual columns of stylolites are almost always parallel to the direction of compression. The overall plane of dissolution normally follows existing weaknesses such as bedding interfaces. Thus, in undisturbed strata in original horizontal position, the stylolite columns are at right-angle to the bedding interfaces, but in inclined strata like limbs of folds where dissolution occurs after the tilting of layers, they are not (Figure 3, Stockdale, 1922).

Figure 2. Three-dimensional block diagram of a layered rock with a horizontal stylolite. The front half of the upper rock mass has been removed to show the columns of the lower block in 3D. The side view at right shows the two-dimensional profile of the stylolite. From L. B. Railsback's web site at http://www.gly.uga.edu/railsback/, modified from John V. Smith (2000).

Figure 3. Diagram of a stylolite along an inclined bedding plane, where the position of the individual columns is vertical, instead of at right angles to the plane of stratification. Observed in the steeply inclined strata of the 'Niagara domes' of northern Indiana by Professor E.R. Cumings. From Stockdale (1922).

Trurnit (1968) suggested that the relative solubility on the two sides determines the shape of the solution seams. The sutured stylolites are characteristic of contact of equal solubility on both sides and smooth contacts are characteristic of unequal solubility materials where only the more soluble material or layer dissolves. However, Guzetta (1984) proposed that distributed insoluble grains or crystals enhance the roughness of dissolution surfaces or seams. A thick zone of insoluble residue acts like a more insoluble solution partner and favors smooth solution seams.

Renard et al. (2004) separated 12 bed-parallel stylolites into two adjacent surfaces in limestone (Figure 4) and measured the three-dimensional geometry of the surfaces using a laser profilometer (Figure 5).

Figure 4. Pictures of pressure solution seam surfaces in limestones showing two different morphologies and peak amplitudes. The two rock bodies of pressure solution seams are separated without damaging the peaks. The roughness of sample S0-8 is typically on the order of 2 millimeters. The roughness of sample S12A, from Vercors Mountains, is up to 5 millimeters. From Renard et al (2004).

Figure 5. Measured profiles of four pressure solution seams, arranged with increasing roughness from bottom to top. Sample S0_8 and S12A are shown in figure above. From Renard et al. (2004).

These authors calculated the statistical characteristics of the surfaces and concluded that all the stylolites have self-affine fractal roughness with a well-characterized crossover length scale separating two self-affine scaling regimes (Figure 6). Strikingly, the characteristic length scale falls with a very narrow range for all the stylolites studied, regardless of the size of microstructures. To explain the data, the authors proposed a continuous phenomenological model that postulates that the complex morphology of stylolites is the result of competition between the long-range elastic redistribution of local stress fluctuations responsible for the surface roughness and surface tension along the interface which tends to smooth it, corresponding to the two scaling regions in Figure 6.

Figure 6. Wavelet spectrum of the stylolite measured with a laser profilometer. The statistics were calculated for profiles in two directions (two sets of perpendicular cuts through the surface in planes perpendicular to the plane of the stylolite). From Renard et al. (2004).