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Sedimentary Rocks

Sedimentary Rocks

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Sedimentary Rocks

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  1. Sedimentary Rocks Sediments form layers, called strata

  2. 6_11 Ions weather out of rock, are transported by groundwater to sediment layers below Sediment grains moved to ocean by streams Ions transported to lake or ocean Water enters pore spaces between sediment grains Ion-rich ground- water LITHIFICATION Compaction and Cementation Dissolved ions precipitate to form cement between sediment grains

  3. Sedimentary environment determines roundness sorting, mineral diversity 6_5 Character of detrital sediments depends on time, distance, and energy. For example, in streams: Particles are large and irregular, and consist of a variety of lithologies, including the least resistant. Particles are mid-sized and of intermediate sphericity, and include resistant and nonresistant lithologies. Particles are small and nearly spherical, and consist mainly of the most resistant lithologies, such as quartz. HIGHLANDS LOWLANDS NEAR-COASTAL

  4. Examination of minerals present and their sorting, angularity, size and cements reveal the distance from the source and local chemistry

  5. Remember: sand 0.063 – 2mm Gravels coarser Muds less

  6. Remember: sand 0.063 – 2mm Gravels coarser Muds less

  7. 500 mm = .5 mm log base 2 of .5 is -1 -1 x -1 is 1

  8. Wadell roundness

  9. Dott: % mud matrix

  10. Dott: % mud matrix

  11. Dott: % mud matrix

  12. Micrite is a term used to describe lime mud, carbonate of mud grade. The term is also used in the Folk classification to describe a carbonate rock dominated by fine-grained calcite. Folk: bound by mud or precipitated calcite Sparry cement: clear, relatively coarse-grained calcite in the interstices of any sedimentary rock

  13. restrain or regulate When bound:

  14. When bound:

  15. Maturity • Blatt and Tracy page 232 • Recall that weathering makes smaller grains, and chemical weathering, especially hydrolysis, dissolves feldspars and micas leaving clays, metal ions, and dissolved silica. • Quartz is not effected except for fragmentation and rounding. • Thus a granite or granodiorite, or compositionally equivalent metamorphic rock, leaves behind quartz and clay. Ions and molecules in aqueous solution are the raw materials fro cements

  16. Transport • The clays are FLAT microcrystals and spend much of their time in suspension. Most end in deep marine sediments over the long run. DEMO sheet of paper • Quartz fragments generally travel in the bed load, so they are deposited, and frequently buried, in a variety of non-marine and marine environments, preserving less common environments Without rift valleys, we wouldn’t know much about our own past.

  17. Angularity Using these ratios then you can apply a name according to Folk (1974): Under 0.60 very elongate 0.60 to 0.63 elongate 0.63 to 0.66 sub-elonate 0.66 to 0.69 intermediate shape 0.69 to 0.72 sub-equant 0.72 to 0.75 equant Over 0.75 very equant

  18. Grains become more rounded with time in transport

  19. Fine sediments need still water to settle

  20. What fills the spaces between grains?

  21. Graywacke with matrix Sandstone with hematite cement

  22. Compaction (some dewatering) and cementation

  23. Diagenesis:

  24. poikilotopic a crystalline sedimentary rock having multisized crystals, the larger of which enclose small ones, often of a different mineral Micritic – very fine grained; Acicular – needle like; Drusy - aggregate of minute crystals coating a surface

  25. Cements often control appearance

  26. Sometimes Quartz from volcanic rocks (e.g. from Rhyolites) does NOT have undulatory extinction because it was not strained One crystal in the grain

  27. Two or more crystals in the grain


  29. Chert grain - The subangular grain on the left is a chert fragment. Internally it is made up of a mosaic of very small crystals ('cryptocrystalline') of quartz. As you rotate this grain under polarized lights, there is a shimmering or flickering effect as each subgrain of quartz goes in and out of extinction Chert: many tiny “cryptocrystals” in the grain

  30. Low cement SS are often very friable Blue filter light shining through gaps with no cement

  31. Here the cement completely fills gaps between grains

  32. Chalcedony: microcrystalline Quartz

  33. Quartz Overgrowths- In this quartzite the individual quartz grains are cemented together with a quartz (silicate) cement. Note that cement that precipitated onto a quartz grain grew in 'optical continuity' with its substrate. For example, the right quartz grain with the 'q' label seems to be a big white blob. However, there is a faint dark line that outlines where the original, rounded, grain boundary is. The outer most part of this grain is composed of quartz overgrowth that is also white in this photo. If you rotated this slide under polarized light you'd see that the entire grain, the ghost and its overgrowth, go extinct together.

  34. Two phases of cementation - This is a quartz sandstone, seen under plain light. The quartz grains have quartz overgrowths (labeled 'o'), reflecting a first phase of cementation, and then, later, a period of carbonate cementation (c) seen as a brownish material filling the pore space.

  35. Calcite cemented sandstone (specifically, a quartz arenite). Well-rounded quartz grains (Q) have euhedral quartz overgrowths beyond their original grain edges (arrows). Calcite (C) has filled remaining pore space. Note very high interference color of the Calcite.

  36. Calcite corrosion of Quartz - The light yellowish blue colored material here is calcite cement. The black and speckled black blobs are quartz grains. The edges of the quartz grains are corroded and filled in with the calcite cement. Note that calcite tends to precipitate under high pH conditions, but that quartz tends to dissolved under these conditions. So with high pH conditions this type of erosion of the quartz grains with cementation of calcite can occur.

  37. Volcanic lithic grain (Lv) - This grain, seen under plain light, is nicely rounded and internally you can see many, euhedral (i.e. with sharp angular crystal faces) microphenocrysts of plagioclase (note the twinning on these euhedral grains). This is a classic Lv grain, probably from a volcanic arc.

  38. Folk Classification R = rock fragments

  39. Embayed plagioclase and calcite cement - The plagioclase feldspars here (note twinning) do not have the nice perfectly round, smooth shape that they probably had originally. Instead the grains have been 'embayed' (partially dissolved away) and calcite cement (bright bluish mosaic) has filled in the embayment. This happens under high pH conditions, where calcite is precipitated and silicates are dissolved. Imagine there were high pH conditions (leading to dissolving the feldspar grains) but no carbonate rich ground water around (so no cement). You can see how this would develop porosity in the sandstone while it was buried underground. Such 'secondary porosity' can be abundant and makes those seeking hydrocarbons quite excited.