Mudstones form 65% of all sedimentary rocks. A mudstone is any rock that consists of rock fragments less than 0.062 mm in size and include shale, mudstone, and claystone. When we reflect back on the process of chemical weathering we can see why this is so. Most of the igneous and metamorphic rocks are composed of about 20% quartz and 80% other silicates. Quartz is the only mineral that is stable under most surface conditions, so the other minerals weather into smaller particles, or into new minerals, like the clay minerals. The clay minerals are the most common mineral group among sedimentary rocks, so it's no wonder that mudstones are the most common sedimentary rocks.
Sandstones are the second most abundant of the sedimentary rocks. They form around 20% to 25% of the sedimentary rocks. Sandstones are those rocks that are formed from rock fragments between 2 mm and 0.062 mm in size. The sandstones are characteristic of highly kinetic environments; environments where moving fluids are common. If you have ever walked along a beach, or visited a desert with large sand dunes, then you've seen an environment with moving fluids; anyplace where water or wind can move large rock fragments. Sandstones are almost entirely composed of quartz and other large rock fragments.
The last of the major groups are the carbonate rocks. These are the limestones and dolostones and make up about 10% - 15% of the sedimentary rocks. Carbonate rocks are composed of the hard parts of marine organisms, the shells of animals like bivalves, mollusks, and other animals. These shells are composed of calcium carbonate (CaCO3) and as the organisms die, their shells accumulate on the bottoms of oceans or lakes. Most of the calcium carbonate comes from microscopic animals like phytoplankton, but many limestones contain shells of larger animals. One thing about calcium carbonate is that it is easily altered by weathering, so many times the shells that are deposited become recrytallized hiding the evidence of their organic origin.
Go To Page: 1 2
