Introduction to Sedimentary Rock Texture and Composition in Thin Section
A Review of Sedimentary Rock Basics
Framework, Matrix, Cement and Porosity
There are four key properties used to describe and classify sedimentary rocks: framework grains, matrix, cement and porosity or FMCP. Framework grains are also known as clasts or allochems (carbonates) and make up the majority of sandstones and gravelstones (conglomerates or breccia). Matrix is the mud-sized sediment fraction that occurs between the framework grains. Cement is formed from crystals that precipitated out of solution. The most common cements are calcite and quartz. Hematite cement is less common, but indicative of an iron-rich environment – there is an example included in this lab. Porosity refers to the volume of empty interstitial spaces. Porosity will decrease with compaction and cementation.
Detrital Sedimentary Rock Classification
Grain Size
Detrital sedimentary rocks are classified first by grain size using the Udden-Wentworth scale (table 1) sandstones are further classified by composition. The table below has additional information about the common roles of detrital grains in rocks. When asked to classify grain size use the “finer” divisions of the Udden-Wentworth scale.
Major / Minor / Size(in mm) / Size
(in φ) / Usual Role
Gravel / Boulders / >256 / <-8 / Coarse Grains (similar to phenocrysts in igneous rocks)
Cobbles / 64-256 / -8 to -6
Pebbles / 4-64 / -6 to -2
Granules / 2-4 / -2 to -1
Sand / Very Coarse Sand / 1-2 / -1 to 4 / Coarse Grains
Coarse Sand / 0.5-1
Medium Sand / 0.25-0.5
Fine Sand / 0.125-0.25
Ver Fine Sand / .0625-0.125
Mud / Silt / .004-.0625 / 4 to 8 / Matrix (similar to groundmass in igneous rocks)
Clay / <.004 / >8 / Matrix
Table 1: Summary of the Udden-Wentworth scale for the classification of detrital grain sizes.
Grain size can tell us a lot about the energy of the depositional environment. In general, if the energy is high, only coarse particles would deposit and fine particles would be carried away. When energy is lower, the lighter particles can be deposited. More on this in the section on sorting below.
Grain Shape
Shape is described by the roundness (how angular or smooth) of a grain or how spherical the grain is (figure 1 below).
Shape can tell us a lot about the transportation of a grain. Only transport by a fluid media will round particles. The degree of rounding is proportional to the duration of transport. A grain that has travelled a shorter distance from its host will tend to be angular, whereas one that travelled a long distance will tend to be rounder. Sphericity is a function of both mineralogy (inherent weaknesses) and type of weathering. Note:well rounded grains may not necessarily be highly spherical, but may have undergone as much transport and re-working as a well rounded highly spherical grain.The original shape of the grain can account for differences in sphericity e.g. prismatic grains will create low sphericity even for well rounded grains as opposed to equant grains that will always have a high sphericity.
Figure 1: Classification of rounding and sphericity.
Sorting
For this lab, we will be using sorting in a qualitative way (figure 2). Sorting can be evaluated quantitatively by measuring and calculating grain size distribution. Sorting descriptions vary depending on your reference frame. A conglomerate with roughly the same size of framework grains might be reasonably described as well sorted if only the framework grains are being described or it could also be accurately described as a poorly sorted if a mud matrix is included. It is important to specify the reference frame. In addition, if you are describinga relatively clean sandstone (i.e. an arenite)a little bit of mud does not equal poor sorting. Try to focus on the big picture. Obviously when describing sandstones with a high mud content such as wackes (or clastic carbonates with high micrite), the mud is a significant component of the rock and should therefore be included. Cement is separate from sorting.
Sorting tells us about the consistency of energy in an environment. Generally, sorting decreases with increasing energy and variability of that energy. High energy can transport a wide variety of grain sizes. A sudden drop in energy would deposit all of those grains, resulting in poor sorting. For example, a creek with low velocity will lack any significantly large grains, but a sudden storm discharge could help the creek entrain much larger particles, which would be evident after the event subsided. Poor sorting can indicate episodic energy, a transport mechanism that is non-selective e.g. glaciers or very short transport. Good sorting is indicative of consistent energy (e.g. the beach), a very selective transport mechanism (e.g. wind), very long transport or multiple cycles of re-working.
Figure 2: Basic qualitative classification of sorting (Arculus et al., 2015).
Maturity
Ultimately, the combination of texture and composition can determine the maturity of the rock. A texturally mature rock would have low mud content with good sorting. A compositionally mature rock would lack lithic fragments, minerals that are less resistant to weathering and mud fraction. While compositionally mature rocks will tend to also be texturally mature that may not always be the case so be mindful of making observations before you make interpretations.
