Sedimentary Rocks: Formation and Properties

Key Concepts

• Water is crucial for all sedimentary rock formation.
• Weathering, both chemical and mechanical, transforms bedrock into sediment.
• Sedimentary rocks are divided into clastic rocks (weathered bedrock pieces) and chemical rocks (precipitates).
• Analysis of sedimentary structures offers insight into Earth's history and depositional environments.
• Sedimentary rocks, formed through weathering, erosion, and lithification, are vital for understanding Earth Science.
• Water is essential in creating minerals found in chemical sedimentary rocks.
• Sedimentary rocks and sediments make up a majority of the Earth's surface.

The Unique Properties of Water

• Water molecules have polarity because of their atomic arrangement, with positively charged hydrogens on one side and a negatively charged oxygen on the other.
• Water's polarity leads to adhesion, allowing it to stick to other substances.
• Cohesion, where water is attracted to itself, results in surface tension and droplet formation.
• Liquid water is denser than solid water, a rare property due to hydrogen bonds formed by polarity.
• Water is densest at 4°C, and less dense above and below that temperature.
• Water expands when it freezes due to molecules moving apart to fit into a crystal lattice.
• Ice floats, and 4°C water sinks, preventing oceans from freezing solid from the bottom up.
• Hydrogen bonding gives liquid water the ability to absorb high amounts of energy before turning into vapor or gas.
• Water remains liquid over a wide temperature range (0°C-100°C/32°F-212°F) due to cohesion.
• Water is a universal solvent that uses polarity and hydrogen bonds to dissolve substances, including rocks and minerals.

Weathering and Erosion

• Bedrock is solid rock in the Earth’s outer crust.
• Weathering turns bedrock into sediment via mechanical and chemical processes.
• Erosion transports sediment, usually via water, wind, gravity, or ice.
• Liquid water serves as the main agent of erosion.
• Erosion resistance shapes geological features like cliffs in the Grand Canyon and hoodoos in Bryce Canyon.

Mechanical Weathering

• Mechanical weathering breaks bedrock into smaller pieces through pressure, temperature, freezing/thawing, plants/animals, and salt evaporation.

Pressure Expansion

• Pressure expansion happens when bedrock rises to the surface and experiences a sudden pressure drop, causing it to expand and crack.
• Sheeting or exfoliation is when the rock surface spalls off in layers.
• Spheroidal weathering involves chemical weathering along joints in the bedrock, producing rounded features.

Frost Wedging

• Frost wedging involves water entering cracks, freezing, expanding, and prying rocks apart through repeated cycles.

Root Wedging

• Root wedging occurs when plant roots grow into cracks and split rocks apart.
• Rhizoliths are fossilized roots preserved in the rock record.

Salt Expansion

• Salt expansion occurs in areas with high evaporation, where salts precipitate and expand in rock cracks.
• Salt expansion causes tafoni (holes in rock) and hopper crystals (square imprints in soft sediment).

Chemical Weathering

• Chemical weathering is dominant in warm, humid environments and chemically degrades bedrock minerals into water-soluble ions.
• Higher temperatures increase chemical weathering rates.
• Chemical and mechanical weathering work together because mechanical weathering increases surface area for chemical weathering.

Carbonic Acid and Hydrolysis

• Carbonic acid (H2CO3) forms when carbon dioxide dissolves in water and aids in hydrolysis and dissolution, which are two chemical weathering reactions.
• Hydrolysis involves water molecules or carbonic acid reacting with minerals to form clay minerals.
• Hydrolysis of silicate rock produces clay minerals and dissolved substances that can precipitate into chemical sedimentary rocks.

Dissolution

• Dissolution is a hydrolysis reaction that dissolves minerals, especially evaporites and carbonates.
• Acidic water accelerates dissolution, and biological agents like lichen can enhance it.
• Goldich Dissolution Series: Minerals that crystallize at high temperatures and pressures weather faster.
• Quartz is very resistant to chemical weathering, while olivine and pyroxene weather rapidly.
• Dissolution creates karst topography with sinkholes and caves.

Oxidation

• Oxidation is the chemical reaction of iron atoms in minerals bonding with oxygen.
• Oxidation produces hematite (red/grey), goethite (brown), and limonite (yellow).
• Iron oxides coat and bind mineral grains in cementation, coloring rocks and creating voids.

Erosion

• Erosion mechanically removes sediment via water, gravity, wind, or ice.
• Liquid water is the primary erosion agent.
• Erosion resistance shapes geological features.

