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Dozens of sedimentary rock classification schemes have been proposed over the decades, and their diversity can be bewildering, especially to the novice.
The diversity of schemes reflects, in part, the complication of sedimentary rocks, but also the fact that different classifications accomplish different purposes. Often when a geologist works on a problem it is only a small part of the realm of sedimentary processes they are interested in, not the total diversity of rocks, so special, more narrow schemes are devised that make the problem easier to solve.
Still, for each group of sedimentary rocks (e.g. clastic, chemical, biochemical, etc.) generalized strategies of classification exist that are modified for special purposes.
Our goal here is to present a few classification schemes ranging from very general and basic, to more sophisticated and specific. Each is briefly introduced below to help you choose one best for your purposes.
A Basic Classification:
Key - pdf version
This classification explores the total diversity of sedimentary rocks. It is a good classification for someone wanting to learn to recognize and identify all the basic sedimentary rocks - without the technical details.
The key provided is pretty much self explanatory, and can be used to identify sedimentary rocks without understanding how or why these rocks are related to each other.
However, the theory behind the classification, based on the simple ideal model, explores the processes of sedimentary rock formation, and how they are related.</td> Clastic Classification:
Key to QFL identification
QFL ternary system
Clastic rocks form from weathering products that do not dissolve in water - clasts. It includes conglomerates, sandstones, and shales.
These classifications require the ability to identify clasts in the rock - quartz, feldspar, and lithics (or QFL).
The key to QFL identification is simple, but deals with relatively clear cut examples of each of the rocks.
The QFL ternary system is the most sophisticated, and the one geologists use. It requires learning to read a ternary diagram, but once that is done the system is powerful and easy to use. Carbonate Classification:
Carbonate Key Carbonates are not easy to classify, and except for advanced geology majors most people never get involved with their classification. The basic key deals with carbonates at about the level most people encounter them.
Carbonates are so much richer than that, however - equally as rich as clastics. For example, where clastics have quartz, feldspar, and lithics, carbonates have allochems (fossils, oolites, pellets, and intraclasts).
In any event, a key that introduces these is provided here. Chemical/Biochemical Class:
Chemical/Biochemical Key Chemical and biochemical sedimentary rocks form a hodge podge of rocks with very different origins. There is no neat way to classify them. But if you can decide that a rock is not clastic or carbonate then it is likely one of these.
An Introduction to Sedimentary Rocks: The Simple Ideal Model
Alphabetic Encyclopedia of Sedimentary Rocks
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