Stylolites: A Geologic Structure
An introduction to stylolites: what they are and how they form
What is a Stylolite?
Greek pillar? Not exactly. . . Image from pixabay.com (2019)
Stylolites take their name from the Greek word stylus, meaning "pillar," and the title couldn't be more fitting (Merino, 1992). Indeed, stylolites do somewhat resemble iconic Greek architecture - though, admittedly on a much smaller scale.
In rocks, stylolites appear as irregular seams or wavy lines. These lines may somewhat resemble a set of shark's teeth, or even the pulse on a heart rate monitor; however, they are actually a result of minerals left behind after a pressure dissolution (Fossen, 2016). Image from pixabay.com (2019).
Pressure dissolution (also known as pressure solution): "ions move in fluid films and pore fluid (chemically and stress controlled)" (Fossen, 2016).
Note how the the stylolite on the right mimics a heart ekg. Stylolites maintain this wavy pattern; though, some are more jagged than others. Stylolites may be horizontal, inclined, or even found in combination with other stylolites. Photo by James St. John (2007).
Pressure Dissolution and Stylolite Formation
Stylolites form as compressive features during pressure dissolution, where ions are dissolved and deposited in another location: this process is accelerated by high stresses (Fossen, 2016). As Merino (1992) explains, stylolites begin forming in areas of high porosity and stress, then finish forming in areas of low stress and porosity. Dissolution occurs in these high porosity areas, and thus results in a high concentration of solutes. Low porosity areas have a lower solute concentration, so the dissolved ions naturally move towards these areas. It is here in these areas of low porosity that the dissolved minerals are deposited and the stylolite is formed (Merino, 1992). When pressures are high, porosity is lowered and the process of dissolution is accelerated, thus increasing the likelihood of stylolite formation (Fossen, 2016).
How Stress Affects Stylolites:
When rocks are compressed by the weight of overlying material, the resulting stress creates stylolites that are parallel to the bedding plane. Conversely, when tectonic stress becomes involved, stylolites are typically at an angle to the bedding plane (Merino, 1992). See figure 1 for stylolite examples.
Figure 1: Depending on the stress involved, stylolites will form at different angles to the bedding plane. Pictured above are a few possible examples of stylolite types. Image from geocaching.com (2019)
Stylolite Classification
Although it is tempting to describe stylolites with a single definition, this conclusion would be an oversimplification of these structures. As with many geologic features, there are are a number of different stylolite types that exist.
Stylolites are classified in terms of their geometry as opposed to their mode of formation. That being said, there is no single classification system for stylolites. The classification system that will be discussed here is the system derived by Park in Schot (1968). Park and Schot describe 6 stylolite types that are based off of both stylolite geometry and the relationship of the stylolites to the bedding plane.
Geometric Classification:
Pictured right: geometric stylolite classification based off of Park and Schot (1968). These types are as follows: wave type, sutured type, down-peak type, up-peak type, seismogram type, and sharp-peak type. These types are based off of the shape of the stylolites themselves, and not how they pertain to the bedding plane. It is also important to note that these types are not mutually exclusive, and stylolites may be a combination of multiple stylolite classes (Park and Schot, 1968). Image from geocaching.com (2019).
Examples of Stylolites:
Image from gly.uga.edu
Photo by Doug Sherman (2008)
Image from geol.umd.edu (2019)
Stylolite Classification based on Bedding Plane Relationships:
Pictured right is a diagram describing the 6 geometric classification systems based on stylolite relationships to the bedding plane as described by Park and Schot (1968). Image from Park and Schot (1968).
Type 1: Stylolite seams run parallel to the bedding plane and have experienced little tectonic stress (Park and Schot, 1968).
Type 2: Unlike type 1 stylolites, type two stylolites are inclined and may or may not have experienced tectonic stress (Park and Schot, 1968).
Type 3: Type 3 stylolites are somewhat of a fusion between types 1 and 2. In this class, horizontal stylolites displace inclined stylolites. Typically, the horizontal seams are more prominent than their inclined counterparts. This stylolite combination most likely forms as a product of multiple pressure systems (Park and Schot, 1968).
Type 4: Type 4 stylolites are essentially the inverse of type 1 stylolites, as they are vertical. These stylolites are “formed by pressures acting at right angles to the bedding” (Park and Schot, 1968).
Type 5: Type 5 stylolites represent stylolite networks. This type may be further subdivided into types 5a and 5b. Type 5a tends more horizontal and has higher amplitudes. Conversely, type 5b has a variety of inclinations and is typically smaller (Park and Schot, 1968).
Type 6: Type 6 stylolites are similar to type 3 stylolites; however, in this case vertical stylolites displace inclined stylolites. As with type 3 stylolites, the inclined and vertical stylolites of type 6 likely formed at separate times (Park and Schot, 1968).
Where are Stylolites Found?
Image from pixabay.com (2019)
Stylolites can be found across the globe and in a number of different rock types. This being said, they are most often found in rocks that have been subject to diagenesis and/or tectonic strain (Koehn et.al, 2016). Stylolites are particularly common in limestone, though they are by no means limited to this rock type (Koehn et. al, 2016).
This map displays some of the areas researched by the authors referenced in this project. Stylolites can be found across the globe!
Are Stylolites Fractures?
Figure 2: Types of fractures described by Fossen (2016). Although a Mode IV is listed, this mode is not actually a type of fracture, but rather stylolites. Image from Fossen (2016).
Although stylolites are often discussed alongside fractures, they do not actually fall into this category. In order to be considered a fracture, stylolites would have to experience loss of cohesion and a gain of permeability (Fossen, 2016). Stylolites exhibit neither of these characteristic; however, it is not uncommon to see them described as contraction fractures. This commonality is likely the reason that the diagram to the left lists stylolites as Mode IV fractures. That being said, it is important that they not become confused with actual fractures. Interestingly, where many fractures experience a gain of permeability, stylolites often experience a loss permeability (Fossen, 2016). This loss of permeability may affect fluid flow in rocks and becomes an important detail when working in environmental and petroleum-related fields.
Why are Stylolites Important?
Stylolites. Image by Callan Bentley (2014)
Stylolites are important for a number of reasons. As discussed earlier, stylolites may affect permeability, which in turn influences fluid flow. If you are working in the environmental industry, where fluid flow influences both water supply and pollutant transport, it is important to know how structures may encourage or inhibit fluid movement. Likewise, these concepts are also important in the petroleum industry. Outside of these fields, stylolites are also important in the study of structural geology, as they work to help indicate the amount of compaction a rock has experienced and as well as the the amount amount of stress (Koehn, et. al, 2016).
References
Fossen, Haakon. Structural Geology. Second Edition ed., Cambridge University Press, 2016.
Koehn et al., 2016 D. Koehn, M.P. Rood, N. Beaudoin, P. Chung, P.D. Bons, E. Gomez-Rivas. "A new stylolite classification scheme to estimate compaction and local permeability variations Sediment." Geol., 346 (2016), pp. 60-71
Merino, Enrique. “Self-Organization in Stylolites.” American Scientist, vol. 80, no. 5, 1992, pp. 466–473. JSTOR, www.jstor.org/stable/29774727.
Park, Won C.; Schot, Erik H. “Stylolites; their nature and origin.” Journal of Sedimentary Research (1968) 38 (1): 175–191.DOI: 10.1306/74D71910-2B21-11D7-8648000102C1 865D
