by Harold L. Levin
Chapter 4 - page 7
Rocks and Minerals: Documents that Record Earth's History
Types of Sedimentary Rocks
Sedimentary rocks are classified or named on the basis of their texture and composition.
Texture means the size, shape, sorting, and arrangement of the sedimentary grains.
Overview of types of sedimentary rocks:
- Clastic Sedimentary Rocks (also called terrigenous or detrital)
- Conglomerate or Breccia
- Shale or Claystone
- Chemical/biochemical Sedimentary Rocks
- Carbonate sedimentary rocks (limestones and dolostone)
- Siliceous sedimentary rocks
- Organic Sedimentary Rocks (Coal)
Clastic Sedimentary Rocks (also called terrigenous or detrital)
Clastic sedimentary rocks are derived from the weathering of pre-existing rocks, which have
been transported to the depositional basin.
They have a clastic (broken or fragmental) texture consisting of:
- Clasts (larger pieces, such as sand or gravel).
- Matrix (mud or fine-grained sediment surrounding the clasts).
- Cement (the chemical "glue" that holds it all together).
Types of cement:
- Iron oxide
Clastic sedimentary rocks are classified according to their texture
- Gravel: Grain size greater than 2 mm.
- If rounded clasts = conglomerate
- If angular clasts = breccia
- Sand: Grain size 1/16 to 2 mm.
- Sandstone (various types)
Graywacke, a type of sandstone
- Silt: Grain size 1/256 to 1/16 mm (gritty).
- Clay: Grain size less than 1/256 mm (smooth).
- Shale (if fissile)
- Claystone (if massive)
Note: Mud is technically a mixture of silt and clay. It forms a rock called mudstone (or
mudshale if fissile).
Shale with a fern fossil.
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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA
September 10, 2005