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EES 450: Sedimentary Geology

EES 450: Sedimentary Geology. GENERATING SEDIMENT. EES 450: Sedimentary Geology. GENERATING SEDIMENT ● Igneous and metamorphic rocks are not stable at Earth’s surface. Why? - Generally, rocks and minerals are stable at the conditions under which they form.

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EES 450: Sedimentary Geology

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  1. EES 450: Sedimentary Geology GENERATING SEDIMENT

  2. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Igneous and metamorphic rocks are not stable at Earth’s surface. Why? - Generally, rocks and minerals are stable at the conditions under which they form. - Igneous and metamorphic rocks form under high-pressure, high-temperature conditions, and hence are not stable under low-pressure, low-temperature Earth surface conditions. - These rocks also form under conditions containing less O2, CO2, H2O and organic matter, so when brought to Earth’s surface by tectonic, isostatic or erosional forces, they become unstable and begin to breakdown (weather).

  3. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Relationship between conditions of mineral formation (Bowen’s reaction series) and mineral stability  Honeycomb weathering – Yehliu, Taiwan

  4. EES 450: Sedimentary Geology GENERATING SEDIMENT ●After weathering, the new materials which form are stable at Earth’s surface, but are very different from the original parent material. - Igneous and metamorphic rocks = ~20% quartz + ~80% other silicate minerals. Quartz sand (quartz is stable at surface conditions) (2 mm – 0.062 mm) Unstable, mostly go to clays + ions or mud (<0.062 mm, silt and clay size)

  5. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Weathering products examples: Quartz  Quartz 2KAl Si3O8 + 2H+ + 9H2O  H4Al2SiO9 + 4H4SiO4 + 2K+ (K-feldspar) (clay) (silicic acid + K ions in solution) NaAlSi3O8or CaAl2Si2O8 (other feldspars) clay + ions Ca2Na(Mg, Fe++)4(Al, Fe+++,Ti) (Al, Si)8O22(OH)2 (hornblende) clay + ions + Fe oxides K(Mg, Fe)3 (AlSi3O10)(OH)2 (biotite) clay + ions + Fe oxides

  6. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Weathering products:

  7. EES 450: Sedimentary Geology GENERATING SEDIMENT ●What is the source of calcium carbonate? H2O + CO2 H2CO3  H++ HCO3- Ca+++ 2(HCO3-) CaCO3 + H2O + CO2 ●Feldspar is the most abundant mineral on Earth. Yet, quartz is usually predominant in sediment and sedimentary rocks. Why? - ~95% of all sedimentary rocks are sandstones, mud rocks, and carbonates: 65% mud rocks 20-25% sandstones 10-15% carbonates 5% other sed. rocks Carbonic acid – H2CO3

  8. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Simple ideal model of sediment generation: (1) Weathering produces three end products: quartz sand, clay and ions in solution. (2) At weathering site, sediments begin as a mixture of any/all of the following: lithic fragments, feldspar, other semi-stable mineral grains, clay, stable iron oxides, quartz and ions in solution. (3) Sedimentary grains are sorted by transporting fluids: Only fluids? - Minerals dissolving into ions in solution leave first. - Clay and iron oxides leave next, usually kept in suspension. - Sand grains move by bouncing/rolling along bottom (saltation). - Larger mineral, lithic grains move only with high water velocity. (4) Sand size and larger grains  roundness and  size down-stream.

  9. EES 450: Sedimentary Geology GENERATING SEDIMENT

  10. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Physical or Mechanical Weathering: Examples - Physical break-up changes surface area to volume ratio. Why is this important?

  11. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Physical or Mechanical Weathering: Examples - Unloading (exfoliation). - Is this form of weathering more common for some rock types? - Can you think of multiple scenarios that would lead to exfoliation?

  12. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Physical or Mechanical Weathering: Examples - Frost wedging (water expands by ~10% upon freezing). - Where might this be important? What kinds of rock are susceptible?

  13. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Physical or Mechanical Weathering: Examples - Biological processes (root wedging, lichens, mosses, etc.).

