L3 - Sedimentary Rocks: Classifications and Characteristics

Overview of Sedimentary Rocks

• Sedimentary rocks are formed from sediments derived from pre-existing rocks or biological processes.

• Different classifications exist for sediments and sedimentary rock types.

Types of Sediments

• Clastic Sediments:

  • Formed from fragments of pre-existing rocks.

  • Examples:

  • Conglomerate: Comprised of rounded particles (pebbles).

  • Breccia: Composed of angular particles.

  • Sandstone: Made up of sand-sized grains.

  • Mudstone: Contains fine particles and clay.

• Biogenic/Biochemical Sediments:

  • Originated from biological activity or organic matter.

  • Example: Chalk, formed from microcalcifiers in the ocean.

  • Other types include limestone, dolostone, cherts, phosphates, coals, and shales.

• Chemical Deposits:

  • Minerals that precipitate from solutions.

  • Examples include evaporites and banded iron formations.

• Volcanic Clastics:

  • Composed of fragments from volcanic rocks, e.g., ignimbrites and tuffs.

  • Not covered in this course, but acknowledged as sedimentary.

Understanding Sediment Characteristics

• It is crucial to describe and classify sediments to infer environmental conditions and transport processes.

• Consider grain size, shape, packing, and spatial relationships.

Key Terms and Measurements

• Grains/Clasts: Particles found in sedimentary rocks; often measured in millimeters.

• Pore Space: Spaces between grains; can be filled with gas or liquid.

• Cement: New minerals that form in pore spaces and help bind sediments together.

• Matrix: Finer materials filling gaps between larger grains.

Techniques for Describing Sediments

• Measuring Grain Size: Use major axis length or Udden-Wentworth classification for numerical descriptions.

  • Coarse grains range from gravel to sand (over 2 mm) to fine grains (silt, clay).

• Sorting and Skewness: Analyze grain size distributions for uniformity and shape average.

  • Well Sorted: Similar shapes and sizes.

  • Poorly Sorted: A wide variety of grain sizes.

  • Moderate Sorting: Intermediate between well and poorly sorted.

Classifying Sedimentary Rocks

• Utilize a ternary diagram to classify rock based on gravel, sand, and mud percentages.

  • Mixed samples can create terms like "muddy conglomerate" or "sandy conglomerate."

• Textural Maturity:

  • Immature: High proportion of matrix, poorly sorted, angular grains.

  • Mature: Low matrix, well sorted, rounded grains.

Summary of Practical Techniques

• In the upcoming practical sessions, observe, sketch, and describe sediment samples using hand lenses.

• Measure grain size distributions, classify using diagrams, and identify cement types.

• Pay attention to grain shape (roundness, sphericity) to assess transport history and environmental context.

Importance of Sediment Analysis

• Understanding sorting and distribution helps infer past environmental conditions.

• This knowledge aids in interpreting geological records and current processes involved in sediment transport and deposition, influencing broader geologic and climatic studies.