How To Identify A Sedimentary Rock

How to Identify Sedimentary Rocks: A Comprehensive Guide

Sedimentary rocks, formed from the accumulation and cementation of sediments, represent a significant portion of the Earth's crust and hold a wealth of information about our planet's history. Identifying these rocks requires a keen eye for detail and an understanding of their unique characteristics. This comprehensive guide will equip you with the knowledge and tools to confidently identify various types of sedimentary rocks.

Understanding Sedimentary Rock Formation

Before delving into identification techniques, it's crucial to grasp the fundamental processes involved in sedimentary rock formation. This understanding will provide context for recognizing key features.

The Sedimentary Rock Cycle:

The journey of a sedimentary rock begins with the weathering and erosion of pre-existing rocks. These processes break down larger rocks into smaller fragments, or sediments. These sediments are then transported by various agents like wind, water, or ice, a process known as transportation. Eventually, these sediments are deposited in layers, often in basins like oceans, lakes, or riverbeds. Over time, the weight of overlying sediments compresses the lower layers, a process called compaction. Finally, dissolved minerals precipitate from groundwater, filling the spaces between sediment grains and cementing them together—this is cementation. This cemented mass then becomes a sedimentary rock.

Types of Sediments:

The type of sediment significantly influences the resulting sedimentary rock. Sediments can be classified into three primary categories:

• Clastic Sediments: These are fragments of pre-existing rocks, ranging in size from microscopic clay particles to large boulders. The size of the clasts is a key identifier in classifying sedimentary rocks.
• Chemical Sediments: These sediments precipitate from solutions, often in water bodies. Examples include evaporites (like halite and gypsum) formed from the evaporation of saltwater and precipitates from chemically-rich groundwater.
• Biochemical Sediments: These are formed from the remains of organisms. Examples include limestone composed of fossilized shells and coral reefs and coal formed from compressed plant matter.

Key Characteristics for Identification

Identifying sedimentary rocks involves carefully observing several key characteristics:

1. Texture:

Texture refers to the size, shape, and arrangement of the sediment grains. Several aspects of texture are crucial for identification:

• Grain Size: This is perhaps the most obvious characteristic. Rocks with coarse grains (e.g., gravel-sized) are easily distinguished from those with fine grains (e.g., clay-sized). Common size classifications include:
  • Boulder: >256 mm
  • Cobble: 64-256 mm
  • Pebble: 2-64 mm
  • Sand: 0.0625-2 mm
  • Silt: 0.0039-0.0625 mm
  • Clay: <0.0039 mm
• Grain Shape: Grain shape can be rounded, angular, or subangular. Rounded grains suggest extensive transportation, while angular grains indicate shorter transport distances.
• Sorting: Sorting refers to the uniformity of grain size. Well-sorted rocks contain grains of similar size, while poorly sorted rocks have a wide range of grain sizes.
• Grain Packing: This refers to how tightly the grains are packed together. Close packing results in denser rocks.

2. Composition:

The mineral composition of the rock provides valuable clues to its origin and type. Common minerals found in sedimentary rocks include quartz, feldspar, calcite, and clay minerals. Identifying these minerals can often be done with a hand lens and basic knowledge of mineral properties.

3. Color:

While color alone is not a definitive identifier, it can be a helpful indicator. For example, red coloration often suggests the presence of iron oxides, indicating an oxidizing environment during formation. Gray or black coloration can be indicative of organic matter.

4. Bedding and Layering:

Sedimentary rocks are often characterized by distinct layers or beds, reflecting changes in depositional environments over time. Observe the thickness, uniformity, and orientation of these layers. Cross-bedding, where layers are inclined at an angle to the main bedding plane, is a strong indicator of deposition by wind or water currents.

5. Fossils:

The presence of fossils is a strong indicator that the rock is sedimentary. Fossils are the preserved remains or traces of ancient organisms, and their presence can reveal information about the age and environment of the rock's formation. Identifying fossils requires specialized knowledge, but their presence is a critical identification characteristic.