Soil

• Soil consists of air, water, minerals, and organic matter at the interface of the biosphere and geosphere.
• Weathering breaks down bedrock into sediment, which then interacts with organisms to form soil.
• Soil is a reservoir for organic components needed by plants, animals, and microorganisms.
• Humus, the organic part of soil, is a source of bioavailable nitrogen due to nitrogen-fixing bacteria.
• Soil productivity depends on water and nutrient content.
• Soil characteristics are determined by parent material mineralogy, topography, weathering, climate, and inhabiting organisms.
• Soil horizons are distinct layers in well-formed soils.
  • O Horizon: Organic material actively decaying into humus.
  • A Horizon: Topsoil with humus and mineral sediment; Horizon E is a leaching zone in wet climates.
  • B Horizon: Subsoil with sediment and humus.
  • C Horizon: Substratum with physically broken bedrock fragments.
  • R Horizon: Unweathered parent bedrock.
• USDA soil classification: Xoxisols (tropical, nutrient-poor, aluminum ore source)
• Ardisols (dry climates, hardened calcite layers)
• Andisols (volcanic ash origin)
• Alfisols (silicate clay minerals).
• Black soils are anoxic, red are oxygen-rich, and green are oxygen-poor.
• Human activities can damage soil, such as the Dust Bowl disaster caused by poor farming practices.

Sedimentary Rocks

• Sedimentary rock are divided into clastic and chemical.
• Clastic rocks come from weathered bedrock pieces.
• Chemical sedimentary rocks come from water precipitates.

Lithification and Diagenesis

• Lithification turns sediment into clastic rock through deposition, compaction, and cementation.
• Diagenesis is a low-temperature metamorphism that alters sediments chemically.
• Diagenesis can transform minerals (e.g., aragonite to calcite) and reduce pore space.
• Diagenesis encompasses the processes that turn organic material into fossils.

Detrital Sedimentary Rocks (Clastic)

• Detrital/clastic sedimentary rocks come from weathered bedrock pieces.

Grain Size

• Detrital rock classification depends on sediment grain size using the Wentworth scale.
• Grain size ranges from boulders to clay, with sand between 2 mm and 0.0625 mm.
• Silt grains can be felt but not seen.

Sorting and Rounding

• Sorting defines the range of grain sizes (well-sorted has narrow range; poorly sorted has wide range).
• Sorting indicates erosion, transport processes, and deposition energy.
• Rounding happens when corners are removed from sediment during transport.
• Rounding reveals erosion time, transport distance, or energy.

Composition and Provenance

• Composition describes the mineral components.
• Quartz is the most common sediment mineral.
• Lithic fragments are pieces of fine-grained bedrock.
• Provenance is the original sediment source determined by mineral composition, fossils, and texture.
• Zircon (ZrSiO4) in quartz sandstone can be used for age-dating the source bedrock.

Classification of Clastic Rocks

• Clastic rocks are classified by grain size.
• Coarse-grained rocks: Conglomerates (rounded clasts) and breccias (angular clasts) in a matrix.
• Medium-grained rocks: Sandstone (or arenite if well sorted) with varied mineral compositions.
  • Quartz sandstone with mostly quartz.
  • Arkose with significant feldspar.
  • Greywacke with muddy matrix or lithic fragments.
• Fine-grained rocks: Mudstone, shale, siltstone, and claystone.
  • Shale separates into thin sheets.
  • Siltstone and claystone are composed of silt or clay sediment, respectively.

Chemical, Biochemical, and Organic

• Chemical sedimentary rocks don't directly involve mechanical weathering.
• Biochemical and organic sediments are clastic but chemically produced.

Inorganic Chemical

• Inorganic chemical sedimentary rocks are made of minerals precipitated without living organisms.
• Water evaporation leads to precipitation of evaporites (salts).
• Deposition order depends on mineral solubility; less soluble minerals precipitate first.
• Calcium carbonate precipitates tufa in saline lakes.
• Cave deposits like stalactites and stalagmites form travertine.
• Banded iron formations formed when oxygenation caused iron to precipitate, alternating with chert layers.
• Chert is produced from silica (SiO2) precipitated from groundwater.
• Oolites form when calcite precipitates around a nucleus in concentric layers.

Biochemical

• Biochemical sedimentary rocks use biological processes to extract dissolved materials.
• Shells are turned into limestone (CaCO3), which reacts with hydrochloric acid.
• Fossiliferous limestone contains visible fossils.
• Coquina is made of loosely cemented shells.
• Chalk contains coccolithophore shells.
• Biogenetic chert forms from microscopic organic shells (ooze) on the deep ocean floor.

Organic

• Organic sedimentary rocks come from organic material that is deposited and lithified.
• Plant and animal remains form coal, limestone, oil shale, and other organic rocks.