Classification
Using the information above, you can classify siliciclastic rocks using the diagrams in Appendix A. Keep in mind that if any mudrock displays fissility, it should be classified as shale.
Activity developed by Dr. Rachel Walters ()
Department of Geological Sciences, University of Florida
Introduction to Sedimentary Rock Texture and Composition in Thin Section
Biochemical and Chemical Sedimentary Rock Classification
Biochemical rocks are also classified based on their texture and composition. However, their composition comes from non-silicate sources. Instead, their composition comes from the accretion and diagenesis of organic matter or from the precipitation of chemicals (that may have been dissolved from what was originally organic matter).
In this lab, we will see:
- Carbonates
- Evaporites
- Siliceous Chemical Sedimentary Rocks
Carbonates
There are two widely used classifications of carbonates: Folk and Dunham. A good resource for the Folk classification scheme : – click on carbonate in the text beneath the menu images and this will pop-up a window with access to a great summary of limestone. Hand samples are usually classified using the Dunham scheme and thin sections by the Folk classification scheme.
Allochems
Framework grains in carbonates: bioclast (fossils), intraclast (grain of partially lithified carbonate sediment), ooid/oolite (small spherical accretionary carbonate grains) and pellet (ovoid or rod-shaped grain composed of carbonate mud). These descriptions are used by the Folk classification. Just as the properties of detrital grains provide information about sedimentary transport and depositional environment, so do allochems (see figure 3 below for the basics and use the website listed above for more detailed information).
Figure 3:Depositional environments based on the type of allochem (
Matrix or Cement
In addition to allochems, the Folk classification also involves the type of matrix or cement – micrite or sparite. Micrite is carbonate mud and may be precipitated chemically or biochemically from seawater, derived from the abrasion of pre-existing calcium grains, or form during disintegration of calcareous green algae (figure above). Micrite accumulates in a variety of settings so interpretation of depositional environment is difficult. Sparite is a cement of crystalline calcite that can form by post-depositional re-crystallization of micrite or precipitation from solution.
Texture and Mud
Texture can be described much like detrital rocks e.g. grain supported or matrix supported; grain size, sorting and rounding. The Dunham classification uses textural aspects of clastic carbonate rocks: the amount of carbonate mud in a limestone and whether or not the framework grains are supporting each other.
Chemical Sedimentary Rocks
Chemical sedimentary rocks are those that have precipitated out of solution and include: evaporites, chert and travertine. These rocks have a crystalline (or micro-crystalline in the case of chert) texture. Chert is siliceous while travertine is calcitic and evaporites have a range of compositions depending on the extent of evaporation: calcite, gypsum, halite and sylvite. Chert may contain fossils and can have an organic origin from the accumulation of siliceous plankton (microscopic organisms that float around in ocean currents that drift to the seabed when they die).
References
Arculus, R.J., Ishizuka, O., Bogus, K., and the Expedition 351 Scientists, 2015
Proceedings of the International Ocean Discovery ProgramVolume 351, doi:10.14379/iodp.proc.351.102.2015
Appendix A: Detrital Rock Classification Diagrams
Dott Classification
Note: the boundary between arenites and wackes is 5% mud matrix.
Reference: based on Blatt and Tracey (p. 257) and partially based on Williams, Turner, and Gilbert (p. 326).
References
Blatt, H. and Tracy, R. J. (1996). Petrology. Igneous, Sedimentary, and Metamorphic, 2nd ed. xix 529 pp. New York, Basingstoke: W. H. Freeman & Co. Price £34.95, US $64.95 (hard covers). ISBN 0 7167 2438 3.
Williams, H., Turner, F. J., and Gilbert, C. M.(1953). Petrography: An introduction to the study of rocks in thin section. 416 pp., 133 illus., W. H. Freeman and Company. San Francisco 1954. Price $6.50 (56/-). ISBN 0716713764.
Appendix B: Carbonate Rock Classification Diagrams
Folk Classification
Image Source:
Dunham Classification
Image Source:
References
Dunham, R. J., 1962,Classification of carbonate rocks according to depositional texture. In: Ham, W. E. (ed.), Classification of carbonate rocks: American Association of Petroleum Geologists Memoir, p. 108-121.
Folk, R.L., 1959,Practical petrographic classification oflimestones: American Association of Petroleum Geologists Bulletin, v. 43, p. 1-38.
Activity developed by Dr. Rachel Walters ()
Department of Geological Sciences, University of Florida