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Chapter 6 sedimentary rocks


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Chapter 6 sedimentary rocks

  1. 1. Essentials of Geology 3 rd Edition Chapter 6 Norton Media Library
  2. 2. Pages of Earth’s Past: Sedimentary Rocks Prepared by: Ronald Parker , Senior Geologist Fronterra Geosciences Houston, Oklahoma City, Denver, Anchorage, Dallas, Midland, Aberdeen, Vienna, Buenos Aires, Neuquén
  3. 3. Sedimentary Cover <ul><li>Earth is covered by a thin ‘veneer’ of sediment. </li></ul><ul><li>The veneer caps ig neous and meta morphic “basement.” </li></ul><ul><li>Sediment cover varies in thickness from 0 to 20 km. </li></ul><ul><ul><li>Thinner (or missing) where ig and meta rocks outcrop. </li></ul></ul><ul><ul><li>Thicker in sedimentary basins. </li></ul></ul>
  4. 4. Sedimentary Rocks <ul><li>Sediments are the building blocks of sedimentary rocks. </li></ul><ul><li>Sediments are diverse, as are the rocks made from them. </li></ul><ul><li>Four classes: </li></ul><ul><ul><li>Clastic – Made from weathered rock fragments (clasts). </li></ul></ul><ul><ul><li>Biochemical – Cemented shells of organisms. </li></ul></ul><ul><ul><li>Organic – The carbon-rich remains of plants. </li></ul></ul><ul><ul><li>Chemical – Minerals that crystallize directly from water. </li></ul></ul>Chemical Clastic Organic Biochemical
  5. 5. Clastic Sedimentary Rocks <ul><li>Clastic sedimentary rocks reflect several processes. </li></ul><ul><ul><li>Weathering – Generation of detritus via rock disintegration. </li></ul></ul><ul><ul><li>Erosion – Removal of sediment grains from rock. </li></ul></ul><ul><ul><li>Transportation – Dispersal by wind, water, and ice. </li></ul></ul><ul><ul><li>Deposition – Settling out of the transporting fluid. </li></ul></ul><ul><ul><li>Lithification – Transformation into solid rock. </li></ul></ul>
  6. 6. <ul><li>Lithification – Transforms loose sediment into solid rock. </li></ul><ul><ul><li>Burial – More sediment is added onto previous layers. </li></ul></ul><ul><ul><li>Compaction – Overburden weight reduces pore space. </li></ul></ul><ul><ul><ul><li>Sand – 10 to 20% </li></ul></ul></ul><ul><ul><ul><li>Clay – 50 to 80% </li></ul></ul></ul><ul><ul><li>Cementation – Minerals grow in pores, “gluing” sediments. </li></ul></ul>Clastic Sedimentary Rocks
  7. 7. Clastic Sedimentary Rocks <ul><li>Classified on the basis of texture and composition. </li></ul><ul><ul><li>Clast (grain) size. </li></ul></ul><ul><ul><li>Clast composition. </li></ul></ul><ul><ul><li>Angularity and sphericity. </li></ul></ul><ul><ul><li>Sorting. </li></ul></ul><ul><ul><li>Character of cement. </li></ul></ul><ul><li>These variables produce a diversity of clastic rocks. </li></ul>
  8. 8. Clastic Sedimentary Rocks <ul><li>Clast (grain) size – The average diameter of clasts. </li></ul><ul><ul><li>Range from very coarse to very fine. </li></ul></ul><ul><ul><li>Boulder, cobble, pebble, sand, silt, and clay. </li></ul></ul><ul><ul><li>With increasing transport, average grain size decreases. </li></ul></ul>
  9. 9. Clastic Sedimentary Rocks <ul><li>Clast composition – The mineral makeup of sediments. </li></ul><ul><ul><li>May be individual minerals or rock fragments. </li></ul></ul><ul><ul><li>Mineral identities provide clues about… </li></ul></ul><ul><ul><ul><li>The source of the sediment. </li></ul></ul></ul><ul><ul><ul><li>The environment of deposition. </li></ul></ul></ul>