  14. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Physical or Mechanical Weathering: Examples - Salt weathering (haloclasty): In addition to halite (NaCl), other salts are effective due to their expansive properties, including sodium sulfate (Na2SO4), magnesium sulfate (MgSO4) and calcium chloride (CaCl2). Where might this occur?

  15. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Physical or Mechanical Weathering: Examples - Insolation weathering (thermal stress): Results from repeated thermal expansion and contraction. - Is intense solar radiation all you need? Is sun the only heat source?

  16. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Chemical Weathering: Changing the chemical composition of rocks and minerals. - Carbonate dissolution-carbonation: The process in which atmospheric carbon dioxide drives dissolution. CaCO3 + H2O + CO2  Ca2+ + 2HCO3- (Ca carbonate) + (water) + (C dioxide)  (Ca ion) + (2 bicarbonate) - What kind of conditions would be ideal for this process and why?

  17. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Chemical Weathering: Examples. - Hydrolysis: Cleavage of chemical bonds by the addition of water. - Water dissociates into H+ (hydrogen cation) and OH- (hydroxyl anion). - H+ displaces other cations in mineral structure. - K+, Na+, Ca2+, Mg+ may combine with OH- or be carried away in solution.

  18. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Chemical Weathering: Examples. - Hydrolysis: Climatic setting matters!

  19. EES 450: Sedimentary Geology Hematite – SEM GENERATING SEDIMENT ●Chemical Weathering: Examples. - Oxidation: Oxygen dissolved in water combines with atoms of metallic elements abundant in silicate minerals. 4Fe2+ + 3O2 2Fe2O3 (4 ferric Iron ions) + (3 Oxygen)  (2 Hematite) Fe2O3 + H2O  2FeOOH (Hematite) + (water)  (2 Goethite) Goethite on Quartz – Cornwall, UK

  20. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Chemical Weathering: Examples. - Organic acids and chelation: Organic acids are ubiquitous at Earth’s surface, and they can assist in the chemical weathering of rocks in more than one way. (1) Reduce pH and increase water aggressiveness. (2) Function as ligands [ions or molecules that bind to a central metal atom to form a coordination complex]. (3) Participate in chelation. Ligands are essential for chelation, which is the formation or presence of two or more separate coordinate bonds between a multiple-bonded ligand and a single central atom. Examples of organic chelates

  21. EES 450: Sedimentary Geology GENERATING SEDIMENT ●What controls rates at which physical or chemical weathering occur? - Water throughput. - Water dissociation. - CO2 concentration. - Temperature. - Based on the figure at right, in what environmental settings would you expect (1) physical weathering to dominate?; (2) chemical weathering to dominate?; (3) physical and chemical weathering to be ~equal? Is this important for us, why or why not?

  22. EES 450: Sedimentary Geology GENERATING SEDIMENT ●What controls rates at which physical or chemical weathering occur? - The nature of the rock being weathered (not composition), such as the presence of joints, and their orientation. Why orientation?

  23. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Weathering often proceeds in a “regular” progression…

  24. EES 450: Sedimentary Geology GENERATING SEDIMENT ●How can we classify weathering?

  25. EES 450: Sedimentary Geology GENERATING SEDIMENT ●Soil formation: Transition or interface medium Regolith and soil = interfaces/transitions between atmosphere and lithosphere = examples of early stages in making of sedimentary rocks. Soil – Why is it stratified? Weathering Weathering & Erosion Regolith – Where is the parent rock? Source rock Soil Sediment

  26. Breccia Conglomerate Diamictite Sandstone Siltstone Shale Mechanical Weathering Clastic or Detrital EES 450: Sedimentary Geology New Clay Minerals Shale Source Rocks Limestone Chert Diatomite Biochemical Solution Evaporites Chert Limestone Ironstone Chemical Weathering Chemical Peat Coal Bitumen Resins Plant Extraction Organic Crustal, upper mantle melts Explosive Eruption Tuff, Bentonite Agglomerate Volcaniclastic

  27. EES 450: Sedimentary Geology LITHIFICATION ●Transforming sediment into rock, which results in porosity destruction through compaction and cementation. - Generally, three processes involved: (1) Compaction (2) Cementation (3) Recrystallization

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