ERTH 101: Exploring Planet Earth

ERTH 101: Exploring Planet Earth

Week 6: Sedimentary Rocks

Instructor: Dr. Alan Rempel

Overview of Sedimentary Rocks

  • Sedimentary rocks are primarily formed through processes that involve the breakdown, transportation, and lithification of sediments.

  • The cycle of sedimentary rock formation is outlined in five steps:

    1. Weathering

    2. Erosion

    3. Transportation

    4. Deposition

    5. Lithification

Weathering

  • Definition: Weathering refers to the combination of processes that break up and corrode solid rock.

  • Subtypes of Weathering:

    • Physical Weathering: Involves the mechanical breakdown of rocks without chemical changes.

    • Chemical Weathering: Involves the chemical alteration of minerals within rocks.

Physical Weathering Processes

  • Jointing: Natural cracks formed in rocks.

    • Causes include thermal expansion/contraction, tectonic unloading, and pre-existing planes of weakness.

  • Exfoliation: The peeling away of outer rock layers due to thermal expansion and pressure release.

  • Root Wedging: Plant roots can exert pressure and lead to the breakdown of rocks.

  • Frost Wedging: The expansion of water as it freezes can fracture rocks.

  • Frost Cracking: Ice growth segregates and propagates cracks, often occurring under saturated conditions.

Chemical Weathering Processes

  • Dissolution: Minerals dissolve in water, particularly when carbonic acid is present (common in limestone).

    • Example: Carbonic acid in groundwater dissolves limestone, leading to features like stalactites and stalagmites.

  • Hydrolysis: A reaction involving water that alters minerals.

    • Example: Potassium feldspar reacting with hydrogen ions and water transforms into kaolinite and releases potassium ions and silica.

    • Reaction: 2KAlSi<em>3O</em>8+2H++H<em>2OightarrowAl</em>2Si<em>2O</em>5(OH)<em>4+2K++4SiO</em>22KAlSi<em>3O</em>8 + 2H^+ + H<em>2O ightarrow Al</em>2Si<em>2O</em>5(OH)<em>4 + 2K^+ + 4SiO</em>2

Biological Weathering Agents

  • Organisms play a significant role in weathering by:

    • Producing organic acids that help dissolve minerals.

    • Physically breaking down rock through root systems, fungi, lichens, and bacteria.

  • Different minerals weather at different rates influenced by temperature and pressure.

    • Order of weathering: Olivine (1400°C), Pyroxene (1050°C), Amphibole, Biotite, K-Feldspar (800°C), Muscovite, Quartz (low temperature).

Erosion

  • Definition: Erosion is the process of separating detritus (particles and clasts) from their original substrate.

  • Agents of erosion include wind, water, ice, and gravity.

  • Example: An intact rock can become talus through erosion.

Transportation

  • Definition: The movement of eroded materials away from their source.

  • The effectiveness of transportation depends on the velocity and viscosity of the transporting medium (e.g., water in rivers).

Deposition

  • Definition: Deposition occurs when the transported detritus settles out from the transporting medium.

Lithification

  • Definition: The transformation of loose detritus into solid rock, involving two primary processes:

    1. Compaction – Reduction of pore space due to pressure.

    2. Cementation – Minerals precipitate from water and fill voids between clasts, binding them together.

Summary of Sedimentary Rock Formation

  1. Weathering

  2. Erosion

  3. Transportation

  4. Deposition

  5. Lithification

Types of Sedimentary Rocks

  • Four main classes of sedimentary rocks:

    1. Clastic Sedimentary Rocks: Formed from clasts/cemented fragments.

    2. Biochemical Sedimentary Rocks: Composed of biological remnants like shell debris.

    3. Organic Sedimentary Rocks: Form from organic material, example: coal.

    4. Chemical Sedimentary Rocks: Form from the precipitation of minerals from solution.

Clastic Sedimentary Rocks

  • Composition: Made of clasts or detritus that is cemented together.

Characteristics Used to Classify Clastic Rocks

  1. Clast/Grain Size

  2. Angularity and Sphericity

  3. Sorting

  4. Sediment Maturity

  5. Clast Composition

  6. Cement Character

Clast Size Classification

  • Classification Table:

    • Coarse to very coarse (>2 mm): Conglomerate, Breccia, Diamictite

    • Medium to coarse (0.06-2 mm): Sandstone (Quartz sandstone, Arkose, Lithic sandstone)

    • Fine (0.004-0.06 mm): Siltstone

    • Very fine (<0.004 mm): Shale (if it breaks into platy sheets) or Mudstone (if it doesn't)

Clast Composition

  • Examples:

    • Quartz rich: Quartz sandstone

    • Feldspar: Arkose

    • Mixed fragments in clay: Wacke

    • Calcium carbonate: Conglomerate

Angularity and Sorting

  • Angularity:

    • Types: Angular, Subangular, Subrounded, Rounded.

  • Sorting: Describes the uniformity of grain size in sedimentary rocks.

    • Well-sorted: All grains are similar in size.

    • Poorly-sorted: A mix of different grain sizes.

Sediment Maturity

  • Refers to the degree to which sediment has evolved based on grain size, sorting, shape and composition.

Lithification Examples

  • Feldspar-rich sand converts into Arkose sandstone.

  • River gravel can lithify into Conglomerate.

  • Shale forms from mud.

Biochemical Sedimentary Rocks

  • Characteristics:

    • Contains calcium carbonate (CaCO3).

    • Forms in tropical, shallow waters and consists mostly of shell debris.

Classification of Biochemical Rocks

  • Examples:

    • Micrite and Fossiliferous limestone based on the Dunham classification (Mudstone, Wackestone, Packstone, Grainstone, Boundstone).

Organic Sedimentary Rocks

  • Composed of organic carbon.

    • Examples: Coal (derived from vegetation), Oil shale (heat-altered organic matter).

Chemical Sedimentary Rocks

  • Formed from the precipitation of minerals from solutions.

  • Examples:

    • Travertine (calcium carbonate formations), Evaporites (gypsum, halite).

Sedimentary Structures

  • Types:

    • Layers (bedding and stratification)

    • Sorting (arrangement of grains)

    • Impressions (e.g., footprints, scours, dehydration cracks).

Bedding and Stratification

  • Definitions:

    • Bed: A single layer of sediment

    • Bedding Plane: Boundary between two beds

    • Strata: Multiple beds together

    • Stratigraphy: Overall arrangement of sediment and beds.

Causes of Bedding

  • Changes in transporting media or sediment sources can affect grain sizes and compositions, leading to layering.

Depositional Environments

  • Sediments can accumulate in different environments resulting in varied rock types.

  • Terrestrial Environments: Conglomerates, Arkosic sandstones, etc.

  • Marine Environments: Carbonates and lime mud.

  • Transgression and Regression: Inward and outward migration of shorelines due to sea level changes.

Key Takeaways

  • Sedimentary rocks vary widely based on clast size, mineral composition, sorting, angularity, and depositional environments.

  • Understanding these concepts is essential for interpreting geological history and processes in Earth sciences.