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Presentation on theme: "GL4 E1 Key Idea 2 b iii SEDIMENTARY ROCKS"— 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 Horizontal layers Credit: British Geological Survey

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

9 The effects of siltation in rivers and streams
Credit: U.S. Geological Survey 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. 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 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 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 CaSO4.2H2O (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
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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