Clastic sedimentary rocks are formed from fragments, known as clasts, of pre-existing rocks that have been weathered, eroded, transported, and cemented together. Classification is fundamental for geologists, providing a standardized way to describe these rocks. This systematic method allows for the interpretation of the rock’s history and the environment in which it was deposited.
Classification by Grain Size
Grain size provides the primary basis for classifying clastic sedimentary rocks. This classification uses the Udden-Wentworth scale, a standardized system that categorizes clast diameters. The size of the grains directly relates to the energy of the environment that transported them, as higher energy environments carry larger particles.
The scale establishes three overarching size groups that determine the rock’s basic name. Gravel is the coarsest group, including particles larger than 2 millimeters, and rocks in this range are termed rudites. Sand-sized clasts fall between 0.0625 and 2 millimeters, forming rocks called arenites. The finest categories are silt and clay, both smaller than 0.0625 millimeters, and rocks made from these fine grains are known as lutites or mudrocks.
Classification by Composition and Matrix
Beyond size, the composition of the grains offers a second layer of classification, especially for sand-sized rocks. Composition is defined by the relative abundance of three primary components: quartz, feldspar, and lithic fragments (pieces of other rocks). Quartz is the most common mineral in siliciclastic rocks because it resists chemical weathering better than most other minerals.
Feldspar’s presence indicates that the sediment was not transported long distances or was buried quickly, preventing complete chemical breakdown. Lithic fragments, which are small rock chips, also suggest shorter transport times or a nearby source rock. The material filling the spaces between the larger grains, called the matrix, further refines the classification. A rock with a significant amount of fine-grained clay or silt matrix is chemically less “clean” than one held together primarily by a chemical cement like silica or calcite.
Classification by Grain Shape and Sorting
Secondary physical characteristics, such as grain shape and size uniformity, provide valuable clues about the sediment’s transport history. Grain shape is described by angularity or roundness, reflecting the amount of abrasion the clast has undergone. Angular clasts suggest the sediment was not moved far from its source or was transported by a medium like ice that causes little abrasion.
Conversely, well-rounded clasts indicate a long history of transport by water or wind, where repeated impacts have smoothed the edges. Sorting refers to the range of grain sizes present in the rock.
A well-sorted rock contains grains all of a similar size, typical of environments like beaches or dunes where currents are consistent and selective. A poorly sorted rock, containing a wide mix of large and small grains, suggests rapid deposition from a high-energy, non-selective event, such as a debris flow.
Identifying the Final Rock Types
Combining the classification criteria results in the final, specific names used by geologists. For coarse-grained rudites, the physical shape of the clasts is the distinguishing factor. A rock made of rounded, gravel-sized fragments is named a Conglomerate, indicating prolonged transport history. If the gravel-sized fragments are highly angular, the rock is called a Breccia, suggesting a shorter, less abrasive journey.
The sandstones, or arenites, are named primarily based on their composition. A Quartz Arenite is a mature sandstone containing 90% or more quartz grains, signifying extensive weathering and transport. If the rock contains more than 25% feldspar, it is classified as an Arkose, suggesting fast deposition from a granitic source. A Litharenite contains more rock fragments than feldspar, pointing to a source of mixed rock types.
Lastly, the fine-grained lutites are named based on their dominant size and layering. Siltstone is mainly silt. A Mudrock with fine laminations that breaks into sheets is called a Shale. Both Siltstone and Shale form in very low-energy environments like deep oceans or lake beds.