Sediments and Sedimentary Rocks

Learning Objectives

  • Sediments

    • Describe clastic sediment in terms of size, sorting, and roundness.

    • Explain where and how chemical sediment is formed.

    • Explain where and how biogenic sediment is formed.

  • Sedimentary Rocks

    • Relate the appearance of sedimentary rock to its mode of formation.

    • Summarize three processes that lead to the lithification of sediment.

    • Identify the most common clastic, chemical, and biogenic sedimentary rock types.

    • Explain how features such as ripple marks, cracks, and fossils can tell geologists about the environment in which a rock originated.

    • Describe the concept of sedimentary facies.

Importance of Studying Sediments and Sedimentary Rocks

  • Definition of Sedimentary Rock

    • Composed of compacted grains that have become cemented and lithified, making up approximately 75% of all rocks exposed at the Earth's surface.

  • Reasons for Studying

    • Understand processes of modern environments to decipher ancient environments.

    • Study early life.

    • Locate and determine aquifers for groundwater.

    • Discover economic mineral deposits.

    • Source of energy.

    • Importance in decarbonization strategies.

Definitions

  • Sediment

    • Unconsolidated matter transported on or near the Earth's surface and deposited.

  • Sedimentary Rock

    • Formed by lithification of sediment through compaction and cementation.

Characteristics of Sediments

  • Found as loose particles including sand, silt, and clay.

  • Clastic Sediments

    • Physically deposited by processes such as running water, wind, or ice.

    • Examples lead to the formation of sandstone, siltstone, and shale through lithification.

  • Chemical and Biochemical Sediments

    • Form via precipitation from water after supersaturation is attained.

The Process of Lithification

  • Deposition

    • Weathering products are transported and eventually accumulate or are precipitated from solutions.

  • Burial and Diagenesis

    • Sediments undergo transformation into rock through increasing pressure and temperature, which may lead to dissolution of existing minerals.

    • Dissolved minerals can precipitate and help in cementing individual grains together, with common cementing agents including silica and calcite.

Formation of Organic Materials

  • Fine-grained sedimentary rocks like shale may contain organic matter, which can be transformed into oil or gas through maturation.

  • Oil Generation Temperature

    • Occurs between 40°C to 150°C; higher temperatures cause oil to become unstable and break down into natural gas.

Clastic Sedimentary Rocks

  • Originating from products of mechanical weathering of pre-existing rocks, transported by gravity, water, ice, or wind.

  • Common Types

    • Sandstone

    • Siltstone

    • Shale

Characteristics of Clastic Sediments and Rocks

  • Grain Size Sorting

    • As currents change speed, they lose the largest grain sizes, leading to more uniform sediment sizes.

    • Sorting defined as the tendency for sediment to segregate by size.

    • Well-sorted: Subjected to prolonged water or wind action.

    • Poorly-sorted: Either close to the source or deposited by glaciers.

  • Roundness and Porosity

    • Grain shape indicates transport distance; longer transport results in more rounded grains.

    • Well-rounded grains do not pack as tightly as mixed assemblages, affecting porosity.

Chemical Sediments

  • Composition of Seawater

    • Nearly constant in composition, dominated by Na+ and Cl- with minor SO4²-, Mg²+, Ca²+, and HCO3-.

    • Density of seawater: 1.03 g/cm³ and salinity of 35‰.

  • Precipitation Process

    • Limestone Formation

    • Ca²+ + CO₃²- ⇌ CaCO₃, usually as aragonite or calcite.

    • Precipitation begins when water concentration reaches 40-60‰ and density reaches 1.10 g/cm³.

  • Halite Formation

    • Na+ + Cl- ⇌ NaCl at concentrations of 340-360‰ with a density of 1.22 g/cm³.

Banded Iron Formations (BIF)

  • Indicative of major transitions in Earth's history, particularly related to oxidation events.

  • Consists mainly of iron-rich minerals and SiO₂, with significant formation during the Archean to Paleoproterozoic periods

  • Redox Reactions

    • Reactions involving reduced elements reacting with oxygen, triggering the formation of ferric iron-bearing minerals.

Biological Sediments

  • Historically composed mainly of microbial life; significant examples include stromatolites.

  • Diatoms

    • Organisms contributing to siliceous oozes; the ocean is typically undersaturated with respect to Si.

  • Calcium Carbonate Formation

    • Formed through biological processes involving organisms such as foraminifera and coccolithophores.

Significant Fossil Sites

  • Gunflint Chert

    • Contains ancient microfossils, indicative of early life forms, including cyanobacteria.

  • Historical significance due to its age of 1.9 billion years and being one of the oldest preserved fossil discoveries.