Introduction
Stylolites are rough dissolution seams that occur in a variety of rock types. They are very common in limestones and appear as dark surfaces that show a roughness on several scales (Dunnington, 1954; Heald, 1955; Park & Schot, 1968; Guzzetta, 1984; Merino, 1992; Railsback, 1993; Karcz & Scholz, 2003). Distinct features include stylolite “teeth”, column like structures with stripes on the sides and quite often a fossil on the top (Fig. 1, 2). During a stylolite’s growth an increasing amount of material of the host-rock is dissolved at the interface and material that dissolves slower or not at all is collected and produces the dark or light seams that are characteristic for these structures (Fig. 2).
The dissolution of material at the stylolite interface is induced by the weight of the overlying sediments in sedimentary basins or by tectonic stresses (Rutter, 1983). In addition, chemical effects may play a key role in the dissolution process (Aharonov, E., Katsman, R., 2009). In order for a stylolite to occur the dissolution will have to take place localized at the stylolite surface (Bathurst, 1987). If no localization takes place the dissolution is pervasive and no distinct dissolution seam develops. The reason for the ocurrance of this localization is still debated (Fletcher and Pollard, 1981; Merino, 1992; Merino et al., 2006; Katsman et al., 2006; Aharonov and Katsman, 2009). The simplest answer is the existence of a heterogeneity in the host-rock, for example a bedding plane. In addition chemical effects that are induced by the collected dark material within the dissolution seam may lead to a feedback mechanism (Aharonov and Katsman, 2009). Stress-concentrations may also lead to a feedback (Fletcher and Pollard, 1981), however the stress-feedback is debated recently (Katsman et al., 2006).
Current models of the roughness development of stylolites assume that the roughening itself is induced by pinning “particles” that dissolve slower than the surrounding rock (Koehn et al., 2007). These particles could be quartz grains, oxides or micas in a limestone, or simply fossils. A stylolite can be visualized as an interface between two pieces of rock that are pushed into each other. If a pinning particle meets the interface it will “push” into the rock on the other side and a spike develops. The development of the spike can be stopped by several effects. One possibility is that the particle meets another pinning particle on the other side of the interface. This could lead to a build up of stresses locally and the originally pinning particle may be dissolved. In addition two forces work against the pinning, surface energy favours a flat surface and leads to the destruction of spikes and elastic energies will also concentrate at spike tops and may destroy them (Schmittbuhl et al., 2004; Renard et al. 2004; Koehn et al., 2007). Roughening due to pinning and the destruction of pinning particles leads to the development of the typical stylolite pattern.
Elastic and surface energies scale differently at the stylolite interface so that elastic energy is dominant on the large scale whereas surface energy is dominant on the small scale (Schmittbuhl et al., 2004; Renard et al. 2004; Koehn et al., 2007). Therefore small stylolites will look different compared to large stylolites or the roughness of a stylolite will look different when you look at it in a hand specimen compared to what it looks like in a thin-section. The most important input parameter is the size of the pinning particles that produce the roughness (Ebner et al., 2009). This noise is probably characterized by the grain-size in most cases. A stylolite with large grains will reach the elastic energy dominated regime much earlier than a stylolite with small grains that will grow first in the surface energy dominated regime (Koehn et al., 2007; Ebner et al., 2009). Recent numerical models have shown that the growth of a stylolite may be non-linear and that the non-linearity depends on whether or not the stylolite grows in the surface energy dominated or the elastic energy dominated regime (Koehn et al., 2007).
This article treats the growth of the roughness of a stylolite and assumes that it starts to grow from an initial heterogeneity in the host-rock, for example a bedding plane. A numerical model is used to study the roughness evolution of different stylolites and will illustrate the differences of the surface versus elastic energy dominated growth regimes.