Sedimentary rocks

Sedimentary rocks are the product of the erosion of existing rocks. Eroded material accumulates as sediment, either in the sea or on land, and is then buried, compacted and cemented to produce sedimentary rock (a process known as diagenesis).

There are two major groupings of sedimentary rocks:

Clastic sedimentary rocks
The fragments of pre-existing rocks or minerals that make up a sedimentary rock are called clasts. Sedimentary rocks made up of clasts are called clastic (clastic indicates that particles have been broken and transported). Clastic sedimentary rocks are primarily classified on the size of their clasts.

Clast size in clastic rocks

NameGradeSize range (mm)Comments
Boulder > 200Clasts should be identifiable.
GravelVery coarse60 – 200
Coarse20 – 60
Medium6 – 20
Fine2 – 6
SandCoarse0.6 – 2Clasts visible to the naked eye. Grains often identifiable.
Medium0.2 – 0.6
Fine0.06 – 0.2
Mud < 0.002 – 0.06Clasts not visible to the naked eye. Feels smooth.

Clast shape, or the degree of rounding of clasts, is important in differentiating some sedimentary rocks. Clasts vary in shape from rounded to angular, depending on the distance they have been transported and / or the environment of deposition, e.g. rounded clasts are generally the product of long transportation distances and / or deposition in high energy environments (beaches, rivers).

Clasts in sandstone

The degree of sorting of clasts can be an important indicator of depositional environment. In water, larger clasts are generally not transported great distances, and they settle faster. For example, in a mixture of mud and sand being transported in a river to the sea, the sand (larger clast size, heavier) would begin to deposit as soon as the river's energy dissipated, while the mud (fine, light-weight) would be transported far off shore. Therefore, a well sorted (clasts of approximately the same size), coarse sandstone indicates deposition in a reasonably high energy environment (near-shore) probably close to the source of the sand. Conversely, a mudstone generally indicates deep water deposition (low energy environment, far off shore).

Structures produced during deposition, e.g. bedding and cross-bedding, can give clues as to depositional environment. So can structures produced by re-working by tidal or storm-generated currents, e.g. ripple marks, rip-up clasts.

Non-clastic sedimentary rocks
These sedimentary rocks occur when minerals / mineraloids are precipitated directly from water, or are concentrated by organic matter / life. Components have not been transported prior to deposition. No clasts are present.