Classification of Chemical Sedimentary Rocks

• Chemical, bochemical, and organic sedimentary rocks are classified by mineral composition.
• Rocks made of halite are called rock salt.
• Limestone (calcite) has subclassifications.
  • The Folk Classification deals with rock grains and usually requires a specialized, petrographic microscope.
  • The Dunham Classification is based on rock texture, which is visible to the naked eye or using a hand lens and is easier for field applications.
• Most carbonate geologists use the Dunham system.

Sedimentary Structures

• Sedimentary structures are visible textures in rocks that help geologists understand rock formation and environments using uniformitarianism.

Bedding Planes

• Bedding planes are planes separating layers in sedimentary and volcanic rocks, representing changes in sediment deposition.
• Each layer is a bed or stratum.
• Varves are bedding planes in repetitive cycles.

Graded Bedding

• Graded bedding has increasingly coarse or fine sediment layers.
• Bouma sequences are observed in turbidite.

Flow Regime and Bedforms

• Bedforms are sedimentary structures created by fluid systems on sandy sediment.
• Flow regime are the flow pattern and velocity.
• Plane beds are flat, parallel layers.
• Ripples are ridges or undulations.
• Dunes are large versions of ripples with cross bedding.
• Herringbone cross bedding happens in tidal areas with changing flows.
• Hummocky cross stratification forms during strong storm winds.
• Antidunes form in fast-flowing upper flow regimes.

Bioturbation

• Bioturbation is the disruption of bedding layers by organisms burrowing through sediment.

Mudcracks

• Mudcracks form in clay-rich sediment that dries out.

Sole Marks

• Sole marks are small features at the base of a bed that indicate flow direction.
  • Flute casts/scour marks are grooves carved by fluid flow.
  • Groove casts are regular and aligned shapes caused by debris.
  • Tool marks come from objects embossed in the sediment.
  • Load casts are indentations of coarse sediment into finer sediment.

Raindrop Impressions

• Raindrop impressions are pits or bumps in soft sediment.

Imbrication

• Imbrication is a stack of clasts that are aligned in the direction of fluid flow.

Geopetal Structures

• Geopetal structures indicate the original "up" direction.
  • Vugs are small voids filled in stages.
  • Cross bedding is when one bed interrupts another below.
  • Ripples/dunes are differentiated by crests and troughs.
  • Fossils in life position are useful for recognizing their original positions.

Depositional Environments

• Stratigraphy studies aim to understand depositional environments.

Marine

• Marine depositional environments are submerged in seawater.
  • Abyssal: Fine-grained sediment (calcareous oozes, siliceous oozes, pelagic clay) on the abyssal plain.
  • Submarine Fans: Turbidite deposits with graded Bouma sequences.
  • Continental Slope: Contourites from deep-water currents.
  • Lower Shoreface: Finely laminated sediment with hummocky cross-stratification.
  • Upper Shoreface: Well-sorted sand and planar bedding.

Transitional coastline environments

• Sea-level fluctuations cause transgression (rising) and regression (falling). Main environments consist of littoral zones, tidal flats, reefs, lagoons, and deltas.
  • Littoral: Highly weathered, well-sorted sand beaches.
  • Tidal Flats: Regularly flooded mud and sand with mudcracks and ripple marks.
  • Reefs: Topographic barriers with fine-grained sediment, mostly carbonate.
  • Lagoons: Fine-grained sediment in small seawater bodies located inland, often include bioturbation marks or coal deposits.
  • Deltas: River-dominated, wave-dominated, and tide-dominated river deposits, located where rivers enter lakes or oceans,

Terrestrial

• Terrestrial depositional environments include fluvial, alluvial, lacustrine, paludal, aeolian, and glacial systems.
  • Fluvial: Channels of water. (either meandering or braided streams).
  • Alluvial: Intermittent water flow of sediment from dry valleys into open areas with alluvial fans.
  • Lacustrine: Lake systems of thinly laminated fine grained sediment.
  • Paludal: Bogs, marshes, swamps, in humid, low-lying zones which consists of coastal environments.
  • Aeolian: Windblown sediments (fine dust to sand), eolian systems consist of formations like large dunes or loess from glaciers.
  • Glacial: Poorly-sorted sediment including some rock flour and giant erratic boulders.

Facies

• Facies (lithofacies) classify sedimentary rock by depositional characteristics.
• Sedimentary facies analyzes sedimentary rock to interpret the original deposition of envionrment. Biological facies examples include fossils.

Summary

• Sedimentary rocks are split into clastic/detrital and chemical.
• Clastic/detrital rocks are sediment that is lithified to create a solid, while being classified by grain size.
• Chemcial sedimentary comes from minerals that have been precipitated.
• Chemical sedimentary is classified according to it's mineral composition.
• Sedimentary structures provide detail on the rocks dispositional histories.
• To determine the paleogeographic history of a region, Geologists analyze rock records, sedimentary structures, and depositional environment