6.2 Classification of sedimentary rocks

Classification of Sedimentary Rocks

Sedimentary rocks record the conditions of Earth's surface at the time they formed. By classifying them, geologists can reconstruct ancient landscapes, climates, and tectonic settings. The three main categories are clastic, chemical, and organic-rich sedimentary rocks, each classified by different criteria.

Classification of clastic sedimentary rocks

Clastic (or detrital) sedimentary rocks form from fragments of pre-existing rocks that have been weathered, transported, and deposited. They're classified primarily by grain size, but composition and texture also matter.

Grain Size

Grain size is the single most important factor in naming a clastic rock. The standard size categories are:

• Gravel (> 2 mm) — produces two different rock types depending on shape:
  • Conglomerate contains rounded gravel clasts, indicating significant transport by rivers or waves (think river rocks or beach pebbles)
  • Breccia contains angular gravel clasts, suggesting short transport distances (talus slopes, fault zones)
• Sand (1/16 mm to 2 mm) — forms sandstone, one of the most common sedimentary rocks. Found in beaches, river channels, and deserts. The Navajo Sandstone of the American Southwest is a classic example of ancient wind-deposited sand.
• Silt (1/256 mm to 1/16 mm) — forms siltstone. Silt grains feel gritty between your teeth but are too small to see individually. Common in river floodplains and wind-blown loess deposits.
• Clay (< 1/256 mm) — the finest grain size, producing two rock types:
  • Claystone is massive (no layered splitting) and forms in mud flats or deep marine settings
  • Shale is fissile, meaning it splits into thin layers. This is the most abundant sedimentary rock on Earth, common in offshore marine environments.

Composition

The mineral makeup of a sandstone reveals where the sediment came from:

• Quartz arenite contains over 90% quartz grains. This high purity means the sand was either heavily weathered or recycled through multiple cycles of erosion and deposition. Typical of mature beach sands.
• Arkose is rich in feldspar (typically 25% or more), indicating rapid erosion of granitic or metamorphic source rocks before feldspar could break down. Often found near fault-bounded mountain ranges.
• Lithic arenite (graywacke) contains abundant rock fragments, reflecting nearby volcanic or tectonically active sources like island arcs.

Texture

Texture describes the physical characteristics of the grains and how they fit together:

• Sorting refers to grain size uniformity. Well-sorted rocks have grains of similar size (beach sand), while poorly-sorted rocks contain a wide mix (glacial till). Good sorting usually means prolonged transport or wave/wind action.
• Rounding ranges from angular to well-rounded. More rounding means more transport distance and abrasion.
• Matrix-supported rocks have fine-grained sediment filling the space around larger clasts (typical of mudflows and debris flows).
• Clast-supported rocks have gravel-sized clasts touching each other, with less fine matrix (typical of river channels with strong currents).

Types of chemical sedimentary rocks

Chemical sedimentary rocks form when dissolved minerals precipitate out of water, either through inorganic processes or with the help of organisms (biochemical). They're classified by mineral composition.

Limestone

• Composed of calcite ($CaCO_3$), either precipitated directly from seawater or built up from the shells and skeletons of marine organisms like corals, foraminifera, and mollusks.
• The classic field test: limestone fizzes vigorously when you apply dilute hydrochloric acid ($HCl$), releasing $CO_2$ gas as bubbles.
• Most common in shallow, warm tropical seas. The Bahamas platform is a modern example. Limestone also forms cave features like stalactites and stalagmites through dissolution and reprecipitation.

Dolostone

• Composed of the mineral dolomite ($CaMg(CO_3)_2$), which forms when magnesium ions replace some of the calcium in limestone after deposition (a process called dolomitization).
• Slightly harder than limestone and reacts more slowly with dilute $HCl$. You often need to powder it first to see fizzing.
• Associated with evaporative tidal flat environments, such as the sabkhas of the Persian Gulf.

Evaporites

Evaporites form when bodies of water in arid climates evaporate, concentrating dissolved minerals until they precipitate. They require restricted basins where water input can't keep up with evaporation.

• Gypsum ($CaSO_4 \cdot 2H_2O$) — a soft mineral (hardness 2) that forms swallowtail twins and desert rose crystal clusters. Precipitates before halite as water evaporates.
• Halite ($NaCl$) — rock salt. You can identify it by its salty taste and cubic crystal form. It dissolves readily in water. The Great Salt Lake is a modern evaporite-forming environment.
• Sylvite ($KCl$) — an important source of potassium for fertilizers, often mined from ancient evaporite deposits.

