Clastic Sedimentary Rocks: How Are They Classified?

Let's dive into the fascinating world of sedimentary rocks! Specifically, we're going to explore clastic sedimentary rocks and how geologists classify them. If you've ever wondered what makes a sandstone different from a conglomerate, or a shale different from a breccia, you're in the right place. Get ready to uncover the secrets hidden within these fragment-filled formations.

Understanding Clastic Sedimentary Rocks

To truly grasp how these rocks are classified, let's start with the basics. What exactly are clastic sedimentary rocks? In simple terms, they are rocks formed from the accumulation and cementation of fragments derived from pre-existing rocks or minerals. Think of it like nature's recycling process, where bits and pieces of older rocks are broken down, transported, deposited, and then glued back together to form something new.

The journey of clastic sediments often begins with weathering, where rocks are broken down by physical and chemical processes. This weathering produces fragments of various sizes, from microscopic clay particles to massive boulders. These fragments are then transported by wind, water, or ice to a new location, where they eventually settle out and accumulate in layers. Over time, the weight of overlying sediments compresses the lower layers, and minerals precipitate from groundwater to cement the fragments together, forming a solid rock.

The key characteristic that distinguishes clastic sedimentary rocks from other types of sedimentary rocks (like chemical or organic sedimentary rocks) is their fragmental nature. They're essentially made up of bits and pieces of other rocks, making them a kind of geological mosaic. Now that we know what they are, let's explore how scientists classify them. Remember, understanding the basis of classification is crucial for identifying and interpreting the history of these rocks.

The Primary Classification Criterion: Size of Rock Fragments

So, what's the main factor that geologists use to classify clastic sedimentary rocks? The answer is the size of the rock fragments that make up the rock. This might seem simple, but it's a fundamental principle in geology. The size of the grains tells us a lot about the energy of the environment where the sediment was deposited. For instance, large, heavy fragments require high-energy environments like fast-flowing rivers or turbulent shorelines to be transported, while fine-grained sediments can settle out in low-energy environments like lakes or deep ocean basins.

Imagine a rushing river carrying a mix of sediment. The larger pebbles and cobbles will be dragged along the bottom, while the finer sand and silt will be carried in suspension. When the river slows down, the larger fragments will settle out first, followed by the smaller ones. This process of sorting by size is a crucial factor in the formation of different types of clastic sedimentary rocks. Therefore, geologists use grain size as the primary basis for classifying these rocks.

The most common size classifications include gravel, sand, silt, and clay. Gravel-sized fragments are larger than 2 millimeters in diameter, sand-sized fragments range from 0.0625 to 2 millimeters, silt-sized fragments range from 0.0039 to 0.0625 millimeters, and clay-sized fragments are smaller than 0.0039 millimeters. Each of these size ranges corresponds to a different type of clastic sedimentary rock. For example, rocks composed primarily of gravel-sized fragments are called conglomerates or breccias, while rocks composed primarily of sand-sized fragments are called sandstones.

Common Types of Clastic Sedimentary Rocks

Now that we know that grain size is king, let's look at some of the most common types of clastic sedimentary rocks and how they fit into this classification scheme.

• Conglomerate: This rock is composed of rounded, gravel-sized fragments cemented together. The rounded shape of the fragments indicates that they have been transported a significant distance and have been abraded by water or wind. Conglomerates often form in high-energy environments like river channels or alluvial fans.
• Breccia: Similar to conglomerate, breccia is also composed of gravel-sized fragments, but the fragments are angular instead of rounded. This angularity indicates that the fragments have not been transported very far and have not been significantly abraded. Breccias often form in environments like landslides or fault zones.
• Sandstone: This rock is composed of sand-sized fragments cemented together. Sandstones are one of the most common types of sedimentary rocks and can form in a variety of environments, including beaches, dunes, and river channels. The composition of the sand grains can vary, but quartz is the most common mineral.
• Siltstone: As the name implies, this rock is composed of silt-sized fragments. Siltstones are finer-grained than sandstones and often form in low-energy environments like floodplains or lakes.
• Shale: This rock is composed of clay-sized fragments. Shales are the most abundant type of sedimentary rock and often form in very low-energy environments like deep ocean basins or lagoons. Shales are typically dark in color due to the presence of organic matter.

Other Factors in Classification

While grain size is the primary factor in classifying clastic sedimentary rocks, it's not the only one. Geologists also consider other characteristics, such as the composition of the fragments, the degree of sorting, and the type of cement that binds the fragments together.

The composition of the fragments can tell us a lot about the source rock from which the sediment was derived. For example, a sandstone composed primarily of quartz grains likely came from a granite or gneiss source, while a sandstone composed primarily of feldspar grains likely came from a volcanic source.

The degree of sorting refers to the range of grain sizes present in the rock. A well-sorted rock has a narrow range of grain sizes, while a poorly sorted rock has a wide range of grain sizes. Well-sorted rocks typically form in environments where the sediment has been transported a long distance and has been winnowed by wind or water. Poorly sorted rocks typically form in environments where the sediment has not been transported very far and has not been significantly winnowed.

The type of cement that binds the fragments together can also provide clues about the rock's history. Common types of cement include quartz, calcite, and iron oxide. The type of cement can affect the rock's color, hardness, and permeability.

Why Classification Matters

So, why is it so important to classify clastic sedimentary rocks? Well, classification helps us to understand the origin and history of these rocks. By knowing the grain size, composition, sorting, and cementation of a clastic sedimentary rock, we can infer the type of environment in which it formed, the source of the sediment, and the processes that transported and deposited the sediment. This information can be invaluable for understanding the geological history of an area, finding natural resources, and assessing environmental hazards.

For example, the presence of a thick layer of sandstone might indicate the presence of an ancient beach or dune system, which could be a potential source of groundwater or petroleum. Similarly, the presence of a layer of shale might indicate the presence of an ancient lake or ocean basin, which could be a source of natural gas or oil shale. Geologists use the clues in sedimentary rocks to piece together the history of the earth.

In Conclusion

So, to recap, the phrase that best describes a feature of clastic sedimentary rocks is grouped by size of rock fragments. While color, chemical weathering, and organic matter can all play a role in the characteristics of these rocks, it is the size of the constituent particles that forms the basis for their classification. This simple principle allows us to unlock a wealth of information about the Earth's past and the processes that have shaped our planet.

Want to expand your knowledge? Visit the USGS website for more in-depth information. USGS.