  10. 10. Clastic Sedimentary Rocks <ul><li>Angularity and sphericity – Indicate degree of transport. </li></ul><ul><li>Fresh detritus is usually angular and non-spherical. </li></ul><ul><li>Grain roundness and sphericity increases with transport. </li></ul><ul><ul><li>Well-rounded – Long transport distances. </li></ul></ul><ul><ul><li>Angular – Negligible transport. </li></ul></ul>
  11. 11. Clastic Sedimentary Rocks <ul><li>Sorting – The uniformity of grain size. </li></ul><ul><ul><li>Well-sorted – Uniform grain sizes. </li></ul></ul><ul><ul><li>Poorly sorted – Wide variety of grain sizes. </li></ul></ul><ul><li>Sorting becomes better with distance from source. </li></ul>
  12. 12. Clastic Sedimentary Rocks <ul><li>Cement – Minerals that fill sediment pores. </li></ul><ul><ul><li>Fluids with dissolved solids flush through pore system. </li></ul></ul><ul><ul><li>Dissolved ions slowly crystallize and fill pores. </li></ul></ul><ul><li>Cementation varies from weak to strong. </li></ul><ul><li>Common cements: </li></ul><ul><ul><li>Quartz </li></ul></ul><ul><ul><li>Calcite </li></ul></ul><ul><ul><li>Hematite </li></ul></ul><ul><ul><li>Clay minerals </li></ul></ul>
  13. 13. Clastic Sedimentary Rocks <ul><li>Coarse clastics – Composed of gravel-sized clasts. </li></ul><ul><ul><li>Breccia – Comprised of angular fragments. </li></ul></ul><ul><ul><ul><li>Angularity indicates a lack of transport processing. </li></ul></ul></ul><ul><ul><ul><li>Deposited relatively close to source. </li></ul></ul></ul>
  14. 14. Clastic Sedimentary Rocks <ul><li>Coarse clastics – Composed of gravel-sized clasts. </li></ul><ul><ul><li>Conglomerate – Comprised of rounded gravel. </li></ul></ul><ul><ul><ul><li>Indicates water transport. </li></ul></ul></ul><ul><ul><ul><ul><li>Clasts bang together forcefully in flowing water. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Collisons round angular corners and edges of clasts. </li></ul></ul></ul></ul><ul><ul><ul><li>Conglomerates are deposited at a distance from the source. </li></ul></ul></ul>
  15. 15. Clastic Sedimentary Rocks <ul><li>Sandstone – Clastic rock made of sand-sized particles. </li></ul><ul><ul><li>Forms in many depositional settings. </li></ul></ul><ul><ul><li>Quartz is, by far, the dominant mineral in sandstones. </li></ul></ul><ul><li>Sandstone varieties. </li></ul><ul><ul><li>Arkose – Contains abundant feldspar. </li></ul></ul><ul><ul><li>Quartz sandstone – Almost pure quartz. </li></ul></ul>
  16. 16. Clastic Sedimentary Rocks <ul><li>Fine clastics - Composed of silt and clay. </li></ul><ul><ul><li>Silt-sized sediments are lithified to form siltstone. </li></ul></ul><ul><ul><li>Clay-sized particles form shale. </li></ul></ul><ul><li>Fine clastics are deposited in quieter waters. </li></ul><ul><ul><li>Floodplains, lagoons, mudflats, deltas, deep-water basins. </li></ul></ul><ul><ul><li>Organic-rich shales are the source of petroleum. </li></ul></ul>
  17. 17. <ul><li>These are sediments derived from living organisms. </li></ul><ul><ul><li>Biochemical – Hard mineral skeletons. </li></ul></ul><ul><ul><li>Organic – Cells of plants, algae, bacteria and plankton. </li></ul></ul>Biochemical and Organic Rocks
  18. 18. Biochemical Rocks <ul><li>Biochemical limestone – CaCO 3 skeletal (shell) remains. </li></ul><ul><ul><li>Warm, tropical, shallow, clear, O 2 -rich, marine water. </li></ul></ul><ul><ul><li>Diverse organisms (plankton, corals, clams, snails, etc.). </li></ul></ul><ul><ul><li>Many textural varieties. </li></ul></ul><ul><ul><ul><li>Reefs. </li></ul></ul></ul><ul><ul><ul><li>Shell debris. </li></ul></ul></ul><ul><ul><ul><li>Lime mud (micrite). </li></ul></ul></ul>