Organic-rich sedimentary rock formation

Organic-rich rocks form from the accumulation and burial of biological material, primarily plant matter or microscopic organisms.

Coal

Coal forms through a progressive series of changes called coalification, driven by increasing burial depth, temperature, and pressure:

1. Plant material accumulates in swamps or marshes faster than it can decompose (the Carboniferous Period coal forests are the classic example)
2. Shallow burial compresses the plant matter into peat, a soft, spongy material that's not yet rock
3. With deeper burial and mild heating, peat converts to lignite (brown coal), the lowest rank
4. Further burial and higher temperatures produce bituminous coal, the most commonly mined rank
5. At the highest temperatures and pressures, bituminous coal becomes anthracite, which has the highest carbon content and energy value

Each step increases carbon concentration and decreases moisture and volatile content. Note that anthracite borders on metamorphic rock since it requires significant heat and pressure.

Oil Shale

• A fine-grained sedimentary rock containing solid organic matter called kerogen, which is insoluble in normal solvents.
• Deposited in oxygen-poor (anoxic) environments where organic material from algal blooms or marine upwelling zones is preserved rather than decomposed.
• When heated to high temperatures (a process called retorting), kerogen breaks down and releases oil and gas. The Green River Formation in the western U.S. is one of the world's largest oil shale deposits.

Identification of sedimentary rocks

When you're handed an unknown sedimentary rock in lab, work through these steps systematically:

Step 1: Determine if it's clastic, chemical, or organic-rich

• Does it have visible grains or fragments? Likely clastic.
• Does it fizz with acid, taste salty, or appear crystalline? Likely chemical.
• Is it black, lightweight, or combustible? Likely organic-rich.

Step 2: For clastic rocks, assess grain size

• Use a grain size card or comparator to measure average grain diameter
• Gravel (> 2 mm), sand (1/16 to 2 mm), silt (1/256 to 1/16 mm), or clay (< 1/256 mm)
• If grains are too fine to see, try the feel test: silt feels gritty between your teeth, clay feels smooth

Step 3: Examine composition and texture

• Identify the dominant minerals (quartz, feldspar, rock fragments)
• Note any fossils, plant debris, or unusual minerals
• Assess sorting (uniform vs. mixed grain sizes) and rounding (angular vs. rounded)
• Determine if the rock is matrix-supported or clast-supported

Step 4: For chemical and biochemical rocks, use diagnostic tests

• Limestone fizzes vigorously with dilute $HCl$ and may contain visible fossils
• Dolostone reacts slowly with acid (try powdering it) and tends to be harder
• Gypsum is very soft (scratched easily with a fingernail) and halite tastes salty

Step 5: For organic-rich rocks, check physical properties

• Coal is black, low-density, and combustible, often with fractures called cleats
• Oil shale is dark, fine-grained, and may give off a petroleum-like odor when freshly broken

Sedimentary Rock Formation and Environments

Depositional environment indicators

The characteristics of a sedimentary rock serve as clues to the environment where it formed. Here's how to read those clues for each rock category.

Clastic rocks

Grain size, sorting, and composition together paint a picture of transport distance, energy level, and tectonic setting:

• Coarse-grained, poorly-sorted sediments point to high-energy environments close to the sediment source, such as alluvial fans and braided rivers.
• Fine-grained, well-sorted sediments indicate low-energy settings far from the source, like offshore marine basins or deep lakes.
• Quartz-rich sands suggest stable continental interiors (cratons) with prolonged chemical weathering, or recycled sediments on passive continental margins.
• Feldspar-rich arkoses indicate rapid erosion of uplifted igneous and metamorphic rocks, often near fault-block mountains where erosion outpaces chemical breakdown.
• Lithic-rich sediments point to active tectonic settings with nearby volcanic sources, such as island arcs or continental rifts.

Chemical and biochemical rocks

• Limestone forms in shallow, warm marine environments like coral reefs and carbonate platforms (the Bahamas today).
• Dolostone is associated with restricted, evaporative tidal flats and lagoons.
• Evaporites accumulate in arid-climate restricted basins with high evaporation rates, such as the salt flats of Death Valley.

Organic-rich rocks

• Coal originates in humid-climate swamps and marshes where plant material is buried rapidly before it fully decomposes.
• Oil shale forms in anoxic marine or lacustrine (lake) environments with high organic productivity, such as upwelling zones or stagnant lagoons.