Sediments and Sedimentary Rocks

1. Sediments and Sedimentary Rocks

2. Sedimentary rocks • Volume of upper crust: 5% • Area of continents: 75% • Record of geological events: e.g., Himalayas will someday be sediment

3. Sediment stages 1. Weathering 2. Erosion 3. Transportation via water, glaciers and wind 4. Deposition 5. Burial and compaction 6. Diagenesis Fig. 8.4

4. Diagenesis • Diagenesis: The physical and chemical changes • Heat • Pressure • Chemical reactions by which buried sediments are lithified and become sedimentary rocks.

5. Diagenetic Processes Sediments are buried compacted and lithified at shallow depths in the Earth’s crust…. …or subducted where they are subjected to high pressure and temperature Fig 8.11

6. Types of sedimentary rock • Clastic: Rock resulting from the consolidation of loose sediment that has been derived from previously existing rocks and accumulated in layers. Clastic particles: Physically transported rock fragments produced by the weathering of pre-existing rocks. Clastic sediments: Accumulations of clastic particles. • (b) Chemical: Rock formed by the precipitation of minerals from solution by either organic or inorganic processes.

7. Diagenetic Processes Compaction squeezes out the water Precipitation: addition of new minerals cements the sediment particles Compaction squeezes out the water Fig. 8.11

8. Types of sediments

9. Types of sediments

10. Clastic rocks Fig. 8.14

11. Relative Abundance of Sedimentary Rocks Fig. 8.13

12. Sedimentary Environments Fig. 8.4

13. Clastic sedimentary environment Fig. 8.4

14. Chemical and biochemicalsedimentary environments

15. Transport affects the type of sediment • Movement of sediment by • wind (e.g., dunes) • ice (e.g., glaciers) • or water (e.g., rivers, streams). • Mode of transport produces distinctive deposits. • Most sediments are carried by rivers (25 billion tons/year)

16. Transport affects the type of sediment • Strong currents: • Faster than 50cm/s • Carry gravel + finer material • Moderately strong currents: • 20-50cm/s • Carry sand • Weak currents • Slower than 20cm/s • Carry mud and silt • Faster flow = Carry larger particles

17. Sorting • Sorting: Measure of the variation in grain sizes in a clastic rock or sediment. • Well-sorted sediments indicate that they have been subjected to prolonged water or wind action. • Poorly-sorted sediments are either not far-removed from their source or are deposited by glaciers. Fig. 8.2

18. Roundness • Roundness: Measure of how rounded the corners are. • Angular grains = close to source • Rounded grains = transported for a great distance Fig. 8.3

19. Sedimentary structures • Bedding or stratification: Parallel layers of grains of different size or compositions. Bedding ranges from mm to m thick. Generally horizontal at the time of deposition (not cross-bedding). • What’s up: • Cross bedding: Sets of bedded material deposited at angles as large as 35o. • Graded bedding: Coarse grains at base and progressively finer grains towards the top. • Ripples: Small dunes of sand or silt whose long dimension is at right angles with the current. • Bioturbation: Remnants of burrows and tunnels by clams, worms, etc.

20. Cross bedding Grains are deposited on the lee side of the dune Fig. 8.6

21. Cross bedding Fig. 8.5

22. Ripples • Very small dunes. • Long axis is at right angle to the prevailing current. • Can be symmetric or asymmetric. Fig. 8.8

23. Ripples

24. Modern rippled sand Fig. 8.7

25. Ripple Marks (Ancient Rippled Sand) Fig. 8.7

26. Making ripples

27. Bioturbation Reworking of existing sediment by burrowing or tunneling organisms, like clams and worms. Fig. 8.9

28. Continental slope

29. Turbidity currents • Suspension of water, sand, and mud that moves downslope (often very rapidly) due to its greater density than that of the surrounding water. • Often triggered by earthquakes. • Speed of turbidity currents first appreciated in 1920 (breaking of phone lines on the Atlantic seafloor). Also gave indication of distance traveled by a single current.

30. Graded Bedding • Found in continental slope environments. • Beds can be cm to m. • Each layer has coarse grains at bottom, fine grains at top (due to decrease in current that deposited the grains).

31. River sediment Fig. 8.10

32. Classification of (bio)chemical sediments

33. Chemical and biochemical sedimentary rocks Fig. 8.17

34. Chemical and biochemical sedimentary rocks • Evaporites: Precipitations due to evaporating seawater. • arid climate • freshwater supply from rivers is low • connections to open sea are restricted • Silica sediment: chert • source of silica: diatoms or other silicate microorganisms • - diagenetic alteration • Iron oxide: • banded iron formation; iron ore • older formed when the atmosphere had less oxygen • ocean contained oxygen producing microorganism oxidizing Fe • Phosphorites • cold deep ocean water rising along the continental margin • deep ocean water rich in phosphorous

35. Carbonate platforms Fig. 8.16

36. Within the reef lagoon, growth of carbonate-secreting organisms, including forminifera, coral, algae and mollusks, 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.

37. Coral Reefs and Atolls Bora Bora atoll, South Pacifc

38. Coral Reefs and Atolls Process first described by Charles Darwin

39. Evaporites 1. During Miocene, the Mediterranean became a shallow basin 2. Reduced exchange with open sea 3. Evaporation removed water 4. Fresh water inflow was limited 5. Gypsum and halite crystallize, forming evaportites Fig. 8.19