Classification of Sedimentary Rocks

Classification of Sedimentary Rocks, Its Origins and Properties


The cliffs of Dover consist of sedimentary rock. Here is a guide to what sedimentary rocks are:

When considering the properties of sedimentary rocks it is important to understand its colour,
texture, minerology and fossils. Sedimentary rock colour is determined by the amount of iron which
is present either as iron II or iron III oxide. The grey or green rocks are the result of Iron II oxide
which is formed when the surroundings have a low oxygen content. On the other hand, iron III oxide
occurs when oxygen is found in larger content and is reddish /brownish in nature. When organic
material is present the rocks could take a rock black or grey colour. The organic material occurs due
to dead plants and organisms. The texture is due to the size, form and orientation of clasts. The
texture determines the density, permeability and porosity. The texture can be of different forms, the
surface texture describes the general shape, rounding is how the smoothness of the shape of grain is
and grain form describes the 3-D structure of the grain. On analysing the sedimentary rocks, it
contains either quartz or calcite. In contrast to metamorphic or igneous rocks, the sedimentary rocks
have only very few major minerals. Fossils are the most visibly found in sedimentary rock. In case of
sedimentary rocks, mica and feldspar are generally reduced to clay minerals like kaolinite, illite or

Classification of sedimentary rocks is based on its origin. The formation is based on four groups
which is responsible for their formation, the clastic sedimentary rocks, chemical sedimentary rocks,
biochemical sedimentary rocks and rocks formed due to volcanism.

The clastic sedimentary rocks are formed due to rock fragments which are cemented by silicate
minerals. It is mainly composed of feldspar, quartz, mica and clay minerals. The clastic sedimentary
rocks fragment into gravel, sand and mud. The tripartite subdivision is mirrored by the categories of
arenites, rudites and lutites. The subdivision happens due to differences in clast shape, composition
and the texture and grain size.

The biochemical sedimentary rock formation happens when organisms use materials which are
dissolved in air or water to build tissues. Many limestone types are formed from skeletons of
organisms such as mollusks, corals and foraminifera. Chert is formed when there is accumulation of
siliceous skeletons of microscopic creatures like diatoms. Coal is formed from plants which has
utilised carbon from atmosphere to build tissues.

The chemical sedimentary rocks form when mineral constituents are in solution and gets super
saturated and precipitate out. The common sedimentary rocks include oolitic limestone and
evaporate minerals like gypsum and barite. The remaining other sedimentary rocks are a result of
volcanic breccias and pyroclastic flows.

When the process of sedimentation occurs, an older rock gets buried deeper as a result.