6.2 Clastic, chemical, and organic sedimentary rocks

Sedimentary rocks tell Earth's story through layers of deposited material. From sandy beaches to deep ocean floors, these rocks form in diverse environments and are classified by how they form: clastic (from rock fragments), chemical (from dissolved minerals precipitating out of solution), or organic (from remains of living things). Each type offers clues about past environments and life on Earth.

Sedimentary Rock Types and Formation

Types of sedimentary rocks

The three categories are defined by the source of the material that makes up the rock.

• Clastic sedimentary rocks form from pre-existing rock fragments that have been weathered, transported by wind, water, or ice, and then deposited. Their composition reflects the source material. Sandstone is a common example.
• Chemical sedimentary rocks precipitate from mineral-rich solutions, often driven by evaporation or changes in water chemistry. Their composition depends on which minerals were dissolved. Limestone is a classic example.
• Organic sedimentary rocks (also called biogenic) accumulate from carbon-rich remains of organisms, typically through processes like plant decay or the buildup of marine shells. Coal is the most familiar example.

Classification of clastic rocks

Clastic rocks are classified two ways: by grain size and by mineral composition. Grain size is the primary way most geologists name them.

By grain size (largest to smallest):

• Conglomerate: rounded clasts larger than 2 mm (pebble-sized and up). Think river gravels that have been tumbled smooth during transport.
• Sandstone: sand-sized grains, 1/16 to 2 mm. Beach sand is the go-to example.
• Siltstone: silt-sized particles, 1/256 to 1/16 mm. Common in calm settings like lake bottoms.
• Mudstone/Shale: clay-sized particles, smaller than 1/256 mm. Shale specifically refers to mudstone that splits into thin layers (fissility). These form in very low-energy environments like deep ocean floors.

By composition (what minerals the grains are made of):

• Quartz sandstone: more than 95% quartz grains. Quartz dominates because it resists chemical weathering better than most minerals. Desert dune sands are a typical source.
• Arkose: contains more than 25% feldspar grains, which signals a nearby granitic source that weathered quickly before feldspar could break down. Often found near alluvial fans.
• Graywacke: a "dirty" sandstone with abundant fine-grained matrix and rock fragments. Common in deep marine settings where turbidity currents dump poorly sorted sediment.
• Breccia: angular clasts larger than 2 mm. The angular shape tells you the fragments weren't transported far, so they didn't get rounded. Talus slopes at the base of cliffs are a classic source.

Formation of chemical sediments

Chemical sedimentary rocks form when dissolved ions in water combine and precipitate as solid minerals. The specific rock that forms depends on water chemistry and environmental conditions.

Limestone is primarily composed of calcium carbonate ($CaCO_3$):

1. Can precipitate directly from warm, shallow seawater when it becomes supersaturated with $CaCO_3$
2. More commonly accumulates from the shells and skeletons of marine organisms (corals, foraminifera, mollusks)
3. Comes in several varieties: oolitic limestone (tiny rounded grains formed by rolling in shallow water), fossiliferous limestone (visible fossil fragments), and micritic limestone (very fine-grained lime mud)

Dolostone consists of the mineral dolomite ($CaMg(CO_3)_2$):

1. Typically forms when magnesium-rich fluids percolate through existing limestone, replacing some calcium with magnesium in a process called dolomitization
2. This alteration changes the crystal structure of the original rock
3. Dolostone is rarely deposited directly; it's almost always a secondary replacement of limestone

Evaporites form when mineral-rich water evaporates, concentrating dissolved ions until they precipitate:

1. Most common in arid environments or restricted basins with limited water circulation (think desert lakes or isolated arms of the sea)
2. Minerals precipitate in a predictable sequence as water evaporates: calcite first, then gypsum ($CaSO_4 \cdot 2H_2O$), then halite ($NaCl$, or rock salt), and finally potassium salts like sylvite ($KCl$)
3. This sequence creates distinctive layered deposits, with the least soluble minerals at the bottom

Organic sedimentary rock characteristics

Organic sedimentary rocks are made primarily from the remains of organisms. The two most important examples differ dramatically in composition.

Coal forms from compressed and chemically altered plant material:

1. Dead plant matter accumulates in oxygen-poor swamps or peat bogs, where it decays slowly
2. Burial under more sediment causes compaction and heating over millions of years
3. Progressive alteration increases carbon content, producing a rank sequence: peat → lignite (soft, brown) → bituminous (black, most commonly mined) → anthracite (hard, glossy, highest carbon content and energy value)

Chert is composed of microcrystalline quartz ($SiO_2$):

1. Often forms from the accumulated siliceous skeletons of microscopic organisms like radiolarians and diatoms
2. Appears in several forms: flint (dark gray/black, found in chalk), jasper (red, colored by iron oxide), and agate (banded, forms in cavities)
3. Occurs either as nodules embedded within limestone or as bedded layers deposited on the deep ocean floor