Chapter 7 – Sedimentary Environments

1. Chapter 7 – Sedimentary Environments Sedimentary rocks are composed of sediment. Sediment forms at or near earth’s surface through the processes of weathering, transportation, deposition, and lithification. Importance of sedimentary rocks include: Energy resources Petroleum Uranium Coal

2. Sedimentary Resources Importance of sedimentary rocks include: Mineral resources Limestone: cement Rock salt: salt Rock gypsum: plaster Importance of sedimentary rocks include: Environment of deposition Oceans and seas Lakes and swamps River valleys and flood plains Deltas Sand dunes

3. Every Rock Tells a Story Because sedimentary rocks form at earth’s surface, they tell the geologist what was happening on the surface when that rock was deposited. The rocks tell the story of Earth. For example . . . Limestone generally only grows in sea water. If a geologist finds limestone in Kansas, the geologist concludes that Kansas was once underwater.

4. Sedimentary Rocks • Sedimentary rocks are composed of sediment. • Sediment can be either clastic (pieces) or chemical. • Clastic sediment is pieces of pre- existing rock like gravel, sand, silt and clay. • Chemical sediment is chemicals dissolved in water.

5. Sediment • Sediment comes in two varieties. • Sediment can be either clastic (pieces) or chemical. • Clastic sediment is pieces of pre- existing rock like gravel, sand, silt and clay. • Chemical sediment is chemicals dissolved in water.

6. Clastic Sediment • Clastic sediment forms from the mechanical, physical and chemical breakdown of rock. • The process of breaking rock down into smaller pieces aids further breakdown by chemical weathering.

7. Chemical Sediment • Chemical sediment forms by the chemical dissolution and alteration of rocks at or near earth’s surface. • Chemical sediment is transported by stream waters to lakes and the oceans.

8. Weathering Chapter 15

9. Mechanical Weathering • Frost wedging • Alternate freezing and thawing • Temperate mountainous climates • 9% expansion as water freezes • Up to 80,000 pounds per square inch pressure

10. Talus Slope • Generally form a large pile of rock at the base of mountains called talus slope.

11. Unloading • As the weight of overlying rock is weathered and eroded away the rock expands and produces fractures (or joints) parallel to the topography. Exfoliation.

12. Joints • Vertical joints (cracks without displacement) form by expansion and contraction.

13. Joints from Top with Striations • Columnar joints with glacial striations. Devils Postpile National Monument.

14. Thermal Expansion Joints • Results from the daily cycle of temperature changes producing daily expans-ion and contraction. Most prominent in hot deserts.

15. Biological Activity • Plants and burrowing animals cause much joint expansion.

16. Biological Activity • The greatest mechanical weathering is now produced by catepillars.

17. Biological Activity • The greatest mechanical weathering is now produced by catepillars.

18. Mechanical Weathering Enhances Chemical Weathering • As mechanical weathering progresses, it enhances chemical weathering by increasing the surface area to volume ratio.

19. Comparison of Surface Area to Volume Ratio • The volume of a cube 2 cm on a side is: • 2 x 2 x 2 = 8 cm3 • The surface area of one side is 2 x 2 = 4 cm • There are six sides, so the total surface area is • 4 cm x 6 = 24 cm2

20. Comparison of Surface Area to Volume Ratio • If the 2 cm cube is bisected by 3 joints how does this affect the surface area? • Each face of the 1 cm cube has a surface area of 1 cm x 1 cm = 1 cm2. There are six sides to each cube so the surface area would be 6 x 1 cm2 = 6 cm2. But there are a total of 8 little cubes so the total surface area is now • 6 cm2 x 8 = 48 cm2 • Compare this to the previous surface area of 24 cm2. The surface area has doubled!

21. Chemical Weathering • The most important chemical causing chemical weathering is H2O-the universal solvent. • However H2O in contact with CO2 gas in the atmosphere reacts to form carbonic acid. • H2O + CO2 = H2CO3 with a pH of about 5.7; a weak acid.

22. Dissolution Weathering • When water precipitates (rains) on halite it will dissolve. NaCl + H2O = Na+ + Cl- + H2O

23. Cave Formation (Karst Topography) • When carbonic acid infiltrates into limestone or marble rock joints it begins to dissolve the calcite. CaCO3 + H2CO3 = Ca+ + HCO3- + CO2 • Solid rock dissolves into calcium ions, bicarbonate ions and carbon dioxide gas. All dissolved in water which carries these products away leaving a cave. • Cave roofs collapse to form cenotes or sinkholes.