6. Cement:

The cementing material that binds sediment grains together can provide clues to the rock's formation. Common cements include calcite, silica, and iron oxides. The type and abundance of cement affect the rock's overall strength and durability.

Common Sedimentary Rock Types

Let's examine some of the most common sedimentary rocks and their identifying features:

Clastic Sedimentary Rocks:

• Conglomerate: A coarse-grained rock composed of rounded gravel-sized clasts cemented together. The clasts are usually well-rounded indicating significant transport.
• Breccia: Similar to conglomerate, but with angular clasts, indicating shorter transportation distances.
• Sandstone: Composed of sand-sized grains, usually quartz. Sandstones can be well-sorted or poorly sorted, depending on the depositional environment. Variations in color reflect different cementing materials and impurities.
• Siltstone: Composed of silt-sized particles. Often appears finely layered and may feel slightly gritty.
• Shale: Composed of clay-sized particles. Typically fissile, meaning it breaks easily along parallel planes. Often dark-colored due to organic matter.

Chemical Sedimentary Rocks:

• Limestone: Composed primarily of calcite (calcium carbonate). Often forms in marine environments from the accumulation of shells and other skeletal remains. Can be fossiliferous.
• Dolostone: Similar to limestone but contains dolomite, a magnesium-rich carbonate mineral.
• Chert: A very hard, fine-grained rock composed of microcrystalline silica. Often forms nodules within other sedimentary rocks.
• Evaporites (e.g., Rock Gypsum, Rock Salt): Formed by the evaporation of saltwater, these rocks contain minerals like gypsum and halite. Often exhibit crystalline textures.

Biochemical Sedimentary Rocks:

• Coquina: A limestone composed of fragmented shells and shell debris.
• Chalk: A soft, white limestone composed of microscopic coccoliths (calcareous plates from marine plankton).
• Coal: A black or dark-brown sedimentary rock composed of compressed plant matter. Formed in swampy environments.

Tools and Techniques for Identification

Identifying sedimentary rocks accurately requires the use of specific tools and techniques:

• Hand Lens: A hand lens magnifies the rock's texture, allowing for better observation of grain size, shape, and arrangement.
• Rock Hammer: Used to break apart rocks, revealing internal structures and textures. Safety glasses should always be worn when using a rock hammer.
• Acid Test: Dilute hydrochloric acid can be used to test for the presence of calcite (a key component of many limestones and dolomites). A positive reaction involves fizzing or bubbling.
• Field Guide: A comprehensive field guide to rocks and minerals is an invaluable resource, providing descriptions, images, and identification keys.
• Geological Maps and Literature: Studying geological maps and literature about the specific region where you collected a rock sample can provide important context.

Putting it All Together: A Step-by-Step Approach

To confidently identify a sedimentary rock, follow these steps:

1. Observe the Overall Appearance: Note the rock's color, texture (coarse, fine, etc.), and any visible layering or bedding.
2. Examine the Texture Closely: Use a hand lens to assess grain size, shape, sorting, and packing.
3. Assess the Composition: Try to identify the dominant minerals using a hand lens and your knowledge of mineral properties. Consider conducting an acid test if you suspect the presence of calcite.
4. Look for Fossils: The presence and type of fossils can provide valuable clues about the rock's age and environment of formation.
5. Consider the Context: The location where you found the rock can provide important clues about its origin. Consider the surrounding geology and any known formations in the area.
6. Compare with Field Guides: Use a field guide to compare your observations with descriptions and images of known sedimentary rock types.

By following these steps and practicing regularly, you'll gradually develop the skills to accurately and confidently identify a wide range of sedimentary rocks. Remember that experience is key—the more rocks you examine, the better you'll become at recognizing subtle differences and making accurate identifications. Remember safety first when handling rocks and using tools. Always wear safety glasses when using a rock hammer.