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Geology/Formation can be sedimentary only ?



Dear Sir, I was wondering and often get confused that why is it that we always hear of sedimentary formations and not of igneous and metamorphic ones.
1. Can igneous and metamorphic be the formations?
2. If yes, then why don't we hear of them that often?

Best Regards,



1)  Igneous and Metamorphic rocks are defined as Units.  The trouble is that they do not occur in uniform layers like sediments do.  Sediments result form deposition by water or wind and result in wide spread layers, like the sand in the desert, or the sand and muds of a river delta.  They are somewhat easy to map and are wide spread. they are defined as Formations within lithostratigraphy.  They form part of the geologic lithochronology or progression of time through the rock column and are broken down into members and put into larger collections called groups.  These can then be correlated world wide by matching groupings of formations by time based on fossil assemblages and the use of other marker beds.  

Metamorphic and igneous rocks are more problematic.

Now the problem with igneous and metamorphic rocks is that they are NOT widespread and they do not occur in nice definable layers like sediments.  For instance, a granite, might be known by a name and is readily identifiable, but they only form in batholiths, which are large bodies of magma that have cooled.  They form large bodies, of rock underneath the ground and are only revealed to us when erosion uncovers them or they get pushed up above the surface by mountain building.

You might even have a large body of granite occurring within layers of sediments where it invaded and cooled.   Then, the heat of the cooling granite alters the sandstone and shales, the sediments around it.  We call that contact metamorphism.  If the alteration is sufficient, the halo of alteration might be given a formation name, or the original formation name might be altered such as Sandstone B Contact Metamorphism Zone A, B , C.  Depending on the level of alteration.

Since the alteration is limited in area it might not be worthy of a formation name, and not fit into Stratigraphic nomenclature rules to be a Member either.

An identifiable body of rock such as a granite will be called a Unit.  Some confusion results when authors used Unit and Formation interchangeably when they should not be.  They have different meanings.

So a granite might form a mountain and is referred to as being composed of Granite of the XY Unit.

Outside of Fredricksburg Texas is a granite mountain called Enchanted Rock, it is an exposed batholith.  In the region there are others, but this on was holy to the Comanche Indians who controlled the area.  The mountain moans at night due to the differential cooling between the not days and cool nights.  They thought they were ghosts so eventually it was turned into a state park.  My point is the granite is called the Capital Granite, because the pink granite from other batholiths in the area was used to face the state capital building in Austin, the state capital.  We do not call it the Capital Granite Unit, just the Capital Granite.

Now for extrusive igneous rocks such as a large spread basalt flow, where it becomes an integral part of the sedimentary stratigraphic progression it might very well be called a formation or a Unit.   Many pyroclastic deposits of ash are referred to as formations where they blanket large areas.  These are still igneous deposits as they represent a unit of volcanic ejecta, that over time became consolidated into a Tuff, or other type of lithology.

In metamorphic rocks we have the same problems.  The boundary of some of the metamorphic units is gradational, due to changes in the pressure and temperature of the metamorphism.  So a Unit of metamorphic rocks might have gradational changes.  These are called "Complexes" due to their chaotic nature and complex boundary associations.

Due to then variable nature of metamorphism a whole or only part of an existing formation might undergo alteration.  So that confuses thing even more.  

So check out the attached links.  The first is for metamorphic rock naming

This is for volcanic rocks:

It might be a good idea to clarify things for you if you read this:


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Keith Patton


I can answer questions concerning physical and historical geology, environmental geology/hydrology, environmental consulting, remote sensing/aerial photo interpretation, G&G computer applications, petroleum exploration, drilling, geochemistry, geochemical and microbiological prospecting, 3D reservoir modeling, computer mapping and drilling.I am not a geophysicist.


I have 24 years experience split between the petroleum and environmental industries. I have served as an expert witness in remote sensing, developmental geologist, exploration geologist, enviromental project manager, and subject matter expert in geology and geophysical software development.

American Association of Petroleum Geologists
American Association of Photogrammetrists and Remote Sensing

Bachelor and Master of Science
Registered Geologist in State of Texas

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