Lecture 2.4 Sedimentary rocks

Introduction to Sedimentary Rocks

• Sedimentary rocks and processes (weathering, erosion, lithification) are crucial to Earth science.

• Majority of Earth's surface is composed of sedimentary rocks and sediments, formed primarily by water.

• Goals for study:

  • List and describe categories of sedimentary rocks.

  • Discuss processes converting sediment into sedimentary rock.

Formation of Sedimentary Rocks

• Origin: Formed from preexisting rocks or organic remains.

• Deposits: Accumulate on Earth's surface, often characterized by distinctive layering and bedding.

• Common Types: Sandstone, limestone, shale. Originates from sediments deposited by rivers in lakes/oceans.

• Lithification: Involves three steps to transform sediment into clastic sedimentary rock:

  1. Deposition: Accumulation of sediment when transport forces (friction/gravity) lag.

  2. Compaction: Pressure from accumulating layers compresses sediments, expelling water.

  3. Cementation: Minerals (calcite, silica, oxides) coat and glue sediment grains into solid rock.

Classification of Sedimentary Rocks

• Two main categories: Clastic and Chemical.

Clastic Sedimentary Rocks

• Definition: Composed of fragments from pre-existing rocks, formed mainly by mechanical weathering.

• Characteristics: Classified by grain shape, size, and sorting.

  • Grain Size:

    • Very coarse: Boulders

    • Coarse: Cobbles

    • Medium: Granules

    • Fine: Sand (2mm to limit of visibility)

    • Very fine: Silt

  • Grain Shape:

    • Conglomerate: Rounded gravel-sized particles, indicating long transport.

    • Breccia: Angular fragments, suggesting minimal transport.

    • Sandstone: Dominated by sand-size particles.

Chemical Sedimentary Rocks

• Formation: Created from dissolved minerals precipitating from water, not involving mechanical processes directly.

• Classification: Based on mineral composition:

  • Inorganic Chemical: Formed without biological influence (e.g., mineral evaporation).

  • Biochemical: Involves biological processes, extracts dissolved materials, creating rocks like limestone.

• Examples:

  • Travertine: Formed from calcite precipitation in caves.

  • Chalk: Biochemical limestone composed of tiny organisms' hard parts.

Importance of Water in Sedimentary Processes

• Role of Water:

  • Main agent in the formation of sediments and sedimentary rocks.

  • Unique properties (polarity, cohesion, universal solvent) facilitate weathering and sediment transport.

Organic Sedimentary Material

• Formation: Organic materials undergo burial, compaction, and diagenesis.

• Coal Formation:

  • Peat -> Lignite (brown coal) -> Bituminous coal -> Anthracite coal (highest grade).

• Coal Distribution:

  • Bituminous is most common in the U.S. (mainly Wyoming); anthracite found in Appalachian region.

Classification of Sedimentary Rock Types

• Monomineralic: Mostly composed of one mineral.

• Exceptions: Limestone has special classifications (Folk, Dunham) based on grain texture and composition.

Sedimentary Structures

• Definition: Textures or arrangements of sediments help interpret formation processes and environments.

• Basic Unit: Bedding planes, separating layers (strata) indicating changes in sediment deposition.

• Stratigraphy: Study of sedimentary layering is crucial to understand geological history.