Sedimentary Rocks and the Production of Sediment

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Sedimentary Rocks and the Production of Sediment

• Sediment and Rocks Formed at the Earths 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

• 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

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Sedimentary Rocks Formation

• 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 Earths 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

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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

• Three types of sediment

• TTerrigenous
• 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

IO Impure orthochemical IA Impure allochemical
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Sedimentary Rocks Weathering

• Weathering
• Surface portion of Earths 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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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, Ca2, Mg2, K, Si4 More Soluble
• Fe 3, Al3 Less soluble

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Chemical Weathering

• Oxidation loss of electrons by a metal
• 4Fe2 302 ? 2Fe32O3
• 2Fe2SiO3 (1/2)O2 ? Fe2O3 2SiO2
• Pyroxene Oxygen ? Hematite Quartz
• Produces iron (and other) oxide(s)

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Chemical Weathering

• Hydrolysis
• 4KAlSi3O8 4H H2O ? 4K Al4Si4O10(OH)8
  8SiO2(aq)
• K-spar acid water ? Kaq Kaolinite
  (clay) silica

• Produces secondary minerals and residual
  (resistate) particles
• Releases ions into solution (leaching)

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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