  19. 19. <ul><li>Chert – Rock made of cryptocrystalline quartz. </li></ul><ul><ul><li>Formed from opalline silica (SiO 2 ) skeletons. </li></ul></ul><ul><ul><ul><li>Diatoms. </li></ul></ul></ul><ul><ul><ul><li>Radiolarians. </li></ul></ul></ul><ul><ul><li>Opalline silica added to bottom sediments dissolves. </li></ul></ul><ul><ul><li>Silica pore fluids solidify to form chert nodules or beds. </li></ul></ul>Biochemical Rocks
  20. 20. <ul><li>Made from organic carbon. </li></ul><ul><ul><li>Coal – Altered remains of fossil vegetation. </li></ul></ul><ul><ul><ul><li>Accumulates in lush tropical wetland settings. </li></ul></ul></ul><ul><ul><ul><li>Requires deposition in the absence of oxygen. </li></ul></ul></ul><ul><ul><li>Oil shale – Shale with heat altered organic matter. </li></ul></ul>Organic Rocks
  21. 21. Chemical Sedimentary Rocks <ul><li>Comprised of minerals precipitated from water solution. </li></ul><ul><li>Evaporites – Created from evaporated seawater. </li></ul><ul><ul><li>Evaporation triggers deposition of chemical precipitates. </li></ul></ul><ul><ul><li>Examples include halite (rock salt) and gypsum. </li></ul></ul>
  22. 22. Chemical Sedimentary Rocks <ul><li>Travertine – Calcium carbonate (CaCO 3 ) precipitated from groundwater where it reaches the surface. </li></ul><ul><ul><li>Dissolved calcium (Ca 2+ ) reacts with bicarbonate (HCO 3 - ). </li></ul></ul><ul><ul><li>CO 2 expelled into the air causes CaCO 3 to precipitate. </li></ul></ul><ul><ul><ul><li>Thermal (hot) springs. </li></ul></ul></ul><ul><ul><ul><li>Caves. </li></ul></ul></ul>
  23. 23. Chemical Sedimentary Rocks <ul><li>Dolostone – Limestone altered by Mg-rich fluids. </li></ul><ul><ul><li>CaCO 3 altered to dolomite CaMg(CO 3 ) 2 by Mg 2+ -rich water. </li></ul></ul><ul><ul><li>Dolostone looks like limestone, except… </li></ul></ul><ul><ul><ul><li>It has a sugary texture and a pervasive porosity. </li></ul></ul></ul><ul><ul><ul><li>It weathers to a buff, tan color. </li></ul></ul></ul>
  24. 24. Chemical Sedimentary Rocks <ul><li>Replacement chert – Nonbiogenic in origin. </li></ul><ul><li>Many varieties. </li></ul><ul><ul><li>Flint – Black or gray from organic matter. </li></ul></ul><ul><ul><li>Jasper – Red or yellow from Fe-oxides. </li></ul></ul><ul><ul><li>Petrified wood – Wood grain preserved by silica. </li></ul></ul><ul><ul><li>Agate – Concentrically layered rings. </li></ul></ul>
  25. 25. Sedimentary Structures <ul><li>Features imparted to sediments at or near deposition. </li></ul><ul><ul><li>Layering. </li></ul></ul><ul><ul><li>Surface features on layers. </li></ul></ul><ul><ul><li>Arrangement of grains. </li></ul></ul><ul><li>Help decipher conditions at or near time of deposition. </li></ul>
  26. 26. Sedimentary Structures <ul><li>Sedimentary rocks are usually layered or “stratified.” </li></ul><ul><ul><li>Arranged in planar, close-to-horizontal “beds.” </li></ul></ul><ul><ul><li>Bedding is often laterally continuous for long distances. </li></ul></ul><ul><ul><li>Beds are often similar in composition, color and texture. </li></ul></ul>
  27. 27. Sedimentary Structures <ul><li>Bedding reflects changing conditions during deposition. </li></ul><ul><li>These can be changes in… </li></ul><ul><ul><li>Energy conditions, and hence, grain size. </li></ul></ul><ul><ul><li>Disturbance by organisms. </li></ul></ul><ul><li>Bedding may also reflect non-deposition or erosion. </li></ul>
  28. 28. Sedimentary Structures <ul><li>A series of beds are referred to as strata. </li></ul><ul><li>Formation: Strata recognized on a regional scale. </li></ul><ul><li>Geologic maps display the distribution of formations. </li></ul><ul><ul><li>i.e. Coconino Formation </li></ul></ul>Note the prominent white band of Coconino Sandstone clearly visible across Grand Canyon.
