IDENTIFICATION OF ROCKS

SEDIMENTARY ROCKS - Sedimentary rocks are formed from the breakdown of preexisting rocks. Broken, worn, pieces of other rock can be deposited, compressed, and cemented together. Other sedimentary rocks are formed by chemical or biochemical means.

Sedimentary rocks formed of particles cemented together are called detrital sedimentary rocks Detrital sedimentary rocks are classified by the size of their particles.

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Detrital Sedimentary Rocks

Particle size	Rock name	Features
coarse ( > 2 mm)	CONGLOMERATE	rounded fragments
	BRECCIA	angular fragments
medium (1/16 to 2 mm)	SANDSTONE	quartz
	ARKOSE	quartz & feldspar usually angular grains
	GREYWACKE	quartz & feldspar w clay very unsorted
fine	SILTSTONE	silt grains 1/16 to 1/256 mm
	SHALE	clay particles (smaller than silt)

Chemical or Biochemical Sedimentary Rocks

The chemical or biological sedimentary rocks may have a variety of forms. Sometimes they are deposited like detrital rocks but their ultimate origin is in a chemical or biological process. The chemical/biological sedimentary rocks are classified by the process of formation and their composition.

Identification of rocks starts with deciding whether it is Igneous, Metamorphic, or Sedimentary.

Sedimentary rocks are nearly always layered. The detrital rocks are always cemented together and do not have interlocking crystals. The biochemical rocks are also usually layered but may be crystallline in some cases.

Layered, not crystalline = sedimentary

Photo credits: Duncan Heron; Frantisek Tvrz; Joel Tassin

(Click on the link for picture)

• Conglomerate has coarse grains in a hand sample and in a thin section



• Sandstone has medium grains in a hand sample and in a thin section



• Shale has small flat grains of clay minerals. They are too small to see in a hand sample but can be seen in a thin section. More on shale

Sorting and rounding

  Detrital sedimentary rocks give clues to how much erosion they have experienced by the amount of rounding of the grains and by the sorting of the grains. The more rounded the more erosion. The less rounded , the less time or distance has occurred.

The sorting can tell about the energy of the cause of the erosion. A large particles suggest a high energy source such as a fast stream. Small particles suggest a lower energy such as a slow river or a lake. If they are well sorted, all the same size, the energy of the agent must be relatvely constant. if the size varies, the source energy varies as well and the deposit may be poorly sorted

A river, for instance, may deposit large particles in the spring, when it floods, but small particles in the summer when it has less water. A glacier, by contrast, has so much energy it deposits all sizes all mixed together with the largest particle being very large and the smallest being very fine.

Limestone may have many appearances because there are many ways it can form. Most limestone is created by animals which make up a fossiliferous limestone.

Limestone may have large fossils or the fossils may be very small as in this fine grained limestone

Fossils may be microscopic as in chalk or there might be no fossils as in this travertine which is deposited by hydrothermal processes.

Now you should now know enough to answer the questions about sedimentary rocks on the question sheet. You can then move on to metamorphic rocks. or return to the main menu

For more information on sedimentary rocks take a look at this link from James Madison University. JMU_Fichter