Sandstone Classification

Previously we discussed how to accurately describe sandstones and conglomerates in the field. (See my article Sandstones and Conglomerates.) While a good field description is critical to understanding the history and environment that produced the sandstone, it does little to help us actually name the rock we are describing. The classification of sandstones can be as simple or as complex as a geologist needs, and there are several different methods I have found to classify sandstones. I will present here a fairly simple model for the classification of sandstones.

To begin our classification we must return to the ternary diagrams we first used in our classification of igneous rocks. (Refer to my article Igneous Rock Identification (Advanced) to learn how to read a ternary diagram.) The apexes for our sedimentary rock ternary diagram are Quartz, Feldspar, and Lithic (or rock fragments) (QFL). Nearly every sandstone (siliciclastic) classification system uses a variation on the QFL ternary diagram, though the each differs slightly. The QFL diagram at the right is one I have modified from several other diagrams and gives you a basic method for classifying sandstones. Quartz (Q) is located at the top apex, feldspar (F) at the left apex, and lithic (L) at the right apex of the diagram. The diagram has seven areas for the classification of a rock sample and is based on the relative percentage of quartz, feldspar or lithic fragments found in the specimen. Note that the names arkose and feldspathic are synonymous with each other and are often used interchangeably by geologists.

Determining the name of the sandstone is only one part of the actual classification of the rock. Two other aspects must also be considered to complete the name. (It seems like a lot of work to go through to give a name to a rock, but after a few tries much of this becomes intuitive.) We must also consider the matrix of the rock. The matrix is the material that cements the sand grains together. The matrix can be of any type, commonly we see calcareous, dolomitic, or siliceous matrixes, but for our purposes here the actual composition of the matrix is not as important as the percentage of matrix present. (A detailed analysis of the matrix is best made in the lab.) If the rock has more than 10% matrix we call it a wacke. If it has less than 10% matrix, it is referred to as an arenite.

The last step in the classification of our sandstone is determining its texture. Texture helps us to determine the maturity of the sandstone. The simple texture diagram at right can help determine the rocks texture. Notice we've replaced QFL with sand, silt, and clay. This diagram is separated into five basic areas, though there are no actual names for each area. Instead we determine the relative abundance or percentage of the different grain sizes in the rock. We then give the rock a textural name based on this percentage, from least abundant to most abundant. For example, if we had a rock with 10% clay, 40% silt and 50% sand we would call it a silty sandstone. (Notice I didn't include the clay. For simplicity, abundances of 10% or less are not mentioned.)

How does the texture help determine the maturity of the sandstone? Generally, from field studies, geologists have learned that sandstones that have more clay are young (or immature) sandstones, and as you lose clay (and gain more sand) the rock becomes more mature until sandstone with all sand is considered supermature.

Now that we have our three pieces of our classification it's time to put them together into a name for our rock. First we use the name we derived from the QFL diagram. We then add the matrix name (wacke or arenite), and finally the textural name. It's helpful to think of the name as and equation: [QFL] + [matrix] + [texture] = rock name. So lets say we have a rock that had 80% quartz and 20% lithic fragments, had less than 10% matrix, and had 45% sand and 55% silt. We would then call this rock a sub-lithic arenite sandy siltstone.

The use of ternary diagrams for sandstones based on the QFL and the texture become invaluable tools for the field geologist to classify and name the rocks. While this system may appear cumbersome and confusing at first, the use of these classifications will not only help you to remember what a rock was once you get back to the lab, but will also tell other geologists with certainty what kind of sandstone you are describing.

For a detailed review of the QFL classification systems and to see a slightly different classification method, check out the Clastic Sedimentary Rock Classification web page by Lynn S. Fichter who teaches at James Madison University in Virginia.