Composition

Sedimentary rocks can vary widely in composition as a result of their constituent grains (ex.  sandstones are mostly composed of sand-sized quartz and feldspar grains) or the soultions from which they were precipitated (ex. limestone is made of CaCO3).

Texture

Texture is a sedimentary rock’s appearance with respect to it’s grain size, shape, and arrangement.  Sedimentary rocks can be broadly classified as clastic or non-clastic.

Clastic: rocks formed through the cementation of previously existing rock fragments. Clastic sedimentary rocks have variable grain sizes, shapes and void spaces.

Non-Clastic: rocks formed through the deposition of minerals from solution. Non-clastic sedimentary rocks have an interlocking arrangement of crystals (crystalline texture)

Grain Size

arkose sandstone grains

Sediment particles that make up sedimentary rocks are classified based on their size.  Grain size influences how easily grains can be transported and can be used to interpret the original depositional environment.  The sample of sandstone shown here has sand-sized grains.

Sorting

Sorting refers to whether grains are separated according to size and can indicate the transportation mechanism.  For example, glaciers deposit grains of all sizes and result in poorly sorted sediment.  Rivers produce well-sorted sediment.

The sample of sandstone shown here has a high degree of sorting.

Clast Shape

Clast shape is an important characteristic which classifies the degree of rounding of the clasts within a clastic sedimentary rock.

Rounding occurs as grains are transported and round grains are thought to have travelled further than angular grains.

The image of breccia shown here has angular grains.

Packing

Packing refers to the arrangement of clasts within a sedimentary rock.

Framework or clast supported: the sedimentary rock is mainly composed of large clasts that are in contact with one another.

Matrix Supported: grains in sedimentary rocks are held in place by a matrix of finer grains