  29. 29. Sedimentary Structures <ul><li>Water flowing over loose sediment creates bedforms. </li></ul><ul><li>Bedforms are linked to flow velocity and sediment size. </li></ul><ul><ul><li>Ripples, cm-scale ridges, and troughs, indicate flow. </li></ul></ul><ul><ul><ul><li>Asymmetric ripples – Unidirectional flow. </li></ul></ul></ul><ul><ul><ul><li>Symmetric ripples – Wave oscillation. </li></ul></ul></ul><ul><ul><ul><li>Ripples are commonly preserved in sedimentary rocks. </li></ul></ul></ul>
  30. 30. Bedforms <ul><li>Cross beds – Created by ripple and dune migration. </li></ul><ul><ul><li>Sediment moves up the gentle side of a ripple or dune. </li></ul></ul><ul><ul><li>Sediment piles up, then slips down the steep face. </li></ul></ul><ul><ul><ul><li>The slip face continually moves downstream. </li></ul></ul></ul><ul><ul><ul><li>Added sediment forms sloping “cross-bedded” layers. </li></ul></ul></ul>
  31. 31. Bedforms <ul><li>Dunes – Similar to ripples except much larger. </li></ul><ul><ul><li>Form from wind-blown sand in desert or beach regions. </li></ul></ul><ul><ul><li>Often preserve large internal cross-laminations. </li></ul></ul>
  32. 32. Bedforms <ul><li>Turbidity currents. </li></ul><ul><ul><li>Sediment moves on a slope as a pulse of turbid water. </li></ul></ul><ul><ul><li>As pulse wanes, water loses velocity and grains settle. </li></ul></ul><ul><ul><li>Coarsest material settles first, medium next, then fines. </li></ul></ul><ul><li>This process forms graded beds in turbidite deposits. </li></ul>
  33. 33. Bed-Surface Markings <ul><li>Occur after deposition while sediment is still soft. </li></ul><ul><ul><li>Mudcracks – Polygonal desiccation features in wet mud. </li></ul></ul><ul><ul><ul><li>Indicate alternating wet and dry conditions. </li></ul></ul></ul><ul><ul><ul><li>Necessitate deposition in a terrestrial setting. </li></ul></ul></ul><ul><ul><li>Scour marks – Troughs eroded in soft mud by current flow. </li></ul></ul><ul><ul><li>Fossils – Evidence of past life. </li></ul></ul><ul><ul><ul><li>Footprints. </li></ul></ul></ul><ul><ul><ul><li>Shell impressions. </li></ul></ul></ul>
  34. 34. <ul><li>Locations where sediment accumulates. They differ in… </li></ul><ul><ul><li>Energy regime. </li></ul></ul><ul><ul><li>Sediment delivery, transport, and depositional conditions. </li></ul></ul><ul><ul><li>Chemical, physical and biological characteristics. </li></ul></ul><ul><li>Environments range from terrestrial to marine. </li></ul>Depositional Environments
  35. 35. Depositional Environments <ul><li>Terrestrial environments – Deposited above sea level. </li></ul><ul><ul><li>Glacial – Due to movement of ice. </li></ul></ul><ul><ul><ul><li>Ice carries and dumps every grain size. </li></ul></ul></ul><ul><ul><ul><li>Creates glacial till; poorly sorted gravel, sand, silt, and clay. </li></ul></ul></ul>
  36. 36. Depositional Environments <ul><li>Terrestrial environments – Deposited above sea level. </li></ul><ul><ul><li>Mountain streams. </li></ul></ul><ul><ul><ul><li>Water carries large clasts during floods. </li></ul></ul></ul><ul><ul><ul><li>During low flow, these cobbles and boulders are immobile. </li></ul></ul></ul><ul><ul><ul><li>Course conglomerate is characteristic of this setting. </li></ul></ul></ul>
  37. 37. Depositional Environments <ul><li>Terrestrial environments – Deposited above sea level. </li></ul><ul><ul><li>Alluvial fan - Sediments that pile up at a mountain front. </li></ul></ul><ul><ul><ul><li>Rapid drop in stream velocity creates a cone-shaped wedge. </li></ul></ul></ul><ul><ul><ul><li>Sediments are immature conglomerates and arkoses. </li></ul></ul></ul>
