Sedimentation and Stratigraphy Geology 5142 Dr. Thieme

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Sedimentation and StratigraphyGeology 5142Dr.
Thieme

• Lecture 28 Sedimentary Geochemistry and
  Carbonate Chemical Stratigraphy

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Sedimentary Geohemistry

• The bulk chemical composition of sedimentary
  rocks depends upon -
• the source area(s) for clastic detritus
• the composition of waters entering the
  sedimentary basin
• surficial weathering history (soil forming
  interval, climate, biota), if the rocks were
  subaerially exposed
• groundwater, brine, hydrocarbon, etc.. held in
  pore spaces

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Composition of Siliciclastic Rocks

• traditionally neglected in favor of mineral
  composition, the basis for sandstone
  classifications
• present chemical composition may not accurately
  reflect the composition at the time of deposition
• chemical compositions change as new minerals
  crystallize during sediment burial and diagenesis
• chemical analyses perceived as expensive in both
  time and money

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Composition of Siliciclastic Rocks

• compared to the parent source rocks from which
  they derive, siliciclastic sedimentary rocks are
• enriched in Si
• depleted in Fe, Mg, Ca, Na, and K
• resistance to chemical weathering
• stability at surface temperature and pressure
  favors secondary quartz, chert, etc...

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Composition of Siliciclastic Rocks

• SiO2 ranges from 30 to over 90 percent
• Al2O3 to upwards of 15 percent, typically due to
  either feldspars or clay minerals
• Fe, Mg, Ca, Na, and K typically less abundant
  than Al
• Bulk rock chemistry often provides information on
  matrix clay mineralogy and diagenic cements

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During sediment burial, porosity is reduced from
approximately 50 percent at the surface to
virtually zero at 5000 m. Mechanical compaction
and pressure solution occur mainly above 2000 m,
quartz cementation below.
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Composition of Shales

• SiO2 is the most abundant constituent, followed
  by Al2O3
• FeO and Fe2O3 may be up to 8 percent, supplied by
  iron oxides, biotite, etc...
• K2O, and MgO are next in abundance, related
  mainly to clay minerals, although some K is from
  feldspars and Mg from dolomite.
• CaO from calcite and plagioclase
• NaO from feldspar and clay minerals

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Composition of Carbonates

• Dominated by only four elements Ca, Mg, C, O
• Minor concentrations of Si, Al, K, Na, and Fe
• Trace elements of importance include B, Be, Ba,
  Se, Cl, Co, Cr, Cu, Ga, Ge, and Li

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Carbonate rock compositions are dominated by only
four elements Ca, Mg, C, O
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Carbonate minerals sum to nearly 100 of many
carbonate rocks.
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Phanerozoic Carbonates

• carbonate ooids in modern seas are all aragonite,
  whereas those in many sedimentary rocks are
  calcite
• formerly attributed to diagenesis, Sandberg
  (1975) argued that ocean chemistry formerly
  favored precipitation of calcite over aragonite
• "Aragonite Sea" has high Mg/Ca ratio compared to
  "Calcite Sea" with lower Mg/Ca

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Aragonite Seas are mostly known from "icehouse"
conditions of the Neogene, late Paleozoic, and
late Proterozoic.
Calcite Seas occurred under "greenhouse"
conditions of the early and middle Paleozoic, and
the Jurassic through Eocene.
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Phanerozoic Carbonates

• Sandberg attributed the ocean chemistry cycle to
  global climate change
• high pCO2 drives down carbonate saturation in the
  oceans
• "greenhouse" oceans were still capable of calcite
  precipitation but not aragonite
• Stanley and Hardie (1998, 1999) attribute the
  ocean chemistry instead to seafloor spreading
  rates

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Aragonite Sea
Calcite Sea
High rates of seafloor spreading result in
Calcite Seas because Mg is stored in basalts and
brines are Mg-depleted. Reduced consumption of Mg
at mid-ocean ridges raises the Mg content in
Aragonite Seas.
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Phanerozoic Carbonates

• the chemical composition of carbonates is also
  controlled by local as opposed to global changes
  in ocean chemistry
• carbonate composition changes as pore water
  chemistry changes during burial and diagenesis
• carbonate composition changes if rocks are
  uplifted and exposed to subaerial weathering

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Locally, much of the Mg found in carbonate rocks
is due to dolomitization.