24. Cave • When.

25. Oregon Cave • When.

26. Hydrolysis of Granite 1 • The most abundant rock in earth’s crust is granite. The most abundant mineral in granite is feldspar. Feldspar hydrolyses to kaolinite clay + bicarbonate + soluble silica.

27. Hydrolysis of Granite 2 • Feldspar + Carbonic acid = Kaolinite clay + Potassium + Sodium + Calcium + Soluble Silica (SiO2) • Soluble silica is important because it becomes the silica cement that makes sand into a sandstone.

28. White Sandstone • sandstone.

29. Hydrolysis of Granite 3 • This weathering of feldspar in granite then releases the quartz grains to form sand that will become sandstone. The kaolinite clay becomes the clay mineral that makes shale. • Shale is the most abundant sedimentary rock.

30. Oxidation of Biotite • Biotite mica in granite reacts with carbonic acid to make kaolinite clay and releases iron that can become the hematite cement that binds sand grains together to form sandstone.

31. Red Sandstone • sandstone.

32. Factors that influence Weathering • Minerals present • Bowen’s Reaction Series • Available liquid water • Water is the most important weathering agent • Tropics • Antarctica • Temperature • As temperature rises rate of chemical reactions increases.

33. Bowen’s Reaction Series • Minerals present • Bowen’s Reaction Series • Available liquid water • Water is the most important weathering agent • Tropics • Antarctica • Temperature • As temperature rises rate of chemical reactions increases.

34. Liquid Water • Minerals present • Bowen’s Reaction Series • Available liquid water • Water is the most important weathering agent • Tropics • Antarctica • Temperature • As temperature rises rate of chemical reactions increases.

35. Cold and Dry Weathering • Minerals present • Bowen’s Reaction Series • Available liquid water • Water is the most important weathering agent • Tropics • Antarctica • Temperature • As temperature rises rate of chemical reactions increases.

36. Sedimentary Rock Pictures • Chemical sedimentary rock. • Gravel clasts.

37. Clastic Sedimentary Rocks • Clastic sediment, composed of rock and/or mineral fragments from pre-existing rocks. • A convenient method to classify clastic material is by its clast (particle) size.

38. Clastic Sedimentary Rock Classification

39. Sedimentary Environments - Shale • Shale • Most abundant sedimentary rock • Deposition in quiet, non-turbulent water. • Particles tend to form in thin lamellae. • Deep sea environment or far away from high energy. • Offshore and away from high-energy surf zone.

40. Shale Pictures

41. Shale Importance • The dark colors of shale usually are caused by the deposition of small amounts of incompletely decayed organic material. • Oil shale • Methane shale

42. Sedimentary Environments – Quartz Sandstone • Quartz Sandstone: Quartz is highly resistant to weathering so it is what is left over when all the other common minerals have weathered away. Mature. • Traveled the farthest. • Sand dunes. • Beach sand.

43. Sedimentary Environments – Arkose Sandstone • Arkose Sandstone: Contains roughly equal amounts of quartz and feldspar. Less mature than quartz sandstone and the presence of feldspar indicates deposition closer to source area (mountains). • Much of the sand size material in the upper portions of the Santa Ana River near the the San Gabriel and San Bernardino Mountains is akose.

44. Arkose Sandstone Picture

45. Sedimentary Environments – Graywacke Sandstone • Graywacke Sandstone: Usually dark colored rock composed of quartz, feldspar, rock fragments, mafic minerals and a matrix of fine clays and silts. Very immature. • Common as turbidity currents fromed from submarine landslides associated with the edge of the continental shelf or trenches.

46. Graywacke Sandstone Picture

47. Sandstone textures • Two important textures in sandstones. • Rounding: the degree of rounding determines the transportation distance with more rounding indicating a longer transport distance.

48. Sandstone Textures continued • Sorting: the range of clast sizes. Usually determined by the means of transportation. • Well sorted = most clasts are the same size. • Poorly sorted = wide range of particle sizes.

49. Sandstone Textures continued again • Three transporting agents. • Wind: very fine sorting. All sand grains nearly the same size. • Water: intermediate sorting. • Ice: very poor sorting. Clay size to boulders transported together.

50. Sedimentary Environments – Conglomerate • Conglomerates form in high energy environments. • By floods • Near mountains • Steep terrains • Usually poorly sorted. • Nine foot boulder in back yard. • Angular clasts indicate deposition very near source. Breccia.