Sedimentary Rocks

• Sedimentary rocks constitute a relatively thin veneer on Earth's crust, with igneous and metamorphic rocks making up the majority by volume.
• These rocks provide evidence of past events, preserving fossils, ripple marks, and mud cracks, while also allowing reconstruction of ancient landscapes.

Formation Process

• Granite transforms into sedimentary rock through weathering and erosion, followed by transportation, deposition, and lithification.

Basic Categories

• Detrital (Clastic) sedimentary rocks form from fragments of pre-existing rocks (e.g., sandstone, shale, conglomerate).
• Chemical sedimentary rocks form through mineral precipitation from water solutions (e.g., limestone, rock salt, chert).
• Organic (Biochemical) sedimentary rocks form from accumulated organic matter (e.g., coal, some limestones).

Detrital Sedimentary Rocks

• Clay minerals and quartz are the most abundant minerals.
• Shale/mudstone is dominated by clay minerals.
• Sandstone is dominated by quartz.
• Particle size is the primary way to classify detrital rocks.
• Mineral composition provides additional information about the source rock and the formation environment.

Expanded Formation Process

• Physical weathering breaks down rocks into smaller pieces without altering their chemical composition through freeze-thaw cycles and abrasion.
• Chemical weathering breaks down rocks via chemical reactions (dissolution, oxidation, hydrolysis), changing the mineral composition.

Erosion Agents

• Water transports sediments through rivers, streams, and ocean currents.
• Wind carries fine particles like sand and dust.
• Ice transports a wide range of sediment sizes through glaciers.
• Gravity causes landslides and rockfalls.

Deposition Environments

• Rivers form floodplains, deltas, and channel deposits.
• Lakes accumulate fine-grained sediments.
• Oceans form beaches, shallow marine deposits, and deep-sea sediments.
• Deserts accumulate windblown sand and dust.
• Lithification turns loose sediment into solid rock through compaction and cementation.
• Compaction reduces pore space via the weight of overlying sediments.
• Cementation involves minerals precipitating from groundwater, filling pore spaces. Calcite, silica, and iron oxides are common cementing minerals.

Key Takeaways

• Sedimentary rock formation is a continuous cycle of weathering, erosion, transportation, deposition, and lithification.
• Detrital rocks are classified primarily by particle size.
• Chemical and organic rocks are classified by mineral composition or organic content.

Sediment Sorting

• Sediments are sorted during transportation with larger, heavier particles settling first, and smaller, lighter particles carried further.
• Poorly sorted sediments result from strong currents or sudden slowing, indicative of rapid changes in transport energy.

Conglomerate vs. Breccia

• Conglomerates have rounded particles, indicating longer transport distances.
• Breccia have angular particles, indicating minimal transport from a nearby source.

Biochemical vs. Inorganic Sediments

• Biochemical sediments, formed by organisms, include limestone from marine organisms and dead animals (shells and skeletal remains of calcium carbonate).
• Inorganic sediments, formed without organisms, include rock salt from evaporation.

Limestone, Dolostone, and Chert

• Limestone is composed of calcium carbonate (CaCO₃) and reacts with acid.
• Dolostone is composed of calcium-magnesium carbonate (CaMg(CO₃)₂ and forms through dolomitization.
• Chert is composed of microcrystalline quartz (SiO₂) and is notably hard.

Evaporites

• Evaporites form via the evaporation of water, leaving dissolved minerals behind (e.g., Death Valley, CA).
• Halite (NaCl) is common salt.
• Gypsum (CaSO₄·2H₂O) is used in plaster and drywall.
• Sylvite (KCl) is potassium chloride, used in fertilizers.

Explanation and Elaboration

• Well-sorted sediment consists of particles of similar size, typically found in consistent energy environments such as beaches.
• Poorly sorted sediment contains a wide range of particle sizes, common in rapid-change environments like glacial deposits.
• River channels and shorelines often contain conglomerates where strong currents can transport and round large clasts.
• Breccias are found at the base of cliffs, produced by mechanical weathering or tectonic activity.
• Biochemical sediments often contain fossils for understanding past life and environments.
• Inorganic sediments are created via physical or chemical processes.

Limestone

• Limestone is a very common sedimentary rock, often forming in marine environments and is is used in construction, agriculture, and cement production.

Dolostone

• Dolostone is less common than limestone but forms through alteration of limestone by magnesium-rich fluids.

Chert

• Chert is durable and was used by early humans for tools.
• Chert can form as nodules in limestone or layers in marine sediments.

Evaporites

• Evaporites are economically important as sources of salt, gypsum, and potash and provide insights into past climates and evaporation basins.
• Evaporite formation is complex, depending on water composition, evaporation rate, and temperature.