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GL4 E1  Key Idea 2 b iii  SEDIMENTARY ROCKS  The mineralogy and texture of sedimentary rocks are controlled by processes of weathering, erosion and.

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Presentation on theme: "GL4 E1  Key Idea 2 b iii  SEDIMENTARY ROCKS  The mineralogy and texture of sedimentary rocks are controlled by processes of weathering, erosion and."— Presentation transcript:

1 GL4 E1  Key Idea 2 b iii  SEDIMENTARY ROCKS  The mineralogy and texture of sedimentary rocks are controlled by processes of weathering, erosion and deposition

2 GL4 E1 KI2 b (iii) Deposition  DEPOSITION, selectively concentrates products in particular environments – grain size related to energy of depositional environment; dominance of quartz and muscovite in coarse fraction and clay minerals in fine fraction; flocculation; precipitation; concentration of biogenic material in particular environments

3 Horizontal layers Credit: British Geological Survey Horizontal layers

4 What is deposition?  Transporting agents can only carry sediment when they have energy  When energy is reduced, some of load must be dropped (deposited)  Energy lost all of a sudden = load dumped, no sorting  Energy lost gradually = load gradually dumped, largest particles first then smaller (graded bedding)

5 Hjulstrom’s diagram

6 Grain size/energy current  Big link between these!  Hence the grain sizes found in a rock tell us the energy current which existed just before it was deposited

7 Depositional environments  River flowing into a lake  Precipitation from evaporation of sea water  Glacier melting  River flood plain  Beach  Estuary delta  Seabed  Desert

8 River deposition Profile along a river from source to mouthSource Mouth Particles deposited here are large and irregular and consist of a variety of lithologies, including the least resistant Particles deposited here are mid-sized and of intermediate sphericity, and include resistant and non-resistant lithologies Particles deposited here are small and nearly spherical, and consist mainly of the most resistant lithologies

9 The effects of siltation in rivers and streams Siltation is a leading pollution problem. Over the short term: silt can kill fish, destroy spawning beds, increase water turbidity (and reduce rates of photosynthesis). Over the long term: unchecked siltation can alter habitat and affect aquatic life. Credit: U.S. Geological Survey Average annual concentrations of suspended sediment, nitrogen, and phosphorus input into the reservoir system and output to the Chesapeake Bay.

10 Rivers  Flowing water (current flow depends on gradient)  Deposition in lower course of river  Meanders (outside = erosion; inside = deposition)  Levees and flood plain – “flood dumping”  Alluvial fans (change in slope  energy change)  Deltas (build out into still body of water – prograde)

11 River deposition features  Asymmetric ripple marks  Cross lamination  Point bar deposits  (Alignment due to river flow)  Gradual rounding of fragments downriver Credit: British Geological Survey

12 Desert  Aeolian (wind) transport  Small range of grain sizes transported (~ 0.3mm)  Frosted grains (wind blasted) Credit: U.S. Geological Survey

13 Desert deposition features  Large scale cross bedding  Red colour (oxidised iron matrix)  Flash floods bring in larger material  Lakes dried up quickly – evaporation features (eg. mud cracks) Credit: British Geological Survey

14 Land environment  Mainly erosional, not depositional  Swamp lands  Glacial deposition

15 Land depositional features  Glaciers – erratics, drumlins, till/boulder clay, outwash sands and gravels  Swamps – coal deposits Pleistocene glacial deposits, Thurstaston, Wirral

16 Sea/coasts  Storm beach – high energy sea (large fragments)  Wave action/long shore drift – rolling of particles – rounding  Lagoons – conditions more still, and may also be evaporation  Limestone reefs  Turbidity currents

17 Sea/coast deposition features  Graded bedding  Finest laminations far offshore, clay/shale deposition Credit: British Geological Survey

18 Other features  Sole marks/groove casts – pebbles/animals bouncing on a sediment surface, marks left infilled by sediment deposited on top  Scour marks/flute casts – erosion of surface by current, infilled  Flame structures/mud volcanoes – sand deposited on top of mud, sand weighs down and mud erupts up through sand layer

19 What else?  dominance of quartz and muscovite in coarse fraction and clay minerals in fine fraction;  Go back to Bowen’s reaction series for the reason why quartz is resistant/survives  Clay minerals from broken down feldspar

20 Other processes  flocculation; precipitation;

21 Flocculation  Fine particles begin to stick together (due to addition of a catalyst – e.g. when freshwater meets saltwater in an estuary)  Flocs are heavy so sink

22 Precipitation  Evaporates  Carbonates

23 Evaporates  Salt water  High temperatures, little rainfall  Calcium carbonate first to precipitate (least soluble)  In shallow lagoons the following can precipitate after limestone:  Gypsum CaSO 4.2H 2 O (must lose 80% of water for this to occur)  Then Halite (NaCl) (90% water gone)  Finally potassium and magnesium salts

24 Limestone  Calcium carbonate  Reef structures (calcium carbonate built by shelled creatures)  Lagoons behind reef collect fine calcium “muds”

25 Evaporates 2  300cm depth of seawater yields just 5cm of salts  Calculate how much water was evaporated to form the Stassfurt sequence in Germany which is 1000m thick

26 Right, the maths bit!!  1000m x 100cm = 100,000cm  100,000cm ÷ 5cm = 20,000 units  20,000 units each of 3m of water = 60,000m of sea water = 60km of sea depth Is this feasible????? How deep are our oceans?

27 Seas aren’t deep enough!  Refill/top-up theory – sea basin (like the Mediterranean) evaporates and then is refilled

28 Carbonate Compensation Depth  Carbonate dissolves when water is deeper than 4km

29 Concentration of biogenic material  Only in selected environments  Trees (coal) – swamps (and delta tops), rapid burial essential (to preserve material by excluding oxygen to stop biogenic material rotting) Credit: British Geological Survey

30  Shells – where current collects material (eg. only one valve from bivalves due to eating habits or a winnowing current) Credit: British Geological Survey


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