The Principal Cause of Grain-Size Variations in Clastic Sedimentary Rocks

Understanding the process of sedimentation and its impact on grain size in clastic sedimentary rocks is crucial for geologists and earth scientists. Clay, silt, sand, and everything larger known as gravel, collectively known as clastic sediments, form through the weathering and erosion of rocks and their subsequent redeposition via various physical processes. This article delves into the principal cause of grain size variations in clastic sedimentary rocks and explores the role of sedimentation patterns in rock formation.

What Determines Grain Size in Clastic Sedimentary Rocks?

The primary determinant of grain size in clastic sedimentary rocks is closely tied to the dynamics of the depositional environment. Grains are sorted based on their size by various forces such as gravity, water currents, and wind. Water is the most common agent of transport and sorting, and the rate of sedimentation often dictates the grain size observed in the resulting rock layer.

Sedimentation Due to Size/Mass Differences

The rate of sedimentation, resulting from size or mass differences among particles, is the principal cause of grain-size variations in sedimentary rocks. Did you know that vertical sedimentation can also result in sorted grain sizes? Even in a vertical environment, where particles are moved from the upper to the lower portion of a water column, size and mass differences play a significant role.

For instance, clastic sediments in fast-moving rivers are typically made up of coarse materials such as gravel and sand. As water slows down, the finer materials such as silt and clay are deposited, forming layers of sorted grains. This sorting process can be observed in cross-sections of sedimentary layers, where coarser materials are found at the base and finer materials towards the top.

Factors Affecting Grain Size Variation

Several factors influence the grain size variation in clastic sedimentary rocks:

Transporting Medium: The physical characteristics of the transporting agent (e.g., water, wind) determine how well the particles are sorted. Water currents are particularly effective at sorting and carrying particles based on their size. Depositional Environment: The dynamics of the depositional environment play a crucial role. Shallow marine environments, for example, often yield better-sorted sediments compared to deeper or more dynamic settings. Wave and Current Action: Wave and current patterns significantly contribute to the sorting of sediments. Waves in coastal areas can generate layers of sorted sediments, while currents in river systems can sort materials based on particle size.

Special Cases: Ungraded Sediments

Exercise for the student: Under what circumstances do totally ungraded sediments accumulate and become rock?

Unlike typical clastic sedimentary rocks that show distinct grain-size layers, ungraded sediments lack significant sorting. These sediments are characterized by a mixed composition of grains without any particular order in size. Ungraded sediments are typically deposited in environments where the transporting agent is very powerful, such as in massive floods or rapid underwater landslides. Such deposits are often found in river terraces, coastal areas, and submarine fans.

Conclusion

The principal cause of grain-size variations in clastic sedimentary rocks is the depositional process, primarily controlled by the rate of sedimentation due to size/mass differences. Understanding these processes is vital for identifying depositional environments and reconstructing the geological history of the area. Whether it's the windblown dunes of arid regions or the river systems shaping the vast landscapes, the science of sedimentation continues to be a fascinating field of study in earth sciences.

References

[1] Hamilton, W. B. (2004). Sedimentology. Springer.

[2] French, S. W. (2009). Handbook of Sedimentary Rocks. Wiley.

[3] Kinsley, M. J. (1996). Sedimentary Basin Analysis. Chapman and Hall.