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Sedimentary Rocks

Sedimentary Rocks. Sedimentary Rocks. Products of mechanical and chemical weathering Account for about 5% of Earth’s crust Contain evidence of past environments Texture Fossils. Sedimentary Rocks. Texture and composition - the keys to classification. Texture: Clastic or detrital

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Sedimentary Rocks

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  1. Sedimentary Rocks

  2. Sedimentary Rocks • Products of mechanical and chemical weathering • Account for about 5% of Earth’s crust • Contain evidence of past environments • Texture • Fossils

  3. Sedimentary Rocks • Texture and composition - the keys to classification. • Texture: • Clastic or detrital • Non-clastic • Chemical (or crystalline) • Biochemical

  4. Sedimentary Rocks • Clastic or detrital rocks are composed of particles of pre-existing rocks which have been weathered, eroded, transported, deposited, and cemented together. • Grain size (energy) • Rounding - energy and length of transport • Sorting - uniformity of grain size

  5. Sedimentary Rocks • Chemical sedimentary rocks: precipitated from solution. • Texture also referred to as crystalline- readily recognized in moderately coarse-grained examples. • Very fine-grained chemical rocks appear massive. • Some limestone, halite, sulfates, etc. • Evaporites are a common source. • Also caves, hot springs.

  6. Sedimentary Rocks • Biochemical rocks: formed by the processes of organisms • Calcite or silica shells or skeletons. • Form directly (coral reefs) or • Cemented together after organisms die & accumulate • Energetic wave action may break up shells and produce clastic textures • = clastic or biochemical?? • We'll consider it biochemical because the fragments are of biochemical origin • If shell fragments are clearly recognizable, the texture is called skeletal

  7. Sedimentary Rocks

  8. Sedimentary Rocks Clastic/Detrital Rocks Shale- fine grained (mud, silt, clay) Fissile (splits in layers) Fissile Shale Shale with leaf fossils • Low-energy environment: • Offshore shallow marine • Lake

  9. Sedimentary Rocks Clastic/Detrital Rocks Siltsone/mudstone- fine grained (mud, silt, clay) Massive- breaks in clumps • Low-energy environment: • Offshore shallow marine • Lake Fissile Shale Shale with leaf fossils

  10. Sedimentary Rocks Thin-section of sandstone under the petrographic microscope Clastic/Detrital Rocks • Sandstone-Composed of sand-sized particles • Well-sorted: water or wind • Forms in a variety of environments: beach, floodplain… • Quartz is the predominant mineral

  11. Sedimentary Rocks Clastic/Detrital Rocks • Graywacke • “Immature” sandstone • Contains lots of easily-weathered material: • Mafic minerals, volcanic glass/ash Typically occurs in subsiding basin near an active volcanic arc (uplift/erosion and burial > chemical weathering)

  12. Sedimentary Rocks Clastic/Detrital Rocks • Conglomerate- • Pebbles/cobbles • Well-rounded: long transport • High-energy environments: • Beach, River

  13. Sedimentary Rocks Clastic/Detrital Rocks • Breccia- • Pebbles/cobbles • Poorly-rounded: short transport • High-energy environments: • Beach, River, Fault zone Breccia

  14. Sedimentary Rocks Chemical Rocks Some Limestone is precipitated directly from water Also true for some dolostone (Ca-Mg-carbonate) and chert (SiO2)

  15. Sedimentary Rocks Chemical Rocks Some Limestone is precipitated directly from water Travertine is typically precipitated from groundwater and is seen in caves Oolitic limestone contains small spherical ooids

  16. Sedimentary Rocks Chemical Rocks Evaporites are precipitated directly from seawater when a large quantity evaporates Rock salt Rock Gypsum

  17. Sedimentary Rocks Biochemical Rocks Most Limestone is organic: precipitated as shells, reefs, and even as tiny shells from planktonic life Fossiliferous Limestones

  18. Sedimentary Rocks Biochemical Rocks Most Limestone is organic: precipitated as shells, reefs, and even as tiny shells from planktonic life Chalk fossils under electron microscope Chalk Cliffs Limestone Reef

  19. Sedimentary Rocks Biochemical Rocks Coquina is composed of broken-up shell fragments due to marine wave action in the near-tidal zone It also qualifies as a detrital rock

  20. Sedimentary Rocks Biochemical Rocks Chert, like chalk, is composed of tiny shells from planktonic life, but the organisms involved secrete SiO2 shells. Diatoms and radiolaria have microscopic SiO2 shells

  21. Sedimentary Rocks Biochemical Rocks Coal is organic: buried and un-oxidized plant remains

  22. Sedimentary Rocks Biochemical Rocks Coal is organic: buried and un-oxidized plant remains

  23. Sedimentary Rocks Sedimentary Environments Using sedimentary rocks to interpret Earth history

  24. Sedimentary Rocks Sedimentary Structures Large-scale cross-beds: wind-blown sands in an arid environment Many sedimentary structures may be used to determine original upward direction in vertical or overturned strata Ripple-marks: running water- beach or river Using sedimentary rocks to interpret Earth history

  25. Sedimentary Rocks Sedimentary Structures Graded beds- offshore turbidites Using sedimentary rocks to interpret Earth history

  26. Sedimentary Rocks Sedimentary Structures Mud-cracks and raindrop imprints Using sedimentary rocks to interpret Earth history

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