  38. 38. Depositional Environments <ul><li>Terrestrial Environments– Deposited above sea level. </li></ul><ul><ul><li>Sand dunes – Wind-blown piles of well-sorted sand. </li></ul></ul><ul><ul><ul><li>Dunes move according to the prevailing winds. </li></ul></ul></ul><ul><ul><ul><li>Result in uniform sandstones with gigantic cross beds. </li></ul></ul></ul>
  39. 39. Depositional Environments <ul><li>Terrestrial environments– Deposited above sea level. </li></ul><ul><ul><li>Rivers – Channelized flow transports sediment. </li></ul></ul><ul><ul><ul><li>Sand and gravel fill concave-upward channels. </li></ul></ul></ul><ul><ul><ul><li>Fine sand, silt, and clay are deposited on nearby floodplains. </li></ul></ul></ul>
  40. 40. Depositional Environments <ul><li>Terrestrial environments– Deposited above sea level. </li></ul><ul><ul><li>Lake – Large ponded bodies of water. </li></ul></ul><ul><ul><ul><li>Gravels and sands trapped near shore. </li></ul></ul></ul><ul><ul><ul><li>Well-sorted muds deposited in deeper water. </li></ul></ul></ul><ul><ul><ul><li>Often capped with wetland muds. </li></ul></ul></ul>
  41. 41. Depositional Environments <ul><li>Marine environments – Deposited at or below sea level. </li></ul><ul><ul><li>Deltas – Sediments dropped where a river enters the sea. </li></ul></ul><ul><ul><ul><li>Sediment carried by the river is dumped when velocity drops. </li></ul></ul></ul><ul><ul><ul><li>Deltas grow over time, building out into the basin. </li></ul></ul></ul><ul><ul><ul><li>Often develop a topset – foreset – bottomset geometry. </li></ul></ul></ul>
  42. 42. Depositional Environments <ul><li>Marine environments – Deposited at or below sea level. </li></ul><ul><ul><li>Coastal beaches – Surf zone. </li></ul></ul><ul><ul><ul><li>Sediments are constantly being processed by wave attack. </li></ul></ul></ul><ul><ul><ul><li>A common result? Well-sorted, well-rounded medium sand. </li></ul></ul></ul><ul><ul><ul><li>Beach sandstones may preserve oscillation ripples. </li></ul></ul></ul>
  43. 43. Depositional Environments <ul><li>Marine environments – Deposited at or below sea level. </li></ul><ul><ul><li>Shallow marine – Finer version of beach sediment. </li></ul></ul><ul><ul><ul><li>Fine silts and muds turn into siltstones and mudstones. </li></ul></ul></ul><ul><ul><ul><li>Usually support an active biotic community. </li></ul></ul></ul>
  44. 44. Depositional Environments <ul><li>Marine environments – Deposited at or below sea level. </li></ul><ul><ul><li>Shallow water carbonates – Tropical. </li></ul></ul><ul><ul><ul><li>Skeletons of marine invertebrates. </li></ul></ul></ul><ul><ul><ul><li>Born in the carbonate factory. </li></ul></ul></ul><ul><ul><ul><li>Warm, clear, shallow, normal salinity, marine water. </li></ul></ul></ul>
  45. 45. Depositional Environments <ul><li>Marine environments – Deposited at or below sea level. </li></ul><ul><ul><li>Deep marine – Fines predominate far from land sources. </li></ul></ul><ul><ul><ul><li>Skeletons of planktonic organisms make chalk or chert. </li></ul></ul></ul><ul><ul><ul><li>Fine silts and clays turn to shale. </li></ul></ul></ul>
