Sedimentary Rocks, Part III

GL101 General Geology I

Lecture of Friday, 10/31/25

SEDIMENTARY ROCKS, Part III

Outline

  • Siliciclastic Sediments and Sedimentary Rocks (continued)

  • Biological Sediments and Sedimentary Rocks

Reading

  • Text: Chapter 5

SILICICLASTIC SEDIMENTS AND SEDIMENTARY ROCKS (continued)

Particle Size, Shape, and Sorting

In addition to particle size, two other factors are useful indicators of time and distance in transport, and of medium of transport: particle shape and particle sorting.

Particle Shape

  • Sphericity: A measure of how spherical a particle is, which can range from:

    • Very Angular

    • Angular

    • Subangular

    • Poorly Rounded

    • Rounded

    • Very Rounded

  • Observations:

    • Sharp, angular particles may indicate a short time and distance in transport before they were deposited.

    • Smooth, rounded particles suggest a long time and distance in transport.

Particle Sorting

  • Sorting Definition: Sorting refers to the degree to which the individual particles in an accumulation of sedimentary grains are similar to one another.

  • Sorting Classification:

    • Very Well Sorted

    • Well Sorted

    • Moderately Sorted

    • Poorly Sorted

    • Very Poorly Sorted

  • Skewness: A measure of whether or not a particular grain size is favored. Distributions can be multi-modal.

  • Kurtosis: A measure of the flatness or peakedness of the size distribution curve.

Implications of Particle Characteristics

If the individual particles come in a variety of sizes and shapes, for example, ranging from large to small and from sharp & angular to smooth & rounded, this indicates transport by ice (glaciers) or gravity (landslides). Conversely, if the individual particles in an accumulation of sedimentary grains are of a similar size and shape, this indicates transport by water.

Glacial Till

  • Definition: Till is the assorted mixture of fine sediments of rock, sand, pebbles, and boulders deposited by a glacier.

BIOLOGICAL SEDIMENTS AND SEDIMENTARY ROCKS

Definition

  • Biological sediments commonly are accumulations of shells, teeth, or bones.

Examples

  • Common examples are accumulations of shells that typically consist of calcite or aragonite in marine depositional environments.

  • Invertebrate shells and skeletons largely consist of CaCO3 (calcium carbonate). Examples include corals, lithic plankton, clams, and oysters.

  • Notably, corals and coral reefs provide spectacular examples of these accumulations.

Biological Sedimentation Process

When biological sediments undergo lithification, they form biological sedimentary rocks such as biological limestones.

Specific Biological Sedimentary Rocks

  • Coquina: A type of limestone composed mainly of shell fragments.

  • Calcarenite (Skeletal Limestone): Composed of carbonate sands often derived from mollusks and foraminifera.

  • Chalk: Composed primarily of the calcareous remains of microscopic organisms.

  • Micrite: A fine-grained carbonate rock composed mainly of calcium carbonate.

Fossil Record of Coral

  • Corals have a long fossil record that extends back approximately 500 million years into the geological past, representing significant periods in various geological eras, including the Precambrian, Paleozoic, Mesozoic, and Cenozoic periods.

Coral Reefs and Paleoclimate Indicators

  • Reef Limestone/Coral Limestone: Features coral fossils preserved in their life position, and represents tropical or subtropical shallow water marine deposits, reacting with dilute hydrochloric acid. Corals also formed the living upper part of a reef complex.

  • Thus, ancient coralline and related limestones serve as paleoclimate indicators, revealing that the area where they formed had a warm subtropical or tropical climate, akin to modern Bahama Islands.

Environmental Conditions for Biological Sediments

  • Biological sediments often form under specific climatic conditions.

  • For instance, coral reefs predominantly develop in subtropical and tropical environments.

Plant Remains in Biological Sediments

  • Biological sediments may additionally include accumulations of plant remains or residues, with an important example being peat in peat bogs.

  • Under suitable geological conditions, the mechanisms of heat and pressure from deep burial lead to the conversion of peat into the biological sedimentary rock known as coal:

    1. Peat formation: Initial accumulation of decaying plants.

    2. Layering: Overlying sediments press down on peat.

    3. Transformative Pressure & Heat: Over many years, pressure and heat convert peat into coal, resulting in various coal types such as lignite, bituminous, sub-bituminous, and anthracite.

Coal Formation from the Carboniferous Period

  • Through this transformation process, the extensive coal swamp forests of the Carboniferous Period, around 300 million years ago, have contributed to the vast majority of the Earth's coal deposits.

Biological Sediments’ Climate Associations

  • Many biological sediments tend to manifest in distinctive climates, specifically coral reefs in tropical and subtropical settings.

Example of a Biological Sedimentary Rock

  • As a concluding thought, an example of a biological sedimentary rock is chalk.

Visual References

  • Illustrations and diagrams such as coral anatomy and sedimentary structures aids in conceptual understanding.

Conclusion

These sedimentary processes provide insights into geological history and climate change over time, making them critical components of geological studies.

Note: These structured notes can serve as a comprehensive resource and reference for studying Siliciclastic and Biological Sediments and their associated sedimentary rocks in GL101, General Geology I.