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Sedimentary rocks

Sedimentary rocks. Geology 103. Making sediment. Weathering = rock breakdown into smaller rock, or minerals, or chemicals Sediment = result of weathering of rocks Erosion = movement of sediment. Sediment stages. 1. Weathering. 2. Erosion. 3. Transportation via water, glaciers and wind.

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Sedimentary rocks

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  1. Sedimentary rocks Geology 103

  2. Making sediment • Weathering = rock breakdown into smaller rock, or minerals, or chemicals • Sediment = result of weathering of rocks • Erosion = movement of sediment

  3. Sediment stages 1. Weathering 2. Erosion 3. Transportation via water, glaciers and wind 4. Deposition 5. Burial and compaction 6. Diagenesis

  4. Physical weathering • Physical: Breaking apart of rocks by a physical force • Also called “mechanical” weathering

  5. Chemical weathering • A chemical reaction alters the composition of the minerals in the rocks (e.g., dissolving halite or altering feldspars into clay minerals) • Dissolving limestone leads to karst topography and caves

  6. Caves are the product of varying water tables and limestone dissolution; cave formations (speleothems) are the result of calcite precipitation

  7. Or a combination • Physical weathering may break up bedrock, then chemical weathering may break down the pieces into a soil

  8. A quick word on soils • Soils are the weathered material at the surface that include both organic and mineral components • Soils differ due to the parent material, time of weathering and water content

  9. Sedimentary rocks become lithified when they are compacted then cemented Lithification

  10. Diagenesis is also a term used to describe the production of sedimentary rocks from sediments Precipitation: addition of new minerals cements the sediment particles Compaction squeezes out the water

  11. Classification of sed rx • Clastic = “broken”; sed rx made of broken-up parts of other rocks • Chemical sed rx are made from the precipitation of chemicals from water (“evaporites”) or the oxidation of chemicals • Biological sed rx are “born”; that is, they derive from the remains of creatures

  12. Relative abundance of sediments

  13. Clastic sed rx • Classified by dominant grain size • Scale: “boulder”, “cobble”, “pebble”, “granule”, “sand”, “silt”, “clay” • Texture and composition are secondary considerations (e.g., “shale”)

  14. Texture: Grain roundedness • The roundedness (that is, how sharply defined the “corners” of individual grains are) is used to determine the transport distance • Proximal = near; distal = far

  15. Texture: Grain sorting • The variation between coarse and fine particles in the sediment is called sorting • Sorting is used to infer transport distance; well-sorted sediments have come far

  16. Mineralogy of sandstone and depositional environments

  17. Layering in clastic sed rx • Layers are called beds, unless they are thin, in which case they are called laminae • Beds and laminae represent distinct depositional events, like floods

  18. Chemical sed rx • Evaporites: limestone, rock salt • Oxidation product: taconite (iron ore)

  19. How evaporites are deposited 1. During the Miocene epoch, the Mediterranean Sea became a shallow evaporitebasin when the Strait of Gibraltar was above sea level 3. Evaporation removed water 4. Fresh water inflow was limited 2. Reduced water exchange with the Atlantic 5. Gypsum and halite crystalize first forming evaporites

  20. The Mediterranean dried up repeatedly between 5.6 and 5.33 Ma; the Zanclean Flood refilled the basin within two years Garcia-Castellanos et al., “Catastrophic flood of the Mediterranean after the Messinian salinity crisis”, Nature462 (2009), 778-781

  21. Biological sed rx • Plant remains: coal • Animal remains: limestone, chert

  22. Most biological sed rocks are deposited on (and make) carbonate platforms

  23. Carbonate platform formation Within the reef lagoon, growth of carbonate-secreting organisms, including forminefera, coral, algae and molusks, is rapid, and carbonate sediments form quickly Eventually a carbonate platform grows with steep sides towards the sea If sea level rises, the reef continues to grow towards the light and lagoon sedimentation outpaces sedimentation in the open ocean

  24. Coral reefs can form an atoll around a mid-ocean volcano Bora Bora atoll, South Pacifc

  25. Coral reefs and atolls creation Process first described by Charles Darwin

  26. Water energy • The speed of the depositing medium (usually water) can be inferred from clastic and some other sed rx • This is because coarser grains settle first in a suspension

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