GEO CH.7 - Sedimentary Rocks & Sedimentary Environments

Sedimentary Rocks and Sedimentary Environments

Introduction to Sedimentary Rocks

  • Sedimentary Rocks: Products of mechanical and chemical weathering.

    • A geologist interprets sedimentary rocks as surface layers covering an underlying metamorphic basement.

Processes from Sediment to Rock

  • Key Processes Involved:

    • Weathering: Disintegration of bedrock into grains or sediments.

    • Erosion: Involves processes such as abrasion, falling, and plucking.

    • Transportation: Gravity, wind, water, and ice carry sediments away.

    • Deposition: Sediments settle out of water or other mediums.

    • Diagenesis & Lithification: Transformation of loose sediments into solid rock.

    • Abrasion Definition: The process of scraping or wearing away.

Lithification Process

  • Lithification: Involves several sub-processes:

    • Compaction: Caused by overburden pressure.

    • Weight of overlying material compresses sediments, reducing their volume.

    • Cementation: Mineral-rich fluids (like calcite, silica, and iron oxide) fill spaces between sediment grains, binding them together.

    • Diagenesis: Refers to the chemical, physical, and biological changes that occur to sediments after they are deposited, altering them into solid rock.

Effects of Diagenesis

  • Effects include:

    • Grains being squeezed together, leading to…

    • Reduction in Porosity:

      • Space between grains, especially in silt and clay, is significantly reduced during compaction (from about 50% down to about 5-10%).

    • Cementation: Fluids that are rich in minerals start to solidify the sediment into rock.

Classification of Sedimentary Rocks

A) Based on Textures

  • Textures:

    • Clastic: Grains are held together by cement, which can include calcite, silica, and iron oxide.

    • Nonclastic: Composed of interlocking crystals.

B) Based on Sediment Transport

  • Detrital Rocks: Formed from sediment transported as solid particles.

  • Chemical Rocks: Formed from sediments that were dissolved in solution.

Common Types of Detrital Rocks

  • Shale: Mud-sized particles; most common.

  • Sandstone: Comprised primarily of sand-sized grains, typically dominated by quartz.

  • Conglomerate: Contains particles greater than 2 mm in diameter.

    • Conglomerate: Rounded particles.

    • Breccia: Angular particles.

Particle Size of Detrital Sedimentary Rocks

  • Size Classification (in mm):

    • > 256 mm: Boulder

    • 64-256 mm: Cobble

    • 4-64 mm: Pebble (Gravel as Conglomerate or Breccia)

    • 2-4 mm: Granule

    • 1/16-2 mm: Sand

    • 1/256-1/16 mm: Silt (Mud)

    • < 1/256 mm: Clay

Chemical Sedimentary Rocks

  1. Limestone: Composed of calcite ($CaCO_3$) and forms from a variety of processes:

    • Organic Limestone: Forms from biological processes such as coral reefs or broken shells (e.g., coquina).

    • Inorganic Limestone: Forms through precipitation of $CaCO_3$ from water, such as travertine and oolitic limestone.

  2. Dolostone: Formed from limestone ($CaCO3$) which is replaced by dolomite ($CaMg(CO3)_2$).

  3. Chert: Includes types such as:

    • Replacement Chert: Silica replaces calcite in limestone.

    • Chert from Petrified Wood: Silica replaces cellulose.

    • Chert (Agate): Siliceous matter fills voids in rocks.

  4. Evaporite: Result from evaporation which leads to deposition of solutes; includes rock salt and rock gypsum.

  5. Coal: Organic rock with composition from 50% to 90% Carbon; forms through stages:

    • Plant Material -> Peat -> Lignite -> Bituminous -> Anthracite (high energy).

Summary of Detrital and Chemical Sedimentary Rocks

  • Detrital Sedimentary Rocks: Characterized by clastic texture, described by particle size.

  • Chemical Sedimentary Rocks: Involve compositions regarding the origin of sediments (e.g., crystalline), such as limestone and various forms of coal.

Sedimentary Environments

1) Continental Environments

  • Types of deposits include:

    • Glacial Deposits: Till from glaciers.

    • Mountain Stream Deposits: Generally turbulent erosive and depositional environments.

    • Alluvial Fans: Wedge-shaped deposits formed in arid regions due to water transport.

    • Wind Deposits: Sand dune formation (Aeolian deposits).

    • River and Lake Deposits: Sedimentary accumulations in fluvial and lacustrine systems.

2) Marine Environments

  • Comprises shallow marine (up to ~200 m depth) and deep marine deposits beyond continental shelves.

3) Transitional Sedimentary Environments

  • Shoreline Features:

    • Tidal Flats: Coastal areas that are alternately submerged and exposed.

    • Lagoons: Quiet waters sheltered by land, often leading to calcite mud and sand deposits.

    • Deltas: Areas where rivers discharge sediments into larger bodies of water.

Sedimentary Structures

  • Definition: Layering within sedimentary rocks indicates deposition clues about environments.

  • Types of Sedimentary Structures:

    1. Ripple Marks: Elongated ridges formed by water currents that indicate direction of flow.

    2. Dunes: Larger versions of ripple marks, shaped by wind.

    3. Bedding Planes: Boundaries between different layers of sediment.

    4. Cross-Bedding: Inclined layers formed due to sediment movement on ripples or dunes.

    5. Grading: Fine grains at the top and coarser grains at the bottom, often due to turbidity currents.

    6. Mud Cracks: Cracks formed in drying mud layers, typically exhibiting a hexagonal pattern.

Sedimentary Basins

  • Definition: Areas where sediments accumulate as the lithosphere sinks.

Types of Basins:

  1. Rift Basins: Formed at divergent boundaries due to tectonic stretching.

  2. Intercontinental Basins: Initially rift zones located deep inland.

  3. Foreland Basins: Result from crustal flexing in responses to plate collisions.

  4. Passive Margin Basins: Develop as a result of subsidence in regions far from active plate boundaries.