Siliciclastic rocks, also known as terrigenous, detrital, or clastic rocks, are a group of sedimentary rocks consisting of fragments of minerals and rocks derived from pre-existing rocks. Such fragments are termed clasts, regardless of their composition and size. Clasts originate due to weathering and subsequent erosion of the rocks exposed on the surface of the Earth. Weathering, acting on rocks as physical and chemical alteration processes, disintegrates rocks to loose fragments (detrital particles) producing new minerals that are stable in sedimentary environments, like clay minerals. Erosion by agents such as wind, water, ice, or gravity, removes the products of weathering and transport them to the site of deposition. Deposition occurs where the velocity of the transporting current becomes too low, allowing clasts to settle to the bottom of a sedimentary basin. Clasts accumulated in a basin, called sediments, pile up layer after layer, increasing the weight, heat, and pressure, on the older, underlying sediments. As a result, sediments progressively compact at depth, expelling the water trapped in their pore spaces. The compacting sediments undergo several chemical and physical transformations (diagenesis) that bound sedimentary particles together producing lithified sedimentary rocks.
Classification of clastic sediments
Sedimentary processes (weathering and erosion) produce a variety of clasts with different grain size (= average diameter) that we classify according to the Udden-Wentworth logarithmic scale into four main classes:
– gravel (> 2 mm), which includes pebbles (2 – 64 mm), cobbles (64 – 256 mm) and boulders (> 256 mm);
– sand (0.0625 – 2 mm);
– silt (0.004 – 0.0625 mm);
– clay (<0.004 mm).
The scale divides further these classes in sub-classes (e.g. fine sand, very fine sand). These names can be used to refer to single clasts (e.g. a gravel clast) or a sedimentary deposit of loose particles with a specific grain size (e.g. gravel deposit or simply, gravel). The logarithmic scale arbitrarily sets the limits between classes in order to define standards, but some of these numbers are useful for classification purposes. In particular, the 0.0625 mm boundary corresponds to the limit of the human eye to distinguish objects: we can recognize very fine sand, but silt and clay particles are too small and are not recognizable in the field. For this reason, field geologists normally group clay and silt together as mud, comprising grain sizes that are smaller than 0.0625 mm.
Grain size is the primary parameter that allows to classify clastic sedimentary rocks. However, it does not fully define clastic textures alone. There are other four major parameters that geologists use to describe clastic rocks and that allow to understand the sedimentary environment where they deposited, their source area, and the type of transport they experienced:
grain shape: the shape of clasts.
sorting: the variability in grain size in a clastic sedimentary rock.
roundness: how much the external outline of a clast has been rounded during transport.
packing: how much grains are close together, with respect to one another and the surrounding matrix.
You can learn more about these parameters in the dedicated pages.
Classification of siliciclastic rocks
Clastic sedimentary rocks are named in agreement with the grain size of the sediments that constitute them. There are four main classes of siliciclastic rocks:
– conglomerate/breccia, consisting of gravel-sized sediments. Conglomerates have rounded/smoothed clasts, whereas breccias contain angular clasts with sharp edges;
– sandstone, composed by sand particles;
– siltite, consisting of silt;
– claystone, consisting of clay.
Siltite and claystone are indistinguishable in the field and we refer to them collectively as mudstone.
Mixed terrigenous sediments and rocks
Many siliciclastic rocks contains non-negligible proportions of all grain sizes. Many efforts have been made through the years to define the boundaries between different clastic sedimentary rocks, resulting in the publication of several classification diagrams, where hybrid names are assigned depending on the proportion of the grain sizes present. One of the most widely used is the gravel-sand-mud diagram by Folk (1980), shown below (slide to see the corresponding sediment and rock names). Detailed classifications are available on the pages of specific siliciclastic rocks.
Compton, R. R. (1962). Manual of field geology. Soil Science, 93(4), 295.
Dott, R. H. (1964). Wacke, graywacke and matrix; what approach to immature sandstone classification?. Journal of Sedimentary Research, 34(3), 625-632.
Folk, R. L. (1980). Petrology of sedimentary rocks. Hemphill publishing company.
Lewis, D. W., & McConchie, D. (2012). Analytical sedimentology. Springer Science & Business Media.
Powers, M. C. (1953). A new roundness scale for sedimentary particles. Journal of Sedimentary Research, 23(2), 117-119.
Wentworth, C. K. (1922). A scale of grade and class terms for clastic sediments. The journal of geology, 30(5), 377-392.