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 copyright of the article Sandstone Classification in Everyday Geology is owned by Geoff Habiger. Permission to republish Sandstone Classification in print or online must be granted by the author in writing.

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