  46. 46. Sedimentary Basins <ul><li>Sediments vary in thickness across Earth’s surface. </li></ul><ul><ul><li>Thin to zero edge where non-sedimentary rocks outcrop. </li></ul></ul><ul><ul><li>Thicken to 10 to 20+ km in sedimentary basins. </li></ul></ul><ul><li>Subsidence – Sinking of the land during sedimentation. </li></ul><ul><ul><li>Due to crustal flexure and faulting. </li></ul></ul><ul><ul><li>Compounded by the weight of added sediments. </li></ul></ul><ul><li>Basins are important locations for natural resources. </li></ul><ul><ul><li>Coal. </li></ul></ul><ul><ul><li>Petroleum. </li></ul></ul><ul><ul><li>Natural gas. </li></ul></ul><ul><ul><li>Uranium. </li></ul></ul>
  47. 47. Sedimentary Basins <ul><li>Basins form where tectonic activity creates space. </li></ul><ul><ul><li>Rift basins – Divergent (pull-apart) plate boundaries. </li></ul></ul><ul><ul><ul><li>Crust thins by stretching and rotational normal faulting. </li></ul></ul></ul><ul><ul><ul><li>Thinned crust subsides. </li></ul></ul></ul><ul><ul><ul><li>Sediment fills the down-dropped basin. </li></ul></ul></ul>
  48. 48. Sedimentary Basins <ul><li>Basins form where tectonic activity creates space. </li></ul><ul><ul><li>Passive margin basins – Non-plate-boundary continental edge. </li></ul></ul><ul><ul><ul><li>Underlain by crust thinned by previous rifting. </li></ul></ul></ul><ul><ul><ul><li>Thinned crust </li></ul></ul></ul><ul><ul><ul><li>subsides as it cools. </li></ul></ul></ul>
  49. 49. Sedimentary Basins <ul><li>Basins form where tectonic activity creates space. </li></ul><ul><ul><li>Intracontinental basins – Interiors far from margins. </li></ul></ul><ul><ul><ul><li>Result from differential thermal subsidence. </li></ul></ul></ul><ul><ul><ul><li>May be linked to failed crustal rifts. </li></ul></ul></ul>
  50. 50. Sedimentary Basins <ul><li>Basins form where tectonic activity creates space. </li></ul><ul><ul><li>Foreland basins – Craton side of collisional mountain belt. </li></ul></ul><ul><ul><ul><li>Flexure of the crust from loading creates a downwarp. </li></ul></ul></ul><ul><ul><ul><li>Fills with debris eroded off of the mountains. </li></ul></ul></ul>
  51. 51. Sedimentary Basins <ul><li>Sea-level changes. </li></ul><ul><ul><li>Sedimentary deposition is strongly linked to sea level. </li></ul></ul><ul><ul><li>Changes in sea level are commonplace geologically. </li></ul></ul><ul><ul><ul><li>Depositional belts shift landward or seaward in response. </li></ul></ul></ul><ul><ul><ul><li>Layers of strata record deepening or shallowing upward. </li></ul></ul></ul><ul><ul><li>Transgression – Flooding due to sea-level rise. </li></ul></ul><ul><ul><ul><li>Sediment belts shift landward; strata “deepen” upward. </li></ul></ul></ul>
  52. 52. Sedimentary Basins <ul><li>Regression – Exposure due to sea level fall. </li></ul><ul><ul><li>Depositional belts shift seaward; strata “shallow upward.” </li></ul></ul><ul><ul><li>Regression tied to erosion; less likely to be preserved. </li></ul></ul><ul><li>Sea-level rise and fall creates a predictable pattern. </li></ul>Transgression – Regression Animation
  53. 53. Diagenesis <ul><li>Physical, chemical, and biological changes to sediment. </li></ul><ul><ul><li>Bioturbation. </li></ul></ul><ul><ul><li>Lithification. </li></ul></ul><ul><ul><li>Dissolution. </li></ul></ul><ul><ul><li>Mineral precipitation. </li></ul></ul><ul><li>Temps between burial and metamorphism (~300 o C). </li></ul><ul><li>Integrates changes across the entire sediment history. </li></ul>
  54. 54. W. W. Norton & Company Independent and Employee-Owned <ul><li>This concludes the Norton Media Library PowerPoint Slide Set for Chapter 6 </li></ul><ul><li>Essentials of Geology </li></ul><ul><li>3 rd Edition (2009) </li></ul><ul><li>by Stephen Marshak </li></ul>