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Sedimentary Rocks and the Production of Sediment. Sediment and Rocks Formed at the Earth’s Surface . Sedimentary Rocks : Introduction. Occurrence: Cover ~66% of continental surface & most of ocean floor Average thickness of 1800 m on continents Importance: Numerous types of ore deposits:

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Sedimentary Rocks and the Production of Sediment

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Sedimentary Rocks and the Production of Sediment

Sediment and Rocks Formed at the Earth’s Surface


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Sedimentary Rocks: Introduction

  • Occurrence:

    • Cover ~66% of continental surface & most of ocean floor

    • Average thickness of 1800 m on continents

  • Importance:

    • Numerous types of ore deposits:

      • Placer (Au, Ag, Pt)

      • BIF (Fe)

      • Laterite (Fe, Al)

    • Building materials

      • Cement (limestone)

      • Plaster (gypsum)

      • Salt (halite)

    • Host oil, gas, and groundwater


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Sedimentary Rocks: Review

  • With your neighbors, discuss:

    • What processes are involved in the production of sediment?

      A. What is the difference between weathering and erosion?

      B. Describe the two main types of weathering.

    • What processes are involved in the creation of sedimentary rock from sediment?

    • How do processes involved in making sedimentary rocks represent yet another aspect of fractionation?


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Sedimentary rocks are those rocks which form at or near the earth's surface primarily through:

Deposition of weathered silicate material by water, wind, or ice (detrital, terrigenous)

Direct inorganic chemical precipitation from water

Precipitation by organic processes

Sedimentary Rocks


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Sediment Source

Provenance (place of origin)

Weathering (physical and chemical) of pre-existing igneous, sedimentary, or metamorphic rocks

Depositional Basin

Low-lying area in which sediment is deposited

Oceans = largest basins (70% of Earth’s surface)

Diagenesis

= physical, chemical, and biological changes affecting sediment and sedimentary rock

Compaction squeezes grains together, reducing pore space

Recrystallization by dissolution and precipitation of minerals

Cementation in which new minerals precipitated in pore spaces bind sediment together, forming rock

New minerals may replace minerals in original rock

Sedimentary Rocks: Formation


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Sedimentary Rocks: Weathering

  • Surficial fractionation/differentiation of crustal material

Sandstone

SiO2

Quartz

Approximate composition

Of “Granite”

Carbonate

Minerals

Clay Minerals

Carbonates

Al2O3

Mudstone

CaCO3 & CaMg(CO3)2


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Sedimentary Rocks

  • T=Terrigenous

    • Residual and secondary weathering products (siliciclastic)

    • Shale, sandstone, conglomerate

  • O= Orthochemical

    • Primary chemical precipitation from dissolved ions

    • Chert, evaporites

  • A= Allochemical

    • Biochemical particles, shell fragments

    • Most limestones

  • Three types of sediment:

IO= Impure orthochemical

IA= Impure allochemical


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Sedimentary Rocks: Weathering

  • Weathering:

    • Surface portion of Earth’s great “recycling” machine

    • Physical and chemical processes that break down rocks to form

      • “Regolith” from pre-existing

        • Igneous rocks

        • Metamorphic rocks

        • Sedimentary rocks

        • Exposed at the surface


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Weathering: Physical

  • Physical (mechanical) weathering

    • Rather minor producer of sediment particles

    • Reduces grain size of materials

    • Increases surface area of rock

    • Major types:

      • Frost Wedging - expansion/contraction by water freezing/thawing

      • Unloading - sheeting, exfoliation

      • Jointing – growth of regularly spaced fractures

      • Thermal Cycling – expansion & contraction due to temperature change

      • Biological activity


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Weathering: Chemical

  • Chemical weathering

    • Chemical reactions that break down rocks to form ions in solution and new minerals

    • Rocks are not in equilibrium with surface conditions

    • Main agents are water and weak acids formed in water

    • Major types:

      • Oxidation/Reduction – change in valence of ion (commonly Fe)

      • Hydrolysis – H+ or OH- replaces ion in the mineral

      • Leaching – ions removed by dissolving in water

      • Dehydration – removal of OH- ion from mineral

      • Complete dissolution – all of the mineral dissolved in water


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Least stable (high temperature minerals)

Olivine Ca plagioclase feldspar

Pyroxene

Amphibole

Biotite Na plagioclase feldspar

Potassium feldspar

Muscovite

Quartz

Most stable (low temperature minerals)

Chemical Weathering

  • Rocks are not in equilibrium with surface conditions

    • Lower T and P conditions

    • More free water and O2

  • Goldich weathering stability series

    • Na+, Ca+2, Mg+2, K+, Si+4 : More Soluble

    • Fe +3, Al+3: Less soluble


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Oxidation: loss of electrons by a metal

4Fe+2 + 302 2Fe+32O3

2Fe+2SiO3 + (1/2)O2 Fe2O3 + 2SiO2

Pyroxene + Oxygen  Hematite + Quartz

Produces iron (and other) oxide(s)

Chemical Weathering


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Produces secondary minerals and residual (resistate) particles

Releases ions into solution (leaching)

Chemical Weathering

  • Hydrolysis

    • 4KAlSi3O8 + 4H+ + H2O  4K+ + Al4Si4O10(OH)8 + 8SiO2(aq)

    • K-spar + acid + water  K+aq + Kaolinite (clay) + silica


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Chemical Weathering

  • Dissolution

    • CaCO3 + H2CO3 Ca2+ + 2HCO3-

    • Calcite + Carbonic acid  Calcium ion + Bicarbonate ion

    • Produces ions in solution

    • Reactions typically aided by weakly acidic ground water:

      • Acid = solution with abundant free H+ ions

      • Most common weak acid in surface waters is carbonic acid, produced by reaction of water with carbon dioxide

      • CO2 + H2O  H2CO3 (Carbonic Acid)

      • H2CO3 H+ + HCO3- (acid + bicarbonate)

      • HCO3- H+ + CO3- (acid + carbonate ion)


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Weathering of Common Rocks

  • Clay minerals, oxides, and quartz are most common byproducts of chemical weathering and most commonly found in sediment and soil


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Mineralogy of Sedimentary Rocks

  • Allogenic: formed elsewhere and transported into area of deposition

    • Unstable: Olivine, pyroxene, Ca-plag, hornblende, epidote, magnetite, garnet

    • Stable: Muscovite, Na-plag, K-spar, clay, quartz, zircon

  • Authigenic – form at site of deposition by chemical precipitation or diagenesis

    • Unstable during diagenesis: gypsum, carbonates, zeolite

    • Stable: quartz, clay, chlorite, K-spar, muscovite

  • Mineralogical maturity

    • Mature sediments consist of minerals stable at surface conditions

    • Immature sediments consist of a high proportion of unstable minerals

    • Maturity reflects how long sediment has been in the weathering and erosion cycles


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